The genus Commiphora (Burseraceae) comprises over 200 species of trees native to the seasonally-dry tropics of Africa and India. The taxonomic distinction between this genus and its putative sister, Bursera, a genus of ~100 species with an entirely Neotropical distribution, is based on one petal aestivation character. On the basis of this and other morphological and ecological similarities, several authors have hypothesized that these two genera may form a paraphyletic grade. Here we present a preliminary sectional phylogeny of Commiphora based on sequences of the nuclear rDNA external transcribed spacer region (ETS) including representative sampling of Bursera and Boswellia. Additionally, we test the phylogenetic placement of three species of Bursera recently transferred to Commiphora: B. inaguensis, B. leptophloeos, and B. tecomaca. While multiple copies of ETS exist within each individual, they coalesce at the species-level and, therefore, are appropriate for phylogenetic reconstruction at the supraspecific level. Contrary to the details of previous hypotheses, Commiphora is sister to Bursera sect. Bursera and not to Bursera sect. Bullockia. As a result, Bursera is paraphyletic but not Commiphora. We also find that B. leptophloeos is embedded within Commiphora and is basal to the clade containing sects. Abyssinicae, Africanae, Campestres, and Opobalsameae, thus corroborating the recent taxonomic placement of this species. The presence of Commiphora within the New World, the equivalent distributions of burseraceous genera between the Old and New World tropics, and fossil remains of the family from more northerly latitudes suggests a vicariant biogeographical history for this family that can be explained, in part, by the boreotropics hypothesis.

Key words: boreotropics hypothesis, Burseraceae, Commiphora, ETS