Phylogeny and taxonomy for Mesembryanthemum subg. Volkeranthus (Aizoaceae-Mesembryanthemoideae)

Abstract

We present a species-level phylogeny for Mesembryanthemum subg. Volkeranthus (Gerbaulet) Klak (Aizoaceae-Mesembryanthemoideae) based on four DNA markers. Previously M. subg. Volkeranthus comprised only two species, Mesembryanthemum longistylum DC. and Mesembryanthemum aitonis Jacq., which are short-lived, succulent, herbaceous perennials found in the three Cape Provinces of South Africa. Our molecular and morphological investigations suggest that the broad species concept put forward for M. aitonis Jacq. by Gerbaulet should be rejected and Mesembryanthemum paulum (N.E.Br.) L.Bolus and Mesembryanthemum clandestinum Haw., previously considered as conspecific with M. aitonis, are reinstated. Morphologically both species are distinct from M. aitonis, so that we now recognise four species in M. subg. Volkeranthus. Among the four species, the distributions of sister species overlap only marginally, but they prefer similar soils: clays for M. longistylum and M. paulum, and sandy/alluvial soils for M. clandestinum and M. aitonis. Although M. clandestinum is known in all three Cape Provinces, within this very large area it is restricted to moist sites along the banks of rivers. In contrast, M. paulum is restricted to the south-western Cape and the Agulhas Plain. Noteworthy differences between M. clandestinum and the other species in M. subg. Volkeranthus include its habit of forming large mats that survive for many years and its ability to propagate vegetatively. The inconspicuousness of its flowers and their few stamens suggest that M. clandestinum may be self-fertile. We provide an amended key to the subgenera of Mesembryanthemum and a key to M. subg. Volkeranthus, as well as synonymy and revised descriptions for its four species.