Reproductive barriers and fertility of two neotropical orchid species and their natural hybrid

Different pre- and postzygotic isolating mechanisms may prevent interspecific gene exchange in secondary contact zones. Due to the different nature of each isolating barrier, which may act in different life history stages, multidisciplinary approaches are crucial to investigate the evolution of reproductive isolation (RI) in contact zones. In this study, we analysed seven different pre- and postzygotic RI mechanisms and reproductive success of two neotropical orchid species with contrasting pollination strategies, the nectarless food-deceptive Epidendrum denticulatum and the nectar rewarding E. orchidiflorum. The two species occur sympatrically in the coastal vegetation of Southeastern Brazil and share habitats with their natural hybrid E. x purpureum. Our aim was to test the contribution of pre and postzygotic reproductive barriers to species cohesion, examining potential asymmetries among RI mechanisms. Our results indicate habitat isolation as an important prezygotic barrier, strongly influenced by the contrasting habitat preferences found between the parental species. Hybrid sterility was also important, though incomplete, to prevent species collapse in this hybrid zone. This latter barrier was likely shaped by strong differences in chromosome numbers found between parental species (E. denticulatum 2n = 52, E. orchidiflorum 2n = 156). Indeed, hybrids showed lower levels of fertility when compared to parental species, probably due to meiotic abnormalities found in hybrid plants. However, contrary to our expectations, hybrid plants are still able to attract flower visitors during the day and night, and natural pollination success was comparable to one of the parental species, suggesting sexual reproduction of hybrid plants may contribute to the persistence of this hybrid zone. This study highlights the importance of studying hybrid zones between species diverging in several morphological and ecological traits, where the balance of hybridization is still unpredictable and almost unknown.