Reproductive versus floral isolation among morphologically similar Serapias L. species (Orchidaceae)

Flowers of the Mediterranean orchid genus Serapias L. form small, dark tubes that vary among taxa in diameter and depth. Visiting insects use the floral tube as shelter and act as pollinators if they touch the sticky viscidium at the rear of the tube and remove the pollinarium. It has been assumed that floral tube size and shape limit access to the flowers and thus may act as a barrier to gene flow between different Serapias species. Here we investigated floral characters and nuclear microsatellite markers in populations belonging to three morphologically similar Serapias species to test whether these species show evidence for floral or reproductive isolation. We found strong overlap of floral traits between two species, suggesting that floral isolation is nonexistent between them. Microsatellite markers applied to the same populations were highly polymorphic and revealed clear genetic differentiation among all three species. These results suggest that reproductive isolation exists, despite the lack of floral isolation between two of the species. In contrast to morphological characters, diagnostic microsatellite alleles were found for all Serapias species. The microsatellite markers could thus provide a useful tool to identify Serapias species and further investigate evolutionary relationships in this fascinating orchid lineage.