Testing evolutionary hypotheses for the world’s main hotspot of temperate alpine flora using Saussurea (Compositae) as a model

The Tibet-Himalaya-Hengduan region (THH) harbours the world's richest temperate alpine flora. To investigate the main evolutionary and ecological processes underlying this outstanding biodiversity, we tested multiple hypotheses focusing in Saussurea (Compositae), a representative genus of ca. 450 species that extensively diversified in THH and adjacent regions. We built a highly-resolved phylogeny based on hundreds of nuclear loci and a broad sampling (70 % of genus richness). We integrated phylogenetic, ecological, geographic, topographic and climatic data with biogeographic and diversification modelling methods. Our results suggest that Hengduan Mountains acted both as evolutionary cradle and museum. Repeated bidirectional biogeographic exchanges between Asian regions substantially contributed to current biodiversity patterns. Uplift and mountain building established favourable conditions for later evolutionary radiations by providing high topographic heterogeneity. However, orogeny alone does not explain the evolutionary radiation of Saussurea and its diversification burst in the Late Miocene with a double-fold increase in speciation rate. Besides orogeny, the main factors associated with current biodiversity are: (1) the interplay of steep altitudinal gradients and topographic heterogeneity with moderate climatic oscillations during the Late Miocene; (2) relative climatic stability during Pleistocene dramatic climatic oscillations, which may have reduced extinction rates; and (3) sufficient annual precipitation (>400 mm).