Acantheae are Acanthaceae s.s. (i.e., plants with seeds borne on retinacula) that share the apomorphy of monothecous anthers and lack the morphological synapomorphies that mark the remaining plants belonging to Acanthaceae s.s. (e.g., cystoliths. porate pollen). The lineage includes 21 genera with a total of ca. 500 species, and is distributed primarily in the American and African tropics. This group has been shown to be monophyletic in our earlier work but details of relationships within it have not been previously addressed. We have used DNA sequence data from three loci (nr-ITS, cp trnL-F and rps16 intron) to test monophyly of Acantheae and to determine how New and Old World plants are related to each other. Do they each comprise monophyletic sublineages or are some Old World plants (e.g., Stenandriopsis) in fact more closely related to New World groups? Finally, we examine the phylogenetic status of the larger genera of Acantheae. In the New World, morphological differences among genera are subtle at best and our work provides insight into placement of a number of enigmatic taxa (e.g., Aphelandra verticillata is more closely related to species of Holographis than to other Aphelandra whereas Geissomeria and Neriacanthus are nested within Aphelandra). This work provides insight into the biogeographic processes that might have yielded present distribution patterns and also helps to understand morphological evolution. The only spinescent Acanthaceae are members of this lineage (i.e., most Old World Acantheae, one group of New World Aphelandra) and many Old World species have remarkable floral morphologies involving extreme modification of the upper corolla lip.

Key words: Acanthaceae, Acantheae, Biogeography, Phylogeny