There and back again: molecular phylogenetics of the Brazilian endemic Psyllocarpus (Rubiaceae: Spermacoceae) supports a circumscription of the genus based on its original concept

The Spermacoce clade, found primarily in the Americas, poses taxonomic challenges, notably in the unclear boundaries of Borreria and Spermacoce. These genera intertwine with smaller, morphologically distinct ones, including Psyllocarpus, a Brazilian endemic redefined into two sections. Psyllocarpus sect. Psyllocarpus, being based on the original genus delineation, encompasses nine species in the Cerrado and campo rupestre of eastern Brazil, and P. sect. Amazonica includes three species in the Amazonian campinas, while Psyllocarpus intermedius remains unclassified. Our phylogenetic study, sampling extensively across the Spermacoce clade for nuclear ribosomal (ETS and ITS) and plastid (rps16 and trnL-trnF) DNA regions and using a variety of approaches to analyse our dataset, revealed that Psyllocarpus is not monophyletic. Notably, P. campinorum (representing P. sect. Amazonica) and P. intermedius are distinct lineages in the Spermacoce clade but fall outside Psyllocarpus. Conversely, P. sect. Psyllocarpus forms a well-supported clade, closely related to Staelia. Therefore, Psyllocarpus has to be circumscribed based on its original concept, excluding P. sect. Amazonica and P. intermedius. This defines the genus as a distinct, easily diagnosable taxon. We provide a synoptic list of names and nomenclatural types of Psyllocarpus to formalize our results, with an updated description of the genus.