Ménage à trois-two endemic species of deceptive orchids and one pollinator species

In the sexually deceptive orchid genus Ophrys, reproductive isolation is based on the specific attraction of males of a single pollinator species by mimicking the female species-specific sex pheromone. Changes in the odor composition can lead to hybridization and speciation by the attraction of a new pollinator that acts as an isolation barrier toward other sympatrically occurring Ophrys species. On Sardinia, we investigated the evolutionary origin of two sympatrically occurring endemic species, Ophrys chestermanii and O. normanii, which are both pollinated by males of the cuckoo bumblebee Bombus vestalis. Chemical and electrophysiological analyses of floral scent and genetic analyses with amplified fragment length polymorphisms and plastid-markers clearly showed that O. normanii is neither a hybrid nor a hybrid species. The two species evolved from different ancestors, viz. O. normanii from O. tenthredinifera and O. chestermanii from O. annae, and converged to the same pollinator attracted by the same bouquet of polar compounds. In spite of sympatry, pollinator sharing and overlapping blooming periods, no evidence has been obtained for gene flow between O. chestermanii and O. normanii indicating an unusual case among sexually deceptive orchids in which postmating rather than premating reproductive isolation mechanisms strongly prevent interspecific gene flow.