Phylogeographic and demographic effects of Quaternary climate oscillations in Hexinia polydichotoma (Asteraceae) in Tarim Basin and adjacent areas

In order to investigate the genetic diversity and influence of climate oscillations on evolutionary processes of organisms in Northwest China, we selected Hexinia polydichotoma, a species endemic to China, and examined the phylogeographic structure and historical factors that influenced the evolutionary history of this species in its entire cover range, Tarim Basin and adjacent areas. In the study, 17 haplotypes were identified in H. polydichotoma on the basis of two chloroplast DNA sequences (trnH-psbA and ycf6-psbM). Shown in the network, the two common haplotypes, A and D, respectively, mainly distribute along the northern and southern rims of the basin. The analyses of molecular variance analysis suggest that genetic variation primarily occurs among populations, and all populations were subdivided into five groups by SAMOVA. Geographic range expansion along the southern and northern rims of the basin was supported by the significant value for Tajima's D and by the unimodal mismatch distribution. It is possible that during the interglacial period of the middle Pleistocene, a large amount of snow and glacial ice melted from the mountains surrounding Tarim Basin. This increased water, the expanding desert, and the dispersal ability of H. polydichotoma were important factors driving not only geographic range expansion, but also the current phylogeographic structure of this species. It is possible that during the middle Pleistocene, the climatic fluctuations resulted in expansion and contraction cycles of river systems and oases, and may consequently have caused population fragmentation.