Identifying a Contact Zone between Two Phylogeographic Lineages of Clematis sibirica (Ranunculeae) in the Tianshan and Altai Mountains

Tianshan and Altai Mountains, located on the peripheries of the Dzungarian Basin in arid northwestern China, are parts of the mountain system of the Central and High Asia regions. The forest flora of these mountains forms the “rear edge” of the Eurasian forest, which is profoundly susceptible to the influence of variations in the local arid climate.

Clematis sibirica (L.) Mill., a widespread species in the Eurasian forest area, has a continuous distribution covering almost the whole of the forested area in the Tianshan and Altai Mountains. It prefers to occur under coniferous forests. The widespread distribution of this species and its particular ecological niche are examined to elucidate possible routes of recolonization along the mountain ranges. In addition, maternally inherited chloroplast DNA (cpDNA), owing to a low frequency of genetic recombination, shows more geographic structure than the nuclear genome in most angiosperms. It has been shown that cpDNA markers are of primary usefulness for phylogeography.

To illustrate how the taxon responded in the area of the Tianshan and Altai Mountains to the Pleistocene climatic fluctuations, Prof. ZHANG Mingli and his student investigated the phylogeographic structure of C. sibirica in the Tianshan and Altai Mountains based on a cpDNA fragment. The study focused on the following two issues: (i) inference of the spatial phylogeographic pattern of C. sibirica and the locations of past glacial refugia; and (ii) reconstruction of its postglacial demographic history and possible recolonization routes.

The chloroplast intergenic spacer psbA-trnH was sequenced for 125 individuals from 28 populations, and a total of eight chlorotypes were identified. The presence of definite phylogeographic structure was detected for the species (N_ST > G_ST, P < 0.001), and phylogenetic analysis indicated that the eight chlorotypes were clustered into two divergent lineages. They split at approximately 550–690 ka BP, according to coalescence analysis, coincident with the Pleistocene maximum glacial stage in these mountains, which suggests the restriction of these lineages to separate refugia at that time. Spatial analysis of molecular variance likewise divided the sampled populations into two associations, an Altai and eastern Tianshan group (populations 1–17), and a western Tianshan group (populations 18–28). Low levels of genetic diversity and unimodal mismatch distributions were obtained for both of these groups, suggesting postglacial range expansions. During the course of these expansions, mountain ranges surrounding the Dzungarian Basin probably served as migration corridors.

The result has been published on Journal of Systematics and Evolution, 2012, 50 (4): 295–304. The paper can be downloaded from http://onlinelibrary.wiley.com/doi/10.1111/j.1759-6831.2012.00198.x/full.