Evolution and development of floral diversity of Melianthaceae, an enigmatic southern African family

Melianthus has strongly modified and monosymmetric (zygomorphic) flowers, a nd the ontogenetic derivation and evolutionary origins of these highly modi fied flowers are sought in a comparative study of the floral ontogeny and a natomy of two species of Melianthus and one species of Bersama. The develop ment of monosymmetry in the family can be observed as a stepwise process li nked with displacements of organ primordia and selective losses of petals a nd stamens. Flowers are weakly (Bersama) to strongly (Melianthus) monosymme tric. Sepal initiation is divided into two time sequences: the three outer sepals arise simultaneously with the anterior sepals behaving as a shieldin g unit before the two inner sepals emerge. Similarities and differences bet ween the two genera are shown to be related to the trend to monosymmetry an d are linked to specific pollination strategies. In Melianthus, an abaxial petal primordium is initiated, but it aborts at an early stage of developme nt; in Bersama, all petals are present, but the incipient monosymmetry is v isible at the onset of initiation. In Bersama, four stamens are the rule, b ut flowers pentamerous in all whorls are occasionally found. The nectary de velops in a pouch formed between the adaxial sepal and the androecium. Ther e is little ontogenetic evidence for a staminodial nature of the "beak" for med between the posterior stamens; it develops as a protective extension of the filament tube. A reduction sequence in the number of ovules can be see n by the progressive size limitation of the placentas. In Bersama, only one ovule arises on a marginal placenta and becomes displaced in a central pos ition 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 Bersa ma, the nectary is partly supplied by an odd adaxial trace. Our data suppor t a relationship of Melianthaceae with Greyiaceae and Francoaceae, concorda nt with macromolecular results. It may be suggested that polysymmetric flow ers of Bersama are secondarily derived from monosymmetric ones.