Relationships, circumscription, and biogeography of Arcytophyllum (Rubiaceae) based on evidence from cpDNA

A cladistic analysis was performed using nucleotide sequence variation in the rps16 intron and the trnL–F region (plastid DNA). Arcytophyllum belongs in a subclade of the tribe Spermacoceae (s.l.) together with the American species presently classified in the genera Hedyotis and Houstonia. This subclade is morphologically characterized by cymbiform seeds. Arcytophyllum is the sister group of all American Hedyotis and Houstonia and it is suggested that these latter would be most conveniently treated as a single genus, the correct name of which would be Houstonia. Arcytophyllum should be circumscribed such that it excludes A. serpyllaceum, which is not a member of the Arcytophyllum–Houstonia clade but more closely related to Bouvardia. The phylogeny that was reconstructed suggests that the ancestral area of the Arcytophyllum–Houstonia clade is the South American tectonic plate.