A mitochondrial DNA analysis of vicariant speciation in two lineages in the Drosophila mulleri subgroup

Hypothesis: Biogeography and patterns of host cactus use in two ancestral-derived species pairs of cactophilic Drosophila suggest that recent divergence and speciation in both lineages were triggered by the same ecological/geological event in North America, the northward spread of the Sonoran Desert, isolating western coastal populations from the main distribution of each ancestral species. Organisms: Two pairs of species in the Drosophila repleta species group: D. aldrichi and D. wheeleri of the D. mulleri cluster, and D. longicornis and D. mainlandi of the D. longicornis cluster. Analytical methods: We analysed sequences of the mitochondrial cytochrome oxidase subunit II (cox2) and NADH dehydrogenase subunit 3 (nad3) from both species pairs, as well as members of relevant outgroups, to determine whether molecular evidence is consistent with concurrent speciation in both lineages. Based on long-term collecting records, we documented patterns of host cactus use throughout the ranges of all four species. Results: Pairwise sequence comparisons between members of each species pair showed ∼1% sequence difference. This difference was no greater than pairwise intraspecific comparisons within D. aldrichi consistent with recent evidence that D. aldrichi may be composed of more than one species. The interspecific differences we observed could also represent ancient polymorphisms, rather than species-specific divergences. We estimated an upper limit on the time of divergence by constructing a linearized tree based on transversion substitutions for nine species in the D. repleta species group. The results suggest that these species pairs arose no more than 0.2 million years ago, and may be much more recent.