Evolution and Development of Floral Diversity of Melianthaceae, an Enigmatic Southern African Family

Abstract

Melianthus has strongly modified and monosymmetric (zygomorphic) flowers, and the ontogenetic derivation and evolutionary origins of these highly modified flowers are sought in a comparative study of the floral ontogeny and anatomy of two species of Melianthus and one species of Bersama. The development of monosymmetry in the family can be observed as a stepwise process linked with displacements of organ primordia and selective losses of petals and stamens. Flowers are weakly (Bersama) to strongly (Melianthus) monosymmetric. Sepal initiation is divided into two time sequences: the three outer sepals arise simultaneously with the anterior sepals behaving as a shielding unit before the two inner sepals emerge. Similarities and differences between the two genera are shown to be related to the trend to monosymmetry and are linked to specific pollination strategies. In Melianthus, an abaxial petal primordium is initiated, but it aborts at an early stage of development; in Bersama, all petals are present, but the incipient monosymmetry is visible at the onset of initiation. In Bersama, four stamens are the rule, but flowers pentamerous in all whorls are occasionally found. The nectary develops in a pouch formed between the adaxial sepal and the androecium. There is little ontogenetic evidence for a staminodial nature of the “beak” formed between the posterior stamens; it develops as a protective extension of the filament tube. A reduction sequence in the number of ovules can be seen by the progressive size limitation of the placentas. In Bersama, only one ovule arises on a marginal placenta and becomes displaced in a central position during development. The vasculature of different species is compared. The development of vascular traces is strictly linked with the initiation and external development of the floral organs. In both Melianthus and Bersama, the nectary is partly supplied by an odd adaxial trace. Our data support a relationship of Melianthaceae with Greyiaceae and Francoaceae, concordant with macromolecular results. It may be suggested that polysymmetric flowers of Bersama are secondarily derived from monosymmetric ones.