Phylogeographic and Demographic Effects of Quaternary Climate Oscillations in Hexinia polydichotoma (Asteraceae) in Tarim Basin and Adjacent Areas

Climate oscillations during the Quaternary had a profound impact on the genetic diversity and distribution of plants in the Northern Hemisphere. In order to understand the impact of these climate shifts, researchers have investigated the phylogeographic history of groups of plants for a long time.

Tarim Basin, with an area of 530,000 km², is the largest inland basin in Northwest China. It is also one of the particular interest regions for investigating the effect of climatic changes on organisms. However, few discussions were conducted on the ogeographic history in this region.

Hexinia polydichotoma is a perennial and drought-tolerant herb, and grows in the gullies of the Gobi desert and areas surrounding the Taklimakan Desert. Given its restricted distribution to Tarim Basin and adjacent areas, H. polydichotoma is a useful species to address phylogeographic patterns in this area.

To investigate the genetic diversity and influence of climate oscillations on evolutionary processes of organisms, SU Zhihao et al. examined the phylogeographic structure and historical factors that influenced the evolutionary history of H. polydichotoma in Tarim Basin and adjacent areas. In this study, 17 haplotypes were identified in H. polydichotoma on the basis of two chloroplast DNA sequences (trnH–psbA and ycf6–psbM).

The result showed that the two common haplotypes, A and D, mainly distributed along the northern and southern rims of the basin. The analyses of molecular variance analysis suggested that genetic variation primarily occurred 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 was 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. The increased water, 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 was 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.

This study was provided by the CAS Important Direction for Knowledge Innovation Project (no. KZCX2-EW-305) and Xinjiang Institute of Ecology and Geography, CAS. The result was published in Plant Systematics and Evolution on November 2012, 298(9): 1767-1776.

Haplotype medianjoining network of Hexinia polydichotoma. The circle size is proportional to haplotype frequencies. The number of inferred steps between haplotypes is shown near the corresponding branch section. Haplotypes in the network shown in different colors represent the following four geographical areas. The yellow circles (mv1 and mv2) represent the missing or inferred haplotypes. (Image by XIEG)