Gender Dimorphism in Tetradium daniellii (Rutaceae): Floral Biology, Gametogenesis, and Sexual System Evolution

We investigated the sexual system, floral biology, sporogenesis, and gametogenesis of Tetradium daniellii (Bennett) Hartley, which possessed distinct male and monoecious sexual phenotypes. Male individuals produced almost exclusively male flowers. In the years 2000–2002, the protandrous monoecious individuals had male and female flowering phases completely separated in time within and between the plants, while in 2003 and 2004, slight overlap of male and female functions occurred. During early floral development, all flowers were hermaphroditic. Dimorphic unisexual flowers resulted from later selective abortion of the gynoecium or androecium. Male flowers and their pollen in the two sexual morphs did not significantly differ in structure. Pollen grains from male flowers on both male and monoecious plants were fertile and could germinate on the stigmas of female flowers on monoecious plants. Maturation of female gametophytes in monoecious plants coincided with the release of the three-celled pollen in the male plant. We propose that the sexual system of T. daniellii has arisen from hermaphroditism and is likely to lead to dioecism.