The hypothesis of gene flow between species with large differences in chromosome numbers has rarely been tested in the wild, mainly because species of different ploidy are commonly assumed to be reproductively isolated from each other because of instantaneous and strong postzygotic barriers. In this study, a broad-scale survey of molecular variation was carried out between two orchid species with different ploidy levels: Epidendrum fulgens (2n = 2x = 24 chromosomes) and Epidendrum puniceoluteum (2n = 4x = 52 chromosomes). To test the strength of their reproductive barriers, we investigated the distribution of genetic variation in sympatric and allopatric populations of these two species and conducted crossing experiments. Nuclear and plastid microsatellite loci were used to genotype 463 individuals from eight populations across the geographical range of both species along the Brazilian coastal plain. All six sympatric populations analysed presented hybrid zones, indicating that hybridization between E. fulgens and E. puniceoluteum is a common phenomenon. Bayesian assignment analysis detected the presence of F 1 and F 2 individuals and also signs of introgression, demonstrating a high potential for interspecific gene flow. Introgression occurs preferentially from E. fulgens to E. puniceoluteum. Pure parental individuals of both species display strong genotype-habitat associations, indicating that environment-dependent selection could be acting in all hybrid zones. This study suggests that hybridization and introgression are evolutionary processes playing a role in the diversification of Epidendrum and indicates the importance of investigations of hybrid zones in understanding reproductive barriers and speciation processes in Neotropical orchid species.

Aim Phylogeographical studies in the Brazilian Atlantic Forest (BAF) have mostly included species associated with forest habitats, whereas taxa associated with grassland and sand-dune plant communities have so far been largely overlooked. This study examines the phylogeography of the orchid Epidendrum fulgens, which occurs on coastal sand dunes and granitic outcrops, in order to identify major genetic divergences or disjunctions across the range of the species and to investigate the genetic signatures of past range contractions and expansions.Location Southern and south-eastern seashore vegetation along the BAF biome, and granitic and arenitic outcrops that occur in the subtropical grassland plant communities located south of the BAF.Methods Nine nuclear and four plastid microsatellite loci were used to genotype 424 individuals from 16 populations across the distributional range of E. fulgens. For both sets of markers, we estimated genetic diversity and population differentiation, testing for a north-south gradient of genetic diversity. The plastid haplotype network and a Bayesian assignment analysis of nuclear markers were used to infer population structure. Past demographic changes were investigated using a coalescence approach.Results A deep disjunction was found between northern populations within the BAF and southern populations outside the BAF that occur on granitic and arenitic outcrops. Recent demographic reductions were detected in northern populations on coastal sands. Such demographic changes were not expected for those populations, as previous studies with forest species had found evidence of population expansion in the same areas. Higher genetic diversity was found in southern populations on granite, in contrast to patterns observed in previous studies of forest species. Main conclusionsThe results are consistent with the long-term persistence of E. fulgens. Bottlenecks were detected in populations from areas where population expansion events have been detected in other plant (and animal) species, suggesting that forest expansion after the Last Glacial Maximum played a role in the population fragmentation and decrease in genetic diversity in E. fulgens. A substantial genetic division in E. fulgens corresponds to the 'Portal de Torres', a region that demarcates the northern limits of subtropical grassland plant communities and the southern limits of the BAF.

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BackgroundBecause of their fragmented nature, inselberg species are interesting biological models for studying the genetic consequences of disjoint populations. Inselbergs are commonly compared with oceanic islands, as most of them display a marked ecological isolation from the surrounding area. The isolation of these rock outcrops is reflected in the high number of recorded endemic species and the strong floristic differences between individual inselbergs and adjacent habitats. We examined the genetic connectivity of orchids Epidendrum cinnabarinum and E. secundum adapted to Neotropical inselbergs of northeastern Brazil. Our goals were to identify major genetic divergences or disjunctions across the range of the species and to investigate potential demographic and evolutionary mechanisms leading to lineage divergence in Neotropical mountain ecosystems.ResultsBased on plastid markers, high genetic differentiation was found for E. cinnabarinum (FST = 0.644) and E. secundum (FST = 0.636). Haplotypes were not geographically structured in either taxon, suggesting that restricted gene flow and genetic drift may be significant factors influencing the diversification of these inselberg populations. Moreover, strong differentiation was found between populations over short spatial scales, indicating substantial periods of isolation among populations. For E. secundum, nuclear markers indicated higher gene flow by pollen than by seeds.ConclusionsThe comparative approach adopted in this study contributed to the elucidation of patterns in both species. Our results confirm the ancient and highly isolated nature of inselberg populations. Both species showed similar patterns of genetic diversity and structure, highlighting the importance of seed-restricted gene flow and genetic drift as drivers of plant diversification in terrestrial islands such as inselbergs.

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