Out-of-India dispersal of Paliurus (Rhamnaceae) indicated by combined molecular phylogenetic and fossil evidence
Paliurus (Rhamnaceae) is a small genus with a narrow and disjunct distribution in East Asia and the Mediterranean. Fossil evidence from the Late Cretaceous to the present suggests it once had a broad geographic distribution, encompassing parts of India, North America, Asia, and Europe. To reconstruct the evolutionary history of Paliurus and understand the origin of the disjunction observed today in the Northern Hemisphere, phylogenetic and biogeographical reconstructions were performed based on ITS, trnL-F, and rbcL sequences. Paliurus is shown to be a monophyletic genus, which could be divided into two clades as suggested by previous studies. Biogeographical inference integrated with fossil information indicates that the origin and dispersal pattern of the genus coincide with the "Out-of-India" hypothesis. The genus may have originated in India or other parts of Gondwana and then dispersed to East Asia after the collision of the Indian subcontinent with Eurasia, and to North America via the Bering land bridge. Paliurus appears to have reached the Mediterranean region in the late Oligocene following closure of the Turgai Strait. The uplift of the Qinghai-Tibet Plateau and the advent of drying and cooling climates during the Miocene may have fragmented the distribution of the genus, shaping the biogeographical patterns observed today. Paliurus populations in different regions either became extinct or adapted to changes in local ecological conditions following global climatic shifts through geological time.
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