The genera Byttneria and Ayenia (Sterculiaceae or Malvaceae s.l.) are characterized by their unusual fly-pollinated flowers, with five petals that are clawed at the base and apically adnate to the top of the staminal column. The only character that consistently can be used to distinguish these two genera from each other is the number of anther thecae (two in the pantropical Byttneria and three in the exclusively neotropical Ayenia). Although basic construction of the flowers remains remarkably constant, habit and leaf morphology show extraordinary variation. Phylogenetic analyses of chloroplast DNA sequences indicate that Ayenia is monophyletic and nested within paraphyletic Byttneria. These results have important implications for the evolution of stamen number and further demonstrate the plasticity of this character within Byttnerioideae. This study contributes to a growing body of empirical work that examines the biogeography of tropical plants in a phylogenetic context. In the case of Byttneria and Ayenia, changes in growth form coincide with radiations into new geographical areas and habitats.

Key words: Ayenia, Byttneria, Malvaceae, Sterculiaceae