A new concept of Leptadenia (Apocynaceae)

We revise the concept of Leptadenia. The closely related, small genus Orthanthera (Ceropegieae-Asclepiadoideae-Apocynaceae) contains four species, all with tubular flowers. Our phylogenetic reconstructions show that the type of Orthanthera, namely the little-known Himalayan endemic O. viminea, is deeply nested within Leptadenia, where it is most closely related to the widespread L. pyrotechnica, a species with rotate flowers. Consequently, we deduce that the tubular flowers of Orthanthera viminea evolved separately from those in the other three species of Orthanthera, which are all from Africa south of the equator. These African species form a separate clade, which is sister to the remainder of Leptadenia. We transfer all four species of Orthanthera to Leptadenia to render Leptadenia monophyletic once more. Leptadenia consists of two subgenera: subg. Leptadenia contains the four species previously in Leptadenia and L. viminea (Orthanthera viminea); subg. Barrowia contains the three species L. albida (Orthanthera albida), L. gossweileri (O. gossweileri) and L. jasminiflora (O. jasminiflora) from Africa south of the equator. We highlight the fact that the range from flat to tubular flowers, well-known in Ceropegia, is also found in each of the three subtribes that are sister to and basal to Ceropegia. A lectotype is selected for Leptadenia and lectotypes and neotypes are selected for several names.