Genetic and phenotypic differentiation suggests incipient speciation within Drosophila arizonae (Diptera: Drosophilidae)

The cactophilic sister species Drosophila arizonae and Drosophila mojavensis, distributed across southwestern USA, Mexico and Guatemala, constitute a well-suited model to study ecology, genetic divergence and speciation. Although D. arizonae is the more broadly distributed of the two, its evolutionary biology has been less intensively investigated than D. mojavensis. Given the important role of genitalic variation in reproductive isolation, we explored the relationship between male aedeagus morphology and genetic differentiation of geographically distinct strains of D. arizonae. We used mitochondrial cytochrome c oxidase subunit I sequence data to establish the phylogenetic relationships among different D. arizonae strains and compared morphological variation, using elliptic Fourier descriptors, among five populations. Our results indicate that the Tuxtla Gutiérrez, Chiapas, population may be at the early stages of lineage divergence. Tuxtla Gutiérrez was the only locality sampled south of the Trans-Mexican Volcanic Belt and the Isthmus of Tehuantepec, both of which have been implicated as gene flow barriers in other organisms. Our results suggest that D. arizonae is an emerging model system for studies of incipient speciation.