Perianth symmetry in sexually differentiated flowers of Akebia quinata (Lardizabalaceae)

Sexual differentiation of monoecious plants usually involves differentiation in the size of female and male flowers produced on the same individuals. In the nectarless Akebia quinata (Houtt.) Decne (Lardizabalaceae), the trimeric, actinomorphic female flowers are larger than the males, which is explained as an adaptive trait to prevent self-pollination, as conspicuous female flowers are usually visited by pollinators earlier than smaller male flowers of the same individuals. This results in the plants being cross-pollinated rather than geitonogamously pollinated. However, it is also known that the development of the perianth in this species is genetically associated with the ontogeny of the petaloid sepals. These are thus developmentally linked to the ontogeny of the stamens. Therefore, it is possible that female flowers lacking fertile stamens also have less developmental control over the perianth ontogeny. Consequently, our study investigated whether female and male flowers of A. quinata differ in their overall shape features, in the amounts of variation among flowers, as well as in the extent of different types of asymmetry in perianth shapes. Geometric morphometric analyses of triradial perianth symmetry based on the generalised Procrustes analysis of a complete symmetry group of perianth shapes showed that female flowers were indeed significantly more variable in all different subspaces of their symmetric and asymmetric shape variation. This included the differences among individual flowers, their rotational and bilateral symmetry as well as the asymmetry among sepals within flowers. These results indicate that developmental control over perianth shape is systematically weaker in female flowers compared to male flowers of A. quinata. It is therefore likely that this phenomenon is related to the presence or absence of fertile male reproductive organs, whose development is linked to the ontogeny of the perianth and the maintenance of its trimeric symmetry.