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Background Microsatellite markers were developed for distylous Linum suffruticosum and tested in the monomorphic sister species Linum tenuifolium. These species are perennial herbs endemic to the western and northwestern Mediterranean, respectively, with a partially overlapping distribution area.Methods and resultsWe developed 12 microsatellite markers for L. suffruticosum using next generation sequencing, and assessed their polymorphism and genetic diversity in 152 individuals from seven natural populations. The markers displayed high polymorphism, with two to 16 alleles per locus and population, and average observed and expected heterozygosities of 0.833 and 0.692, respectively. All loci amplified successfully in the sister species L. tenuifolium, and 150 individuals from seven populations were also screened. The polymorphism exhibited was high, with two to ten alleles per locus and population, and average observed and expected heterozygosities of 0.77 and 0.62, respectively. Conclusions The microsatellite markers identified in L. suffruticosum and tested in L. tenuifolium are a powerful tool to facilitate future investigations of the population genetics, mating patterns and hybridization between both Linum species in their contact zone.

Supergenes govern multi-trait-balanced polymorphisms in a wide range of systems; however, our understanding of their origins and evolution remains incomplete. The reciprocal placement of stigmas and anthers in pin and thrum floral morphs of distylous species constitutes an iconic example of a balanced polymorphism governed by a supergene, the distyly S-locus. Recent studies have shown that the Primula and Turnera distyly supergenes are both hemizygous in thrums, but it remains unknown whether hemizygosity is pervasive among distyly S-loci. As hemizygosity has major consequences for supergene evolution and loss, clarifying whether this genetic architecture is shared among distylous species is critical. Here, we have characterized the genetic architecture and evolution of the distyly supergene in Linum by generating a chromosome-level genome assembly of Linum tenue, followed by the identification of the S-locus using population genomic data. We show that hemizygosity and thrum-specific expression of S-linked genes, including a pistil-expressed candidate gene for style length, are major features of the Linum S-locus. Structural variation is likely instrumental for recombination suppression, and although the non-recombining dominant haplotype has accumulated transposable elements, S-linked genes are not under relaxed purifying selection. Our findings reveal remarkable convergence in the genetic architecture and evolution of independently derived distyly supergenes, provide a counterexample to classic inversion-based supergenes, and shed new light on the origin and maintenance of an iconic floral polymorphism.

Erica australis and Erica arborea are morphologically and ecologically similar heather species. Erica australis is restricted to the western Mediterranean Basin where it overlaps with the westernmost distribution of E. arborea. Here we investigate the role of the Strait of Gibraltar (SG) as a potential biogeographical barrier to dispersal and/or as glacial refugium in these two Erica spp. in the western Mediterranean (WMed) region with contrasting geographical origins and distributions. Samples were collected from 55 and 54 populations of E. australis and E. arborea, respectively. One individual each of 52 and 45 populations of E. australis and E. arborea, respectively, were sequenced for plastid DNA regions, and 1304 and 1214 individuals from 44 and 42 populations of E. australis and E. arborea, respectively, were genotyped using nuclear microsatellites (SSRs). Plastid DNA sequences were used to estimate divergence time of lineages and to construct haplotype networks. SSR data helped to infer population genetic diversity and fixation indices and genetic structure patterns through Bayesian analysis, analysis of molecular variance and isolation by distance. Plastid haplotype diversity of E. australis was higher in the SG than in the WMed area, whereas the opposite was found in E. arborea. SSRs revealed high genetic diversity within populations of both species and population genetic structure patterns were consistent with those retrieved from plastid DNA. The SG was identified as the likely area of origin and diversification for E. australis in the late Pliocene-Pleistocene, where it survived the Last Glacial Maximum (LGM) and expanded northwards into the western Iberian Peninsula. In contrast, two separate evolutionary lineages were found for E. arborea in the Iberian Peninsula. The SG and southern WMed areas represent the western Mediterranean expansion limit of an E. arborea lineage from East Africa/Arabia in the late Pliocene-Pleistocene, whereas the northern WMed

An integrative taxonomic approach, including molecular phylogenetic reconstructions based on plastid rps4-trnF and nuclear ITS sequences, statistical analysis of morphological-anatomical characters, and classical taxonomy, indicates that the reduction of 13 Amphidium species to three in a recent morphological revision represents a case of too extensive lumping. Instead, six Amphidium species can be distinguished based on molecular and morphological data, the widespread Amphidium lapponicum, A. mougeotii, and A. tortuosum, as well as the Macaronesian endemic A. curvipes, the North American endemic A. californicum, and a newly discovered species from Central Asia (southern Siberia and northern Mongolia), A. asiaticum sp. nov. Diagnostic morphological characters for all six species are discussed. The present data confirm that species diversity of Amphidium is highest in the Holarctic, where all six species occur.

Shifts in sexual systems are one of the key drivers of species diversification. In contrast to angiosperms, unisexuality prevails in bryophytes. Here, we test the hypotheses that bisexuality evolved from an ancestral unisexual condition and is a key innovation in liverworts. We investigate whether shifts in sexual systems influence diversification using hidden state speciation and extinction analysis (HiSSE). This new method compares the effects of the variable of interest to the best-fitting latent variable, yielding robust and conservative tests. We find that the transitions in sexual systems are significantly biased toward unisexuality, even though bisexuality is coupled with increased diversification. Sexual systems are strongly conserved deep within the liverwort tree but become much more labile toward the present. Bisexuality appears to be a key innovation in liverworts. Its effects on diversification are presumably mediated by the interplay of high fertilization rates, massive spore production and long-distance dispersal, which may separately or together have facilitated liverwort speciation, suppressed their extinction, or both. Importantly, shifts in liverwort sexual systems have the opposite effect when compared to angiosperms, leading to contrasting diversification patterns between the two groups. The high prevalence of unisexuality among liverworts suggests, however, a strong selection for sexual dimorphism.

Paleontological evidence and current patterns of angiosperm species richness suggest that European biota experienced more severe bottlenecks than North American ones during the last glacial maximum. How well this pattern fits other plant species is less clear. Bryophytes offer a unique opportunity to contrast the impact of the last glacial maximum in North America and Europe because about 60% of the European bryoflora is shared with North America. Here, we use population genetic analyses based on approximate Bayesian computation on eight amphi-Atlantic species to test the hypothesis that North American populations were less impacted by the last glacial maximum, exhibiting higher levels of genetic diversity than European ones and ultimately serving as a refugium for the postglacial recolonization of Europe. In contrast with this hypothesis, the best-fit demographic model involved similar patterns of population size contractions, comparable levels of genetic diversity and balanced migration rates between European and North American populations. Our results thus suggest that bryophytes have experienced comparable demographic glacial histories on both sides of the Atlantic. Although a weak, but significant genetic structure was systematically recovered between European and North American populations, evidence for migration from and towards both continents suggests that amphi-Atlantic bryophyte population may function as a metapopulation network. Reconstructing the biogeographic history of either North American or European bryophyte populations therefore requires a large, trans-Atlantic geographic framework.