Whole genome duplication is considered to be a significant contributor to angiosperm speciation due to accumulation of rapid, strong interploid reproductive isolation. However, recent work suggests ...that interploid reproductive isolation may not be complete, especially among higher order cytotypes. This study evaluates postzygotic reproductive isolation among three cytotypes within a polyploid complex.
We conducted reciprocal crosses using two diploid and two hexaploid populations each crossed to tetraploid populations spanning the geographic and phylogenetic range of the Campanula rotundifolia polyploid complex. Interploid and intrapopulation crosses were scored for fruit set, seed number, germination proportion and pollen viability. Postzygotic isolation was calculated for each cross as the product of these fitness components. A subset of offspring was cytotyped via flow cytometry.
Postzygotic isolation was significantly lower in tetraploid–hexaploid crosses than diploid–tetraploid crosses, mostly due to substantially higher germination among tetraploid–hexaploid crosses. Tetraploid–hexaploid crosses produced pentaploids exclusively, whereas diploid–tetraploid crosses produced both triploids and tetraploids in high frequencies.
Postzygotic isolation was weaker among higher order polyploids than between diploids and tetraploids, and unreduced gametes may facilitate diploid–tetraploid reproduction. This incomplete postzygotic isolation could allow ongoing interploid gene flow, especially among higher order polyploids, which may slow divergence and speciation in polyploid complexes.
Most land plants are now known to be ancient polyploids that have rediploidized. Diploidization involves many changes in genome organization that ultimately restore bivalent chromosome pairing and ...disomic inheritance, and resolve dosage and other issues caused by genome duplication. In this review, we discuss the nature of polyploidy and its impact on chromosome pairing behavior. We also provide an overview of two major and largely independent processes of diploidization: cytological diploidization and genic diploidization fractionation. Finally, we compare variation in gene fractionation across land plants and highlight the differences in diploidization between plants and animals. Altogether, we demonstrate recent advancements in our understanding of variation in the patterns and processes of diploidization in land plants and provide a road map for future research to unlock the mysteries of diploidization and eukaryotic genome evolution.
Whole‐genome duplication is considered an important speciation mechanism in plants. However, its effect on reproductive isolation between higher cytotypes is not well understood. We used backcrosses ...between different ploidy levels and surveys of mixed‐ploidy contact zones to determine how reproductive barriers differed with cytotype across a polyploid complex. We backcrossed F1 hybrids derived from 2X‐4X and 4X‐6X crosses in the Campanula rotundifolia autopolyploid complex, measured backcross fitness, and estimated backcross DNA cytotype. We then sampled four natural mixed‐ploidy contact zones (two 2X‐4X and two 4X‐6X), estimated ploidy, and genotyped individuals across each contact zone. Reproductive success and capacity for gene flow was markedly lower for 2X‐4X than 4X‐6X hybrids. In fact, 3X hybrids could not backcross; all 2X‐4X backcross progeny resulted from neotetraploid F1 hybrids. Further, no 3X individuals were found in 2X‐4X contact zones, and 2X and 4X individuals were genetically distinct. By contrast, backcrosses of 5X hybrids were relatively successful, particularly when crossed to 6X individuals. In 4X‐6X contact zones, 5X individuals and aneuploids were common and all cytotypes were largely genetically similar and spatially intermixed. Taken together, these results provide strong evidence that reproduction is low between 2X and 4X cytotypes, primarily occurring via unreduced gamete production, but that reproduction and gene flow are ongoing between 4X and 6X cytotypes. Further, it suggests whole‐genome duplication can result in speciation between diploids and polyploids, but is less likely to create reproductive barriers between different polyploid cytotypes, resulting in two fundamentally different potentials for speciation across polyploid complexes.
To assess the role of ploidy in determining reproductive isolation and speciation in polyploid contact zones, we used backcrosses between different ploidy levels and surveys of mixed‐ploidy contact zones to determine how reproductive barriers differed with cytotype across a polyploid complex. Reproductive success and capacity for gene flow was markedly higher for 4X‐6X hybrids than 2X‐4X hybrids, which was also seen in natural mixed‐ploidy contact zones. Our results suggest whole‐genome duplication can result in speciation between diploids and polyploids, but is less likely to create reproductive barriers between different polyploid cytotypes, resulting in two fundamentally different potentials for speciation across polyploid complexes.
Premise Of The Study
Both intrinsic and extrinsic factors contribute to a species distribution. Among plants, the extrinsic effects of glaciation and intrinsic effects of whole genome duplication are ...powerful drivers of biogeographical patterns, but the interplay of these factors is poorly understood. Here, we investigate the roles glaciation and whole‐genome duplication have played in the evolution of the widespread polyploid complex Campanula rotundifolia.
Methods
We assessed the cytotype of 37 populations that spanned the geographic and cytotypic range of the C. rotundifolia complex. We constructed a chloroplast phylogeny for these populations and used RAD‐seq to create nuclear phylogenies and networks for a subset of 23 populations; and estimated divergence times of major clades using Bayesian estimation of substitution rates.
Key Results
Campanula rotundifolia originated in south‐central Europe and underwent range expansion throughout much of Europe and North America. Multiple genome duplications have occurred in C. rotundifolia—at least two tetraploid and three hexaploid formations.
Conclusions
Nuclear and chloroplast phylogenies are largely congruent with a history of populations surviving glacial maxima in known Pleistocene refugia in Europe and North America. Divergent European clades are consistent with two disjunct glacial refugia within Europe. North America was colonized by hexaploids derived from Western European lineages. A glacial refugium in Midwestern North America likely facilitated post‐glacial recolonization of North America and limited genetic divergence. These results implicate both glaciation and whole‐genome duplication as contributing factors to the extant biogeography of C. rotundifolia.
When differentiated lineages come into contact, their fates depend on demographic and reproductive factors. These factors have been well-studied in taxa of the same ploidy, but less is known about ...sympatric lineages that differ in ploidy, particularly with respect to demographic factors. We assessed prezygotic, postzygotic, and total reproductive isolation in naturally pollinated arrays of diploid-tetraploid and tetraploid-hexaploid population mixes of Campanula rotundifolia by measuring pollinator transitions, seed yield, germination rate, and proportion of hybrid offspring. Four frequencies of each cytotype were tested, and pollinators consistently overvisited rare cytotypes. Seed yield and F1 hybrid production were greater in 4X-6X arrays than 2X-4X arrays, whereas germination rates were similar, creating two distinct patterns of reproductive isolation. In 2X-4X arrays, postzygotic isolation was near complete (3% hybrid offspring), and prezygotic isolation associated with pollinator preference is expected to facilitate the persistence of minority cytotypes. However, in 4X-6X arrays where postzygotic isolation permitted hybrid formation (44% hybrids), pollinator behavior drove patterns of reproductive isolation, with rare cytotypes being more isolated and greater gene flow expected from rare into common cytotypes. In polyploid complexes, both the specific cytotypes in contact and local cytotype frequency, likely reflecting spatial demography, will influence likelihood of gene exchange.
Premise of the Study
Angiosperm species often shift from self‐incompatibility to self‐compatibility following population bottlenecks. Across the range of a species, population bottlenecks may result ...from multiple factors, each of which may affect the geographic distribution and magnitude of mating‐system shifts. We describe how intercontinental dispersal and genome duplication facilitate loss of self‐incompatibility.
Methods
Self and outcross pollinations were performed on plants from 24 populations of the Campanula rotundifolia polyploid complex. Populations spanned the geographic distribution and three dominant cytotypes of the species (diploid, tetraploid, hexaploid).
Key Results
Loss of self‐incompatibility was associated with both intercontinental dispersal and genome duplication. European plants were largely self‐incompatible, whereas North American plants were intermediately to fully self‐compatible. Within both European and North American populations, loss of self‐incompatibility increased as ploidy increased. Ploidy change and intercontinental dispersal both contributed to loss of self‐incompatibility in North America, but range expansion did not affect self‐incompatibility within Europe or North America.
Conclusions
When species are subject to population bottlenecks arising through multiple factors, each factor can contribute to self‐incompatibility loss. In a widespread polyploid complex, the loss of self‐incompatibility can be predicted by the cumulative effects of whole‐genome duplication and intercontinental dispersal.
The aquatic fern
Salvinia molesta
D.S. Mitch. is an invasive species that can have devastating effects on the freshwater habitats it colonizes. Currently, a lack of clarity surrounding the genomic ...composition and genetic diversity of
S. molesta
impedes eradication efforts.
Salvinia molesta
is a polyploid hybrid with unknown and controversial parentage, first noted in Africa but morphologically similar to South American species. Giant salvinia is also thought to reproduce primarily, perhaps exclusively, through vegetative reproduction, raising the possibility that the global invasion comprises one or a few clonal genotypes. This research focuses on identifying the maternal genome donor of
S. molesta
, determining if this species consists of a single or multiple independently derived lineages, and evaluating invasive-range genotypic diversity. Whole chloroplast genome (plastome) sequencing from field-collected and herbarium specimens was used to quantify genetic diversity in
S. molesta
and the phylogenetic relationships among
Salvinia
species. Phylogenetic analysis revealed that
S. molesta
and
S. herzogii
share the same plastome, although
S. herzogii
is unlikely to be
S. molesta
’s maternal progenitor due to its own hybrid status and odd ploidy. Rather, we conclude that
S. molesta
’s maternal progenitor is either an undescribed or extinct species. The observed plastome diversity within
S. molesta
indicates the presence of multiple divergent genotypes which strongly suggest multiple origins of this hybrid. Additionally, this diversity clearly indicates that a single clone does not dominate the invasive range. This genomic diversity could have direct implications for the successful management of this invasive species, particularly for biological control.
Botany is the root and the future of invasion biology Sutherland, Brittany L.; Barrett, Craig F.; Beck, James B. ...
American journal of botany,
April 2021, 2021-Apr, 2021-04-00, 20210401, Letnik:
108, Številka:
4
Journal Article
Premise
Whole‐genome duplications (WGDs) are prevalent throughout the evolutionary history of plants. For example, dozens of WGDs have been phylogenetically localized across the order Brassicales, ...specifically, within the family Brassicaceae. A WGD event has also been identified in the Cleomaceae, the sister family to Brassicaceae, yet its placement, as well as that of WGDs in other families in the order, remains unclear.
Methods
Phylo‐transcriptomic data were generated and used to infer a nuclear phylogeny for 74 Brassicales taxa. Genome survey sequencing was also performed on 66 of those taxa to infer a chloroplast phylogeny. These phylogenies were used to assess and confirm relationships among the major families of the Brassicales and within Brassicaceae. Multiple WGD inference methods were then used to assess the placement of WGDs on the nuclear phylogeny.
Results
Well‐supported chloroplast and nuclear phylogenies for the Brassicales and the putative placement of the Cleomaceae‐specific WGD event Th‐ɑ are presented. This work also provides evidence for previously hypothesized WGDs, including a well‐supported event shared by at least two members of the Resedaceae family, and a possible event within the Capparaceae.
Conclusions
Phylogenetics and the placement of WGDs within highly polyploid lineages continues to be a major challenge. This study adds to the conversation on WGD inference difficulties by demonstrating that sampling is especially important for WGD identification and phylogenetic placement. Given its economic importance and genomic resources, the Brassicales continues to be an ideal group for assessing WGD inference methods.
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