Sylvainia, a new monospecific genus within the subtribe Cephalanthinae (Rubiaceae, Naucleeae)

Background and aims – The genus Cephalanthus belongs to the tribe Naucleeae together with 26 other genera. Members of Cephalanthus mainly grow in temperate and subtropical wetlands in the Americas and Asia, but there is a single African species that is associated with forest margins and rocky outcrops in moist temperate grasslands. The genus comprises six species and was historically known to have species with a markedly disjunct distribution: three species are from the Americas: C. glabratus, C. occidentalis, and C. salicifolius; two species are from tropical Asia: C. angustifolius and C. tetrandrus, and one species is from tropical Africa: C. natalensis. Recent molecular phylogenetic studies have supported the monophyly of the genus, however, most of the morphological synapomorphies are absent in the only African species, which is sister to the remaining species of the genus. This work aims to provide a short taxonomic revision of the genus Cephalanthus, taking into consideration morphological aspects previously underestimated or not considered, and to describe a new monospecific genus based on comparative morphological analysis. Material and methods – Vegetative and reproductive material from all the species of Cephalanthus were analysed following conventional taxonomy techniques. Key results – We transferred C. natalensis to a new genus, Sylvainia. Morphological similarities and differences between Cephalanthus and the new genus are discussed. Cephalanthus glabrifolius, a poorly known species and previously considered a synonym of C. tetrandrus, is resurrected as a valid species, bringing the number of species in Cephalanthus to six again. All species are described, illustrated, and their known distributions plotted on regional maps. Conclusion – The subtribe Cephalanthinae now has two genera, its type Cephalanthus with six species, and a new monospecific genus Sylvainia endemic to south-eastern Africa. Based on the amended description of Cephalanthus, now absent from Africa, the generic concept is substantially modified, being represented now by three American and three Asian species.