Evolution of succulent Senecioneae (Asteraceae) of Southern Africa
Southern African succulents of tribe Senecioneae are likely to have come from non-succulent inhabitants of neighbour mesic to semi-arid areas of S to SE Africa. Four phyla are believed to have evolved succes sively in situ by colonizing arid regions of SW Africa. The Senecio medley-woodii-phylum must have been the ﬁrst one to have developed the succulent syndrome. Its members are densely pubescent leaf-succulent herbs without special water-storage tissue in their leaves. This phylum seems to have been followed by Othonna-phylum which has mostly developed caudex/bulbous growth habit with annual hemi-succulent to non-succulent shoots. Some Othonna species are stem succulents and a few ones are leaf-succulents. No one species has dense indumentum, however. Curio species and some succulent Senecio ones con stitute the unit alias Curio-phylum. Interrelationships between these species remain still unresolved. The Curio-phylum might have evolved a little bit later than the Othonna-phylum. Contrary to the latter, only 2 representatives of the Curio-phylum are bulbous herbs with annual hemi-succulent shoots and only 1 species is a stem succulent. The 3 species mentioned all occupy territories outside areas of Othonna species of similar growth habits. None of them has indumentum. Most members of the Curio-phylum are glabrate leaf succulents with special water-storage tissue in their leaves. We believe that speciﬁc succulent syndrome of each phylum indicates speciﬁc adaptive zone it occupies in arid regions of SW Africa (though we are unable to characterize these zones distinctively). These differences in succulent syndromes must enable the 3 phyla to coexist in numerous arid areas of SW Africa. Moreover, the differences evidently enable them to “close” competitively these areas to the latest Kleinia-phylum. Then, the species of the Kleinia-phylum inhabit semi-arid areas of SE Africa and semi-arid to arid areas of E & N Africa, Canary Islands and Arabia. Only a few stem succulent Kleinia species live in those arid areas of the SW Africa where there are neither stem succulent othonnas nor stem succulent curios. Evolution of the succulence in Southern African Senecioneae thus outlined ﬁts the Gause’s competitive exclusion principle.