Different but not isolated: absence of reproductive barriers and strong floral divergence between ecotypes of Epidendrum fulgens (Orchidaceae)

Abstract

Reproductive barriers and divergent traits may occur between populations of the same species occurring at different habitat types. Here, we investigated the reproductive compatibility and flower morphology of eight populations of Epidendrum fulgens (Orchidaceae) distributed in two different disjunct environments: coastal sand dunes and inland rock outcrops. We investigated potential reproductive isolation barriers using artificial hand pollination experiments, and considering fruit set and seed viability as proxies for pre- and postzygotic barriers, respectively. For morphology investigation, we analysed the shape of flowers using a geometric morphometric approach. Contrary to our expectations, we found high levels of fruit set (0.85) and seed viability (0.73) in crosses between ecotypes, suggesting a lack of reproductive isolation barriers. Regarding floral morphology, canonical variate analysis revealed all floral parts were significantly different between ecotypes (P < .05), while Procrustes ANOVA on symmetric components of shape indicated lateral sepals (P = .005) and ventral sepal (P = .011) were significantly different. Considering that both ecotypes are found on different habitats, the floral morphological divergence may be affected by specific abiotic conditions affecting flower development and/or different pollinator assemblages. Despite the morphological differences, reproductive barriers between ecotypes are absent, suggesting populations from divergent environments may merge upon secondary contact. Our contrasting results highlight the importance of multidisciplinary approaches for providing insights into the processes operating during the early stages of speciation.