Recent radiation of Brachystelma and Ceropegia (Apocynaceae) across the Old World against a background of climatic change

Abstract

The genera Brachystelma Sims and Ceropegia L. of the Ceropegieae (Apocynaceae-Asclepiadoideae) consist of ±320 species of geophytes and slender climbers with a tendency to stem-succulence in Ceropegia. They occur in and around the semi-arid, mainly tropical parts of the Old World. For 146 species (around half of the total) from most of the geographic range of the genera, we analysed data from two nuclear and five plastid regions. The evolution of Ceropegia is very complex, with at least 13 mostly well-supported lineages, one of which is sister to the ±350 species of stapeliads. Species of Brachystelma have evolved at least four times, with most of them nested within two separate major lineages. So, neither Brachystelma nor Ceropegia is monophyletic. We recover a broad trend, in two separate major lineages, from slender climbers to small, geophytic herbs. Several clades are recovered in which all species possess an underground tuber. Small, erect, non-climbing, geophytic species of Ceropegia with a tuber are nested among species of Brachystelma. Consequently, the distinctive tubular flowers used to define Ceropegia do not reflect relationships. This re-iterates the great floral plasticity in the Ceropegieae, already established for the stapeliads. Both major lineages exhibit a trend from tubular flowers with faint, often fruity odours, pollinated by very small Dipteran flies, to flatter flowers often with a bad odour, pollinated by larger flies. Most of the diversity in Brachystelma and Ceropegia is recent and arose within the last 3my against a background of increased aridification or extreme climatic variability during the Pliocene. In the ingroup, diversity is highest in Southern Africa, followed by Tropical East Africa and other arid parts of Africa, the Arabian Peninsula and India. Many disjunctions are revealed and these are best explained by recent, long distance dispersal. In Africa, the diversity arises from the presence of many different lineages over wide areas but there is also evidence of closely related species growing together with different pollinators.

Keywords: Ceropegieae; DNA sequence-data; Geographical patterns; Molecular dating; Phylogeny; Stapeliads.