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Diversity of MHC IIB genes and parasitism in hybrids of evolutionarily divergent cyprinoid species indicate heterosis advantage. Sci Rep 2021; 11:16860. [PMID: 34413384 PMCID: PMC8376869 DOI: 10.1038/s41598-021-96205-x] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2020] [Accepted: 08/02/2021] [Indexed: 02/07/2023] Open
Abstract
The genes of the major histocompatibility complex (MHC) are an essential component of the vertebrate immune system and MHC genotypes may determine individual susceptibility to parasite infection. In the wild, selection that favors MHC variability can create situations in which interspecies hybrids experience a survival advantage. In a wild system of two naturally hybridizing leuciscid fish, we assessed MHC IIB genetic variability and its potential relationships to hosts' ectoparasite communities. High proportions of MHC alleles and parasites were species-specific. Strong positive selection at specific MHC codons was detected in both species and hybrids. MHC allele expression in hybrids was slightly biased towards the maternal species. Controlling for a strong seasonal effect on parasite communities, we found no clear associations between host-specific parasites and MHC alleles or MHC supertypes. Hybrids shared more MHC alleles with the more MHC-diverse parental species, but expressed intermediate numbers of MHC alleles and positively selected sites. Hybrids carried significantly fewer ectoparasites than either parent species, suggesting a hybrid advantage via potential heterosis.
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Talarico L, Marta S, Rossi AR, Crescenzo S, Petrosino G, Martinoli M, Tancioni L. Balancing selection, genetic drift, and human-mediated introgression interplay to shape MHC (functional) diversity in Mediterranean brown trout. Ecol Evol 2021; 11:10026-10041. [PMID: 34367556 PMCID: PMC8328470 DOI: 10.1002/ece3.7760] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2021] [Revised: 05/14/2021] [Accepted: 05/17/2021] [Indexed: 12/19/2022] Open
Abstract
The extraordinary polymorphism of major histocompatibility complex (MHC) genes is considered a paradigm of pathogen-mediated balancing selection, although empirical evidence is still scarce. Furthermore, the relative contribution of balancing selection to shape MHC population structure and diversity, compared to that of neutral forces, as well as its interaction with other evolutionary processes such as hybridization, remains largely unclear. To investigate these issues, we analyzed adaptive (MHC-DAB gene) and neutral (11 microsatellite loci) variation in 156 brown trout (Salmo trutta complex) from six wild populations in central Italy exposed to introgression from domestic hatchery lineages (assessed with the LDH gene). MHC diversity and structuring correlated with those at microsatellites, indicating the substantial role of neutral forces. However, individuals carrying locally rare MHC alleles/supertypes were in better body condition (a proxy of individual fitness/parasite load) regardless of the zygosity status and degree of sequence dissimilarity of MHC, hence supporting balancing selection under rare allele advantage, but not heterozygote advantage or divergent allele advantage. The association between specific MHC supertypes and body condition confirmed in part this finding. Across populations, MHC allelic richness increased with increasing admixture between native and domestic lineages, indicating introgression as a source of MHC variation. Furthermore, introgression across populations appeared more pronounced for MHC than microsatellites, possibly because initially rare MHC variants are expected to introgress more readily under rare allele advantage. Providing evidence for the complex interplay among neutral evolutionary forces, balancing selection, and human-mediated introgression in shaping the pattern of MHC (functional) variation, our findings contribute to a deeper understanding of the evolution of MHC genes in wild populations exposed to anthropogenic disturbance.
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Affiliation(s)
- Lorenzo Talarico
- Laboratory of Experimental Ecology and AquacultureDepartment of BiologyUniversity of Rome “Tor Vergata”RomeItaly
| | - Silvio Marta
- Department of Environmental Science and PolicyUniversity of MilanMilanItaly
| | - Anna Rita Rossi
- Department of Biology and Biotechnology C. DarwinUniversity of Rome “La Sapienza”RomeItaly
| | - Simone Crescenzo
- Department of Biology and Biotechnology C. DarwinUniversity of Rome “La Sapienza”RomeItaly
| | - Gerardo Petrosino
- Department of Biology and Biotechnology C. DarwinUniversity of Rome “La Sapienza”RomeItaly
| | - Marco Martinoli
- Laboratory of Experimental Ecology and AquacultureDepartment of BiologyUniversity of Rome “Tor Vergata”RomeItaly
- Consiglio per la Ricerca in Agricoltura e l'Analisi dell'Economia Agraria (CREA)Centro di Zootecnia e AcquacolturaMonterotondoItaly
| | - Lorenzo Tancioni
- Laboratory of Experimental Ecology and AquacultureDepartment of BiologyUniversity of Rome “Tor Vergata”RomeItaly
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Vlček J, Štefka J. Association between louse abundance and MHC II supertypes in Galápagos mockingbirds. Parasitol Res 2020; 119:1597-1605. [PMID: 32006226 DOI: 10.1007/s00436-020-06617-3] [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: 09/17/2019] [Accepted: 01/22/2020] [Indexed: 01/30/2023]
Abstract
Major histocompatibility complex class II (MHC II) is an essential molecule triggering the adaptive immune response by the presentation of pathogens to helper T cells. The association between individual MHC II variants and various parasites has become a frequent finding in studies of vertebrate populations. However, although bird ectoparasites have a significant effect on their host's fitness, and the host's immune system can regulate ectoparasitic infections, no study has yet investigated the association between MHC II polymorphism and ectoparasite infection in the populations of free-living birds. Here, we test whether an association exists between the abundance of a chewing louse (Myrsidea nesomimi) and MHC II polymorphism of its hosts, the Galápagos mockingbirds (Mimus). We have found that the presence of two MHC II supertypes (functionally differentiated clusters) was significantly associated with louse abundance. This pattern supports the theory that a co-evolutionary interaction stands behind the maintenance of MHC polymorphism. Moreover, we have found a positive correlation between louse abundance and heterophil/lymphocyte ratio (an indicator of immunological stress) that serves as an additional piece of evidence that ectoparasite burden is affected by immunological state of Galápagos mockingbirds.
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Affiliation(s)
- Jakub Vlček
- Institute of Parasitology, Biology Centre CAS, Branišovská 1160/31, 37005, České Budějovice, Czech Republic. .,Faculty of Science, University of South Bohemia, Branišovská, 1760, České Budějovice, Czech Republic.
| | - Jan Štefka
- Institute of Parasitology, Biology Centre CAS, Branišovská 1160/31, 37005, České Budějovice, Czech Republic.,Faculty of Science, University of South Bohemia, Branišovská, 1760, České Budějovice, Czech Republic
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Stefan T, Matthews L, Prada JM, Mair C, Reeve R, Stear MJ. Divergent Allele Advantage Provides a Quantitative Model for Maintaining Alleles with a Wide Range of Intrinsic Merits. Genetics 2019; 212:553-564. [PMID: 30952668 PMCID: PMC6553829 DOI: 10.1534/genetics.119.302022] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2019] [Accepted: 03/30/2019] [Indexed: 01/28/2023] Open
Abstract
The Major Histocompatibility Complex (MHC) is the most genetically diverse region of the genome in most vertebrates. Some form of balancing selection is necessary to account for the extreme diversity, but the precise mechanism of balancing selection is unknown. Due to the way MHC molecules determine immune recognition, overdominance (also referred to as heterozygote advantage) has been suggested as the main driving force behind this unrivalled diversity. However, both theoretical results and simulation models have shown that overdominance in its classical form cannot maintain large numbers of alleles unless all alleles confer unrealistically similar levels of fitness. There is increasing evidence that heterozygotes containing genetically divergent alleles allow for broader antigen presentation to immune cells, providing a selective mechanism for MHC polymorphism. By framing competing models of overdominance within a general framework, we show that a model based on Divergent Allele Advantage (DAA) provides a superior mechanism for maintaining alleles with a wide range of intrinsic merits, as intrinsically less-fit MHC alleles that are more divergent can survive under DAA. Specifically, our results demonstrate that a quantitative mechanism built from the DAA hypothesis is able to maintain polymorphism in the MHC. Applying such a model to both livestock breeding and conservation could provide a better way of identifying superior heterozygotes, and quantifying the advantages of genetic diversity at the MHC.
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Affiliation(s)
- Thorsten Stefan
- Boyd Orr Centre for Population and Ecosystem Health, Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, G12 8QQ, United Kingdom
- Institute of Applied Mathematics and Statistics, University of Hohenheim, 70593 Stuttgart, Germany
| | - Louise Matthews
- Boyd Orr Centre for Population and Ecosystem Health, Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, G12 8QQ, United Kingdom
| | - Joaquin M Prada
- Boyd Orr Centre for Population and Ecosystem Health, Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, G12 8QQ, United Kingdom
| | - Colette Mair
- Boyd Orr Centre for Population and Ecosystem Health, Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, G12 8QQ, United Kingdom
| | - Richard Reeve
- Boyd Orr Centre for Population and Ecosystem Health, Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, G12 8QQ, United Kingdom
| | - Michael J Stear
- Boyd Orr Centre for Population and Ecosystem Health, Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, G12 8QQ, United Kingdom
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Osborne MJ, Pilger TJ, Lusk JD, Turner TF. Spatio-temporal variation in parasite communities maintains diversity at the major histocompatibility complex class IIβ in the endangered Rio Grande silvery minnow. Mol Ecol 2016; 26:471-489. [PMID: 27864911 DOI: 10.1111/mec.13936] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2016] [Revised: 11/02/2016] [Accepted: 11/14/2016] [Indexed: 01/30/2023]
Abstract
Climate change will strongly impact aquatic ecosystems particularly in arid and semi-arid regions. Fish-parasite interactions will also be affected by predicted altered flow and temperature regimes, and other environmental stressors. Hence, identifying environmental and genetic factors associated with maintaining diversity at immune genes is critical for understanding species' adaptive capacity. Here, we combine genetic (MHC class IIβ and microsatellites), parasitological and ecological data to explore the relationship between these factors in the remnant wild Rio Grande silvery minnow (Hybognathus amarus) population, an endangered species found in the southwestern United States. Infections with multiple parasites on the gills were observed and there was spatio-temporal variation in parasite communities and patterns of infection among individuals. Despite its highly endangered status and chronically low genetic effective size, Rio Grande silvery minnow had high allelic diversity at MHC class IIβ with more alleles recognized at the presumptive DAB1 locus compared to the DAB3 locus. We identified significant associations between specific parasites and MHC alleles against a backdrop of generalist parasite prevalence. We also found that individuals with higher individual neutral heterozygosity and higher amino acid divergence between MHC alleles had lower parasite abundance and diversity. Taken together, these results suggest a role for fluctuating selection imposed by spatio-temporal variation in pathogen communities and divergent allele advantage in maintenance of high MHC polymorphism. Understanding the complex interaction of habitat, pathogens and immunity in protected species will require integrated experimental, genetic and field studies.
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Affiliation(s)
- Megan J Osborne
- Department of Biology and Museum of Southwestern Biology, University of New Mexico, MSC 03-2020, Albuquerque, NM, 87131, USA
| | - Tyler J Pilger
- Department of Biology and Museum of Southwestern Biology, University of New Mexico, MSC 03-2020, Albuquerque, NM, 87131, USA
| | - Joel D Lusk
- U.S. Fish and Wildlife Service, New Mexico Ecological Services, Albuquerque, NM, 87113, USA
| | - Thomas F Turner
- Department of Biology and Museum of Southwestern Biology, University of New Mexico, MSC 03-2020, Albuquerque, NM, 87131, USA
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