Floral morphology in Caesalpinioid legumes: Testing the monophyly of the "Umtiza clade"

The legume subfamily Caesalpinioideae forms a basal grade in the family, within which the subfamilies Mimosoideae and Papilionoideae are nested. Monophyletic groups within Caesalpinioideae include the tribes Cercideae, which is grouped sister to all remaining legumes, and Detarieae s.l., which is monophyletic only if the monospecific genus Umtiza is excluded. Umtiza is endemic to South Africa and has traditionally been included in the Detarieae, but its placement there has long been questioned. We present phylogenetic analyses of data from the chloroplast trnL intron and trnL-F spacer and morphology that resulted in a single most parsimonious tree in which Umtiza is grouped with six other small caesalpinioid genera in an association of taxonomically and biogeographically disparate taxa. Umtiza, Gleditsia, and Gymnocladus form a clade that is sister to Ceratonia, Acrocarpus, Arcoa, and Tetrapterocarpon. Although these seven genera are dissimilar in many respects, they share several potential morphological synapomorphies, one of which is the presence of dioecy, which occurs in Gymnocladus, Gleditsia, Ceratonia, Arcoa, and Tetrapterocarpon. However, the occasional occurrence of apparently bisexual flowers in several of these genera complicates our understanding of the evolutionary history of sexual expression in these plants. Flowers in the Umtiza clade are small and green to white in color (green with a colorful hypanthium in Acrocarpus). Perianth structure has been of interest in this group, especially in Gleditsia and Gymnocladus, which have been portrayed as having a poorly differentiated calyx and corolla. This has been interpreted as evidence that they are among the most archaic members of the family. However, although the calyx and corolla are similarly colored, they are easily distinguished on the basis of size and texture. Thus, the characterization is inaccurate, and the evolutionary interpretation is unsupported. The greenish perianth and other unusual aspects of floral morphology observed in the Umtiza clade are clearly apomorphies and not plesiomorphic features for the family.