Speciation along the elevation gradient: Divergence of Roscoea species within the south slope of the Himalayas.

The Himalayas with dramatic elevation gradient is one of the global biodiversity hotspots. Although origin of biodiversity of the Himalayas is of great concern, the speciation process within the Himalayas is poorly known. Roscoea within the Himalayas serve as a good model system to test the speciation process along an elevation gradient. 32,375 unlinked SNPs were used to reconstruct phylogenetic relationships and introgression analyses in D-statistics and Fastsimicoal2. Species distribution modeling (SDM) was used to simulate habitat shift of Roscoea species during climate changes. Phylogeny suggested that the speciation order, except R. capitata, was from highland to lowland. D-statistics analyses suggested significant bidirectional ancient introgression between elevation-neighboring clades but no introgression between R. capitata and othern clades and no introgression among extant species. Fastsimicoal2 suggested interspecific introgressions were asymmetric. SDM predicted that habitats of Roscoea shifted to low elevation during cooling age. These results suggested that the sudden uplift of the Himalayas likely promoted speciation by vicariance, and climate cooling drove species divergence towards lower elevation. This study provides explanations for the origin of biodiversity within the Himalayas, and an insight to understand speciation along elevation in the mountainous regions.