Abstract
The formation of the Mekong-Salween Divide and climatic oscillations in Pleistocene were the main drivers for the contemporary diversity and genetic structure of plants in the Himalaya-Hengduan Mountains (HHM). To identify the relative roles of the two historical events in shaping population history of plants in HHM, we investigated the phylogeographic pattern of Oxyria sinensis, a perennial plant endemic to the HHM. Sixteen chloroplast haplotypes were identified and were clustered into three phylogenetic clades. The age of the major clades was estimated to be in the Pleistocene, falling into several Pleistocene glacial stages and postdating the formation of the Mekong-Salween Divide. Range expansions occurred at least twice in the early and middle Pleistocene, but the spatial genetic distribution rarely changed since the Last Glacial Maximum. Our results suggest that temporary mountain glaciers may act as barriers in promoting the lineage divergence in O. sinensis and that subsequential range expansions and secondary contacts might reshape the genetic distribution in geography and blur the boundary of population differentiation created in the earlier glacial stages. This study demonstrates that Pleistocene climatic change and mountain glaciers, rather than the Mekong-Salween Divide, play the primary role in shaping the spatial genetic structure of O. sinensis.
Highlights
The formation of the Mekong-Salween Divide and climatic oscillations in Pleistocene were the main drivers for the contemporary diversity and genetic structure of plants in the Himalaya-Hengduan Mountains (HHM)
The relative roles of these two historical forces in generating genetic diversity in this region are inconsistent among previous studies, leading to two contrasting hypotheses: a geological hypothesis emphasizing a major role for geological impacts and a climatic hypothesis highlighting the role of Pleistocene climate changes[10,12,13]
We studied the phylogeography of an alpine plant, Oxyria sinensis Hemsley (Polygonaceae), which diverged from its sole sister species O. digyna in the genus Oxyria between 12 and 14 million years ago (Mya)[27]
Summary
The formation of the Mekong-Salween Divide and climatic oscillations in Pleistocene were the main drivers for the contemporary diversity and genetic structure of plants in the Himalaya-Hengduan Mountains (HHM). Over the past two decades, DNA-based phylogeographic studies have greatly advanced our understanding of demographic dynamics and the evolutionary history of plants in response to historical events at a much finer time and spatial scale These events include geological changes and climate changes, such as the formation of vicariance, the uplift of mountains, and climatic oscillations associated with Pleistocene glaciers. In the former, the estimated divergence time between genealogies from the west and east side of Mekong-Salween Divide fell into the late Miocene, agreeing with the formation of the Mekong-Salween Divide In another case, the present genealogical distribution of the plant species Terminalia franchetii was believed to be geographically structured by the paleo-drainage re-arrangements rather than the modern drainage systems, highlighting the role of historical geologic events in shaping contemporary genetic distribution[13,18]. The Hengduan Mountain region were proposed as an important refugium to the QTP and neighboring areas[10,36], and several case studies have suggested that mountain glaciers in the Pleistocene rather than geological breaks resulted in allopatric divergence and profoundly affected the intraspecific phylogeographic structure[10,37]
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