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

Abstract

The Spermacoceclade, found primarily in the Americas, poses taxonomic challenges, notably in the unclear boundaries of Borreriaand Spermacoce. These genera intertwine with smaller, morphologically distinct ones, including Psyllocarpus, a Brazilian endemic redefined into two sections. Psyllocarpussect. Psyllocarpus, being based on the original genus delineation, encompasses nine species in the Cerrado and campo rupestre of eastern Brazil, and P. sect. Amazonicaincludes three species in the Amazonian campinas, while Psyllocarpus intermediusremains unclassified. Our phylogenetic study, sampling extensively across the Spermacoceclade for nuclear ribosomal (ETS and ITS) and plastid (rps16and trnL-trnF) DNA regions and using a variety of approaches to analyse our dataset, revealed that Psyllocarpusis not monophyletic. Notably, P. campinorum(representing P. sect. Amazonica) and P. intermediusare distinct lineages in the Spermacoceclade but fall outside Psyllocarpus. Conversely, P. sect. Psyllocarpusforms a well-supported clade, closely related to Staelia. Therefore, Psyllocarpushas to be circumscribed based on its original concept, excluding P. sect. Amazonicaand P. intermedius. This defines the genus as a distinct, easily diagnosable taxon. We provide a synoptic list of names and nomenclatural types of Psyllocarpusto formalize our results, with an updated description of the genus.