Evolution of the succulent organization of the Southern African Senecioneae (Asteraceae)

Abstract

Southern African succulents of the tribe Senecioneae are likely to have originated from nonsucculent inhabitants of the neighboring semiarid and more humid areas of southern or southeastern Africa. The formation of succulent organization probably occurred successively in four clades colonizing the arid regions of southwestern Africa. The Senecio medley-woodii clade was the first to have developed the succulent organization. Its members are densely pubescent leaf-succulent herbs without special water-storage tissue in their leaves. This clade was followed by the Othonna clade, which developed caudex or bulbous herbs with annual hemisucculent or nonsucculent shoots. Only some species are stem succulents, and a few ones are leaf-succulents. No one species has dense leaf indumentum, however. Curio species and some succulent Senecio species constitute a group with unresolved phylogenetic links, which emerged a bit later than the Othonna clade. Contrary to the latter, only two Senecio species of this group and one Curio species are bulbous geophyte and stem succulents. They occupy territories outside the areas of the Othonna species with similar growth habits. Most species of this group are leaf succulents with glabrous leaves and special water-storage tissue in the mesophyll. We believe that the specific succulent organization of each clade indicates specific adaptive zones sensu Simpson, but we are unable to characterize these zones distinctively. These differences allow specimens of these clades to coexist in numerous arid areas of southwestern Africa and probably to prevent penetration of the species of the latest Kleinia clade. The species of the Kleinia clade then inhabit the semiarid areas of southeastern Africa and the semiarid to arid areas of eastern and northern Africa, the Canary Islands, and the Arabian Peninsula. Only some stem-succulent Kleinia species live in those arid areas of southwestern Africa, where there are no Othonna, Senecio, or Curio stem-succulent species. The reconstructed emergence of the succulent organization of southern African species of the Senecioneae tribe corresponds with the Gause’s competitive exclusion principle.