Molecular phylogenetic and morphological study of Kohautia (Spermacoceae, Rubiaceae), with the recognition of the new genus Cordylostigma

Kohautia Cham. & Schltdl. belongs to the predominantly herbaceous Rubiaceae tribe Spermacoceae. Species of Kohautia can easily be distinguished from other Spermacoceae by their monomorphic short-styled flowers in which anthers and stigma are included in the corolla tube, with the stigma always positioned below the anthers. Mainly because of this unique floral morphology. Kohautia was considered to be a distinct genus. Molecular data (atpB-rbcL, petD, rps16, trnL-trnF, ETS, ITS) confirm that the genus is biphyletic. Two distantly related clades correspond to the subgenera Kohautia and Pachystigma Bremek. A similar type of floral organisation thus seems to have evolved twice independently, resulting in similar, but distantly related lineages. In order to translate the biphyletic nature of Kohautia into a formal classification, the two subgenera are recognized at generic level. A substitute name, Cordylostigma Groeninckx & Dessein is proposed for K. subg. Pachystigma because of the existence of Pachystigma Hochst. in the Rubiaceae tribe Vanguerieae. Floral, pollen and seed characters were studied to morphologically characterize Kohautia s.str. and Cordylostigma. By optimizing pollination syndromes and pollen characters onto the molecular phylogeny, we investigated pollination shifts and pollen evolution within the two genera. Detailed floral morphological studies show that the nectar guides in the psychophilous species of Kohautia s.str. and Cordylostigma evolved in different ways but result in the same visual effect.