The genetic basis of a species-specific character in the Drosophila virilis species group.

The genetic basis of the species-specific dorsal abdominal stripe of Drosophila novamexicana was examined. The dorsal stripe is present in D. novamexicana and absent in all other members of the Drosophila virilis species group. Interspecific crosses between D. novamexicana and genetically marked D. virilis revealed that all four of the autosomes (except the tiny dot chromosome, which was not marked) and the sex chromosomes (the X and Y chromosome effects could not be disentangled) showed a significant effect on the width of the dorsal stripe. All the autosomes act approximately additively; only minor interactions were detected among them. No significant maternal effects were found. This means that a minimum of five loci are involved in the character difference between the two species, and this is the maximum number that this technique could discern. These results suggest that, based on the number of factors involved in the character difference, the inheritance of this character should be considered polygenic, but because chromosome 2 (the largest chromosome in the species) contributed over half of the variance toward the character difference, it is best to consider the inheritance oligogenic based on effect. The implications of these findings are discussed in light of the importance of macromutation in speciation and the sex chromosome theory of speciation.