Pollination system and evolution of dioecy from distyly in Mussaenda parviflora (Rubiaceae)

The floral morphology and reproductive ecology of a scandent shrub species, Mussaenda parviflora Miq. (Rubiaceae), were studied on Iriomote and Amami Ohshima Islands, Japan. The morphological study demonstrated that M. parviflora was gynodioecious. Crossing experiments, however, revealed that it was functionally dioecious. The flowers were visited mainly by butterflies in the daytime and mainly by crepuscular hawkmoths in the evening. Dioecy in this species is thought to have evolved from distyly. Dioecy of this species, which is pollinated by long‐tongued Lepidoptera, is unusual as the evolution of dioecy from distyly in plants with long corolla tubes is thought to have been triggered by a pollinator shift from long‐ to short‐tongued insects. In this species, however, two floral morphological features may have caused the evolution. The first feature is the presence of many upward‐facing hairs covering the inner surface of the upper part of the corolla tube; these hairs appear to have impeded pollen flow from long‐ to short‐styled flowers. The second feature is the overlap in anther height between the two types of flowers, which may have reduced seed set by clogging stigma with incompatible pollen grains. Accordingly, male sterility in long‐styled plants may have evolved.