Abstract: A new genus and species of Marsdenieae, Xua hongheensis, native to the Red River basin of Yunnan Province, China, is described here for the first time and compared with its close relatives. The plant is a perennial, pubescent, twining herb, and is characterized by having small (ca. 1 cm diam.) flowers and fleshy, erect staminal corona lobes with concave apices, and is endemic to the dry-hot valleys of Yimen, Xinping, and Yuanjiang Counties, between 1000 and 1200 m, in Yunnan Province, China. In this study, we used plastome data to examine the tribal position of Xua within the family, and seven plastid and nuclear loci to further clarify its phylogenetic relationship. Our family-wide phylogenetic analysis confirms that Xua is a member of Marsdenieae. Our subsequent analyses of Marsdenieae further support that Xua is a distinct genus and probably allied to Dolichopetalum, Cionura, Campestigma, Harmandiella, and Gongronema-Dischidanthus-Sarcolobus. Molecular dating analysis indicates that Xua originated at about 10 Ma and Xua and the five probable allies diverged rapidly over a period shorter than 3 million years, which might be facilitated by both the orogeny and the concurrent reinforcement of the East Asian monsoon system during the early Late Miocene. For this new genus and species, a description, illustration, assessment of the conservation status, and its placement in the Marsdenieae phylogenetic tree are provided.

Key words: Apocynaceae, dry-hot valley, Marsdenieae, new taxa, phylogeny, Xua