A taxonomic revision of the tribe Desmodieae (Leguminosae - Papilionoideae)

Abstract

The tribe Desmodieae has a pantropical distribution and is one of the most advanced tribes in the subfamily Papilionoideae. Despite this fact, the tribe was already present in the fossil record of the early Tertiary and so the Papilionoideae appear to be much older than is currently accepted. With its greatest centres of development in tropical Asia and America, Africa is relatively poorly endowed and only four genera comprising sixteen species occur in the Flora of southern Africa. Many of these species are widespread in the old World tropics and the few African endemics appear to be closely related to them. Desmodium is the largest genus with the bulk of species belonging to the Asian subgenus Sagotia. Of the two sections of Sagotia represented in Africa, section Nicolsonia is clearly more derived than section Heteroloma, showing many characters intermediate between Heteroloma and the considerably more derived genera Alysicarpus and Lespedeza. Other Desmodium species either of the Old or New World have often become naturalised as weeds in Africa. Pseudarthria is a genus derived by an aneuploid increase of the basic x = 11 for the tribe rather than an aneuploid decrease as in the case of Alysicarpus and Lespedeza. A re-count of the chromosome number of P. hookeri (2n = 26) matches the count obtained in another species of Pseudarthria for the first time. Flowering strategy and flower structure in the tribe are the result of intense coevolution with the ecological requirements of skilled bee pollinators and although many bees can trip Desmodieae flowers, only a relatively few high energy demanding, traplining bees are able to bring about effective pollination. The overall breeding system represents a fine balance between the need for selfing and the need for outcrossing in order to produce enough seed of sufficient quality to survive in unstable environments.