The upper reaches of the largest river in Southern China as an “evolutionary front” of tropical plants: Evidences from Asia-endemic genus Hiptage (Malpighiaceae)

The biodiversity hotspot at the Guizhou–Yunnan–Guangxi borders is a distribution centre of tropical plants in China. It spans the whole upper reaches of Zhujiang River, the largest river in Southern China. In this paper, I aimed to explore the roles of the river in the spread and diversification of tropical plants in this area, using the Asia-endemic genus Hiptage Gaertn. (Malpighiaceae) as an example. Two diversity and endemism centres of Hiptage are recognized: Indo-China Peninsula and upper reaches of Zhujiang River (UZJ). The area-adjusted endemism index further indicates UZJ as the most important distribution region of endemic species since UZJ has a very small area (~210,000 km 2 ) but six out of the total seven species are narrow endemics. UZJ is located at the northern edge of distribution ranges of Hiptage , which resulted mainly from the north-west–south-east river systems of UZJ promoting northward spreads of this tropical genus. The highly-fragmented limestone landscapes in this region may promote habitat isolation and tends to be the main driving factor for origins of these endemic species. Hiptage is also distinctive for its highly-specialized pollination system, mirror-image flowers, which probably facilitates species diversification via floral and pollination isolation. Other studies also found UZJ as a major diversification centre of the tropical plant families Gesneriaceae and Begoniaceae. Thereafter, it is concluded that UZJ is an “evolutionary front” of tropical plants in China, which contributes significantly to the origin and maintenance of the unique biodiversity in the area.