Ophrys annae and Ophrys chestermanii: an impossible love between two orchid sister species

The biological species concept is founded in the presence of barriers that restrict gene flow, but species delimitation remain a vivid topic in orchids. In sexually deceptive Ophrys, the role of postmating barriers has traditionally been thought to be weak and, despite relevant morphological and ethological differences, only low levels of phylogenetic and genetic differentiation have been recorded. In contrast, not many studies have been focused on the relevance of different barriers in closely related species on islands. Using a morphological approach and experimental crosses to evaluate postmating barriers, we here analyse the distinctness of two sardinian endemic sister-species, Ophrys annae and O. chestermanii, both currently referred to the O. bornmuelleri group. We found that the two endemics differ sustantially in 16 functional morphological traits. We further obtained asymmetric results from manual crosses suggesting that postmating barriers may reinforce the premating barriers. Our results suggest that in closely related species of Ophrys, gene flow is limited also by postmating barriers which contribute significantly to keeping the species reproductively isolated.