Phylogeography of Sophora moorcroftiana Supports Wu’s Hypothesis on the Origin of Tibetan Alpine Flora

<span class="paragraphSection"><div class="boxTitle">Abstract</div>Wu hypothesized that the Tibetan flora originated mostly from the paleotropical Tertiary flora in the Hengduan Mountains by adapting to the cold and arid environments associated with the strong uplift of the Qinghai-Tibet Plateau (QTP). Here, we combine the phylogeographic history of <span style="font-style:italic;">Sophora moorcroftiana</span> with that of <span style="font-style:italic;">Sophora davidii</span> to explore the speciation of <span style="font-style:italic;">S. moorcroftiana</span> to test this hypothesis. We collected 151 individuals from 17 populations and sequenced 2 chloroplast fragments and the internal transcribed spacer of rDNA. Five chlorotypes and 9 ribotypes were detected but no significant phylogeographic structure was revealed. The integrated results of phylogeographic studies of these 2 species clearly support the progenitor–derivative relationship between them. We infer that the western peripheral population of <span style="font-style:italic;">S. davidii</span> migrated westwards from the Hengduan Mountains to the middle reaches of the Yarlung Zangbo River and differentiated from its ancestor in the process of adaptation to increasingly cold and arid environments with the uplift of the QTP and finally evolved into <span style="font-style:italic;">S. moorcroftiana</span> during the Late Pliocene. In addition, our findings shed light on the idea that natural selection, as imposed by climate differentiation (especially mean diurnal range and precipitation seasonality), directly drove this peripatric speciation event after geographic isolation. The speciation of <span style="font-style:italic;">S. moorcroftiana</span> is a strong case supporting Wu's hypothesis about the origin of Tibet's flora.</span>

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