Biomechanics, pollination, and evolution of Catasetum (Catasetinae, Orchidaceae)

Catasetum is a large Neotropical genus of orchids with strong floral sexual dimorphism and extreme morphological variation in its male flowers. The large pollinarium of male Catasetum flowers is forcibly discharged onto its pollinator as it enters the flower and contacts specialized triggering appendages. In this study, the biomechanics of this remarkable pollination system was characterized, as well as the phylogenetics of Catasetum and related genera in the Catasetinae subtribe. The propulsive energy necessary to eject the pollinarium was found to be stored in the bending of the stipe, a membranous strip of tissue that is part of the pollinarium. Moreover, a unique suite of biomechanical characters involved in pollinarium ejection and targeting was found to have enabled the exceptional diversification of male flower shape in Catasetum. The phylogenetic inference revealed substantial evidence of hybridization in Catasetum , which complements previous research on the occurrence of natural hybrids in the genus. An examination of pollinarium expulsion within the phylogenetic framework showed that variation in the key biomechanical parameters of flower opening angle and relative column deflection is not constrained by the degree of relatedness among Catasetum species.