Polyploid formation shapes flowering plant diversity

Evolutionary patterns of variations in chromosome counts and genome sizes show positive correlations with taxonomic diversity in tropical gingers

PREMISE

Cytogenetic traits such as an organism's chromosome number and genome size are taxonomically critical as they are instrumental in defining angiosperm diversity. Variations in these traits can be traced to evolutionary processes such as polyploidization, although geographic variations across cytogenetic traits remain underexplored. In the pantropical monocot family Zingiberaceae (~1500 species), cytogenetic traits have been well documented; however, the role of these traits in shaping taxonomic diversity and biogeographic patterns of gingers is not known.

METHODS

A time-calibrated Bayesian phylogenetic tree was constructed for 290 taxa covering three of the four subfamilies in Zingiberaceae. We tested models of chromosome number and genome size evolution within the family and whether lineage age, taxonomic diversity, and distributional range explain the variations in the cytogenetic traits. Tests were carried out at two taxonomic ranks: within Zingiberaceae and within genus Hedychium using correlations, generalized linear models and phylogenetic least square models.

RESULTS

The most frequent changes in chromosome number within Zingiberaceae were noted to be demi-polyploidization and polyploidization (~57% of the time), followed by ascending dysploidy (~27%). The subfamily Zingiberoideae showed descending dysploidy at its base, while Alpinioideae showed polyploidization at its internal nodes. Although chromosome counts and genome sizes did not corroborate with each other, suggesting that they are not equivalent; higher chromosome number variations and higher genome size variations were associated with higher taxonomic diversity and wider biogeographic distribution.

CONCLUSIONS

Within Zingiberaceae, multiple incidences of polyploidization were discovered, and cytogenetic events appear to have reduced the genome sizes and increased taxonomic diversity, distributional ranges and invasiveness.

Deep reticulation: the long legacy of hybridization in vascular plant evolution

Hybridization has long been recognized as a fundamental evolutionary process in plants but, until recently, our understanding of its phylogenetic distribution and biological significance across deep evolutionary scales has been largely obscure. Over the past decade, genomic and phylogenomic datasets have revealed, perhaps not surprisingly, that hybridization, often associated with polyploidy, has been common throughout the evolutionary history of plants, particularly in various lineages of flowering plants. However, phylogenomic studies have also highlighted the challenges of disentangling signals of ancient hybridization from other sources of genomic conflict (in particular, incomplete lineage sorting). Here, we provide a critical review of ancient hybridization in vascular plants, outlining well-documented cases of ancient hybridization across plant phylogeny, as well as the challenges unique to documenting ancient versus recent hybridization. We provide a definition for ancient hybridization, which, to our knowledge, has not been explicitly attempted before. Further documenting the extent of deep reticulation in plants should remain an important research focus, especially because published examples likely represent the tip of the iceberg in terms of the total extent of ancient hybridization. However, future research should increasingly explore the macroevolutionary significance of this process, in terms of its impact on evolutionary trajectories (e.g. how does hybridization influence trait evolution or the generation of biodiversity over long time scales?), as well as how life history and ecological factors shape, or have shaped, the frequency of hybridization across geologic time and plant phylogeny. Finally, we consider the implications of ubiquitous ancient hybridization for how we conceptualize, analyze, and classify plant phylogeny. Networks, as opposed to bifurcating trees, represent more accurate representations of evolutionary history in many cases, although our ability to infer, visualize, and use networks for comparative analyses is highly limited. Developing improved methods for the generation, visualization, and use of networks represents a critical future direction for plant biology. Current classification systems also do not generally allow for the recognition of reticulate lineages, and our classifications themselves are largely based on evidence from the chloroplast genome. Updating plant classification to better reflect nuclear phylogenies, as well as considering whether and how to recognize hybridization in classification systems, will represent an important challenge for the plant systematics community.

Phylotranscriptomic analyses reveal multiple whole-genome duplication events, the history of diversification and adaptations in the Araceae

BACKGROUND AND AIMS

The Araceae are one of the most diverse monocot families with numerous morphological and ecological novelties. Plastid and mitochondrial genes have been used to investigate the phylogeny and to interpret shifts in the pollination biology and biogeography of the Araceae. In contrast, the role of whole-genome duplication (WGD) in the evolution of eight subfamilies remains unclear.

METHODS

New transcriptomes or low-depth whole-genome sequences of 65 species were generated through Illumina sequencing. We reconstructed the phylogenetic relationships of Araceae using concatenated and species tree methods, and then estimated the age of major clades using TreePL. We inferred the WGD events by Ks and gene tree methods. We investigated the diversification patterns applying time-dependent and trait-dependent models. The expansions of gene families and functional enrichments were analysed using CAFE and InterProScan.

KEY RESULTS

Gymnostachydoideae was the earliest diverging lineage followed successively by Orontioideae, Lemnoideae and Lasioideae. In turn, they were followed by the clade of 'bisexual climbers' comprised of Pothoideae and Monsteroideae, which was resolved as the sister to the unisexual flowers clade of Zamioculcadoideae and Aroideae. A special WGD event ψ (psi) shared by the True-Araceae clade occurred in the Early Cretaceous. Net diversification rates first declined and then increased through time in the Araceae. The best diversification rate shift along the stem lineage of the True-Araceae clade was detected, and net diversification rates were enhanced following the ψ-WGD. Functional enrichment analyses revealed that some genes, such as those encoding heat shock proteins, glycosyl hydrolase and cytochrome P450, expanded within the True-Araceae clade.

CONCLUSIONS

Our results improve our understanding of aroid phylogeny using the large number of single-/low-copy nuclear genes. In contrast to the Proto-Araceae group and the lemnoid clade adaption to aquatic environments, our analyses of WGD, diversification and functional enrichment indicated that WGD may play a more important role in the evolution of adaptations to tropical, terrestrial environments in the True-Araceae clade. These insights provide us with new resources to interpret the evolution of the Araceae.

Genome-wide characterization of ovate family protein gene family associated with number of seeds per silique in Brassica napus

Ovate family proteins (OFPs) were firstly identified in tomato as proteins controlling the pear shape of the fruit. Subsequent studies have successively proved that OFPs are a class of negative regulators of plant development, and are involved in the regulation of complex traits in different plants. However, there has been no report about the functions of OFPs in rapeseed growth to date. Here, we identified the OFPs in rapeseed at the genomic level. As a result, a total of 67 members were obtained. We then analyzed the evolution from Arabidopsis thaliana to Brassica napus, illustrated their phylogenetic and syntenic relationships, and compared the gene structure and conserved domains between different copies. We also analyzed their expression patterns in rapeseed, and found significant differences in the expression of different members and in different tissues. Additionally, we performed a GWAS for the number of seeds per silique (NSPS) in a rapeseed population consisting of 204 natural accessions, and identified a new gene BnOFP13_2 significantly associated with NSPS, which was identified as a novel function of OFPs. Haplotype analysis revealed that the accessions with haplotype 3 had a higher NSPS than other accessions, suggesting that BnOFP13_2 is associated with NSPS. Transcript profiling during the five stages of silique development demonstrated that BnOFP13_2 negatively regulates NSPS. These findings provide evidence for functional diversity of OFP gene family and important implications for oilseed rape breeding.

Cell types as species: Exploring a metaphor

The concept of "cell type," though fundamental to cell biology, is controversial. Cells have historically been classified into types based on morphology, physiology, or location. More recently, single cell transcriptomic studies have revealed fine-scale differences among cells with similar gross phenotypes. Transcriptomic snapshots of cells at various stages of differentiation, and of cells under different physiological conditions, have shown that in many cases variation is more continuous than discrete, raising questions about the relationship between cell type and cell state. Some researchers have rejected the notion of fixed types altogether. Throughout the history of discussions on cell type, cell biologists have compared the problem of defining cell type with the interminable and often contentious debate over the definition of arguably the most important concept in systematics and evolutionary biology, "species." In the last decades, systematics, like cell biology, has been transformed by the increasing availability of molecular data, and the fine-grained resolution of genetic relationships have generated new ideas about how that variation should be classified. There are numerous parallels between the two fields that make exploration of the "cell types as species" metaphor timely. These parallels begin with philosophy, with discussion of both cell types and species as being either individuals, groups, or something in between (e.g., homeostatic property clusters). In each field there are various different types of lineages that form trees or networks that can (and in some cases do) provide criteria for grouping. Developing and refining models for evolutionary divergence of species and for cell type differentiation are parallel goals of the two fields. The goal of this essay is to highlight such parallels with the hope of inspiring biologists in both fields to look for new solutions to similar problems outside of their own field.

Altered Developmental and Metabolic Gene Expression in Basil Interspecific Hybrids

To understand the altered developmental changes and associated gene expression in inter-genomic combinations, a study was planned in two diverse yet closely related species of Ocimum, targeting their hybrid F1 and amphidiploids. The existing developmental variations between F1 and amphidiploids was analyzed through phenotypical and anatomical assessments. The absence of 8330 transcripts of F1 in amphidiploids and the exclusive presence of two transcripts related to WNK lysine-deficient protein kinase and geranylgeranyl transferase type-2 subunit beta 1-like proteins in amphidiploids provided a set of genes to compare the suppressed and activated functions between F1 and amphidiploids. The estimation of eugenol and methyleugenol, flavonoid, lignin and chlorophyll content was correlated with the average FPKM and differential gene expression values and further validated through qRT-PCR. Differentially expressed genes of stomatal patterning and development explained the higher density of stomata in F1 and the larger size of stomata in amphidiploids. Gene expression study of several transcription factors putatively involved in the growth and developmental processes of plants clearly amalgamates the transcriptome data linking the phenotypic differences in F1 and amphidiploids. This investigation describes the influence of interspecific hybridization on genes and transcription factors leading to developmental changes and alleviation of intergenomic instability in amphidiploids.

In vitro octaploid induction of Populus hopeiensis with colchicine

BACKGROUND

Autopolyploids, especially artificial lines, provide model systems for understanding the mechanisms of gene dosage effects on trait variation owing to their relatively uniform genetic background. Here, a protocol for in vitro octaploid induction of Populus hopeiensis from leaf blades with colchicine treatment was established through investigation of the effects of different pre-culture durations, colchicine concentrations, and exposure times.

RESULTS

We found that pre-culture duration, colchicine concentration, and exposure time had significant effects on the survival rate, shoot regeneration rate, and octaploid induction rate of P. hopeiensis leaf blades. The highest octaploid induction rate (8.61%) was observed when leaf blades pre-cultured for 9 days were treated for 4 days with 100 μM colchicine. The ploidy level of all regenerated plantlets was analyzed by flow cytometry and further confirmed by chromosome counting. A total of 14 octaploids were obtained. The stomatal length, width, and density of leaf blades significantly differed between tetraploid and octaploid plants. Compared with diploid and tetraploid plants, octaploids had a slower growth rate, smaller leaf blade size, and shorter internodes.

CONCLUSIONS

We established an effective protocol for inducing octaploids in vitro from autotetraploid P. hopeiensis leaf blades by colchicine treatment.

The chromosome-scale genomes of Dipterocarpus turbinatus and Hopea hainanensis (Dipterocarpaceae) provide insights into fragrant oleoresin biosynthesis and hardwood formation

Dipterocarpaceae are typical tropical plants (dipterocarp forests) that are famous for their high economic value because of their production of fragrant oleoresins, top-quality timber and usage in traditional Chinese medicine. Currently, the lack of Dipterocarpaceae genomes has been a limiting factor to decipher the fragrant oleoresin biosynthesis and gain evolutionary insights into high-quality wood formation in Dipterocarpaceae. We generated chromosome-level genome assemblies for two representative Dipterocarpaceae species viz. Dipterocarpus turbinatus Gaertn. f. and Hopea hainanensis Merr. et Chun. Our whole-genome duplication (WGD) analysis revealed that Dipterocarpaceae underwent a shared WGD event, which showed significant impacts on increased copy numbers of genes related to the biosynthesis of terpene, BAHD acyltransferases, fatty acid and benzenoid/phenylpropanoid, which probably confer to the formation of their characteristic fragrant oleoresin. Additionally, compared with common soft wood plants, the expansion of gene families was also found to be associated with wood formation, such as in CESA (cellulose synthase), CSLE (cellulose synthase-like protein E), laccase and peroxidase in Dipterocarpaceae genomes, which might also contribute to the formation of harder, stronger and high-density timbers. Finally, an integrative analysis on a combination of genomic, transcriptomic and metabolic data from different tissues provided further insights into the molecular basis of fragrant oleoresins biosynthesis and high-quality wood formation of Dipterocarpaceae. Our study contributes the first two representative genomes for Dipterocarpaceae, which are valuable genetic resources for further researches on the fragrant oleoresins and superior-quality timber, genome-assisted breeding and improvement, and conservation biology of this family.

Evolution of AITR family genes in cotton and their functions in abiotic stress tolerance

Abiotic stresses are major environmental factors inhibiting plant growth and development. AITRs (ABA-induced transcription repressors) are a novel family of transcription factors regulating ABA (abscisic acid) signalling and plant responses to abiotic stresses in Arabidopsis. However, the composition and evolution history of AITRs and their roles in the cotton genus are largely unknown. A total of 12 putative AITRs genes were identified in cultivated tetraploid cotton, Gossypium hirsutum. Phylogenetic analysis of GhAITRs in these tetraploid cottons and their closely related species implicate ancient genome-wide duplication occurring after speciation of Gossypium, and Theobroma could generate duplicates of GhAITRs. Duplicated GhAITRs were stably inherited following diploid speciation and further allotetraploidy in Gossypium. Homologous GhAITRs shared common expression patterns in response to ABA, drought and salinity treatments, and drought tolerance induced in transgenic Arabidopsis plants expressing GhAITR-A1. Together, our findings reveal that duplicates in the GhAITRs gene family were achieved by whole genome duplication rather than three individual duplication events, and that GhAITRs function as transcription repressors and are involved in the regulation of plant responses to ABA and drought stress. These results provide insights towards the improvement of abiotic stress tolerance in cotton using GhAITRs.

Polyploidy formation in cancer cells: How a Trojan horse is born

Ploidy increase has been shown to occur in different type of tumors and participate in tumor initiation and resistance to the treatment. Polyploid giant cancer cells (PGCCs) are cells with multiple nuclei or a single giant nucleus containing multiple complete sets of chromosomes. The mechanism leading to formation of PGCCs may depend on: endoreplication, mitotic slippage, cytokinesis failure, cell fusion or cell cannibalism. Polyploidy formation might be triggered in response to various genotoxic stresses including: chemotherapeutics, radiation, hypoxia, oxidative stress or environmental factors like: air pollution, UV light or hyperthermia. A fundamental feature of polyploid cancer cells is the generation of progeny during the reversal of the polyploid state (depolyploidization) that may show high aggressiveness resulting in the formation of resistant disease and tumor recurrence. Therefore, we propose that modern anti-cancer therapies should be designed taking under consideration polyploidization/ depolyploidization processes, which confer the polyploidization a hidden potential similar to a Trojan horse delayed aggressiveness. Various mechanisms and stress factors leading to polyploidy formation in cancer cells are discussed in this review.

Integration of small RNA, degradome and proteome sequencing in Oryza sativa reveals a delayed senescence network in tetraploid rice seed

Seed of rice is an important strategic resource for ensuring the security of China's staple food. Seed deterioration as a result of senescence is a major problem during seed storage, which can cause major economic losses. Screening among accessions in rice germplasm resources for traits such as slow senescence and increased seed longevity during storage is, therefore, of great significance. However, studies on delayed senescence in rice have been based mostly on diploid rice seed to date. Despite better tolerance have been verified by the artificial aging treatment for polyploid rice seed, the delayed senescence properties and delayed senescence related regulatory mechanisms of polyploid rice seed are rarely reported, due to the lack of polyploid rice materials with high seed set. High-throughput sequencing was applied to systematically investigate variations in small RNAs, the degradome, and the proteome between tetraploid and diploid rice seeds. Degradome sequencing analysis of microRNAs showed that expression of miR-164d, which regulates genes encoding antioxidant enzymes, was changed significantly, resulting in decreased miRNA-mediated cleavage of target genes in tetraploid rice. Comparisons of the expression levels of small RNAs (sRNAs) in the tetraploid and diploid libraries revealed that 12 sRNAs changed significantly, consistent with the findings from degradome sequencing. Furthermore, proteomics also showed that antioxidant enzymes were up-regulated in tetraploid rice seeds, relative to diploids.

Polyploids increase overall diversity despite higher turnover than diploids in the Brassicaceae

Although polyploidy is widespread across the plant Tree of Life, its long-term evolutionary significance is still poorly understood. Here, we examine the effects of polyploidy in explaining the large-scale evolutionary patterns within angiosperms by focusing on a single family exhibiting extensive interspecific variation in chromosome numbers. We inferred ploidy from haploid chromosome numbers for 80% of species in the most comprehensive species-level chronogram for the Brassicaceae. After evaluating a total of 94 phylogenetic models of diversification, we found that ploidy influences diversification rates across the Brassicaceae. We also found that despite diversifying at a similar rate to diploids, polyploids have played a significant role in driving present-day differences in species richness among clades. Overall, in addition to highlighting the complexity in the evolutionary consequences of polyploidy, our results suggest that rare successful polyploids persist while significantly contributing to the long-term evolution of clades. Our findings further indicate that polyploidy has played a major role in driving the long-term evolution of the Brassicaceae and highlight the potential of polyploidy in shaping present-day diversity patterns across the plant Tree of Life.

Sequencing and Analyzing the Transcriptomes of a Thousand Species Across the Tree of Life for Green Plants

The 1,000 Plants (1KP) initiative was the first large-scale effort to collect next-generation sequencing (NGS) data across a phylogenetically representative sampling of species for a major clade of life, in this case theViridiplantae, or green plants. As an international multidisciplinary consortium, we focused on plant evolution and its practical implications. Among the major outcomes were the inference of a reference species tree for green plants by phylotranscriptomic analysis of low-copy genes, a survey of paleopolyploidy (whole-genome duplications) across the Viridiplantae, the inferred evolutionary histories for many gene families and biological processes, the discovery of novel light-sensitive proteins for optogenetic studies in mammalian neuroscience, and elucidation of the genetic network for a complex trait (C₄ photosynthesis). Altogether, 1KP demonstrated how value can be extracted from a phylodiverse sequencing data set, providing a template for future projects that aim to generate even more data, including complete de novo genomes, across the tree of life.

Bottlenecks and inbreeding depression in autotetraploids

Inbreeding depression is dependent on the ploidy of populations and can inhibit the evolution of selfing. While polyploids should generally harbor less inbreeding depression than diploids at equilibrium, it has been unclear whether this pattern holds in non-equilibrium conditions following bottlenecks. We use stochastic individual-based simulations to determine the effects of population bottlenecks on inbreeding depression in diploids and autotetraploids, in addition to cases where neo-autotetraploids form from the union of unreduced gametes. With a ploidy-independent dominance function based on enzyme kinetics, inbreeding depression is generally lower in autotetraploids for fully and partially recessive mutations. Due to the sampling of more chromosomes during reproduction, bottlenecks generally reduce inbreeding depression to a lesser extent in autotetraploids. All else being equal, population bottlenecks may have ploidy-dependent effects for another reason-in some cases matings between close relatives temporarily increase inbreeding depression in autotetraploids by increasing the frequency of the heterozygous genotype harboring the most harmful mutations. When neo-autotetraploids are formed by few individuals, inbreeding depression is dramatically reduced, given extensive masking of harmful mutations following whole genome duplication. This effect persists as nascent tetraploids reach mutation-selection-drift balance, providing a transient period of permissive conditions favoring the evolution of selfing.

Macroevolutionary Patterns of Flowering Plant Speciation and Extinction

Species diversity is remarkably unevenly distributed among flowering plant lineages. Despite a growing toolbox of research methods, the reasons underlying this patchy pattern have continued to perplex plant biologists for the past two decades. In this review, we examine the present understanding of transitions in flowering plant evolution that have been proposed to influence speciation and extinction. In particular, ploidy changes, transitions between tropical and nontropical biomes, and shifts in floral form have received attention and have offered some surprises in terms of which factors influence speciation and extinction rates. Mating systems and dispersal characteristics once predominated as determining factors, yet recent evidence suggests that these changes are not as influential as previously thought or are important only when paired with range shifts. Although range extent is an important correlate of speciation, it also influences extinction and brings an applied focus to diversification research. Recent studies that find that past diversification can predict present-day extinction risk open an exciting avenue for future research to help guide conservation prioritization.

Factors promoting polyploid persistence and diversification and limiting diploid speciation during the K-Pg interlude

The large wave of polyploidization following the Cretaceous-Paleogene (K-Pg) mass extinction has been explained by enhanced polyploid persistence arising from adaptive properties of the polyploids themselves, as well as an increase in unreduced gamete production and diploid hybridization. We propose that the demise of diploids afforded opportunities for polyploid establishment and expansion into novel habitats. Augmented polyploid gene pools from diploid and polyploid relatives, in association with their multiple and independent origins (of both autopolyploids and allopolyploids), facilitated their subsequent diversification. Their ability to recruit genetic variation from their diploid relatives or from products of recurrent origins sharing their genome(s) ostensibly contributed to polyploid persistence. Concomitantly, we propose that the number of congeneric diploid species dramatically contracted disproportionally to polyploids during the K-Pg interval (i.e. a diploid trough), resulting in a reduction in the rate of diploid speciation. Accordingly, the preponderance of neopolyploids was likely autopolyploids.

Cladogenetic and Anagenetic Models of Chromosome Number Evolution: A Bayesian Model Averaging Approach

ABSSTRACT

Chromosome number is a key feature of the higher-order organization of the genome, and changes in chromosome number play a fundamental role in evolution. Dysploid gains and losses in chromosome number, as well as polyploidization events, may drive reproductive isolation and lineage diversification. The recent development of probabilistic models of chromosome number evolution in the groundbreaking work by Mayrose et al. (2010, ChromEvol) have enabled the inference of ancestral chromosome numbers over molecular phylogenies and generated new interest in studying the role of chromosome changes in evolution. However, the ChromEvol approach assumes all changes occur anagenetically (along branches), and does not model events that are specifically cladogenetic. Cladogenetic changes may be expected if chromosome changes result in reproductive isolation. Here we present a new class of models of chromosome number evolution (called ChromoSSE) that incorporate both anagenetic and cladogenetic change. The ChromoSSE models allow us to determine the mode of chromosome number evolution; is chromosome evolution occurring primarily within lineages, primarily at lineage splitting, or in clade-specific combinations of both? Furthermore, we can estimate the location and timing of possible chromosome speciation events over the phylogeny. We implemented ChromoSSE in a Bayesian statistical framework, specifically in the software RevBayes, to accommodate uncertainty in parameter estimates while leveraging the full power of likelihood based methods. We tested ChromoSSE's accuracy with simulations and re-examined chromosomal evolution in Aristolochia, Carex section Spirostachyae, Helianthus, Mimulus sensu lato (s.l.), and Primula section Aleuritia, finding evidence for clade-specific combinations of anagenetic and cladogenetic dysploid and polyploid modes of chromosome evolution. [Anagenetic; Bayes factors; chromosome evolution; chromosome speciation; chromoSSE; cladogenetic; dysploidy; phylogenetic models; polyploidy; reversible-jump Markov chain Monte Carlo; whole genome duplication.].

Impact of whole-genome duplication events on diversification rates in angiosperms

PREMISE OF THE STUDY

Polyploidy or whole-genome duplication (WGD) pervades the evolutionary history of angiosperms. Despite extensive progress in our understanding of WGD, the role of these events in promoting diversification is still not well understood. We seek to clarify the possible association between WGD and diversification rates in flowering plants.

METHODS

Using a previously published phylogeny spanning all land plants (31,749 tips) and WGD events inferred from analyses of the 1000 Plants (1KP) transcriptome data, we analyzed the association of WGDs and diversification rates following numerous WGD events across the angiosperms. We used a stepwise AIC approach (MEDUSA), a Bayesian mixture model approach (BAMM), and state-dependent diversification analyses (MuSSE) to investigate patterns of diversification. Sister-clade comparisons were used to investigate species richness after WGDs.

KEY RESULTS

Based on the density of 1KP taxon sampling, 106 WGDs were unambiguously placed on the angiosperm phylogeny. We identified 334-530 shifts in diversification rates. We found that 61 WGD events were tightly linked to changes in diversification rates, and state-dependent diversification analyses indicated higher speciation rates for subsequent rounds of WGD. Additionally, 70 of 99 WGD events showed an increase in species richness compared to the sister clade.

CONCLUSIONS

Forty-six of the 106 WGDs analyzed appear to be closely associated with upshifts in the rate of diversification in angiosperms. Shifts in diversification do not appear more likely than random within a four-node lag phase following a WGD; however, younger WGD events are more likely to be followed by an upshift in diversification than older WGD events.

Genome-Wide Analysis of the NF-YB Gene Family in Gossypium hirsutum L. and Characterization of the Role of GhDNF-YB22 in Embryogenesis

Members of the NF-YB transcription factor gene family play important roles in diverse processes related to plant growth and development, such as seed development, drought tolerance, and flowering time. However, the function of NF-YB genes in cotton remains unclear. A total of 23, 24, and 50 NF-YB genes were identified in Gossypium arboreum (G. arboreum), Gossypium raimondii (G. raimondii), and G. hirsutum, respectively. A systematic phylogenetic analysis was carried out in G. arboretum, G. raimondii, G. hirsutum, Arabidopsis thaliana, cacao, rice and, sorghum, where the 150 NF-YB genes were divided into five groups (α–ε). Of these groups, α is the largest clade, and γ contains the LEC1 type NF-YB proteins. Syntenic analyses revealed that paralogues of NF-YB genes in G. hirsutum exhibited good collinearity. Owing to segmental duplication within the A sub-genome (A_t) and D sub-genome (D_t), there was an expanded set of NF-YB genes in G. hirsutum. Furthermore, we investigated the structures of exons, introns, and conserved motifs of NF-YB genes in upland cotton. Most of the NF-YB genes had only one exon, and the genes from the same clade exhibited a similar motif pattern. Expression data show that most NF-YB genes were expressed ubiquitously, and only a few genes were highly expressed in specific tissues, as confirmed by quantitative real-time PCR (qRT-PCR) analysis. The overexpression of GhDNF-YB22 gene, predominantly expressed in embryonic tissues, indicates that GhDNF-YB22 may affect embryogenesis in cotton. This study is the first comprehensive characterization of the GhNF-YB gene family in cotton, and showed that NF-YB genes could be divided into five clades. The duplication events that occurred over the course of evolution were the major impetus for NF-YB gene expansion in upland cotton. Collectively, this work provides insight into the evolution of NF-YB in cotton and further our knowledge of this commercially important species.

Homoeologous exchanges cause extensive dosage-dependent gene expression changes in an allopolyploid crop

Structural variation is a major source of genetic diversity and an important substrate for selection. In allopolyploids, homoeologous exchanges (i.e. between the constituent subgenomes) are a very frequent type of structural variant. However, their direct impact on gene content and gene expression had not been determined. Here, we used a tissue-specific mRNA-Seq dataset to measure the consequences of homoeologous exchanges (HE) on gene expression in Brassica napus, a representative allotetraploid crop. We demonstrate that expression changes are proportional to the change in gene copy number triggered by the HEs. Thus, when homoeologous gene pairs have unbalanced transcriptional contributions before the HE, duplication of one copy does not accurately compensate for loss of the other and combined homoeologue expression also changes. These effects are, however, mitigated over time. This study sheds light on the origins, timing and functional consequences of homeologous exchanges in allopolyploids. It demonstrates that the interplay between new structural variation and the resulting impacts on gene expression, influences allopolyploid genome evolution.

Karyotypic variation and pollen stainability in resynthesized allopolyploids Tragopogon miscellus and T. mirus

PREMISE OF THE STUDY

Polyploidy has extensively shaped the evolution of plants, but the early stages of polyploidy are still poorly understood. The neoallopolyploid species Tragopogon mirus and T. miscellus are both characterized by widespread karyotypic variation, including frequent aneuploidy and intergenomic translocations. Our study illuminates the origins and early impacts of this variation by addressing two questions: How quickly does karyotypic variation accumulate in Tragopogon allopolyploids following whole-genome duplication (WGD), and how does the fertility of resynthesized Tragopogon allopolyploids evolve shortly after WGD?

METHODS

We used genomic in situ hybridization and lactophenol-cotton blue staining to estimate the karyotypic variation and pollen stainability, respectively, of resynthesized T. mirus and T. miscellus during the first five generations after WGD.

KEY RESULTS

Widespread karyotypic variation developed quickly in synthetics and resembled that of naturally occurring T. mirus and T. miscellus by generation S₄ . Pollen stainability in resynthesized allopolyploids was consistently lower than that of natural T. mirus and T. miscellus, as well as their respective diploid progenitor species. Logistic regression showed that mean pollen stainability increased slightly over four generations in resynthesized T. mirus but remained at equivalent levels in T. miscellus.

CONCLUSIONS

Our results clarify some of the changes that occur in T. mirus and T. miscellus immediately following their origin, most notably the rapid onset of karyotypic variation within these species immediately following WGD.

The effects of genome duplications in a community context

Contents 57 I. 57 II. 59 III. 59 IV. 63 V. 64 VI. 64 VII. 66 66 References 66 SUMMARY: Whole-genome duplication (WGD), or polyploidy, has important effects on the genotype and phenotype of plants, potentially altering ecological interactions with other organisms. Even though the connections between polyploidy and species interactions have been recognized for some time, we are only just beginning to test whether WGD affects community context. Here I review the sparse information on polyploidy and community context and then present a set of hypotheses for future work. Thus far, community-level studies of polyploids suggest an array of outcomes, from no changes in community context to shifts in the abundance and composition of interacting species. I propose a number of mechanisms for how WGD could alter community context and how the emergence of polyploids in populations could also alter the community context of parental diploids and other plant species. Resolving how and when these changes are expected to occur will require a deeper understanding of the connections among WGD, phenotypic changes, and the direct and indirect effects of species interactions.

The evolutionary significance of polyploidy

Polyploidy, or the duplication of entire genomes, has been observed in prokaryotic and eukaryotic organisms, and in somatic and germ cells. The consequences of polyploidization are complex and variable, and they differ greatly between systems (clonal or non-clonal) and species, but the process has often been considered to be an evolutionary 'dead end'. Here, we review the accumulating evidence that correlates polyploidization with environmental change or stress, and that has led to an increased recognition of its short-term adaptive potential. In addition, we discuss how, once polyploidy has been established, the unique retention profile of duplicated genes following whole-genome duplication might explain key longer-term evolutionary transitions and a general increase in biological complexity.

Niche divergence between diploid and autotetraploid Tolmiea

PREMISE OF STUDY

Polyploidy is common in eukaryotes and is of major evolutionary importance over both short and long time-scales. Compared to allopolyploids, autopolyploids remain understudied; they are often morphologically cryptic and frequently remain taxonomically unrecognized, although there is increasing recognition of the high frequency of autopolyploidy in angiosperms. While autopolyploidy can serve as an instant speciation mechanism, little is known about the ecological consequences of this process. We describe the ecological divergence of a diploid-autotetraploid species pair in Tolmiea.

METHODS

We investigated whether abiotic niche divergence has shaped the current allopatric distribution of diploid T. diplomenziesii and its autotetraploid derivative, T. menziesii, in the Pacific Northwest of North America. We employed field measures of light availability, as well as niche modeling and a principal component analysis of environmental space. Within a common garden, we also investigated physiological responses to changes in soil moisture.

KEY RESULTS

Diploid and autotetraploid Tolmiea inhabit significantly different climatic niche spaces. The climatic niche divergence between these two species is best explained by a shift in precipitation availability, and we found evidence of differing physiological response to water availability between these species.

CONCLUSIONS

We found that spatial segregation of T. diplomenziesii and T. menziesii was accompanied by adaptation to changes in climatic regime. Tolmiea menziesii is not a nascent autotetraploid, having persisted long enough to be established throughout the Pacific Northwest, and therefore both polyploidization and subsequent evolution have contributed to the observed differences between T. menziesii and T. diplomenziesii.

Analyses of transcriptome sequences reveal multiple ancient large-scale duplication events in the ancestor of Sphagnopsida (Bryophyta)

The goal of this research was to investigate whether there has been a whole-genome duplication (WGD) in the ancestry of Sphagnum (peatmoss) or the class Sphagnopsida, and to determine if the timing of any such duplication(s) and patterns of paralog retention could help explain the rapid radiation and current ecological dominance of peatmosses. RNA sequencing (RNA-seq) data were generated for nine taxa in Sphagnopsida (Bryophyta). Analyses of frequency plots for synonymous substitutions per synonymous site (Ks ) between paralogous gene pairs and reconciliation of 578 gene trees were conducted to assess evidence of large-scale or genome-wide duplication events in each transcriptome. Both Ks frequency plots and gene tree-based analyses indicate multiple duplication events in the history of the Sphagnopsida. The most recent WGD event predates divergence of Sphagnum from the two other genera of Sphagnopsida. Duplicate retention is highly variable across species, which might be best explained by local adaptation. Our analyses indicate that the last WGD could have been an important factor underlying the diversification of peatmosses and facilitated their rise to ecological dominance in peatlands. The timing of the duplication events and their significance in the evolutionary history of peat mosses are discussed.

Phylogenetic evidence for cladogenetic polyploidization in land plants

PREMISE OF THE STUDY

Polyploidization is a common and recurring phenomenon in plants and is often thought to be a mechanism of "instant speciation". Whether polyploidization is associated with the formation of new species (cladogenesis) or simply occurs over time within a lineage (anagenesis), however, has never been assessed systematically.

METHODS

We tested this hypothesis using phylogenetic and karyotypic information from 235 plant genera (mostly angiosperms). We first constructed a large database of combined sequence and chromosome number data sets using an automated procedure. We then applied likelihood models (ClaSSE) that estimate the degree of synchronization between polyploidization and speciation events in maximum likelihood and Bayesian frameworks.

KEY RESULTS

Our maximum likelihood analysis indicated that 35 genera supported a model that includes cladogenetic transitions over a model with only anagenetic transitions, whereas three genera supported a model that incorporates anagenetic transitions over one with only cladogenetic transitions. Furthermore, the Bayesian analysis supported a preponderance of cladogenetic change in four genera but did not support a preponderance of anagenetic change in any genus.

CONCLUSIONS

Overall, these phylogenetic analyses provide the first broad confirmation that polyploidization is temporally associated with speciation events, suggesting that it is indeed a major speciation mechanism in plants, at least in some genera.

Accounting for genotype uncertainty in the estimation of allele frequencies in autopolyploids

Despite the increasing opportunity to collect large-scale data sets for population genomic analyses, the use of high-throughput sequencing to study populations of polyploids has seen little application. This is due in large part to problems associated with determining allele copy number in the genotypes of polyploid individuals (allelic dosage uncertainty-ADU), which complicates the calculation of important quantities such as allele frequencies. Here, we describe a statistical model to estimate biallelic SNP frequencies in a population of autopolyploids using high-throughput sequencing data in the form of read counts. We bridge the gap from data collection (using restriction enzyme based techniques [e.g. GBS, RADseq]) to allele frequency estimation in a unified inferential framework using a hierarchical Bayesian model to sum over genotype uncertainty. Simulated data sets were generated under various conditions for tetraploid, hexaploid and octoploid populations to evaluate the model's performance and to help guide the collection of empirical data. We also provide an implementation of our model in the R package polyfreqs and demonstrate its use with two example analyses that investigate (i) levels of expected and observed heterozygosity and (ii) model adequacy. Our simulations show that the number of individuals sampled from a population has a greater impact on estimation error than sequencing coverage. The example analyses also show that our model and software can be used to make inferences beyond the estimation of allele frequencies for autopolyploids by providing assessments of model adequacy and estimates of heterozygosity.