1
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Murga-Moreno J, Casillas S, Barbadilla A, Uricchio L, Enard D. An efficient and robust ABC approach to infer the rate and strength of adaptation. G3 (BETHESDA, MD.) 2024; 14:jkae031. [PMID: 38365205 PMCID: PMC11090462 DOI: 10.1093/g3journal/jkae031] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/10/2023] [Revised: 10/10/2023] [Accepted: 01/29/2024] [Indexed: 02/18/2024]
Abstract
Inferring the effects of positive selection on genomes remains a critical step in characterizing the ultimate and proximate causes of adaptation across species, and quantifying positive selection remains a challenge due to the confounding effects of many other evolutionary processes. Robust and efficient approaches for adaptation inference could help characterize the rate and strength of adaptation in nonmodel species for which demographic history, mutational processes, and recombination patterns are not currently well-described. Here, we introduce an efficient and user-friendly extension of the McDonald-Kreitman test (ABC-MK) for quantifying long-term protein adaptation in specific lineages of interest. We characterize the performance of our approach with forward simulations and find that it is robust to many demographic perturbations and positive selection configurations, demonstrating its suitability for applications to nonmodel genomes. We apply ABC-MK to the human proteome and a set of known virus interacting proteins (VIPs) to test the long-term adaptation in genes interacting with viruses. We find substantially stronger signatures of positive selection on RNA-VIPs than DNA-VIPs, suggesting that RNA viruses may be an important driver of human adaptation over deep evolutionary time scales.
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Affiliation(s)
- Jesús Murga-Moreno
- Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85719, USA
| | - Sònia Casillas
- Department of Genetics and Microbiology, Universitat Autònoma de Barcelona, Bellaterra, Barcelona 08193, Spain
- Institute of Biotechnology and Biomedicine, Universitat Autònoma de Barcelona, Bellaterra, Barcelona 08193, Spain
| | - Antonio Barbadilla
- Department of Genetics and Microbiology, Universitat Autònoma de Barcelona, Bellaterra, Barcelona 08193, Spain
- Institute of Biotechnology and Biomedicine, Universitat Autònoma de Barcelona, Bellaterra, Barcelona 08193, Spain
| | | | - David Enard
- Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85719, USA
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2
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Ali F. Patterns of Change in Nucleotide Diversity Over Gene Length. Genome Biol Evol 2024; 16:evae078. [PMID: 38608148 DOI: 10.1093/gbe/evae078] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2023] [Revised: 03/26/2024] [Accepted: 04/03/2024] [Indexed: 04/14/2024] Open
Abstract
Nucleotide diversity at a site is influenced by the relative strengths of neutral and selective population genetic processes. Therefore, attempts to estimate Effective population size based on the diversity of synonymous sites demand a better understanding of their selective constraints. The nucleotide diversity of a gene was previously found to correlate with its length. In this work, I measure nucleotide diversity at synonymous sites and uncover a pattern of low diversity towards the translation initiation site of a gene. The degree of reduction in diversity at the translation initiation site and the length of this region of reduced diversity can be quantified as "Effect Size" and "Effect Length" respectively, using parameters of an asymptotic regression model. Estimates of Effect Length across bacteria covaried with recombination rates as well as with a multitude of translation-associated traits such as the avoidance of mRNA secondary structure around translation initiation site, the number of rRNAs, and relative codon usage of ribosomal genes. Evolutionary simulations under purifying selection reproduce the observed patterns and diversity-length correlation and highlight that selective constraints on the 5'-region of a gene may be more extensive than previously believed. These results have implications for the estimation of effective population size, and relative mutation rates, and for genome scans of genes under positive selection based on "silent-site" diversity.
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Affiliation(s)
- Farhan Ali
- Biodesign Center for Mechanisms of Evolution, Arizona State University, Tempe, AZ 85281, USA
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3
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Matheson J, Masel J. Background Selection From Unlinked Sites Causes Nonindependent Evolution of Deleterious Mutations. Genome Biol Evol 2024; 16:evae050. [PMID: 38482769 PMCID: PMC10972689 DOI: 10.1093/gbe/evae050] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 03/11/2024] [Indexed: 04/01/2024] Open
Abstract
Background selection describes the reduction in neutral diversity caused by selection against deleterious alleles at other loci. It is typically assumed that the purging of deleterious alleles affects linked neutral variants, and indeed simulations typically only treat a genomic window. However, background selection at unlinked loci also depresses neutral diversity. In agreement with previous analytical approximations, in our simulations of a human-like genome with a realistically high genome-wide deleterious mutation rate, the effects of unlinked background selection exceed those of linked background selection. Background selection reduces neutral genetic diversity by a factor that is independent of census population size. Outside of genic regions, the strength of background selection increases with the mean selection coefficient, contradicting the linked theory but in agreement with the unlinked theory. Neutral diversity within genic regions is fairly independent of the strength of selection. Deleterious genetic load among haploid individuals is underdispersed, indicating nonindependent evolution of deleterious mutations. Empirical evidence for underdispersion was previously interpreted as evidence for global epistasis, but we recover it from a non-epistatic model.
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Affiliation(s)
- Joseph Matheson
- Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA
- Department of Ecology, Behavior, and Evolution, University of California San Diego, San Diego, CA 92093, USA
| | - Joanna Masel
- Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA
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4
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Buffalo V, Kern AD. A quantitative genetic model of background selection in humans. PLoS Genet 2024; 20:e1011144. [PMID: 38507461 PMCID: PMC10984650 DOI: 10.1371/journal.pgen.1011144] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2023] [Revised: 04/01/2024] [Accepted: 01/19/2024] [Indexed: 03/22/2024] Open
Abstract
Across the human genome, there are large-scale fluctuations in genetic diversity caused by the indirect effects of selection. This "linked selection signal" reflects the impact of selection according to the physical placement of functional regions and recombination rates along chromosomes. Previous work has shown that purifying selection acting against the steady influx of new deleterious mutations at functional portions of the genome shapes patterns of genomic variation. To date, statistical efforts to estimate purifying selection parameters from linked selection models have relied on classic Background Selection theory, which is only applicable when new mutations are so deleterious that they cannot fix in the population. Here, we develop a statistical method based on a quantitative genetics view of linked selection, that models how polygenic additive fitness variance distributed along the genome increases the rate of stochastic allele frequency change. By jointly predicting the equilibrium fitness variance and substitution rate due to both strong and weakly deleterious mutations, we estimate the distribution of fitness effects (DFE) and mutation rate across three geographically distinct human samples. While our model can accommodate weaker selection, we find evidence of strong selection operating similarly across all human samples. Although our quantitative genetic model of linked selection fits better than previous models, substitution rates of the most constrained sites disagree with observed divergence levels. We find that a model incorporating selective interference better predicts observed divergence in conserved regions, but overall our results suggest uncertainty remains about the processes generating fitness variation in humans.
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Affiliation(s)
- Vince Buffalo
- Department of Integrative Biology, University of California, Berkeley, Berkeley, California, United States of America
- Institute of Ecology and Evolution and Department of Biology, University of Oregon, Eugene, Oregon, United States of America
| | - Andrew D. Kern
- Institute of Ecology and Evolution and Department of Biology, University of Oregon, Eugene, Oregon, United States of America
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5
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Cousins T, Tabin D, Patterson N, Reich D, Durvasula A. Accurate inference of population history in the presence of background selection. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.01.18.576291. [PMID: 38313273 PMCID: PMC10838404 DOI: 10.1101/2024.01.18.576291] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 02/06/2024]
Abstract
All published methods for learning about demographic history make the simplifying assumption that the genome evolves neutrally, and do not seek to account for the effects of natural selection on patterns of variation. This is a major concern, as ample work has demonstrated the pervasive effects of natural selection and in particular background selection (BGS) on patterns of genetic variation in diverse species. Simulations and theoretical work have shown that methods to infer changes in effective population size over time (Ne(t)) become increasingly inaccurate as the strength of linked selection increases. Here, we introduce an extension to the Pairwise Sequentially Markovian Coalescent (PSMC) algorithm, PSMC+, which explicitly co-models demographic history and natural selection. We benchmark our method using forward-in-time simulations with BGS and find that our approach improves the accuracy of effective population size inference. Leveraging a high resolution map of BGS in humans, we infer considerable changes in the magnitude of inferred effective population size relative to previous reports. Finally, we separately infer Ne(t) on the X chromosome and on the autosomes in diverse great apes without making a correction for selection, and find that the inferred ratio fluctuates substantially through time in a way that differs across species, showing that uncorrected selection may be an important driver of signals of genetic difference on the X chromosome and autosomes.
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Affiliation(s)
- Trevor Cousins
- Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, USA
| | - Daniel Tabin
- Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, USA
| | - Nick Patterson
- Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, USA
- Broad Institute of MIT and Harvard, Cambridge, MA USA
| | - David Reich
- Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, USA
- Broad Institute of MIT and Harvard, Cambridge, MA USA
- Department of Genetics, Harvard Medical School, Boston, MA, USA
- Howard Hughes Medical Institute, Boston, MA, USA
| | - Arun Durvasula
- Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, USA
- Broad Institute of MIT and Harvard, Cambridge, MA USA
- Department of Genetics, Harvard Medical School, Boston, MA, USA
- Department of Epidemiology, Harvard School of Public Health, Boston, MA, USA
- Center for Genetic Epidemiology, Department of Population and Public Health Sciences, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA
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6
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Schrider DR. Allelic gene conversion softens selective sweeps. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.12.05.570141. [PMID: 38106127 PMCID: PMC10723294 DOI: 10.1101/2023.12.05.570141] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/19/2023]
Abstract
The prominence of positive selection, in which beneficial mutations are favored by natural selection and rapidly increase in frequency, is a subject of intense debate. Positive selection can result in selective sweeps, in which the haplotype(s) bearing the adaptive allele "sweep" through the population, thereby removing much of the genetic diversity from the region surrounding the target of selection. Two models of selective sweeps have been proposed: classical sweeps, or "hard sweeps", in which a single copy of the adaptive allele sweeps to fixation, and "soft sweeps", in which multiple distinct copies of the adaptive allele leave descendants after the sweep. Soft sweeps can be the outcome of recurrent mutation to the adaptive allele, or the presence of standing genetic variation consisting of multiple copies of the adaptive allele prior to the onset of selection. Importantly, soft sweeps will be common when populations can rapidly adapt to novel selective pressures, either because of a high mutation rate or because adaptive alleles are already present. The prevalence of soft sweeps is especially controversial, and it has been noted that selection on standing variation or recurrent mutations may not always produce soft sweeps. Here, we show that the inverse is true: selection on single-origin de novo mutations may often result in an outcome that is indistinguishable from a soft sweep. This is made possible by allelic gene conversion, which "softens" hard sweeps by copying the adaptive allele onto multiple genetic backgrounds, a process we refer to as a "pseudo-soft" sweep. We carried out a simulation study examining the impact of gene conversion on sweeps from a single de novo variant in models of human, Drosophila, and Arabidopsis populations. The fraction of simulations in which gene conversion had produced multiple haplotypes with the adaptive allele upon fixation was appreciable. Indeed, under realistic demographic histories and gene conversion rates, even if selection always acts on a single-origin mutation, sweeps involving multiple haplotypes are more likely than hard sweeps in large populations, especially when selection is not extremely strong. Thus, even when the mutation rate is low or there is no standing variation, hard sweeps are expected to be the exception rather than the rule in large populations. These results also imply that the presence of signatures of soft sweeps does not necessarily mean that adaptation has been especially rapid or is not mutation limited.
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Affiliation(s)
- Daniel R Schrider
- Department of Genetics, University of North Carolina, Chapel Hill, NC 27599
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7
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Panigrahi M, Rajawat D, Nayak SS, Ghildiyal K, Sharma A, Jain K, Lei C, Bhushan B, Mishra BP, Dutt T. Landmarks in the history of selective sweeps. Anim Genet 2023; 54:667-688. [PMID: 37710403 DOI: 10.1111/age.13355] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2023] [Accepted: 08/28/2023] [Indexed: 09/16/2023]
Abstract
Half a century ago, a seminal article on the hitchhiking effect by Smith and Haigh inaugurated the concept of the selection signature. Selective sweeps are characterised by the rapid spread of an advantageous genetic variant through a population and hence play an important role in shaping evolution and research on genetic diversity. The process by which a beneficial allele arises and becomes fixed in a population, leading to a increase in the frequency of other linked alleles, is known as genetic hitchhiking or genetic draft. Kimura's neutral theory and hitchhiking theory are complementary, with Kimura's neutral evolution as the 'null model' and positive selection as the 'signal'. Both are widely accepted in evolution, especially with genomics enabling precise measurements. Significant advances in genomic technologies, such as next-generation sequencing, high-density SNP arrays and powerful bioinformatics tools, have made it possible to systematically investigate selection signatures in a variety of species. Although the history of selection signatures is relatively recent, progress has been made in the last two decades, owing to the increasing availability of large-scale genomic data and the development of computational methods. In this review, we embark on a journey through the history of research on selective sweeps, ranging from early theoretical work to recent empirical studies that utilise genomic data.
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Affiliation(s)
- Manjit Panigrahi
- Division of Animal Genetics, Indian Veterinary Research Institute, Bareilly, India
| | - Divya Rajawat
- Division of Animal Genetics, Indian Veterinary Research Institute, Bareilly, India
| | | | - Kanika Ghildiyal
- Division of Animal Genetics, Indian Veterinary Research Institute, Bareilly, India
| | - Anurodh Sharma
- Division of Animal Genetics, Indian Veterinary Research Institute, Bareilly, India
| | - Karan Jain
- Division of Animal Genetics, Indian Veterinary Research Institute, Bareilly, India
| | - Chuzhao Lei
- College of Animal Science and Technology, Northwest A&F University, Yangling, Shaanxi, China
| | - Bharat Bhushan
- Division of Animal Genetics, Indian Veterinary Research Institute, Bareilly, India
| | - Bishnu Prasad Mishra
- Division of Animal Biotechnology, ICAR-National Bureau of Animal Genetic Resources, Karnal, India
| | - Triveni Dutt
- Livestock Production and Management Section, Indian Veterinary Research Institute, Bareilly, India
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8
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Murga-Moreno J, Casillas S, Barbadilla A, Uricchio L, Enard D. An efficient and robust ABC approach to infer the rate and strength of adaptation. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.08.29.555322. [PMID: 37693550 PMCID: PMC10491248 DOI: 10.1101/2023.08.29.555322] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/12/2023]
Abstract
Inferring the effects of positive selection on genomes remains a critical step in characterizing the ultimate and proximate causes of adaptation across species, and quantifying positive selection remains a challenge due to the confounding effects of many other evolutionary processes. Robust and efficient approaches for adaptation inference could help characterize the rate and strength of adaptation in non-model species for which demographic history, mutational processes, and recombination patterns are not currently well-described. Here, we introduce an efficient and user-friendly extension of the McDonald-Kreitman test (ABC-MK) for quantifying long-term protein adaptation in specific lineages of interest. We characterize the performance of our approach with forward simulations and find that it is robust to many demographic perturbations and positive selection configurations, demonstrating its suitability for applications to non-model genomes. We apply ABC-MK to the human proteome and a set of known Virus Interacting Proteins (VIPs) to test the long-term adaptation in genes interacting with viruses. We find substantially stronger signatures of positive selection on RNA-VIPs than DNA-VIPs, suggesting that RNA viruses may be an important driver of human adaptation over deep evolutionary time scales.
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Affiliation(s)
- Jesús Murga-Moreno
- University of Arizona Department of Ecology and Evolutionary Biology, Tucson, USA
| | - Sònia Casillas
- Department of Genetics and Microbiology, Universitat Autònoma de Barcelona, Bellaterra, Barcelona 08193, Spain
- Institute of Biotechnology and Biomedicine, Universitat Autònoma de Barcelona, Bellaterra, Barcelona 08193, Spain
| | - Antonio Barbadilla
- Department of Genetics and Microbiology, Universitat Autònoma de Barcelona, Bellaterra, Barcelona 08193, Spain
- Institute of Biotechnology and Biomedicine, Universitat Autònoma de Barcelona, Bellaterra, Barcelona 08193, Spain
| | | | - David Enard
- University of Arizona Department of Ecology and Evolutionary Biology, Tucson, USA
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9
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Rougemont Q, Leroy T, Rondeau EB, Koop B, Bernatchez L. Allele surfing causes maladaptation in a Pacific salmon of conservation concern. PLoS Genet 2023; 19:e1010918. [PMID: 37683018 PMCID: PMC10545117 DOI: 10.1371/journal.pgen.1010918] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2022] [Revised: 10/02/2023] [Accepted: 08/11/2023] [Indexed: 09/10/2023] Open
Abstract
How various factors, including demography, recombination or genome duplication, may impact the efficacy of natural selection and the burden of deleterious mutations, is a central question in evolutionary biology and genetics. In this study, we show that key evolutionary processes, including variations in i) effective population size (Ne) ii) recombination rates and iii) chromosome inheritance, have influenced the genetic load and efficacy of selection in Coho salmon (Oncorhynchus kisutch), a widely distributed salmonid species on the west coast of North America. Using whole genome resequencing data from 14 populations at different migratory distances from their southern glacial refugium, we found evidence supporting gene surfing, wherein reduced Ne at the postglacial recolonization front, leads to a decrease in the efficacy of selection and a surf of deleterious alleles in the northernmost populations. Furthermore, our results indicate that recombination rates play a prime role in shaping the load along the genome. Additionally, we identified variation in polyploidy as a contributing factor to within-genome variation of the load. Overall, our results align remarkably well with expectations under the nearly neutral theory of molecular evolution. We discuss the fundamental and applied implications of these findings for evolutionary and conservation genomics.
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Affiliation(s)
- Quentin Rougemont
- Centre d’Ecologie Fonctionnelle et Evolutive, Univ Montpellier, CNRS, EPHE, IRD, Montpellier, France
| | - Thibault Leroy
- GenPhySE, INRAE, INP, ENVT, Université de Toulouse, Auzeville- Tolosane, France
| | - Eric B. Rondeau
- Department of Fisheries and Ocean, Pacific Biological Station, Nanaimo, Canada
| | - Ben Koop
- Department of Biology, University of Victoria, Victoria, Canada
| | - Louis Bernatchez
- Département de Biologie, Institut de Biologie Intégrative et des Systèmes (IBIS), Université Laval, Québec, Canada
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10
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Teterina AA, Willis JH, Lukac M, Jovelin R, Cutter AD, Phillips PC. Genomic diversity landscapes in outcrossing and selfing Caenorhabditis nematodes. PLoS Genet 2023; 19:e1010879. [PMID: 37585484 PMCID: PMC10461856 DOI: 10.1371/journal.pgen.1010879] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2022] [Revised: 08/28/2023] [Accepted: 07/21/2023] [Indexed: 08/18/2023] Open
Abstract
Caenorhabditis nematodes form an excellent model for studying how the mode of reproduction affects genetic diversity, as some species reproduce via outcrossing whereas others can self-fertilize. Currently, chromosome-level patterns of diversity and recombination are only available for self-reproducing Caenorhabditis, making the generality of genomic patterns across the genus unclear given the profound potential influence of reproductive mode. Here we present a whole-genome diversity landscape, coupled with a new genetic map, for the outcrossing nematode C. remanei. We demonstrate that the genomic distribution of recombination in C. remanei, like the model nematode C. elegans, shows high recombination rates on chromosome arms and low rates toward the central regions. Patterns of genetic variation across the genome are also similar between these species, but differ dramatically in scale, being tenfold greater for C. remanei. Historical reconstructions of variation in effective population size over the past million generations echo this difference in polymorphism. Evolutionary simulations demonstrate how selection, recombination, mutation, and selfing shape variation along the genome, and that multiple drivers can produce patterns similar to those observed in natural populations. The results illustrate how genome organization and selection play a crucial role in shaping the genomic pattern of diversity whereas demographic processes scale the level of diversity across the genome as a whole.
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Affiliation(s)
- Anastasia A. Teterina
- Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon, United States of America
- Center of Parasitology, Severtsov Institute of Ecology and Evolution RAS, Moscow, Russia
| | - John H. Willis
- Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon, United States of America
| | - Matt Lukac
- Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon, United States of America
| | - Richard Jovelin
- Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada
| | - Asher D. Cutter
- Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada
| | - Patrick C. Phillips
- Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon, United States of America
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11
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Ali F. Patterns of change in nucleotide diversity over gene length. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.07.13.548940. [PMID: 37503020 PMCID: PMC10369989 DOI: 10.1101/2023.07.13.548940] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 07/29/2023]
Abstract
Nucleotide diversity at a site is influenced by the relative strengths of neutral and selective population genetic processes. Therefore, attempts to identify sites under positive selection require an understanding of the expected diversity in its absence. The nucleotide diversity of a gene was previously found to correlate with its length. In this work, I measure nucleotide diversity at synonymous sites and uncover a pattern of low diversity towards the translation initiation site (TIS) of a gene. The degree of reduction in diversity at the TIS and the length of this region of reduced diversity can be quantified as "Effect Size" and "Effect Length" respectively, using parameters of an asymptotic regression model. Estimates of Effect Length across bacteria covaried with recombination rates as well as with a multitude of fast-growth adaptations such as the avoidance of mRNA secondary structure around TIS, the number of rRNAs, and relative codon usage of ribosomal genes. Thus, the dependence of nucleotide diversity on gene length is governed by a combination of selective and non-selective processes. These results have implications for the estimation of effective population size and relative mutation rates based on "silent-site" diversity, and for pN/pS-based prediction of genes under selection.
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Affiliation(s)
- Farhan Ali
- Biodesign Institute, Arizona State University, Tempe, Arizona
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12
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Booker WW, Ray DD, Schrider DR. This population does not exist: learning the distribution of evolutionary histories with generative adversarial networks. Genetics 2023; 224:iyad063. [PMID: 37067864 PMCID: PMC10213497 DOI: 10.1093/genetics/iyad063] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2023] [Revised: 02/23/2023] [Accepted: 04/05/2023] [Indexed: 04/18/2023] Open
Abstract
Numerous studies over the last decade have demonstrated the utility of machine learning methods when applied to population genetic tasks. More recent studies show the potential of deep-learning methods in particular, which allow researchers to approach problems without making prior assumptions about how the data should be summarized or manipulated, instead learning their own internal representation of the data in an attempt to maximize inferential accuracy. One type of deep neural network, called Generative Adversarial Networks (GANs), can even be used to generate new data, and this approach has been used to create individual artificial human genomes free from privacy concerns. In this study, we further explore the application of GANs in population genetics by designing and training a network to learn the statistical distribution of population genetic alignments (i.e. data sets consisting of sequences from an entire population sample) under several diverse evolutionary histories-the first GAN capable of performing this task. After testing multiple different neural network architectures, we report the results of a fully differentiable Deep-Convolutional Wasserstein GAN with gradient penalty that is capable of generating artificial examples of population genetic alignments that successfully mimic key aspects of the training data, including the site-frequency spectrum, differentiation between populations, and patterns of linkage disequilibrium. We demonstrate consistent training success across various evolutionary models, including models of panmictic and subdivided populations, populations at equilibrium and experiencing changes in size, and populations experiencing either no selection or positive selection of various strengths, all without the need for extensive hyperparameter tuning. Overall, our findings highlight the ability of GANs to learn and mimic population genetic data and suggest future areas where this work can be applied in population genetics research that we discuss herein.
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Affiliation(s)
- William W Booker
- Department of Genetics, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27514-2916, USA
| | - Dylan D Ray
- Department of Genetics, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27514-2916, USA
| | - Daniel R Schrider
- Department of Genetics, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27514-2916, USA
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13
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Yuan S, Shi Y, Zhou BF, Liang YY, Chen XY, An QQ, Fan YR, Shen Z, Ingvarsson PK, Wang B. Genomic vulnerability to climate change in Quercus acutissima, a dominant tree species in East Asian deciduous forests. Mol Ecol 2023; 32:1639-1655. [PMID: 36626136 DOI: 10.1111/mec.16843] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2022] [Revised: 12/30/2022] [Accepted: 01/05/2023] [Indexed: 01/11/2023]
Abstract
Understanding the evolutionary processes that shape the landscape of genetic variation and influence the response of species to future climate change is critical for biodiversity conservation. Here, we sampled 27 populations across the distribution range of a dominant forest tree, Quercus acutissima, in East Asia, and applied genome-wide analyses to track the evolutionary history and predict the fate of populations under future climate. We found two genetic groups (East and West) in Q. acutissima that diverged during Pliocene. We also found a heterogeneous landscape of genomic variation in this species, which may have been shaped by population demography and linked selections. Using genotype-environment association analyses, we identified climate-associated SNPs in a diverse set of genes and functional categories, indicating a model of polygenic adaptation in Q. acutissima. We further estimated three genetic offset metrics to quantify genomic vulnerability of this species to climate change due to the complex interplay between local adaptation and migration. We found that marginal populations are under higher risk of local extinction because of future climate change, and may not be able to track suitable habitats to maintain the gene-environment relationships observed under the current climate. We also detected higher reverse genetic offsets in northern China, indicating that genetic variation currently present in the whole range of Q. acutissima may not adapt to future climate conditions in this area. Overall, this study illustrates how evolutionary processes have shaped the landscape of genomic variation, and provides a comprehensive genome-wide view of climate maladaptation in Q. acutissima.
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Affiliation(s)
- Shuai Yuan
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.,Guangdong Provincial Key Laboratory of Applied Botany, Guangzhou, China.,South China National Botanical Garden, Guangzhou, China
| | - Yong Shi
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.,Guangdong Provincial Key Laboratory of Applied Botany, Guangzhou, China.,South China National Botanical Garden, Guangzhou, China
| | - Biao-Feng Zhou
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.,Guangdong Provincial Key Laboratory of Applied Botany, Guangzhou, China.,South China National Botanical Garden, Guangzhou, China
| | - Yi-Ye Liang
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.,Guangdong Provincial Key Laboratory of Applied Botany, Guangzhou, China.,South China National Botanical Garden, Guangzhou, China
| | - Xue-Yan Chen
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.,Guangdong Provincial Key Laboratory of Applied Botany, Guangzhou, China.,South China National Botanical Garden, Guangzhou, China
| | - Qing-Qing An
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.,Guangdong Provincial Key Laboratory of Applied Botany, Guangzhou, China.,South China National Botanical Garden, Guangzhou, China
| | - Yan-Ru Fan
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.,Guangdong Provincial Key Laboratory of Applied Botany, Guangzhou, China.,South China National Botanical Garden, Guangzhou, China
| | - Zhao Shen
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.,Guangdong Provincial Key Laboratory of Applied Botany, Guangzhou, China.,South China National Botanical Garden, Guangzhou, China
| | - Pär K Ingvarsson
- Department of Plant Biology, Linnean Center for Plant Biology, Uppsala BioCenter, Swedish University of Agricultural Sciences, Uppsala, Sweden
| | - Baosheng Wang
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.,Guangdong Provincial Key Laboratory of Applied Botany, Guangzhou, China.,South China National Botanical Garden, Guangzhou, China
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14
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Joyce LR, Youngblom MA, Cormaty H, Gartstein E, Barber KE, Akins RL, Pepperell CS, Palmer KL. Comparative Genomics of Streptococcus oralis Identifies Large Scale Homologous Recombination and a Genetic Variant Associated with Infection. mSphere 2022; 7:e0050922. [PMID: 36321824 PMCID: PMC9769543 DOI: 10.1128/msphere.00509-22] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2022] [Accepted: 10/17/2022] [Indexed: 11/07/2022] Open
Abstract
The viridans group streptococci (VGS) are a large consortium of commensal streptococci that colonize the human body. Many species within this group are opportunistic pathogens causing bacteremia and infective endocarditis (IE), yet little is known about why some strains cause invasive disease. Identification of virulence determinants is complicated by the difficulty of distinguishing between the closely related species of this group. Here, we analyzed genomic data from VGS that were isolated from blood cultures in patients with invasive infections and oral swabs of healthy volunteers and then determined the best-performing methods for species identification. Using whole-genome sequence data, we characterized the population structure of a diverse sample of Streptococcus oralis isolates and found evidence of frequent recombination. We used multiple genome-wide association study tools to identify candidate determinants of invasiveness. These tools gave consistent results, leading to the discovery of a single synonymous single nucleotide polymorphism (SNP) that was significantly associated with invasiveness. This SNP was within a previously undescribed gene that was conserved across the majority of VGS species. Using the growth in the presence of human serum and a simulated infective endocarditis vegetation model, we were unable to identify a phenotype for the enriched allele in laboratory assays, suggesting a phenotype may be specific to natural infection. These data highlighted the power of analyzing natural populations for gaining insight into pathogenicity, particularly for organisms with complex population structures like the VGS. IMPORTANCE The viridians group streptococci (VGS) are a large collection of closely related commensal streptococci, with many being opportunistic pathogens causing invasive diseases, such as bacteremia and infective endocarditis. Little is known about virulence determinants in these species, and there is a distinct lack of genomic information available for the VGS. In this study, we collected VGS isolates from invasive infections and healthy volunteers and performed whole-genome sequencing for a suite of downstream analyses. We focused on a diverse sample of Streptococcus oralis genomes and identified high rates of recombination in the population as well as a single genome variant highly enriched in invasive isolates. The variant lies within a previously uncharacterized gene, nrdM, which shared homology with the anaerobic ribonucleoside triphosphate reductase, nrdD, and was highly conserved among VGS. This work increased our knowledge of VGS genomics and indicated that differences in virulence potential among S. oralis isolates were, at least in part, genetically determined.
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Affiliation(s)
- Luke R. Joyce
- Department of Biological Sciences, The University of Texas at Dallas, Richardson, Texas, USA
| | - Madison A. Youngblom
- Microbiology Doctoral Training Program, University of Wisconsin-Madison, Madison, Wisconsin, USA
- Department of Medical Microbiology and Immunology, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin, USA
| | - Harshini Cormaty
- Department of Biological Sciences, The University of Texas at Dallas, Richardson, Texas, USA
| | - Evelyn Gartstein
- Department of Biological Sciences, The University of Texas at Dallas, Richardson, Texas, USA
| | - Katie E. Barber
- Department of Pharmacy Practice, University of Mississippi School of Pharmacy, University of Mississippi, Jackson, Mississippi, USA
| | | | - Caitlin S. Pepperell
- Department of Medical Microbiology and Immunology, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin, USA
- Department of Medicine (Infectious Diseases), School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin, USA
| | - Kelli L. Palmer
- Department of Biological Sciences, The University of Texas at Dallas, Richardson, Texas, USA
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15
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Abstract
We discuss the genetic, demographic, and selective forces that are likely to be at play in restricting observed levels of DNA sequence variation in natural populations to a much smaller range of values than would be expected from the distribution of census population sizes alone-Lewontin's Paradox. While several processes that have previously been strongly emphasized must be involved, including the effects of direct selection and genetic hitchhiking, it seems unlikely that they are sufficient to explain this observation without contributions from other factors. We highlight a potentially important role for the less-appreciated contribution of population size change; specifically, the likelihood that many species and populations may be quite far from reaching the relatively high equilibrium diversity values that would be expected given their current census sizes.
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Affiliation(s)
- Brian Charlesworth
- Institute of Ecology and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom
| | - Jeffrey D Jensen
- School of Life Sciences, Arizona State University, Tempe, AZ, USA
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16
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Booker TR, Payseur BA, Tigano A. Background selection under evolving recombination rates. Proc Biol Sci 2022; 289:20220782. [PMID: 35730151 PMCID: PMC9233929 DOI: 10.1098/rspb.2022.0782] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
Background selection (BGS), the effect that purifying selection exerts on sites linked to deleterious alleles, is expected to be ubiquitous across eukaryotic genomes. The effects of BGS reflect the interplay of the rates and fitness effects of deleterious mutations with recombination. A fundamental assumption of BGS models is that recombination rates are invariant over time. However, in some lineages, recombination rates evolve rapidly, violating this central assumption. Here, we investigate how recombination rate evolution affects genetic variation under BGS. We show that recombination rate evolution modifies the effects of BGS in a manner similar to a localized change in the effective population size, potentially leading to underestimation or overestimation of the genome-wide effects of selection. Furthermore, we find evidence that recombination rate evolution in the ancestors of modern house mice may have impacted inferences of the genome-wide effects of selection in that species.
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Affiliation(s)
- Tom R. Booker
- Department of Zoology, University of British Columbia, Vancouver Campus, Vancouver, BC, Canada
| | - Bret A. Payseur
- Laboratory of Genetics, University of Wisconsin - Madison, Madison, WI, USA
| | - Anna Tigano
- Department of Biology, University of British Columbia, Okanagan Campus, Kelowna, BC, Canada
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17
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Harwood MP, Alves I, Edgington H, Agbessi M, Bruat V, Soave D, Lamaze FC, Favé MJ, Awadalla P. Recombination affects allele-specific expression of deleterious variants in human populations. SCIENCE ADVANCES 2022; 8:eabl3819. [PMID: 35559670 PMCID: PMC9106294 DOI: 10.1126/sciadv.abl3819] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/09/2021] [Accepted: 03/29/2022] [Indexed: 06/15/2023]
Abstract
How the genetic composition of a population changes through stochastic processes, such as genetic drift, in combination with deterministic processes, such as selection, is critical to understanding how phenotypes vary in space and time. Here, we show how evolutionary forces affecting selection, including recombination and effective population size, drive genomic patterns of allele-specific expression (ASE). Integrating tissue-specific genotypic and transcriptomic data from 1500 individuals from two different cohorts, we demonstrate that ASE is less often observed in regions of low recombination, and loci in high or normal recombination regions are more efficient at using ASE to underexpress harmful mutations. By tracking genetic ancestry, we discriminate between ASE variability due to past demographic effects, including subsequent bottlenecks, versus local environment. We observe that ASE is not randomly distributed along the genome and that population parameters influencing the efficacy of natural selection alter ASE levels genome wide.
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Affiliation(s)
- Michelle P. Harwood
- Ontario Institute for Cancer Research, Toronto, ON, Canada
- Department of Molecular Genetics, University of Toronto, Toronto, ON, Canada
| | - Isabel Alves
- Université de Nantes, CHU Nantes, CNRS, INSERM, L’Institut du thorax, F-44000 Nantes, France
| | | | | | - Vanessa Bruat
- Ontario Institute for Cancer Research, Toronto, ON, Canada
| | - David Soave
- Ontario Institute for Cancer Research, Toronto, ON, Canada
- Department of Mathematics, Wilfrid Laurier University, Waterloo, ON, Canada
| | - Fabien C. Lamaze
- Ontario Institute for Cancer Research, Toronto, ON, Canada
- Institut universitaire de cardiologie et de pneumologie de Québec, Université Laval, Québec, QC, Canada
| | | | - Philip Awadalla
- Ontario Institute for Cancer Research, Toronto, ON, Canada
- Department of Molecular Genetics, University of Toronto, Toronto, ON, Canada
- Dalla Lana School of Public Health, University of Toronto, Toronto, ON, Canada
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18
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Xu K. The genetic basis of selfing rate evolution. Evolution 2022; 76:883-898. [PMID: 35395695 DOI: 10.1111/evo.14480] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2021] [Revised: 02/02/2022] [Accepted: 02/28/2022] [Indexed: 01/21/2023]
Abstract
Evolution of selfing is common in plant populations, but the genetic basis of selfing rate evolution remains unclear. Although the effects of genetic properties on fixation for mating-unrelated alleles have been investigated, loci that modify the selfing rate (selfing modifiers) differ from mating-unrelated loci in several aspects. Using population genetic models, I investigate the genetic basis of selfing rate evolution. For mating-unrelated alleles, selfing promotes fixation only for recessive mutations, but for selfing modifiers, because the selection coefficient depends on the background selfing rate, selfing can promote fixation even for dominant modifiers. For mating-unrelated alleles, the fixation probability from standing variation is independent of dominance and decreases with an increased background selfing rate. However, for selfing modifiers, the fixation probability peaks at an intermediate selfing rate and when alleles are recessive, because a change of its selection coefficient necessarily involves a change of the inbreeding coefficient, because both depend on the level of inbreeding depression. Furthermore, evolution of selfing involving multiple modifier loci is more likely when selfing is controlled by few large-effect rather than many slight-effect modifiers. I discuss how these characteristics of selfing modifiers have implications for the unidirectional transition from outcrossing to selfing and other empirical patterns.
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Affiliation(s)
- Kuangyi Xu
- Department of Biology, University of North Carolina at Chapel Hill, Coker Hall, 120 South Road, Chapel Hill, North Carolina, 27599, United States
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19
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DeGiorgio M, Szpiech ZA. A spatially aware likelihood test to detect sweeps from haplotype distributions. PLoS Genet 2022; 18:e1010134. [PMID: 35404934 PMCID: PMC9022890 DOI: 10.1371/journal.pgen.1010134] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2021] [Revised: 04/21/2022] [Accepted: 03/04/2022] [Indexed: 01/13/2023] Open
Abstract
The inference of positive selection in genomes is a problem of great interest in evolutionary genomics. By identifying putative regions of the genome that contain adaptive mutations, we are able to learn about the biology of organisms and their evolutionary history. Here we introduce a composite likelihood method that identifies recently completed or ongoing positive selection by searching for extreme distortions in the spatial distribution of the haplotype frequency spectrum along the genome relative to the genome-wide expectation taken as neutrality. Furthermore, the method simultaneously infers two parameters of the sweep: the number of sweeping haplotypes and the “width” of the sweep, which is related to the strength and timing of selection. We demonstrate that this method outperforms the leading haplotype-based selection statistics, though strong signals in low-recombination regions merit extra scrutiny. As a positive control, we apply it to two well-studied human populations from the 1000 Genomes Project and examine haplotype frequency spectrum patterns at the LCT and MHC loci. We also apply it to a data set of brown rats sampled in NYC and identify genes related to olfactory perception. To facilitate use of this method, we have implemented it in user-friendly open source software. Identifying regions of the genome that contain adaptive variation is of fundamental interest in evolutionary biology, providing insight into an organism’s history and biology. When positive selection is recent or ongoing, we expect to find genomic patterns such as high frequency haplotypes and low genetic diversity in the vicinity of the adaptive locus. Here we develop a statistic to identify these regions based on distortions of the haplotype frequency spectrum from a background distribution. We evaluate the performance of this statistic under numerous realistic settings of interest to empiricists and demonstrate its superior performance relative to other haplotype-based selection statistics. We also apply this statistic to real population-genetic data. As a positive control, we explore two well-studied loci, LCT and MHC, in a European and an African human population that show strong evidence for selection. We also apply this statistic to the genomes of an urban brown rat population, where we uncover evidence for adaptation in olfactory perception genes. We release user-friendly software implementing this statistic.
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Affiliation(s)
- Michael DeGiorgio
- Department of Electrical Engineering and Computer Science, Florida Atlantic University, Boca Raton, Florida, United States of America
- * E-mail: (MD); (ZAS)
| | - Zachary A. Szpiech
- Department of Biology, Pennsylvania State University, University Park, Pennsylvania, United States of America
- Institute for Computational and Data Sciences, Pennsylvania State University, University Park, Pennsylvania, United States of America
- * E-mail: (MD); (ZAS)
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20
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Pettie N, Llopart A, Comeron JM. Meiotic, genomic and evolutionary properties of crossover distribution in Drosophila yakuba. PLoS Genet 2022; 18:e1010087. [PMID: 35320272 PMCID: PMC8979470 DOI: 10.1371/journal.pgen.1010087] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2021] [Revised: 04/04/2022] [Accepted: 02/09/2022] [Indexed: 12/14/2022] Open
Abstract
The number and location of crossovers across genomes are highly regulated during meiosis, yet the key components controlling them are fast evolving, hindering our understanding of the mechanistic causes and evolutionary consequences of changes in crossover rates. Drosophila melanogaster has been a model species to study meiosis for more than a century, with an available high-resolution crossover map that is, nonetheless, missing for closely related species, thus preventing evolutionary context. Here, we applied a novel and highly efficient approach to generate whole-genome high-resolution crossover maps in D. yakuba to tackle multiple questions that benefit from being addressed collectively within an appropriate phylogenetic framework, in our case the D. melanogaster species subgroup. The genotyping of more than 1,600 individual meiotic events allowed us to identify several key distinct properties relative to D. melanogaster. We show that D. yakuba, in addition to higher crossover rates than D. melanogaster, has a stronger centromere effect and crossover assurance than any Drosophila species analyzed to date. We also report the presence of an active crossover-associated meiotic drive mechanism for the X chromosome that results in the preferential inclusion in oocytes of chromatids with crossovers. Our evolutionary and genomic analyses suggest that the genome-wide landscape of crossover rates in D. yakuba has been fairly stable and captures a significant signal of the ancestral crossover landscape for the whole D. melanogaster subgroup, even informative for the D. melanogaster lineage. Contemporary crossover rates in D. melanogaster, on the other hand, do not recapitulate ancestral crossovers landscapes. As a result, the temporal stability of crossover landscapes observed in D. yakuba makes this species an ideal system for applying population genetic models of selection and linkage, given that these models assume temporal constancy in linkage effects. Our studies emphasize the importance of generating multiple high-resolution crossover rate maps within a coherent phylogenetic context to broaden our understanding of crossover control during meiosis and to improve studies on the evolutionary consequences of variable crossover rates across genomes and time.
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Affiliation(s)
- Nikale Pettie
- Interdisciplinary Program in Genetics, University of Iowa, Iowa City, Iowa, United States of America
| | - Ana Llopart
- Interdisciplinary Program in Genetics, University of Iowa, Iowa City, Iowa, United States of America
- Department of Biology, University of Iowa, Iowa City, Iowa, United States of America
| | - Josep M. Comeron
- Interdisciplinary Program in Genetics, University of Iowa, Iowa City, Iowa, United States of America
- Department of Biology, University of Iowa, Iowa City, Iowa, United States of America
- * E-mail:
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21
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Charlesworth B. The effects of weak selection on neutral diversity at linked sites. Genetics 2022; 221:6527636. [PMID: 35150278 PMCID: PMC9071562 DOI: 10.1093/genetics/iyac027] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2021] [Accepted: 02/04/2022] [Indexed: 11/15/2022] Open
Abstract
The effects of selection on variability at linked sites have an important influence on levels and patterns of within-population variation across the genome. Most theoretical models of these effects have assumed that selection is sufficiently strong that allele frequency changes at the loci concerned are largely deterministic. These models have led to the conclusion that directional selection for selectively favorable mutations, or against recurrent deleterious mutations, reduces nucleotide site diversity at linked neutral sites. Recent work has shown, however, that fixations of weakly selected mutations, accompanied by significant stochastic changes in allele frequencies, can sometimes cause higher diversity at linked sites when compared with the effects of fixations of neutral mutations. This study extends this work by deriving approximate expressions for the mean conditional times to fixation and loss of mutations subject to selection, and analyzing the conditions under which selection increases rather than reduces these times. Simulations are used to examine the relations between diversity at a neutral site and the fixation and loss times of mutations at a linked site that is subject to selection. It is shown that the long-term level of neutral diversity can be increased over the purely neutral value by recurrent fixations and losses of linked, weakly selected dominant or partially dominant favorable mutations, or linked recessive or partially recessive deleterious mutations. The results are used to examine the conditions under which associative overdominance, as opposed to background selection, is likely to operate.
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Affiliation(s)
- Brian Charlesworth
- Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3FL, United Kingdom
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22
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Friedlander E, Steinrücken M. A numerical framework for genetic hitchhiking in populations of variable size. Genetics 2022; 220:6526396. [PMID: 35143667 PMCID: PMC8893261 DOI: 10.1093/genetics/iyac012] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2021] [Accepted: 12/27/2021] [Indexed: 11/13/2022] Open
Abstract
Natural selection on beneficial or deleterious alleles results in an increase or decrease, respectively, of their frequency within the population. Due to chromosomal linkage, the dynamics of the selected site affect the genetic variation at nearby neutral loci in a process commonly referred to as genetic hitchhiking. Changes in population size, however, can yield patterns in genomic data that mimic the effects of selection. Accurately modeling these dynamics is thus crucial to understanding how selection and past population size changes impact observed patterns of genetic variation. Here, we model the evolution of haplotype frequencies with the Wright-Fisher diffusion to study the impact of selection on linked neutral variation. Explicit solutions are not known for the dynamics of this diffusion when selection and recombination act simultaneously. Thus, we present a method for numerically evaluating the Wright-Fisher diffusion dynamics of 2 linked loci separated by a certain recombination distance when selection is acting. We can account for arbitrary population size histories explicitly using this approach. A key step in the method is to express the moments of the associated transition density, or sampling probabilities, as solutions to ordinary differential equations. Numerically solving these differential equations relies on a novel accurate and numerically efficient technique to estimate higher order moments from lower order moments. We demonstrate how this numerical framework can be used to quantify the reduction and recovery of genetic diversity around a selected locus over time and elucidate distortions in the site-frequency-spectra of neutral variation linked to loci under selection in various demographic settings. The method can be readily extended to more general modes of selection and applied in likelihood frameworks to detect loci under selection and infer the strength of the selective pressure.
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Affiliation(s)
- Eric Friedlander
- Department of Ecology and Evolution, University of Chicago, Chicago, IL 60637, USA,Department of Mathematics, Saint Norbert College, Green Bay, WI 54115, USA
| | - Matthias Steinrücken
- Department of Ecology and Evolution, University of Chicago, Chicago, IL 60637, USA,Department of Human Genetics, University of Chicago, Chicago, IL 60637, USA,Corresponding author: Department of Ecology & Evolution, The University of Chicago, 1101 E. 57th Street, Chicago, IL 60637, USA.
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23
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Novo I, Santiago E, Caballero A. The estimates of effective population size based on linkage disequilibrium are virtually unaffected by natural selection. PLoS Genet 2022; 18:e1009764. [PMID: 35077457 PMCID: PMC8815936 DOI: 10.1371/journal.pgen.1009764] [Citation(s) in RCA: 12] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2021] [Revised: 02/04/2022] [Accepted: 12/21/2021] [Indexed: 11/19/2022] Open
Abstract
The effective population size (Ne) is a key parameter to quantify the magnitude of genetic drift and inbreeding, with important implications in human evolution. The increasing availability of high-density genetic markers allows the estimation of historical changes in Ne across time using measures of genome diversity or linkage disequilibrium between markers. Directional selection is expected to reduce diversity and Ne, and this reduction is modulated by the heterogeneity of the genome in terms of recombination rate. Here we investigate by computer simulations the consequences of selection (both positive and negative) and recombination rate heterogeneity in the estimation of historical Ne. We also investigate the relationship between diversity parameters and Ne across the different regions of the genome using human marker data. We show that the estimates of historical Ne obtained from linkage disequilibrium between markers (NeLD) are virtually unaffected by selection. In contrast, those estimates obtained by coalescence mutation-recombination-based methods can be strongly affected by it, which could have important consequences for the estimation of human demography. The simulation results are supported by the analysis of human data. The estimates of NeLD obtained for particular genomic regions do not correlate, or they do it very weakly, with recombination rate, nucleotide diversity, proportion of polymorphic sites, background selection statistic, minor allele frequency of SNPs, loss of function and missense variants and gene density. This suggests that NeLD measures mainly reflect demographic changes in population size across generations. The inference of the demographic history of populations is of great relevance in evolutionary biology. This inference can be made from genomic data using coalescence methods or linkage disequilibrium methods. However, the assessment of these methods is usually made assuming neutrality (absence of selection). Here we show by computer simulations and analyses of human data that the estimates of historical effective population size obtained from linkage disequilibrium between markers are virtually unaffected by natural selection, either positive or negative. In contrast, estimates obtained by coalescence mutation-recombination-based methods can be strongly affected by it, which could have important consequences for recent estimations of human demography.
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Affiliation(s)
- Irene Novo
- Centro de Investigación Mariña, Universidade de Vigo, Facultade de Bioloxía, Vigo, Spain
- * E-mail:
| | - Enrique Santiago
- Departamento de Biología Funcional, Facultad de Biología, Universidad de Oviedo, Oviedo, Spain
| | - Armando Caballero
- Centro de Investigación Mariña, Universidade de Vigo, Facultade de Bioloxía, Vigo, Spain
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24
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Vecchyo DOD, Lohmueller KE, Novembre J. Haplotype-based inference of the distribution of fitness effects. Genetics 2022; 220:6501446. [PMID: 35100400 PMCID: PMC8982047 DOI: 10.1093/genetics/iyac002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2021] [Accepted: 12/18/2021] [Indexed: 11/13/2022] Open
Abstract
Abstract
Recent genome sequencing studies with large sample sizes in humans have discovered a vast quantity of low-frequency variants, providing an important source of information to analyze how selection is acting on human genetic variation. In order to estimate the strength of natural selection acting on low-frequency variants, we have developed a likelihood-based method that uses the lengths of pairwise identity-by-state between haplotypes carrying low-frequency variants. We show that in some non-equilibrium populations (such as those that have had recent population expansions) it is possible to distinguish between positive or negative selection acting on a set of variants. With our new framework, one can infer a fixed selection intensity acting on a set of variants at a particular frequency, or a distribution of selection coefficients for standing variants and new mutations. We show an application of our method to the UK10K phased haplotype dataset of individuals.
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Affiliation(s)
- Diego Ortega-Del Vecchyo
- Laboratorio Internacional de Investigación sobre el Genoma Humano, Universidad Nacional Autónoma de México, Juriquilla, Querétaro, 76230, México
- Interdepartmental Program in Bioinformatics, University of California, Los Angeles, Los Angeles, California, 90095, United States of America
| | - Kirk E Lohmueller
- Interdepartmental Program in Bioinformatics, University of California, Los Angeles, Los Angeles, California, 90095, United States of America
- Department of Ecology and Evolutionary Biology, University of California, Los Angeles, Los Angeles, California, 90095, United States of America
- Department of Human Genetics, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California, 90095, United States of America
| | - John Novembre
- Department of Human Genetics, University of Chicago, Chicago, Illinois, 60637, United States of America
- Department of Ecology and Evolution, University of Chicago, Chicago, Illinois, 60637, United States of America
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25
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Liang YY, Shi Y, Yuan S, Zhou BF, Chen XY, An QQ, Ingvarsson PK, Plomion C, Wang B. Linked selection shapes the landscape of genomic variation in three oak species. THE NEW PHYTOLOGIST 2022; 233:555-568. [PMID: 34637540 DOI: 10.1111/nph.17793] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/05/2021] [Accepted: 09/27/2021] [Indexed: 06/13/2023]
Abstract
Natural selection shapes genome-wide patterns of diversity within species and divergence between species. However, quantifying the efficacy of selection and elucidating the relative importance of different types of selection in shaping genomic variation remain challenging. We sequenced whole genomes of 101 individuals of three closely related oak species to track the divergence history, and to dissect the impacts of selective sweeps and background selection on patterns of genomic variation. We estimated that the three species diverged around the late Neogene and experienced a bottleneck during the Pleistocene. We detected genomic regions with elevated relative differentiation ('FST -islands'). Population genetic inferences from the site frequency spectrum and ancestral recombination graph indicated that FST -islands were formed by selective sweeps. We also found extensive positive selection; the fixation of adaptive mutations and reduction neutral diversity around substitutions generated a signature of selective sweeps. Prevalent negative selection and background selection have reduced genetic diversity in both genic and intergenic regions, and contributed substantially to the baseline variation in genetic diversity. Our results demonstrate the importance of linked selection in shaping genomic variation, and illustrate how the extent and strength of different selection models vary across the genome.
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Affiliation(s)
- Yi-Ye Liang
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, China
- University of the Chinese Academy of Sciences, Beijing, 100049, China
| | - Yong Shi
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, China
| | - Shuai Yuan
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, China
| | - Biao-Feng Zhou
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, China
| | - Xue-Yan Chen
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, China
| | - Qing-Qing An
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, China
| | - Pär K Ingvarsson
- Department of Plant Biology, Linnean Center for Plant Biology, Uppsala BioCenter, Swedish University of Agricultural Sciences, Uppsala, SE-75007, Sweden
| | | | - Baosheng Wang
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, China
- Center of Conservation Biology, Core Botanical Gardens, Chinese Academy of Sciences, Guangzhou, 510650, China
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26
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Xu K. Mutation accumulation in inbreeding populations under evolution of the selfing rate. J Evol Biol 2021; 35:23-39. [PMID: 34860448 DOI: 10.1111/jeb.13968] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2021] [Revised: 11/17/2021] [Accepted: 11/23/2021] [Indexed: 11/28/2022]
Abstract
It is theoretically established that self-fertilization can facilitate mutation accumulation, thus increasing extinction risk. However, in previous studies, selfing rates are often set as fixed parameters, but in natural systems, evolution of selfing rates and deleterious mutations may mutually affect each other. I carried out simulations to investigate the dynamics of selfing rates and mutation accumulation, by allowing deleterious mutations to coevolve with alleles that modify the selfing rate (selfing modifiers). I found that selfing rates will often fluctuate over time, due to successive invasion of alleles that increase selfing and outcrossing. Since mutation fixation is mainly caused by Muller's ratchet, its rate is sensitive to the change of the selfing rate mutations will accumulate in a punctuated pattern. The dynamics are influenced by several factors, such as recombination and the selfing rate effects of selfing modifier loci. Also, such temporal variation produces variation of selfing rates and mutation accumulation rates between multiple conspecific populations, which can increase the average fitness across populations. As factors, such as the genomic mutation rate of deleterious mutations, can simultaneously influence the selfing rate and mutation fixation, effects of these factors on mutation accumulation rates can be complicated and non-monotonic.
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Affiliation(s)
- Kuangyi Xu
- Department of Biology, University of North Carolina, Chapel Hill, North Carolina, USA
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27
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Charlesworth B, Jensen JD. Effects of Selection at Linked Sites on Patterns of Genetic Variability. ANNUAL REVIEW OF ECOLOGY, EVOLUTION, AND SYSTEMATICS 2021; 52:177-197. [PMID: 37089401 PMCID: PMC10120885 DOI: 10.1146/annurev-ecolsys-010621-044528] [Citation(s) in RCA: 41] [Impact Index Per Article: 13.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Patterns of variation and evolution at a given site in a genome can be strongly influenced by the effects of selection at genetically linked sites. In particular, the recombination rates of genomic regions correlate with their amount of within-population genetic variability, the degree to which the frequency distributions of DNA sequence variants differ from their neutral expectations, and the levels of adaptation of their functional components. We review the major population genetic processes that are thought to lead to these patterns, focusing on their effects on patterns of variability: selective sweeps, background selection, associative overdominance, and Hill–Robertson interference among deleterious mutations. We emphasize the difficulties in distinguishing among the footprints of these processes and disentangling them from the effects of purely demographic factors such as population size changes. We also discuss how interactions between selective and demographic processes can significantly affect patterns of variability within genomes.
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Affiliation(s)
- Brian Charlesworth
- Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3FL, United Kingdom
| | - Jeffrey D. Jensen
- School of Life Sciences, Arizona State University, Tempe, Arizona 85281, USA
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28
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Srikulnath K, Ahmad SF, Singchat W, Panthum T. Why Do Some Vertebrates Have Microchromosomes? Cells 2021; 10:2182. [PMID: 34571831 PMCID: PMC8466491 DOI: 10.3390/cells10092182] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2021] [Revised: 08/17/2021] [Accepted: 08/17/2021] [Indexed: 12/27/2022] Open
Abstract
With more than 70,000 living species, vertebrates have a huge impact on the field of biology and research, including karyotype evolution. One prominent aspect of many vertebrate karyotypes is the enigmatic occurrence of tiny and often cytogenetically indistinguishable microchromosomes, which possess distinctive features compared to macrochromosomes. Why certain vertebrate species carry these microchromosomes in some lineages while others do not, and how they evolve remain open questions. New studies have shown that microchromosomes exhibit certain unique characteristics of genome structure and organization, such as high gene densities, low heterochromatin levels, and high rates of recombination. Our review focuses on recent concepts to expand current knowledge on the dynamic nature of karyotype evolution in vertebrates, raising important questions regarding the evolutionary origins and ramifications of microchromosomes. We introduce the basic karyotypic features to clarify the size, shape, and morphology of macro- and microchromosomes and report their distribution across different lineages. Finally, we characterize the mechanisms of different evolutionary forces underlying the origin and evolution of microchromosomes.
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Affiliation(s)
- Kornsorn Srikulnath
- Animal Genomics and Bioresource Research Center (AGB Research Center), Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand; (S.F.A.); (W.S.); (T.P.)
- Laboratory of Animal Cytogenetics and Comparative Genomics (ACCG), Department of Genetics, Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
- The International Undergraduate Program in Bioscience and Technology, Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
- Special Research Unit for Wildlife Genomics (SRUWG), Department of Forest Biology, Faculty of Forestry, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
- Amphibian Research Center, Hiroshima University, 1-3-1, Kagamiyama, Higashihiroshima 739-8526, Japan
| | - Syed Farhan Ahmad
- Animal Genomics and Bioresource Research Center (AGB Research Center), Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand; (S.F.A.); (W.S.); (T.P.)
- Laboratory of Animal Cytogenetics and Comparative Genomics (ACCG), Department of Genetics, Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
- The International Undergraduate Program in Bioscience and Technology, Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
- Special Research Unit for Wildlife Genomics (SRUWG), Department of Forest Biology, Faculty of Forestry, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
| | - Worapong Singchat
- Animal Genomics and Bioresource Research Center (AGB Research Center), Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand; (S.F.A.); (W.S.); (T.P.)
- Laboratory of Animal Cytogenetics and Comparative Genomics (ACCG), Department of Genetics, Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
- Special Research Unit for Wildlife Genomics (SRUWG), Department of Forest Biology, Faculty of Forestry, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
| | - Thitipong Panthum
- Animal Genomics and Bioresource Research Center (AGB Research Center), Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand; (S.F.A.); (W.S.); (T.P.)
- Laboratory of Animal Cytogenetics and Comparative Genomics (ACCG), Department of Genetics, Faculty of Science, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
- Special Research Unit for Wildlife Genomics (SRUWG), Department of Forest Biology, Faculty of Forestry, Kasetsart University, 50 Ngamwongwan, Chatuchak, Bangkok 10900, Thailand
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29
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Buffalo V. Quantifying the relationship between genetic diversity and population size suggests natural selection cannot explain Lewontin's Paradox. eLife 2021; 10:e67509. [PMID: 34409937 PMCID: PMC8486380 DOI: 10.7554/elife.67509] [Citation(s) in RCA: 37] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2021] [Accepted: 08/16/2021] [Indexed: 12/21/2022] Open
Abstract
Neutral theory predicts that genetic diversity increases with population size, yet observed levels of diversity across metazoans vary only two orders of magnitude while population sizes vary over several. This unexpectedly narrow range of diversity is known as Lewontin's Paradox of Variation (1974). While some have suggested selection constrains diversity, tests of this hypothesis seem to fall short. Here, I revisit Lewontin's Paradox to assess whether current models of linked selection are capable of reducing diversity to this extent. To quantify the discrepancy between pairwise diversity and census population sizes across species, I combine previously-published estimates of pairwise diversity from 172 metazoan taxa with newly derived estimates of census sizes. Using phylogenetic comparative methods, I show this relationship is significant accounting for phylogeny, but with high phylogenetic signal and evidence that some lineages experience shifts in the evolutionary rate of diversity deep in the past. Additionally, I find a negative relationship between recombination map length and census size, suggesting abundant species have less recombination and experience greater reductions in diversity due to linked selection. However, I show that even assuming strong and abundant selection, models of linked selection are unlikely to explain the observed relationship between diversity and census sizes across species.
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Affiliation(s)
- Vince Buffalo
- Institute for Ecology and Evolution, University of OregonEugeneUnited States
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30
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Yengo L, Yang J, Keller MC, Goddard ME, Wray NR, Visscher PM. Genomic partitioning of inbreeding depression in humans. Am J Hum Genet 2021; 108:1488-1501. [PMID: 34214457 DOI: 10.1016/j.ajhg.2021.06.005] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2020] [Accepted: 06/01/2021] [Indexed: 02/05/2023] Open
Abstract
Across species, offspring of related individuals often exhibit significant reduction in fitness-related traits, known as inbreeding depression (ID), yet the genetic and molecular basis for ID remains elusive. Here, we develop a method to quantify enrichment of ID within specific genomic annotations and apply it to human data. We analyzed the phenomes and genomes of ∼350,000 unrelated participants of the UK Biobank and found, on average of over 11 traits, significant enrichment of ID within genomic regions with high recombination rates (>21-fold; p < 10-5), with conserved function across species (>19-fold; p < 10-4), and within regulatory elements such as DNase I hypersensitive sites (∼5-fold; p = 8.9 × 10-7). We also quantified enrichment of ID within trait-associated regions and found suggestive evidence that genomic regions contributing to additive genetic variance in the population are enriched for ID signal. We find strong correlations between functional enrichment of SNP-based heritability and that of ID (r = 0.8, standard error: 0.1). These findings provide empirical evidence that ID is most likely due to many partially recessive deleterious alleles in low linkage disequilibrium regions of the genome. Our study suggests that functional characterization of ID may further elucidate the genetic architectures and biological mechanisms underlying complex traits and diseases.
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31
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Novo I, López-Cortegano E, Caballero A. Highly pleiotropic variants of human traits are enriched in genomic regions with strong background selection. Hum Genet 2021; 140:1343-1351. [PMID: 34228221 PMCID: PMC8338839 DOI: 10.1007/s00439-021-02308-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2021] [Accepted: 06/18/2021] [Indexed: 11/27/2022]
Abstract
Recent studies have shown the ubiquity of pleiotropy for variants affecting human complex traits. These studies also show that rare variants tend to be less pleiotropic than common ones, suggesting that purifying natural selection acts against highly pleiotropic variants of large effect. Here, we investigate the mean frequency, effect size and recombination rate associated with pleiotropic variants, and focus particularly on whether highly pleiotropic variants are enriched in regions with putative strong background selection. We evaluate variants for 41 human traits using data from the NHGRI-EBI GWAS Catalog, as well as data from other three studies. Our results show that variants involving a higher degree of pleiotropy tend to be more common, have larger mean effect sizes, and contribute more to heritability than variants with a lower degree of pleiotropy. This is consistent with the fact that variants of large effect and frequency are more likely detected by GWAS. Using data from four different studies, we also show that more pleiotropic variants are enriched in genome regions with stronger background selection than less pleiotropic variants, suggesting that highly pleiotropic variants are subjected to strong purifying selection. From the above results, we hypothesized that a number of highly pleiotropic variants of low effect/frequency may pass undetected by GWAS.
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Affiliation(s)
- Irene Novo
- Centro de Investigación Mariña, Universidade de Vigo, Facultade de Bioloxía, 36310, Vigo, Spain.
| | - Eugenio López-Cortegano
- Centro de Investigación Mariña, Universidade de Vigo, Facultade de Bioloxía, 36310, Vigo, Spain
- Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, EH9 3FL, UK
| | - Armando Caballero
- Centro de Investigación Mariña, Universidade de Vigo, Facultade de Bioloxía, 36310, Vigo, Spain
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32
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Christmas MJ, Jones JC, Olsson A, Wallerman O, Bunikis I, Kierczak M, Peona V, Whitley KM, Larva T, Suh A, Miller-Struttmann NE, Geib JC, Webster MT. Genetic Barriers to Historical Gene Flow between Cryptic Species of Alpine Bumblebees Revealed by Comparative Population Genomics. Mol Biol Evol 2021; 38:3126-3143. [PMID: 33823537 PMCID: PMC8321533 DOI: 10.1093/molbev/msab086] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Evidence is accumulating that gene flow commonly occurs between recently diverged species, despite the existence of barriers to gene flow in their genomes. However, we still know little about what regions of the genome become barriers to gene flow and how such barriers form. Here, we compare genetic differentiation across the genomes of bumblebee species living in sympatry and allopatry to reveal the potential impact of gene flow during species divergence and uncover genetic barrier loci. We first compared the genomes of the alpine bumblebee Bombus sylvicola and a previously unidentified sister species living in sympatry in the Rocky Mountains, revealing prominent islands of elevated genetic divergence in the genome that colocalize with centromeres and regions of low recombination. This same pattern is observed between the genomes of another pair of closely related species living in allopatry (B. bifarius and B. vancouverensis). Strikingly however, the genomic islands exhibit significantly elevated absolute divergence (dXY) in the sympatric, but not the allopatric, comparison indicating that they contain loci that have acted as barriers to historical gene flow in sympatry. Our results suggest that intrinsic barriers to gene flow between species may often accumulate in regions of low recombination and near centromeres through processes such as genetic hitchhiking, and that divergence in these regions is accentuated in the presence of gene flow.
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Affiliation(s)
- Matthew J Christmas
- Department of Medical Biochemistry and Microbiology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden
| | - Julia C Jones
- Department of Medical Biochemistry and Microbiology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden.,School of Biology and Environmental Science, University College Dublin, Dublin, Ireland
| | - Anna Olsson
- Department of Medical Biochemistry and Microbiology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden
| | - Ola Wallerman
- Department of Medical Biochemistry and Microbiology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden
| | - Ignas Bunikis
- Department of Immunology, Genetics and Pathology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden
| | - Marcin Kierczak
- Department of Cell and Molecular Biology, National Bioinformatics Infrastructure Sweden, Science for Life Laboratory, Uppsala University, Uppsala, Sweden
| | - Valentina Peona
- Department of Organismal Biology-Systematic Biology, Uppsala University, Uppsala, Sweden
| | - Kaitlyn M Whitley
- Department of Biology, Appalachian State University, Boone, NC, USA.,U.S. Department of Agriculture, Agriculture Research Service, Charleston, SC, USA
| | - Tuuli Larva
- Department of Medical Biochemistry and Microbiology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden
| | - Alexander Suh
- Department of Organismal Biology-Systematic Biology, Uppsala University, Uppsala, Sweden.,School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich, United Kingdom
| | | | - Jennifer C Geib
- Department of Biology, Appalachian State University, Boone, NC, USA
| | - Matthew T Webster
- Department of Medical Biochemistry and Microbiology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden
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33
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Comeault AA, Wang J, Tittes S, Isbell K, Ingley S, Hurlbert AH, Matute DR. Genetic Diversity and Thermal Performance in Invasive and Native Populations of African Fig Flies. Mol Biol Evol 2021; 37:1893-1906. [PMID: 32109281 PMCID: PMC7306694 DOI: 10.1093/molbev/msaa050] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
During biological invasions, invasive populations can suffer losses of genetic diversity that are predicted to negatively impact their fitness/performance. Despite examples of invasive populations harboring lower diversity than conspecific populations in their native range, few studies have linked this lower diversity to a decrease in fitness. Using genome sequences, we show that invasive populations of the African fig fly, Zaprionus indianus, have less genetic diversity than conspecific populations in their native range and that diversity is proportionally lower in regions of the genome experiencing low recombination rates. This result suggests that selection may have played a role in lowering diversity in the invasive populations. We next use interspecific comparisons to show that genetic diversity remains relatively high in invasive populations of Z. indianus when compared with other closely related species. By comparing genetic diversity in orthologous gene regions, we also show that the genome-wide landscape of genetic diversity differs between invasive and native populations of Z. indianus indicating that invasion not only affects amounts of genetic diversity but also how that diversity is distributed across the genome. Finally, we use parameter estimates from thermal performance curves for 13 species of Zaprionus to show that Z. indianus has the broadest thermal niche of measured species, and that performance does not differ between invasive and native populations. These results illustrate how aspects of genetic diversity in invasive species can be decoupled from measures of fitness, and that a broad thermal niche may have helped facilitate Z. indianus's range expansion.
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Affiliation(s)
- Aaron A Comeault
- School of Natural Sciences, Bangor University, Bangor, Gwynedd, United Kingdom
| | - Jeremy Wang
- Department of Genetics, University of North Carolina at Chapel Hill, Chapel Hill, NC
| | - Silas Tittes
- Department of Evolution and Ecology, University of California, Davis, Davis, CA
| | - Kristin Isbell
- Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC
| | - Spencer Ingley
- Faculty of Sciences, Brigham Young University, Hawaii, Laie, HI
| | - Allen H Hurlbert
- Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC
| | - Daniel R Matute
- Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC
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34
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Waller DM. Addressing Darwin's dilemma: Can pseudo-overdominance explain persistent inbreeding depression and load? Evolution 2021; 75:779-793. [PMID: 33598971 DOI: 10.1111/evo.14189] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/11/2020] [Revised: 01/06/2021] [Accepted: 01/30/2021] [Indexed: 01/01/2023]
Abstract
Darwin spent years investigating the effects of self-fertilization, concluding that "nature abhors perpetual self-fertilization." Given that selection purges inbred populations of strongly deleterious mutations and drift fixes mild mutations, why does inbreeding depression (ID) persist in highly inbred taxa and why do no purely selfing taxa exist? Background selection, associations and interference among loci, and drift within small inbred populations all limit selection while often increasing fixation. These mechanisms help to explain why more inbred populations in most species consistently show more fixed load. This drift load is manifest in the considerable heterosis regularly observed in between-population crosses. Such heterosis results in subsequent high ID, suggesting a mechanism by which small populations could retain variation and inbreeding load. Multiple deleterious recessive mutations linked in repulsion generate pseudo-overdominance. Many tightly linked load loci could generate a balanced segregating load high enough to sustain ID over many generations. Such pseudo-overdominance blocks (or "PODs") are more likely to occur in regions of low recombination. They should also result in clear genetic signatures including genomic hotspots of heterozygosity; distinct haplotypes supporting alleles at intermediate frequency; and high linkage disequilibrium in and around POD regions. Simulation and empirical studies tend to support these predictions. Additional simulations and comparative genomic analyses should explore POD dynamics in greater detail to resolve whether PODs exist in sufficient strength and number to account for why ID and load persist within inbred lineages.
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Affiliation(s)
- Donald M Waller
- Department of Botany, University of Wisconsin-Madison, Madison, Wisconsin, 53706
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35
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Hartfield M. Approximating the Coalescent Under Facultative Sex. J Hered 2021; 112:145-154. [PMID: 33511984 DOI: 10.1093/jhered/esaa036] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2019] [Accepted: 09/01/2020] [Indexed: 11/14/2022] Open
Abstract
Genome studies of facultative sexual species, which can either reproduce sexually or asexually, are providing insight into the evolutionary consequences of mixed reproductive modes. It is currently unclear to what extent the evolutionary history of facultative sexuals' genomes can be approximated by the standard coalescent, and if a coalescent effective population size Ne exists. Here, I determine if and when these approximations can be made. When sex is frequent (occurring at a frequency much greater than 1/N per reproduction per generation, for N the actual population size), the underlying genealogy can be approximated by the standard coalescent, with a coalescent Ne≈N. When sex is very rare (at frequency much lower than 1/N), approximations for the pairwise coalescent time can be obtained, which is strongly influenced by the frequencies of sex and mitotic gene conversion, rather than N. However, these terms do not translate into a coalescent Ne. These results are used to discuss the best sampling strategies for investigating the evolutionary history of facultative sexual species.
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Affiliation(s)
- Matthew Hartfield
- Institute of Evolutionary Biology, The University of Edinburgh, Edinburgh, UK
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36
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Schrider DR. Background Selection Does Not Mimic the Patterns of Genetic Diversity Produced by Selective Sweeps. Genetics 2020; 216:499-519. [PMID: 32847814 PMCID: PMC7536861 DOI: 10.1534/genetics.120.303469] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2019] [Accepted: 08/04/2020] [Indexed: 12/28/2022] Open
Abstract
It is increasingly evident that natural selection plays a prominent role in shaping patterns of diversity across the genome. The most commonly studied modes of natural selection are positive selection and negative selection, which refer to directional selection for and against derived mutations, respectively. Positive selection can result in hitchhiking events, in which a beneficial allele rapidly replaces all others in the population, creating a valley of diversity around the selected site along with characteristic skews in allele frequencies and linkage disequilibrium among linked neutral polymorphisms. Similarly, negative selection reduces variation not only at selected sites but also at linked sites, a phenomenon called background selection (BGS). Thus, discriminating between these two forces may be difficult, and one might expect efforts to detect hitchhiking to produce an excess of false positives in regions affected by BGS. Here, we examine the similarity between BGS and hitchhiking models via simulation. First, we show that BGS may somewhat resemble hitchhiking in simplistic scenarios in which a region constrained by negative selection is flanked by large stretches of unconstrained sites, echoing previous results. However, this scenario does not mirror the actual spatial arrangement of selected sites across the genome. By performing forward simulations under more realistic scenarios of BGS, modeling the locations of protein-coding and conserved noncoding DNA in real genomes, we show that the spatial patterns of variation produced by BGS rarely mimic those of hitchhiking events. Indeed, BGS is not substantially more likely than neutrality to produce false signatures of hitchhiking. This holds for simulations modeled after both humans and Drosophila, and for several different demographic histories. These results demonstrate that appropriately designed scans for hitchhiking need not consider BGS's impact on false-positive rates. However, we do find evidence that BGS increases the false-negative rate for hitchhiking, an observation that demands further investigation.
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Affiliation(s)
- Daniel R Schrider
- Department of Genetics, University of North Carolina, Chapel Hill, North Carolina 27514
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37
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Zhang XX, Cheng X, Li LL, Wang X, Zhou W, Chen XY, Hu XS. The wave of gene advance under diverse systems of mating. Heredity (Edinb) 2020; 125:253-268. [PMID: 32606419 PMCID: PMC7490428 DOI: 10.1038/s41437-020-0333-9] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2019] [Revised: 06/09/2020] [Accepted: 06/09/2020] [Indexed: 11/09/2022] Open
Abstract
Mating systems will influence gene spread across the natural distribution of a plant species. Existing theories have not fully explored the role of mating systems on the wave of advance of an advantageous gene. Here, we develop a theory to account for the rate of spread of both advantageous and neutral genes under different mating systems, based on migration-selection processes. We show that a complex relationship exists between selfing rate and the speed of gene spread. The interaction of selfing with gametophytic selection shapes the traveling wave of the advantageous gene. Selfing can impede (or enhance) the spread of an advantageous gene in the presence (or absence) of gametophytic selection. The interaction of selfing with recombination shapes the spread of a neutral gene. Linkage disequilibrium, mainly generated by selfing, enhances the traveling wave of the neutral gene that is tightly linked with the selective gene. Recombination gradually breaks down the genetic hitchhiking effects along the direction of advantageous gene spread, yielding decreasing waves of advance of neutral genes. The stochastic process does not alter the pattern of selfing effects except for increasing the uncertainty of the waves of advance of both advantageous and neutral genes. This theory helps us to explain how mating systems act as a barrier to spread of adaptive and neutral genes, and to interpret species cohesion maintained by a low level of adaptive gene flow.
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Affiliation(s)
- Xin-Xin Zhang
- College of Forestry and Landscape Architecture, South China Agricultural University, Guangdong, 510642, China
- Guangdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, South China Agricultural University, Guangdong, 510642, China
| | - Xiang Cheng
- College of Forestry and Landscape Architecture, South China Agricultural University, Guangdong, 510642, China
- Guangdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, South China Agricultural University, Guangdong, 510642, China
| | - Ling-Ling Li
- College of Forestry and Landscape Architecture, South China Agricultural University, Guangdong, 510642, China
- Guangdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, South China Agricultural University, Guangdong, 510642, China
| | - Xi Wang
- College of Forestry and Landscape Architecture, South China Agricultural University, Guangdong, 510642, China
- Guangdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, South China Agricultural University, Guangdong, 510642, China
| | - Wei Zhou
- College of Forestry and Landscape Architecture, South China Agricultural University, Guangdong, 510642, China
- Guangdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, South China Agricultural University, Guangdong, 510642, China
| | - Xiao-Yang Chen
- College of Forestry and Landscape Architecture, South China Agricultural University, Guangdong, 510642, China
- Guangdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, South China Agricultural University, Guangdong, 510642, China
| | - Xin-Sheng Hu
- College of Forestry and Landscape Architecture, South China Agricultural University, Guangdong, 510642, China.
- Guangdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, South China Agricultural University, Guangdong, 510642, China.
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38
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Buffalo V, Coop G. Estimating the genome-wide contribution of selection to temporal allele frequency change. Proc Natl Acad Sci U S A 2020; 117:20672-20680. [PMID: 32817464 PMCID: PMC7456072 DOI: 10.1073/pnas.1919039117] [Citation(s) in RCA: 50] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Rapid phenotypic adaptation is often observed in natural populations and selection experiments. However, detecting the genome-wide impact of this selection is difficult since adaptation often proceeds from standing variation and selection on polygenic traits, both of which may leave faint genomic signals indistinguishable from a noisy background of genetic drift. One promising signal comes from the genome-wide covariance between allele frequency changes observable from temporal genomic data (e.g., evolve-and-resequence studies). These temporal covariances reflect how heritable fitness variation in the population leads changes in allele frequencies at one time point to be predictive of the changes at later time points, as alleles are indirectly selected due to remaining associations with selected alleles. Since genetic drift does not lead to temporal covariance, we can use these covariances to estimate what fraction of the variation in allele frequency change through time is driven by linked selection. Here, we reanalyze three selection experiments to quantify the effects of linked selection over short timescales using covariance among time points and across replicates. We estimate that at least 17 to 37% of allele frequency change is driven by selection in these experiments. Against this background of positive genome-wide temporal covariances, we also identify signals of negative temporal covariance corresponding to reversals in the direction of selection for a reasonable proportion of loci over the time course of a selection experiment. Overall, we find that in the three studies we analyzed, linked selection has a large impact on short-term allele frequency dynamics that is readily distinguishable from genetic drift.
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Affiliation(s)
- Vince Buffalo
- Population Biology Graduate Group, University of California, Davis, CA 95616;
- Center for Population Biology, Department of Evolution and Ecology, University of California, Davis, CA 95616
| | - Graham Coop
- Center for Population Biology, Department of Evolution and Ecology, University of California, Davis, CA 95616
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39
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Recent introgression between Taiga Bean Goose and Tundra Bean Goose results in a largely homogeneous landscape of genetic differentiation. Heredity (Edinb) 2020; 125:73-84. [PMID: 32451423 PMCID: PMC7413267 DOI: 10.1038/s41437-020-0322-z] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2019] [Revised: 05/11/2020] [Accepted: 05/12/2020] [Indexed: 02/06/2023] Open
Abstract
Several studies have uncovered a highly heterogeneous landscape of genetic differentiation across the genomes of closely related species. Specifically, genetic differentiation is often concentrated in particular genomic regions (“islands of differentiation”) that might contain barrier loci contributing to reproductive isolation, whereas the rest of the genome is homogenized by introgression. Alternatively, linked selection can produce differentiation islands in allopatry without introgression. We explored the influence of introgression on the landscape of genetic differentiation in two hybridizing goose taxa: the Taiga Bean Goose (Anser fabalis) and the Tundra Bean Goose (A. serrirostris). We re-sequenced the whole genomes of 18 individuals (9 of each taxon) and, using a combination of population genomic summary statistics and demographic modeling, we reconstructed the evolutionary history of these birds. Next, we quantified the impact of introgression on the build-up and maintenance of genetic differentiation. We found evidence for a scenario of allopatric divergence (about 2.5 million years ago) followed by recent secondary contact (about 60,000 years ago). Subsequent introgression events led to high levels of gene flow, mainly from the Tundra Bean Goose into the Taiga Bean Goose. This scenario resulted in a largely undifferentiated genomic landscape (genome-wide FST = 0.033) with a few notable differentiation peaks that were scattered across chromosomes. The summary statistics indicated that some peaks might contain barrier loci while others arose in allopatry through linked selection. Finally, based on the low genetic differentiation, considerable morphological variation and incomplete reproductive isolation, we argue that the Taiga and the Tundra Bean Goose should be treated as subspecies.
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40
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Gagnaire PA. Comparative genomics approach to evolutionary process connectivity. Evol Appl 2020; 13:1320-1334. [PMID: 32684961 PMCID: PMC7359831 DOI: 10.1111/eva.12978] [Citation(s) in RCA: 21] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2019] [Revised: 04/02/2020] [Accepted: 04/03/2020] [Indexed: 01/01/2023] Open
Abstract
The influence of species life history traits and historical demography on contemporary connectivity is still poorly understood. However, these factors partly determine the evolutionary responses of species to anthropogenic landscape alterations. Genetic connectivity and its evolutionary outcomes depend on a variety of spatially dependent evolutionary processes, such as population structure, local adaptation, genetic admixture, and speciation. Over the last years, population genomic studies have been interrogating these processes with increasing resolution, revealing a large diversity of species responses to spatially structured landscapes. In parallel, multispecies meta-analyses usually based on low-genome coverage data have provided fundamental insights into the ecological determinants of genetic connectivity, such as the influence of key life history traits on population structure. However, comparative studies still lack a thorough integration of macro- and micro-evolutionary scales to fully realize their potential. Here, I present how a comparative genomics framework may provide a deeper understanding of evolutionary process connectivity. This framework relies on coupling the inference of long-term demographic and selective history with an assessment of the contemporary consequences of genetic connectivity. Standardizing this approach across several species occupying the same landscape should help understand how spatial environmental heterogeneity has shaped the diversity of historical and contemporary connectivity patterns in different taxa with contrasted life history traits. I will argue that a reasonable amount of genome sequence data can be sufficient to resolve and connect complex macro- and micro-evolutionary histories. Ultimately, implementing this framework in varied taxonomic groups is expected to improve scientific guidelines for conservation and management policies.
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41
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Johri P, Charlesworth B, Jensen JD. Toward an Evolutionarily Appropriate Null Model: Jointly Inferring Demography and Purifying Selection. Genetics 2020; 215:173-192. [PMID: 32152045 PMCID: PMC7198275 DOI: 10.1534/genetics.119.303002] [Citation(s) in RCA: 82] [Impact Index Per Article: 20.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2019] [Accepted: 03/05/2020] [Indexed: 01/27/2023] Open
Abstract
The question of the relative evolutionary roles of adaptive and nonadaptive processes has been a central debate in population genetics for nearly a century. While advances have been made in the theoretical development of the underlying models, and statistical methods for estimating their parameters from large-scale genomic data, a framework for an appropriate null model remains elusive. A model incorporating evolutionary processes known to be in constant operation, genetic drift (as modulated by the demographic history of the population) and purifying selection, is lacking. Without such a null model, the role of adaptive processes in shaping within- and between-population variation may not be accurately assessed. Here, we investigate how population size changes and the strength of purifying selection affect patterns of variation at "neutral" sites near functional genomic components. We propose a novel statistical framework for jointly inferring the contribution of the relevant selective and demographic parameters. By means of extensive performance analyses, we quantify the utility of the approach, identify the most important statistics for parameter estimation, and compare the results with existing methods. Finally, we reanalyze genome-wide population-level data from a Zambian population of Drosophila melanogaster, and find that it has experienced a much slower rate of population growth than was inferred when the effects of purifying selection were neglected. Our approach represents an appropriate null model, against which the effects of positive selection can be assessed.
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Affiliation(s)
- Parul Johri
- School of Life Sciences, Arizona State University, Tempe, Arizona 85287
| | - Brian Charlesworth
- Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, EH9 3FL, United Kingdom
| | - Jeffrey D Jensen
- School of Life Sciences, Arizona State University, Tempe, Arizona 85287
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42
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Woerner AE, Veeramah KR, Watkins JC, Hammer MF. The Role of Phylogenetically Conserved Elements in Shaping Patterns of Human Genomic Diversity. Mol Biol Evol 2020; 35:2284-2295. [PMID: 30113695 DOI: 10.1093/molbev/msy145] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
Evolutionary genetic studies have shown a positive correlation between levels of nucleotide diversity and either rates of recombination or genetic distance to genes. Both positive-directional and purifying selection have been offered as the source of these correlations via genetic hitchhiking and background selection, respectively. Phylogenetically conserved elements (CEs) are short (∼100 bp), widely distributed (comprising ∼5% of genome), sequences that are often found far from genes. While the function of many CEs is unknown, CEs also are associated with reduced diversity at linked sites. Using high coverage (>80×) whole genome data from two human populations, the Yoruba and the CEU, we perform fine scale evaluations of diversity, rates of recombination, and linkage to genes. We find that the local rate of recombination has a stronger effect on levels of diversity than linkage to genes, and that these effects of recombination persist even in regions far from genes. Our whole genome modeling demonstrates that, rather than recombination or GC-biased gene conversion, selection on sites within or linked to CEs better explains the observed genomic diversity patterns. A major implication is that very few sites in the human genome are predicted to be free of the effects of selection. These sites, which we refer to as the human "neutralome," comprise only 1.2% of the autosomes and 5.1% of the X chromosome. Demographic analysis of the neutralome reveals larger population sizes and lower rates of growth for ancestral human populations than inferred by previous analyses.
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Affiliation(s)
- August E Woerner
- ARL Division of Biotechnology, University of Arizona, Tucson, AZ.,Center for Human Identification, University of North Texas Health Science Center, Fort Worth, TX
| | - Krishna R Veeramah
- Department of Ecology and Evolution, Stony Brook University, Stony Brook, NY
| | | | - Michael F Hammer
- ARL Division of Biotechnology, University of Arizona, Tucson, AZ
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43
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The Temporal Dynamics of Background Selection in Nonequilibrium Populations. Genetics 2020; 214:1019-1030. [PMID: 32071195 DOI: 10.1534/genetics.119.302892] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2019] [Accepted: 01/30/2020] [Indexed: 01/06/2023] Open
Abstract
Neutral genetic diversity across the genome is determined by the complex interplay of mutation, demographic history, and natural selection. While the direct action of natural selection is limited to functional loci across the genome, its impact can have effects on nearby neutral loci due to genetic linkage. These effects of selection at linked sites, referred to as genetic hitchhiking and background selection (BGS), are pervasive across natural populations. However, only recently has there been a focus on the joint consequences of demography and selection at linked sites, and some empirical studies have come to apparently contradictory conclusions as to their combined effects. To understand the relationship between demography and selection at linked sites, we conducted an extensive forward simulation study of BGS under a range of demographic models. We found that the relative levels of diversity in BGS and neutral regions vary over time and that the initial dynamics after a population size change are often in the opposite direction of the long-term expected trajectory. Our detailed observations of the temporal dynamics of neutral diversity in the context of selection at linked sites in nonequilibrium populations provide new intuition about why patterns of diversity under BGS vary through time in natural populations and help reconcile previously contradictory observations. Most notably, our results highlight that classical models of BGS are poorly suited for predicting diversity in nonequilibrium populations.
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44
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Hämälä T, Guiltinan MJ, Marden JH, Maximova SN, dePamphilis CW, Tiffin P. Gene Expression Modularity Reveals Footprints of Polygenic Adaptation in Theobroma cacao. Mol Biol Evol 2020; 37:110-123. [PMID: 31501906 DOI: 10.1093/molbev/msz206] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Separating footprints of adaptation from demography is challenging. When selection has acted on a single locus with major effect, this issue can be alleviated through signatures left by selective sweeps. However, as adaptation is often driven by small allele frequency shifts at many loci, studies focusing on single genes are able to identify only a small portion of genomic variants responsible for adaptation. In face of this challenge, we utilize coexpression information to search for signals of polygenetic adaptation in Theobroma cacao, a tropical tree species that is the source of chocolate. Using transcriptomics and a weighted correlation network analysis, we group genes with similar expression patterns into functional modules. We then ask whether modules enriched for specific biological processes exhibit cumulative effects of differential selection in the form of high FST and dXY between populations. Indeed, modules putatively involved in protein modification, flowering, and water transport show signs of polygenic adaptation even though individual genes that are members of those groups do not bear strong signatures of selection. Modeling of demography, background selection, and the effects of genomic features reveal that these patterns are unlikely to arise by chance. We also find that specific modules are enriched for signals of strong or relaxed purifying selection, with one module bearing signs of adaptive differentiation and an excess of deleterious mutations. Our results provide insight into polygenic adaptation and contribute to understanding of population structure, demographic history, and genome evolution in T. cacao.
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Affiliation(s)
- Tuomas Hämälä
- Department of Plant and Microbial Biology, University of Minnesota, St. Paul, MN
| | - Mark J Guiltinan
- Department of Plant Sciences, The Pennsylvania State University, University Park, PA.,Huck Institutes of the Life Sciences, The Pennsylvania State University, University Park, PA
| | - James H Marden
- Huck Institutes of the Life Sciences, The Pennsylvania State University, University Park, PA.,Department of Biology, The Pennsylvania State University, University Park, PA
| | - Siela N Maximova
- Department of Plant Sciences, The Pennsylvania State University, University Park, PA.,Huck Institutes of the Life Sciences, The Pennsylvania State University, University Park, PA
| | - Claude W dePamphilis
- Huck Institutes of the Life Sciences, The Pennsylvania State University, University Park, PA.,Department of Biology, The Pennsylvania State University, University Park, PA
| | - Peter Tiffin
- Department of Plant and Microbial Biology, University of Minnesota, St. Paul, MN
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Gilbert KJ, Pouyet F, Excoffier L, Peischl S. Transition from Background Selection to Associative Overdominance Promotes Diversity in Regions of Low Recombination. Curr Biol 2019; 30:101-107.e3. [PMID: 31866368 DOI: 10.1016/j.cub.2019.11.063] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2019] [Revised: 10/16/2019] [Accepted: 11/21/2019] [Indexed: 12/16/2022]
Abstract
Linked selection is a major driver of genetic diversity. Selection against deleterious mutations removes linked neutral diversity (background selection [BGS]) [1], creating a positive correlation between recombination rates and genetic diversity. Purifying selection against recessive variants, however, can also lead to associative overdominance (AOD) [2, 3], due to an apparent heterozygote advantage at linked neutral loci that opposes the loss of neutral diversity by BGS. Zhao and Charlesworth [3] identified the conditions under which AOD should dominate over BGS in a single-locus model and suggested that the effect of AOD could become stronger if multiple linked deleterious variants co-segregate. We present a model describing how and under which conditions multi-locus dynamics can amplify the effects of AOD. We derive the conditions for a transition from BGS to AOD due to pseudo-overdominance [4], i.e., a form of balancing selection that maintains complementary deleterious haplotypes that mask the effect of recessive deleterious mutations. Simulations confirm these findings and show that multi-locus AOD can increase diversity in low-recombination regions much more strongly than previously appreciated. While BGS is known to drive genome-wide diversity in humans [5], the observation of a resurgence of genetic diversity in regions of very low recombination is indicative of AOD. We identify 22 such regions in the human genome consistent with multi-locus AOD. Our results demonstrate that AOD may play an important role in the evolution of low-recombination regions of many species.
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Affiliation(s)
- Kimberly J Gilbert
- Institute of Ecology and Evolution, Baltzerstrasse 6, University of Bern, 3012 Bern, Switzerland; Swiss Institute of Bioinformatics, Quartier Sorge-Batiment Amphipole, 1015 Lausanne, Switzerland.
| | - Fanny Pouyet
- Institute of Ecology and Evolution, Baltzerstrasse 6, University of Bern, 3012 Bern, Switzerland; Swiss Institute of Bioinformatics, Quartier Sorge-Batiment Amphipole, 1015 Lausanne, Switzerland
| | - Laurent Excoffier
- Institute of Ecology and Evolution, Baltzerstrasse 6, University of Bern, 3012 Bern, Switzerland; Swiss Institute of Bioinformatics, Quartier Sorge-Batiment Amphipole, 1015 Lausanne, Switzerland
| | - Stephan Peischl
- Swiss Institute of Bioinformatics, Quartier Sorge-Batiment Amphipole, 1015 Lausanne, Switzerland; Interfaculty Bioinformatics Unit, Baltzerstrasse 6, University of Bern, 3012 Bern, Switzerland
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46
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Schmidt JM, de Manuel M, Marques-Bonet T, Castellano S, Andrés AM. The impact of genetic adaptation on chimpanzee subspecies differentiation. PLoS Genet 2019; 15:e1008485. [PMID: 31765391 PMCID: PMC6901233 DOI: 10.1371/journal.pgen.1008485] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/10/2019] [Revised: 12/09/2019] [Accepted: 10/17/2019] [Indexed: 12/25/2022] Open
Abstract
Chimpanzees, humans' closest relatives, are in danger of extinction. Aside from direct human impacts such as hunting and habitat destruction, a key threat is transmissible disease. As humans continue to encroach upon their habitats, which shrink in size and grow in density, the risk of inter-population and cross-species viral transmission increases, a point dramatically made in the reverse with the global HIV/AIDS pandemic. Inhabiting central Africa, the four subspecies of chimpanzees differ in demographic history and geographical range, and are likely differentially adapted to their particular local environments. To quantitatively explore genetic adaptation, we investigated the genic enrichment for SNPs highly differentiated between chimpanzee subspecies. Previous analyses of such patterns in human populations exhibited limited evidence of adaptation. In contrast, chimpanzees show evidence of recent positive selection, with differences among subspecies. Specifically, we observe strong evidence of recent selection in eastern chimpanzees, with highly differentiated SNPs being uniquely enriched in genic sites in a way that is expected under recent adaptation but not under neutral evolution or background selection. These sites are enriched for genes involved in immune responses to pathogens, and for genes inferred to differentiate the immune response to infection by simian immunodeficiency virus (SIV) in natural vs. non-natural host species. Conversely, central chimpanzees exhibit an enrichment of signatures of positive selection only at cytokine receptors, due to selective sweeps in CCR3, CCR9 and CXCR6 -paralogs of CCR5 and CXCR4, the two major receptors utilized by HIV to enter human cells. Thus, our results suggest that positive selection has contributed to the genetic and phenotypic differentiation of chimpanzee subspecies, and that viruses likely play a predominate role in this differentiation, with SIV being a likely selective agent. Interestingly, our results suggest that SIV has elicited distinctive adaptive responses in these two chimpanzee subspecies.
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MESH Headings
- Adaptation, Physiological/genetics
- Adaptation, Physiological/immunology
- Animals
- Demography
- Genetic Drift
- Genetic Speciation
- HIV/genetics
- HIV/immunology
- HIV/pathogenicity
- Humans
- Immunity, Innate/genetics
- Pan troglodytes/genetics
- Pan troglodytes/immunology
- Pan troglodytes/virology
- Polymorphism, Single Nucleotide/genetics
- Receptors, CCR/genetics
- Receptors, CCR3/genetics
- Receptors, CCR5/genetics
- Receptors, CXCR4/genetics
- Receptors, CXCR6/immunology
- Selection, Genetic/genetics
- Simian Immunodeficiency Virus/genetics
- Simian Immunodeficiency Virus/immunology
- Simian Immunodeficiency Virus/pathogenicity
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Affiliation(s)
- Joshua M. Schmidt
- UCL Genetics Institute, Department of Genetics, Evolution and Environment, University College London, London, United Kingdom
- Max Planck Institute for Evolutionary Anthropology, Department of Evolutionary Genetics, Leipzig, Germany
- * E-mail: (JMS); (AMA)
| | - Marc de Manuel
- Institut de Biologia Evolutiva (Consejo Superior de Investigaciones Científicas–Universitat Pompeu Fabra), Barcelona, Spain
| | - Tomas Marques-Bonet
- Institut de Biologia Evolutiva (Consejo Superior de Investigaciones Científicas–Universitat Pompeu Fabra), Barcelona, Spain
- National Centre for Genomic Analysis–Centre for Genomic Regulation, Barcelona Institute of Science and Technology, Barcelona, Spain
- Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain
| | - Sergi Castellano
- Max Planck Institute for Evolutionary Anthropology, Department of Evolutionary Genetics, Leipzig, Germany
- Genetics and Genomic Medicine Programme, Great Ormond Street Institute of Child Health, University College London (UCL), London, United Kingdom
- UCL Genomics, London, United Kingdom
| | - Aida M. Andrés
- UCL Genetics Institute, Department of Genetics, Evolution and Environment, University College London, London, United Kingdom
- Max Planck Institute for Evolutionary Anthropology, Department of Evolutionary Genetics, Leipzig, Germany
- * E-mail: (JMS); (AMA)
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47
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Quilodrán CS, Ruegg K, Sendell‐Price AT, Anderson EC, Coulson T, Clegg SM. The multiple population genetic and demographic routes to islands of genomic divergence. Methods Ecol Evol 2019. [DOI: 10.1111/2041-210x.13324] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
| | - Kristen Ruegg
- Department of Zoology University of Oxford Oxford UK
- Center for Tropical Research Institute of the Environment and Sustainability University of California, Los Angeles Los Angeles CA USA
- Department of Biology Colorado State University Fort Collins CO USA
| | | | - Eric C. Anderson
- Fisheries Ecology Division Southwest Fisheries Science Center National Marine Fisheries ServiceNOAA Santa Cruz CA USA
| | - Tim Coulson
- Department of Zoology University of Oxford Oxford UK
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48
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Buffalo V, Coop G. The Linked Selection Signature of Rapid Adaptation in Temporal Genomic Data. Genetics 2019; 213:1007-1045. [PMID: 31558582 PMCID: PMC6827383 DOI: 10.1534/genetics.119.302581] [Citation(s) in RCA: 32] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2019] [Accepted: 09/22/2019] [Indexed: 11/18/2022] Open
Abstract
The majority of empirical population genetic studies have tried to understand the evolutionary processes that have shaped genetic variation in a single sample taken from a present-day population. However, genomic data collected over tens of generations in both natural and laboratory populations are increasingly used to find selected loci underpinning adaptation over these short timescales. Although these studies have been quite successful in detecting selection on large-effect loci, the fitness differences between individuals are often polygenic, such that selection leads to allele frequency changes that are difficult to distinguish from genetic drift. However, one promising signal comes from polygenic selection's effect on neutral sites that become stochastically associated with the genetic backgrounds that lead to fitness differences between individuals. Previous theoretical work has established that the random associations between a neutral allele and heritable fitness backgrounds act to reduce the effective population size experienced by this neutral allele. These associations perturb neutral allele frequency trajectories, creating autocovariance in the allele frequency changes across generations. Here, we show how temporal genomic data allow us to measure the temporal autocovariance in allele frequency changes and characterize the genome-wide impact of polygenic selection. We develop expressions for these temporal autocovariances, showing that their magnitude is determined by the level of additive genetic variation, recombination, and linkage disequilibria in a region. Furthermore, by using analytic expressions for the temporal variances and autocovariances in allele frequency, we demonstrate that one can estimate the additive genetic variation for fitness and the drift-effective population size from temporal genomic data. We also show how the proportion of total variation in allele frequency change due to linked selection can be estimated from temporal data. Overall, we demonstrate that temporal genomic data offer opportunities to identify the role of linked selection on genome-wide diversity over short timescales, and can help bridge population genetic and quantitative genetic studies of adaptation.
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Affiliation(s)
- Vince Buffalo
- Population Biology Graduate Group, University of California, Davis, California 95616
- Center for Population Biology, Department of Evolution and Ecology, University of California, Davis, California 95616
| | - Graham Coop
- Center for Population Biology, Department of Evolution and Ecology, University of California, Davis, California 95616
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49
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Rougemont Q, Bernatchez L. The demographic history of Atlantic salmon (Salmo salar) across its distribution range reconstructed from approximate Bayesian computations. Evolution 2019; 72:1261-1277. [PMID: 29644624 DOI: 10.1111/evo.13486] [Citation(s) in RCA: 49] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2017] [Accepted: 03/14/2018] [Indexed: 12/18/2022]
Abstract
Understanding the dual roles of demographic and selective processes in the buildup of population divergence is one of the most challenging tasks in evolutionary biology. Here, we investigated the demographic history of Atlantic salmon across the entire species range using 2035 anadromous individuals from North America and Eurasia. By combining results from admixture graphs, geo-genetic maps, and an Approximate Bayesian Computation (ABC) framework, we validated previous hypotheses pertaining to secondary contact between European and Northern American populations, but also identified secondary contacts in European populations from different glacial refugia. We further identified the major sources of admixture from the southern range of North America into more northern populations along with a strong signal of secondary gene flow between genetic regional groups. We hypothesize that these patterns reflect the spatial redistribution of ancestral variation across the entire North American range. Results also support a role for linked selection and differential introgression that likely played an underappreciated role in shaping the genomic landscape of species in the Northern hemisphere. We conclude that studies between partially isolated populations should systematically include heterogeneity in selective and introgressive effects among loci to perform more rigorous demographic inferences of the divergence process.
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Affiliation(s)
- Quentin Rougemont
- Département de biologie, Institut de Biologie Intégrative et des Systèmes (IBIS), Université Laval, G1V 0A6 Québec, Canada
| | - Louis Bernatchez
- Département de biologie, Institut de Biologie Intégrative et des Systèmes (IBIS), Université Laval, G1V 0A6 Québec, Canada
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50
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Matthey‐Doret R, Whitlock MC. Background selection andFST: Consequences for detecting local adaptation. Mol Ecol 2019; 28:3902-3914. [DOI: 10.1111/mec.15197] [Citation(s) in RCA: 51] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2018] [Revised: 06/19/2019] [Accepted: 07/03/2019] [Indexed: 01/03/2023]
Affiliation(s)
- Remi Matthey‐Doret
- Department of Zoology and Biodiversity Research Centre University of British Columbia Vancouver BC Canada
| | - Michael C. Whitlock
- Department of Zoology and Biodiversity Research Centre University of British Columbia Vancouver BC Canada
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