Endosperm-based hybridization barriers explain the pattern of gene flow between Arabidopsis lyrata and Arabidopsis arenosa in Central Europe.

Based on the biological species concept, two species are considered distinct if reproductive barriers prevent gene flow between them. In Central Europe, the diploid species Arabidopsis lyrata and Arabidopsis arenosa are genetically isolated, thus fitting this concept as "good species." Nonetheless, interspecific gene flow involving their tetraploid forms has been described. The reasons for this ploidy-dependent reproductive isolation remain unknown. Here, we show that hybridization between diploid A. lyrata and A. arenosa causes mainly inviable seed formation, revealing a strong postzygotic reproductive barrier separating these two species. Although viability of hybrid seeds was impaired in both directions of hybridization, the cause for seed arrest differed. Hybridization of A. lyrata seed parents with A. arenosa pollen donors resulted in failure of endosperm cellularization, whereas the endosperm of reciprocal hybrids cellularized precociously. Endosperm cellularization failure in both hybridization directions is likely causal for the embryo arrest. Importantly, natural tetraploid A. lyrata was able to form viable hybrid seeds with diploid and tetraploid A. arenosa, associated with the reestablishment of normal endosperm cellularization. Conversely, the defects of hybrid seeds between tetraploid A. arenosa and diploid A. lyrata were aggravated. According to these results, we hypothesize that a tetraploidization event in A. lyrata allowed the production of viable hybrid seeds with A. arenosa, enabling gene flow between the two species.

Non-reciprocal Interspecies Hybridization Barriers in the Capsella Genus Are Established in the Endosperm.

The transition to selfing in Capsella rubella accompanies its recent divergence from the ancestral outcrossing C. grandiflora species about 100,000 years ago. Whether the change in mating system was accompanied by the evolution of additional reproductive barriers that enforced species divergence remained unknown. Here, we show that C. rubella and C. grandiflora are reproductively separated by an endosperm-based, non-reciprocal postzygotic hybridization barrier. While hybridizations of C. rubella maternal plants with C. grandiflora pollen donors resulted in complete seed abortion caused by endosperm cellularization failure, the reciprocal hybridization resulted in the formation of small seeds with precociously cellularized endosperm. Strikingly, the transcriptomic response of both hybridizations mimicked respectively the response of paternal and maternal excess hybridizations in Arabidopsis thaliana, suggesting unbalanced genome strength causes hybridization failure in both species. These results provide strong support for the theory that crosses between plants of different mating systems will be unbalanced, with the outcrosser behaving like a plant of increased ploidy, evoking a response that resembles an interploidy-type seed failure. Seed incompatilibity of C. rubella pollinated by C. grandiflora followed the Bateson-Dobzhansky-Muller model, involving negative genetic interaction of multiple paternal C. grandiflora loci with at least one maternal C. rubella locus. Given that both species only recently diverged, our data suggest that a fast evolving mechanism underlies the post-zygotic hybridization barrier(s) separating both species.

The evolutionary history of the white-rayed species of Melampodium (Asteraceae) involved multiple cycles of hybridization and polyploidization.

PREMISE OF THE STUDY: Polyploidy plays an important role in race differentiation and eventually speciation. Underlying mechanisms include chromosomal and genomic changes facilitating reproductive isolation and/or stabilization of hybrids. A prerequisite for studying these processes is a sound knowledge on the origin of polyploids. A well-suited group for studying polyploid evolution consists of the three species of Melampodium ser. Leucantha (Asteraceae): M. argophyllum, M. cinereum, and M. leucanthum. METHODS: The origin of polyploids was inferred using network and tree-based phylogenetic analyses of several plastid and nuclear DNA sequences and of fingerprint data (AFLP). Genome evolution was assessed via genome size measurements, karyotype analysis, and in situ hybridization of ribosomal DNA. KEY RESULTS: Tetraploid cytotypes of the phylogenetically distinct M. cinereum and M. leucanthum had, compared to the diploid cytotypes, doubled genome sizes and no evidence of gross chromosomal rearrangements. Hexaploid M. argophyllum constituted a separate lineage with limited intermixing with the other species, except in analyses from nuclear ITS. Its genome size was lower than expected if M. cinereum and/or M. leucanthum were involved in its origin, and no chromosomal rearrangements were evident. CONCLUSIONS: Polyploids in M. cinereum and M. leucanthum are of recent autopolyploid origin in line with the lack of significant genomic changes. Hexaploid M. argophyllum also appears to be of autopolyploid origin against the previous hypothesis of an allopolyploid origin involving the other two species, but some gene flow with the other species in early phases of differentiation cannot be excluded.

Multiple Pleistocene refugia and Holocene range expansion of an abundant southwestern American desert plant species (Melampodium leucanthum, Asteraceae).

Pleistocene climatic fluctuations had major impacts on desert biota in southwestern North America. During cooler and wetter periods, drought-adapted species were isolated into refugia, in contrast to expansion of their ranges during the massive aridification in the Holocene. Here, we use Melampodium leucanthum (Asteraceae), a species of the North American desert and semi-desert regions, to investigate the impact of major aridification in southwestern North America on phylogeography and evolution in a widespread and abundant drought-adapted plant species. The evidence for three separate Pleistocene refugia at different time levels suggests that this species responded to the Quaternary climatic oscillations in a cyclic manner. In the Holocene, once differentiated lineages came into secondary contact and intermixed, but these range expansions did not follow the eastwardly progressing aridification, but instead occurred independently out of separate Pleistocene refugia. As found in other desert biota, the Continental Divide has acted as a major migration barrier for M. leucanthum since the Pleistocene. Despite being geographically restricted to the eastern part of the species' distribution, autotetraploids in M. leucanthum originated multiple times and do not form a genetically cohesive group.

Quaternary range dynamics and polyploid evolution in an arid brushland plant species (Melampodium cinereum, Asteraceae).

Pleistocene climatic fluctuations played a principal role for range formation and population history of many biota, including regions not directly affected by glaciations, such as the arid habitats of the southwestern United States and adjacent Mexico. Specifically, drought-adapted species are expected to have persisted during cooler and wetter periods in one or more refugia, resulting in lineage differentiation, from where they reached their current distribution after range expansion in the course of Holocene aridification. Here, we test this hypothesis using Melampodium cinereum (Asteraceae), a morphologically and cytologically variable species of dry brushlands of Texas and adjacent Mexico. In line with the hypothesized presence of several refugia, AFLP data provide strong evidence for the presence of geographically distinct genetic lineages, which, however, only partly agree with current intraspecific taxonomy. Despite multiple origins, tetraploids form a genetically cohesive group. The exclusive occurrence of tetraploids in a range parapatric to that of the diploids likely results from former geographic isolation of cytotypes, lending further support for the presence of Pleistocene refugia. Whereas plastid sequence data show a clear signal for the expected Holocene range and population expansion, they show little geographic structure and high levels of intrapopulational diversity. This may be due to lineage sorting during periods of population separation and/or substantial gene flow among populations via seeds, which has not been sufficient to erode the overall pattern of genetic divergence resulting from geographic isolation.

Molecular phylogenetic analyses of nuclear and plastid DNA sequences support dysploid and polyploid chromosome number changes and reticulate evolution in the diversification of Melampodium (Millerieae, Asteraceae).

Chromosome evolution (including polyploidy, dysploidy, and structural changes) as well as hybridization and introgression are recognized as important aspects in plant speciation. A suitable group for investigating the evolutionary role of chromosome number changes and reticulation is the medium-sized genus Melampodium (Millerieae, Asteraceae), which contains several chromosome base numbers (x=9, 10, 11, 12, 14) and a number of polyploid species, including putative allopolyploids. A molecular phylogenetic analysis employing both nuclear (ITS) and plastid (matK) DNA sequences, and including all species of the genus, suggests that chromosome base numbers are predictive of evolutionary lineages within Melampodium. Dysploidy, therefore, has clearly been important during evolution of the group. Reticulate evolution is evident with allopolyploids, which prevail over autopolyploids and several of which are confirmed here for the first time, and also (but less often) on the diploid level. Within sect. Melampodium, the complex pattern of bifurcating phylogenetic structure among diploid taxa overlain by reticulate relationships from allopolyploids has non-trivial implications for intrasectional classification.

Spatial and temporal determinants of genetic structure in Gentianella bohemica.

The biennial plant Gentianella bohemica is a subendemic of the Bohemian Massif, where it occurs in seminatural grasslands. It has become rare in recent decades as a result of profound changes in land use. Using amplified fragment length polymorphisms (AFLP) fingerprint data, we investigated the genetic structure within and among populations of G. bohemica in Bavaria, the Czech Republic, and the Austrian border region. The aim of our study was (1) to analyze the genetic structure among populations and to discuss these findings in the context of present and historical patterns of connectivity and isolation of populations, (2) to analyze genetic structure among consecutive generations (cohorts of two consecutive years), and (3) to investigate relationships between intrapopulational diversity and effective population size (N(e)) as well as plant traits. (1) The German populations were strongly isolated from each other (pairwise F(ST)= 0.29-0.60) and from all other populations (F(ST)= 0.24-0.49). We found a pattern of near panmixis among the latter (F(ST)= 0.15-0.35) with geographical distance explaining only 8% of the genetic variance. These results were congruent with a principal coordinate analysis (PCoA) and analysis using STRUCTURE to identify genetically coherent groups. These findings are in line with the strong physical barrier and historical constraints, resulting in separation of the German populations from the others. (2) We found pronounced genetic differences between consecutive cohorts of the German populations (pairwise F(ST)= 0.23 and 0.31), which can be explained by local population history (land use, disturbance). (3) Genetic diversity within populations (Shannon index, H(Sh)) was significantly correlated with N(e) (R(S)= 0.733) and reflected a loss of diversity due to several demographic bottlenecks. Overall, we found that the genetic structure in G. bohemica is strongly influenced by historical periods of high connectivity and isolation as well as by marked demographic fluctuations in declining populations.