Population Genetic Structure, Phylogeography, and Demographic History of Platycrater arguta (Hydrangeaceae) Endemic to East China and South Japan, Inferred from Chloroplast DNA Sequence Variation

Platycrater arguta (Hydrangeaceae) is a small deciduous shrub of the Sino-Japanese floristic region, where it occurs in montane sites mostly covered with warm-temperate deciduous forest. This sole representative of its genus contains two varieties dijunctly distributed between East China (var. sinensis) and South Japan (var. arguta). To illuminate the biogeographic and demographic history of this rare species, we conducted a survey of chloroplast DNA (cpDNA) sequence variation (trnD-trnE, trnH-psbA) within and among twelve populations (four from China, eight from Japan, 129 individuals in total) representing the overall distributional range of the species. Based on a total of 19 haplotypes identified, P. arguta was found to harbor surprisingly high levels of haplotype and nucleotide diversity (hT = 0.882; πT = 0.00475), possibly associated with its lon gevolutionary history. Spatial analysis of molecular variance found two regional phylogroups, corresponding to var. sinensis and var. arguta, and supported by genealogical (unrooted network) analysis of haplotypes. Using a coalescent-based model of 'divergence by isolation with migration', the likely vicariant origin of these varieties was dated to the mid-Pleistocene (ca. 0.89 mya). Very similar haplotype mismatch distributions indicate that var. sinensis and var. arguta underwent past demographic growth almost simultaneously (dated to ca. 0.43 and 0.45 mya, respectively), suggesting climate-induced expansion. However, var. sinensis likely experienced a mere demographic expansion in a narrowly circumscribed mountain rante, while var. arguta underwent a spatial northward expansion that might have consisted of a series of bottlenecks, leading to genetically impoverished populations that most likely derived from initial population(s) in southern Kyushu. Our results endorse the recognition of two 'evolutionarily significant units' within P. arguta, corresponding to var. sinensis from East China and var. arguta from South Japan.