Synopsis of the genus <i>Polystemma</i> (Apocynaceae, Asclepiadoideae) and recognition of three new species

Background: Polystemma is characterized by a mixed indumentum of glandular trichomes that turn white, rotate to rotate-campanulate corollas, a gynostegial corona with filiform appendages, and fusiform follicles. The taxon has undergone numerous taxonomic changes in recent years. In current phylogenies, Polystemma has been recovered as paraphyletic and many of its species require a new combination. In this study, we propose the corresponding changes and describe, map, and illustrate three new species within this genus. Questions: Do morphological data support the transfer of Matelea species to Polystemma? Are the plants of Polystemma collected in Durango, Jalisco, and Sinaloa different species? Studied species: Polystemma. Study site and dates: United States, Mexico, Guatemala, El Salvador, Honduras, Nicaragua y Costa Rica, between 2022 and 2024. Methods: More than 3,000 specimens from 20 herbaria and virtual databases were reviewed. Distribution maps were generated, and risk categories were proposed. The cohesive species concept was proposed as an explanatory hypothesis for taxon recognition. Results: A total of 27 species of the genus were recognized, these distributed from the southeastern United States of America to Costa Rica. Eleven new combinations and three lectotypifications are made, and three new species for science are proposed. Identification key, nomenclatural type information, synonyms, habitat, images of all species and distribution maps are also included. Conclusions: The review of this genus allowed for updating and improving the systematics within the subtribe Gonolobinae. Likewise, it highlights Mexico as the center of diversity and endemism for Polystemma.