Reproductive barriers and genetic differentiation between continental and island populations of Epidendrum fulgens (Orchidaceae)

Studies on insular organisms provide an important framework for investigating patterns of genetic differentiation and reproductive isolation. By focusing on populations of the same species, we have the opportunity to study the mechanisms operating during the earliest stages of speciation, as reproductive barriers can be examined among divergent lineages in a geographic context. We investigated the genetic differentiation and the evolution of early stages of intrinsic postmating reproductive isolation between continental and insular populations of Epidendrum fulgens, a neotropical orchid distributed in southeastern Brazil. Genetic diversity and structure were estimated for both nuclear and plastid markers by using genetic differentiation measures and model-based assignment test. Furthermore, two components of reproductive isolation were examined by analyzing fruit set and seed viability in interpopulation crosses. Strong plastid genetic structure (F-ST = 0.679) was found between insular and remaining populations, indicating that E. fulgens reduced gene flow via seed dispersal, although significant nuclear genetic structure was lower (D-ST = 0.179), likely due to the smaller effective population size of the plastid genome. Significant differences in seed viability between self- and cross-pollinated plants were found, indicating the occurrence of inbreeding depression in all populations. Seed viability was significantly lower in crosses between insular and remaining populations, suggesting a late postzygotic reproductive barrier due to low migration associated with genetic drift (i.e., bottleneck) and its consequences. Our results confirm the importance of islands as drivers for populational differentiation and suggest a central role for outbreeding depression during the early stages of lineage diversification.

Automated summary

This study on the neotropical orchid Epidendrum fulgens investigated the genetic differentiation and reproductive isolation between insular and continental populations, finding stronger plastid genetic structure compared to nuclear genetic structure. Seed viability showed significant differences between self-pollinated and cross-pollinated plants, with a lower viability in crosses between insular and continental populations, indicating late postzygotic reproductive barriers due to low migration associated with genetic drift. Islands play a significant role in driving populational differentiation and outbreeding depression is important during the early stages of lineage diversification.