Crossing barriers between the unrewarding Mediterranean orchids Serapias vomeracea and S. cordigera

Understanding the genetic architecture of admixed hybridizing populations helps inevaluating the nature of species boundaries and the levels of gene exchange betweenco-occurring species. In the present study, we examined a contact zone between Serapiasvomeracea and Serapias cordigera, two unrewarding Mediterranean orchid species with anon-specific pollination strategy. Fruit production and seed viability from interspecifichand-pollination treatments pointed out the weaknesses of post-pollination barriers. Theoccurrence of hybridization was molecularly confirmed in the genus Serapias for the firsttime, as parts of plants with a transitional morphology were observed in both alleles ofthe parental LEAFY intron. Amplified fragment length polymorphism analysis revealedthat plants with uncertain morphology and classified as one of the other parental speciesare actually backcrosses, attesting to an extensive interspecific gene exchange. Overall, thecontact zone is more similar to a hybrid zone of Ophrys species, well known for theirhighly specialized pollination, than to a hybrid zone of unspecialized food-deceptiveorchids. Therefore, species boundaries in Serapias are maintained by pre-pollinationmechanisms that need to be better investigated. In light of the intriguing similaritiesbetween Serapias and Ophrys underlined by the present study, we hypothesize that theemission of floral scents could be involved in the maintenance of species boundaries inSerapias.