Origin Area And Glacial Refugia:Chloroplast DNA Diversity In The Arctic-alpine Plant Oxyria Digyna (Polygonaceae)

Qxyria digyna is an ancient Arctic-alpine plant species found throughout the world. It occurs over much of the northern hemisphere, including Arctic North America and Eurasia, the mountain ranges of southern Russia, the Himalayan region, southern Europe, and western North America. Our research about phylogeography of O. digyna could provide important insight into the evolutionary history of Arctic-alpine plants having a similar distribution pattern. To investigate the potential origins and glacial refuge of O. digyna during the Quaternary climatic oscillation, the phylogeography of this species was examined through studies of the chloroplast DNA sequences (atpB-rbcL, trnL-trnF, trnH-psbA and trnT-trnL) of 141 individual specimens from 16 different populations spread over southwestern China, northern Europe and western North America, respectively. The distribution of haplotypes and the genetic diversity of the sampled regions were conferred. Patterns of migration were inferred by haplotype networks and reconstructed bio-geographical history. Our main conclusions are as follows.A total of 26 haplotypes were identified based on 76 variations (48 nucleotide mutations and 28 indels) between the cpDNA sequences. High haplotype diversity and endemic haplotypes were found in the Hengduan Mountains (H-D Mountains) and North America. High haplotype diversity of the two regions are 0.6255 and 0.8962, respectively. A single haplotype 15 was found exclusively in the Arctic region of Northern Europe. The results of this study suggest that O. digyna took refuge on the Tibetan Plateau (TP) western North America and Northern Europe.The AMOVA analyses suggested a larger proportion of the total variation among populations (94.93%) than that within the populations (5.07%). Significant population and phylogeographic structure (GST=0.665±0.0836, NST=0.825± 0.0666; NST>GST, P<0.01) was detected for this species.Relationships and divergence time among Oxyria digyna haplotypes from the combined analysis of psbA-trnH, atpB-rbcL,trnT-trnL and trnL-trnF spacer regions revealed that O. digyna originated in the H-D Mountains, together with geographical distribution pattern of O.sinensis and ancestral pollen type distribution of the Polygonaceae family.Based on the geographic range of O. digyna and another Arctic-alpine geographical distribution pattern for the plant Koenigia islandica, these data suggested the long-distance migration route for this species may have begun in the H-D Mountains in western China, moving at different times, first westward along the Himalayas, then northward across the Tianshan and Altai Mountains and the Central Siberian Plateau to reach the Arctic circumpolar region. From the Tamyr Penninsula the route may then have turned eastward through Beringia to North America and westward along the Arctic Ocean to northern Europe.