Phylogeographic study revealed microrefugia for an endemic species on the Qinghai-Tibetan Plateau: Rhodiola chrysanthemifolia (Crassulaceae)

摘要

Most phylogeographic studies on the Qinghai-Tibetan Plateau and in adjacent regions have focused on tree or shrub species, or on herbaceous species that mainly inhabit alpine meadows or grasslands. The phylogeography of herbaceous species that inhabit woodlands and shrubberies has been largely neglected. Here we investigate the evolutionary history of a woodland- and shrubbery-inhabited herbaceous species, Rhodiola chrysanthemifolia, which grows in southern Tibet and in the mountains of the Hengduan ecoregion. The cpDNA trnL-F, rpl20-rps12 and nrDNA ITS regions of 183 individuals from 13 populations were sequenced. The ITS dataset revealed a significant phylogeographic structure across the distribution range of R. chrysanthemifolia, while the cpDNA dataset showed no significant structure. Besides, analyses of molecular variance showed that among-population variation was described as 96.27 % of the total variation for ITS marker, much higher than that for cpDNA (63.68 %). Limited seed migration among populations together with inbreeding within populations of R. chsysanthemifolia may result in the different phylogeographic and genetic patterns for ITS and cpDNA markers in this species. A high frequency and an even distribution of private cpDNA haplotypes were discovered throughout the distribution range of R. chrysanthemifolia; ITS genotypes formed three main lineages which showed a geographical distribution pattern. Furthermore, populations with above-average gene diversity were evenly spread across the current distribution range of R. chrysanthemifolia, supporting the existence of microrefugia for this species during Last Glacial Maximum, even earlier glaciations, similar to the pattern of sympatric tree and shrub species of the Juniperus tibetica complex and Hippophae tibetana. Allopatric divergence of populations in isolated microrefugia could be responsible for large number of private cpDNA haplotypes across the current distribution range of R. chrysanthemifolia. The putative factors which drove diversification between R. chrysanthemifolia and R. alsia were also discussed.