Role of the Maloti-Drakensberg in the evolution of the southern African temperate flora: The biogeographic history of Helichrysum in the Great Escarpment

• The Succulent Karoo is confirmed as the area of origin for the Helichrysum clade. • The Central summer-rainfall Plateau is the main inferred route northwards to the Maloti-Drakensberg. • Greatest species exchange occurred between the Maloti-Drakensberg and NE Escarpment & Manica Highlands. • Much speciation has occurred within the Maloti-Drakensberg, but no large endemic Helichrysum clade. • The Maloti-Drakensberg may have seeded occasional dispersals to Madagascar and Macaronesia. We explore the role of the Maloti-Drakensberg and other major portions of the Great Escarpment in the diversification of the southern African flora using Helichrysum (Asteraceae, tribe Gnaphalieae), a large genus with centres of diversity in all major temperate habitats of the subregion. We use a dated Bayesian tree based on rDNA sequences from Helichrysum and relatives including 77% of southern African Helichrysum species. We infer local extinction, dispersal rates, and colonisation history of the lineage across a sample of 1000 trees under the DEC model. The Helichrysum clade is inferred to have arisen in the arid western regions of South Africa in the Late Oligocene, and most Succulent Karoo Helichrysum species are descended from the initial divergence events. There is negligible signal of later recolonisation of the arid habitat, suggesting unidirectional adaptive evolution to moister, cooler habitats. The earliest dispersal to the Maloti-Drakensberg is inferred to have occurred in the early Miocene, with the inferred route via the central summer-rainfall plateau. Helichrysum diversified extensively in the greater Maloti-Drakensberg region, and establishment in this area appears to underpin migrations of the lineages out of South Africa, including to the southern and northern Afrotemperate regions. Occasional dispersals to areas outside of Africa from the Maloti-Drakensberg may have seeded some of the diversifications of this clade in Madagascar, Europe, Asia, and the Americas. Our analyses recover a previously unknown role for the Central Plateau of southern Africa in cross-seeded regional radiations, possibly linked to expansion of the cold-temperate habitat during glacial maxima, although we also infer some direct migrations between the Maloti-Drakensberg and both the Succulent Karoo and Fynbos regions. A long history in the northern and eastern parts of the Great Escarpment explains the current diversity of Helichrysum in both the Maloti-Drakensberg region and the North-East Escarpment. While no single large radiation is recovered, there is evidence of considerable parallel in situ speciation in the Maloti-Drakensberg, creating an important centre of Helichrysum diversity and endemism. Rare recolonisation of the Fynbos Biome contributed to this important centre of Helichrysum diversity and endemism. The Maloti-Drakensberg has also played a key role as a recent source of floral diversity for the temperate and montane floras of southern, Central and East Africa.