Petalostelma of Brazil and the initial evolution of Metastelmatinae (Apocynaceae)

The MOG clade is one of the four neotropical lineages of the subfamily Asclepiadoideae (Apocynaceae), and its core group is formed by the subtribes Metastelmatinae, Oxypetalinae, Gonolobinae, Tassadiinae and Topeinae, consisting of about 935 species and 63 genera. These subtribes form well-supported and morphologically distinguishable clades, but the relationships among them are yet to be resolved. Metastelmatinae is composed of approximately 300 species and 12 genera with complex circumscriptions. There is no agreement regarding the sequence of phylogenetic events at the beginning of the Metastelmatinae evolution. Petalostelma stands out for being a genus frequently assigned to the basal grade of Metastelmatinae. In this study, we evaluated the circumscription of Petalostelma , its phylogenetic position in Metastelmatinae and the relationships between the subtribes of the MOG core group, thus offering an overview of the initial diversification of Metastelmatinae. Our results support the monophyly of Petalostelma and a basal grade sequentially formed by Blepharodon s.s., Petalostelma , Barjonia and Minaria within Metastelmatinae; however, alternative hypotheses in which the positions of Petalostelma and Barjonia are interchanged cannot be rejected. The sister clades Metastelmatine–Gonolobinae and Oxypetalinae–Tassadiinae are the most likely within the MOG core group, although without statistical significance. We also present a taxonomic revision of Petalostelma in Brazil, including complete morphological descriptions, an identification key, illustrations, distribution maps, assessments of the conservation status and taxonomic comments for the nine species recognized in the country, including a new species from eastern Brazil, P. atlanticum , endemic to Espírito Santo and critically endangered.