| The climatic oscillation during Quaternary must have important effects on spatial distribution and genetic structure of organisms distributed in the Qinghai-Tibetan Plateau(QTP).However,how species occurred there changed their distribution ranges in response to Quaternary climatic oscillation remains controversial.Here,we report a study of the phylogeography of Hippophae tibetana,an endemic and dominant shrub species in the plateau.In most angiosperms,chloroplast DNA (cpDNA) is maternally inherited,and the variation of the cpDNA is extensively used to study genetic signatures of plants due to the climatic fluctuation.While the internal transcribed spacer(ITS) regions of the nrDNA,because of its greater sequence divergeces between taxa and convenience in sequencing,have also been developed as an valuable molecular marker to resolve phylogenetic relationships at different taxonomic levels.In present survey,we examined sequences variation of both maternally inherited, seed-dispersed chloroplast DNA(trnL-F & trnS-G) and biparentally inherited,seed and pollen-dispersed nuclear ribosomal DNA(Internal transcribed spacer,ITS)of H. tibetana within and among 17 natural populations across the entire range of the species.Nine chlorotypes(H1-H9) and 23 ITS types were identified among 116 individuals surveyed.Phylogenies of cpDNA and ITS were reconstructed and showed a consistent topology,in which two distinct lineages corresponding to east and west geographical distributions(Population 6-17 and population 1-5,respectively) were detected.The two lineages have no common either cpDNA sequences or ITS types. For chlorotypes,at least seven variable sites between the two groups were detected. The analysis of molecular variation(AMOVA) indicated that～90%of total variation was partitioned between the two groups,whereas only～6%was partitioned within populations.Moreover,population divergence was high for cpDNA variation(GST= 0.814,NST=0.944),indicating low levels of gene flow and significant phylogeographic structure(NST＞GST,P＜0.001).Our results indicate very strong differentiation between the two groups,and suggested that at least two different isolated refugia located at high altitude(＞3,500 m) on the plateau were occupied by this species during the LGM.Coalescent simulation of chlorotype genealogies using MESQUITE supported the hypothesis,and also suggested that the split of the two major lineages started before 130,000.In conclusion,the results for H.tibetana suggest a different pattern how the alpine plants in the QTP responded to the climatic changes during the Quaternary that at least some of these cold tolerant species may have also survived centrally on the QTP platform,compared with the previous studies(eg.Juniperus przewalskii) showing that such plants retreated to refugia at the eastern/south-eastern plateau edge during the LGM and/or previous glacial maxima.Therefore,our study provides valuable evidence that the species endemic in the QTP do not share a common phylogeographic history. |
