Floral ontogeny links Dialypetalanthus (Condamineeae) with the floral developmental morphology of other Rubiaceae

Background - Vegetative and fruit characters of the Amazonian genus Dialypetalanthus point to a position in Rubiaceae. However, its floral morphology is so deviant that the genus was often placed in a family of its own. Even relationships outside Gentianales were postulated. Current molecular phylogenetic studies firmly show that Dialypetalanthus belongs to Rubiaceae. Aims - This study aims to understand the idiosyncratic floral morphology in Dialypetalanthus and to compare it with the floral development in two other Condamineeae genera as well as in other Rubiaceae for which ontogenetic data are available. Material and methods - SEM and LM based floral ontogeny in Dialypetalanthus fuscescens, Mussaendopsis beccariana, and Pogonopus exsertus. Results and main conclusions - Flowers in Dialypetalanthus develop a stamen-corolla-calyx tube, which can be considered as a floral morphological link between the genus and the other Rubiaceae. The polyandrous androecium originates from an annular intercalary meristem at the adaxial side of the stamen-corolla-calyx tube.