Petrodesmum, a new genus of the legume tribe Desmodieae from Laos

Recent molecular evidence has revealed that Desmodium (Fabaceae: Desmodieae), as traditionally defined in Legumes of the World, is highly polyphyletic and has now been subdivided into 18 distinct genera. The currently circumscribed Desmodium is primarily distributed in the Americas, with only one species native to Africa and none naturally occurring in Asia. Approximately one-third of the Asian species formerly classified under Desmodium have been reassigned to the recently resurrected genus Grona. The Laotian species Desmodium vidalii, which was originally placed in Desmodium subg. Podocarpium (now treated as Hylodesmum and Monarthrocarpus), was recently transferred to Grona. However, this species exhibits a suite of distinctive morphological and ecological traits—such as a herbaceous habit with short zigzag stems, leaves bearing an exceptionally long petiole and a long rachis of the terminal leaflet, long laxly flowered inflorescences, deeply constricted pods with a stipe, and occurrence in limestone habitats—suggesting that its generic placement requires re-evaluation. Using plastid and nuclear ribosomal DNA sequences, as well as 353 single-copy nuclear genes, and based on comprehensive sampling of all 47 genera and approximately 32% of species in the tribe, our phylogenetic analyses recognize 20 major clades within Desmodieae. Desmodium vidalii constitutes an independent clade, clearly separate from Grona, Hylodesmum, and Monarthrocarpus. Consequently, we propose the new genus Petrodesmum K.W.Jiang & Z.Q.Song to accommodate this species. A key to all 29 genera of Desmodieae occurring in Laos is also provided.