Molecular phylogeny of Vincetoxicum (Apocynaceae, Asclepiadoideae) from Thailand and integrative taxonomy corroborating a new cryptic species within Vincetoxicum kerrii

An updated phylogeny of the genus Vincetoxicum s.l. based on DNA sequences of the nuclear internal transcribed spacer (ITS) region and three plastid markers is presented. In total, 21 accessions newly sequenced from Thailand were added to the dataset of the homologous sequences of 75 other Vincetoxicum taxa downloaded from GenBank. In our analysis, the relationships between the well-supported clades largely correspond to those revealed in previous studies. With some exceptions, the phylogenetic positions of the Thai taxa in relation to other conspecifics and congeners generally reflect the geographic distributions of taxa. Moreover, recent extensive sampling throughout Thailand and in-depth investigation have revealed V. kerrii, a slender twiner widespread from South China to Indo-China, to be a species complex. A combination of molecular, morphological, anatomical, ultrastructural and ecological evidence allowed us to reveal a new cryptic species hidden within V. kerrii, described here under the name V. simplex. A comprehensive description, illustrations, photographs, and comparison with the morphologically similar species are provided. Although V. simplex and V. kerrii s.s. resemble one another in various aspects of vegetative and reproductive structures, the latter is phylogenetically closely related to V. irrawadense, which is much less similar morphologically to both V. simplex and V. kerrii s.s. than the latter two are to each other. In addition to the new cryptic species recognized in the present study, a new record for Thailand, V. microstachys, is also reported.