The development of the superior ovary in Tetraplasandra (Araliaceae)

Tetraplasandra is a small Hawaiian genus of seven species with remarkable diversity in ovary position, ranging from inferior to completely superior. Tetraplasandra gymnocarpa is the only member of the Araliaceae with a fully superior ovary. A comparative study of floral anatomy and development in superior and inferior ovary species of Tetraplasandra revealed that the superior ovary in T. gymnocarpa is unusual in that it develops within an epigynous ground plan. During the course of development, the ovary changes from inferior to secondarily superior primarily by an upward expansion of the ovary from the insertion point of the perianth and androecium to the ovary apex. The superior ovary of T. gymnocarpa, evident in late ontogeny, is a modified inferior ovary; thus it is not structurally homologous to a truly superior ovary. The adaptive significance of the switch from inferior to superior ovary is reexamined. A recent phylogeny of Tetraplasandra and the biogeography of the extant species provide evidence that the change in ovary position may be associated with a shift in pollination strategy that may have occurred as recently as 2.6 million years ago.