Abstract
The tribe Retiniphylleae and its single genus Retiniphyllum Bonpl. contains 22 species of shrubs and small trees that grow in white sand soils mostly in the Guayana Region of South America. The circumscription of the tribe is based on the diagnostic characteristic of two collateral and pendulous ovules per locule, a rare condition in the Rubiaceae. However, for the same reason, its placement within the family has been controversial. The monophyly and systematic position of the tribe Retiniphylleae and Retiniphyllum were tested based on a phylogenetic analysis of trnL-F and rps16 sequence data. The results confirm the monophyly of the tribe and genus Retiniphyllum. The tribe is placed sister to the core members of the subfamily Ixoroideae s. str. The genera Botryarrhena Ducke and Scyphiphora C. F. Gaertn. are not related to the tribe Retiniphylleae, as formerly hypothesized.