The Marsdenieae (Asclepiadoideae, Apocynaceae) comprise approximately 750 species and 26 accepted genera, half of which contain just one or two species. The tribe has been under-represented in phylogenetic studies to date, most of which have focussed on the ornamental paleotropical genus Hoya and related genera. American Marsdenieae are all currently classified in Marsdenia, a heterogeneous genus with about 150 species distributed in the tropics and subtropics and whose broad circumscription has been questioned for a long time. Here, we perform phylogenetic analyses comprising half of the tribe's genera and using two plastid (trnL-F and psbD-trnT) and two nuclear (ITS and ETS) regions. Our analyses show that the American species of Marsdenia are monophyletic but not closely related to M. tinctoria, the type of the genus. Thus, we reinstate Ruehssia H.Karst. to classify this American clade of Marsdenieae. We present a taxonomic revision of the genus in Brazil, with 42 species (one with two varieties) recognised. Other than R. macrophylla, the type of Ruehssia, all names are new combinations. One new synonym is proposed and 10 lectotypes and two neotypes are selected. All species are described and illustrated, together with comments and distribution maps for their occurrence in Brazil.
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