Contrasting pollination ecology of Disepalum species (Annonaceae): evolutionary loss of the floral chamber and partial breakdown of protogyny associated with a shift in pollination system

Disepalum comprises two monophyletic sister subgenera, Enicosanthellum and Disepalum, with strikingly different floral morphologies: the former has two whorls of unfused sepals, forming a partially enclosed floral chamber, whereas the latter possesses a single whorl of congenitally fused petals and lacks a floral chamber. The pollination ecologies of representative species are reported, including assessments of floral phenology, pollinators and floral thermogenesis. Disepalum pulchrum (subgenus Enicosanthellum) has hermaphroditic flowers with a pollination chamber and is protogynous with prolonged anthesis; it is pollinated by nitidulid beetles and drosophilid flies. Disepalum anomalum (subgenus Disepalum) is also hermaphroditic with prolonged anthesis, but has incomplete protogyny due to overlapping pistillate and staminate phases; it is pollinated by meliponine bees, which are attracted by the pollen, but which are only able to transfer pollen to receptive stigmas during the overlap phase. Alternative evolutionary hypotheses are evaluated, including the possibility that the ancestor of the subgenus Disepalum lineage may have experienced a profound genetic mutation, possibly involving genes responsible for organ merism and fusion, resulting in the loss of the pollination chamber and hence favouring different floral visitors. The breakdown in protogyny required for effective pollination is likely to have had significant ramifications on population genetic diversity.