Molecular phylogeography of East Asian Kirengeshoma (Hydrangeaceae) in relation to Quaternary climate change and landbridge configurations

Kirengeshoma comprises two species inhabiting warm temperate-deciduous forests in East China/South Japan (Kirengeshoma palmata) and South Korea (Kirengeshoma koreana). A survey of chloroplast (cp) DNA and inter-simple sequence repeats (ISSRs) variation in Kirengeshoma was carried out to determine the population history of a plant taxon around the East China Sea (ECS). CpDNA and ISSRs revealed lower genetic divergence between China and Japan relative to the other contrasts, in line with intrageneric classification. Molecular dating suggests that K. koreana diverged at the Plio-Pleistocene boundary from the Japanese populations, whereas the latter migrated into China during the early-to-mid Pleistocene via the ECS basin. Vicariant segregation of Chinese and Japanese populations likely occurred during the mid-Pleistocene. Mismatch distributions and neutrality tests indicated that Chinese populations expanded their range during the Late Pleistocene, probably during a cold period, whereas those from Japan showed no significant population growth. We conclude that the current distribution and differentiation of components of presently isolated warm temperate-deciduous forests in China, Japan and Korea likely resulted from a combination of relatively ancient vicariant and immigration events, and those from Japan were particularly sensitive to range fragmentation and long-term refugial isolation throughout the Late Pleistocene.