Diversity on a small scale - phylogeography of the locally endemic dwarf succulent genus Oophytum (Aizoaceae) in the Knersvlakte of South Africa.

Abstract

Background and aims: Oophytum (Aizoaceae) is a locally endemic genus of the extremely fast evolving subfamily Ruschioideae and consists of only two formally accepted species (O. nanum and O. oviforme). Both species are leaf-succulent dwarf shrubs and habitat specialists on quartz fields in the Knersvlakte, a renowned biodiversity hotspot in the arid winter-rainfall Succulent Karoo Biome of South Africa. Quartz fields present specialised patchy habitats with an island-like distribution in the landscape. Oophytum oviforme grows in the south-western part, whereas O. nanum covers most of the remaining Knersvlakte. These species co-occur in a small area but within different quartz islands. We investigated the effects of the patchy distribution, environmental conditions and potential effects of paleoclimatic changes on the genetics of Oophytum.

Methods: Phylogenetic and population genetic analyses of 35 populations of the genus, covering its entire distribution area, were conducted using four cpDNA markers and an AFLP dataset. These were combined with environmental data via a principal component analysis and comparative heatmap analyses.

Key results: The genetic pattern of the Oophytum metapopulation is a tripartite division with a northern, central and western group. This geographical pattern does not correspond to the two-species concept of Oophytum. Only the western O. oviforme populations form a monophyletic lineage, whereas the central populations of O. oviforme are genetic hybrids of O. nanum populations. The highly restricted gene flow often resulted in private gene pools with very low genetic diversity, in contrast to the hybrid gene pools of the central and edge populations.

Conclusions: Oophytum is an exceptional example of an extremely fast-evolving genus that illustrates the high speciation rate of the Ruschioideae and their success as one of the leading plant groups of the drought-prone succulent Karoo Biome. The survival strategy of these dwarf quartz-field endemics is an interplay of adaptation to diverse island habitats, highly restricted gene flow, occasional long-distance dispersal, migration, founder effects and hybridisation events within a small and restricted area caused by glacial and interglacial changing climate conditions from Pleistocene up to Present. These findings have important implications for future conservation management strategies.