Isolation mechanisms between two sympatric Sophronitis (Orchidaceae) species endemic to Northeastern Brazil

Sophronitis sincorana and S. pfisteri (Orchidaceae) are endemic to the campo rupestre vegetation, northeastern Brazil, occurring sympatrically and flowering synchronously. In this work we studied the reproductive biology of both species and investigated reproductive barriers between them. Both species are partially self-compatible, pollinator dependent, and are pollinated by males and worker bees of Bombus brevivillus. Pollinaria are deposited on the anterior portion of the mesonotum of these insects in S. pfisteri, and on the scutellum in S. sincorana. The flowers of both species are deceptive, as they have a dry cuniculus. Morphological differences in the labellum and the gynostemium impede inter-specific pollination of S. pfisteri with pollen from S. sincorana. Very low fruiting results from inter-specific crosses of S. sincorana with pollen from S. pfisteri. Although viable seeds can be produced from experimental inter-specific crosses, no natural hybrids were found. Species integrity appears to be maintained by additive unidirectional reproductive barriers between them.