Palicourea or Mexocarpus? Molecular phylogenetics of a Mesoamerican genus (Palicoureeae, Rubiaceae)

One of the most conflicting groups among Rubiaceae is related to Psychotria-Palicourea, which has always been a subject of debate among taxonomists due to its high number of species, its paraphyly, and its morphological variation. The genus Mexocarpus was described to encompass Palicourea tetragona due to its pyrenes with prominent ridges, atypical within the rest of Palicourea species. Mexocarpus is a genus distributed from southern Mexico to Central America, aside the pyrene morphology is characterized by the long white flowers with nocturnal anthesis and the fruits tetragonal when dry. The species was originally published as Cephaelis, then transferred to Palicourea and finally to Mexocarpus, however, this latter transfer is not accepted by some researchers arguing that the pyrene morphology is variable. We conducted phylogenetic analyses using two nuclear markers (ETS and ITS) and one plastid marker (trnL-F) to test the phylogenetical position of Mexocarpus tetragona. Our molecular phylogeny shows that Mexocarpus tetragonus does not form a separate entity from Palicourea, forming an unresolved clade with Palicourea tomentosa and Palicourea faxlucens. Additionally, the morphological comparison with closely related species indicates that pyrene morphology is highly variable and therefore cannot be used as a diagnostic character to segregate a genus within this complex. Then, based on morphology and molecular evidence our conclusion is not recognize Mexocarpus as a separate genus.