A genealogical view of chromosomal evolution and species delimitation in the Drosophila virilis species subgroup.

Chromosomal arrangement was a historically important character used for defining taxonomic boundaries. The Drosophila virilis species group exhibits a series of chromosomal rearrangements, and the resulting differences among karyotypes were primary characters originally used to define taxa within the group. However, some chromosomally divergent forms have not been sufficiently resolved in phylogenetic reconstructions of DNA sequences from several nuclear genes. Sequences of mitochondrial regions have the potential for finer-scale resolution of closely related taxa; therefore, sequences of two mitochondrial genes were used to examine phylogenetic relationships within the chromosomally variable virilis subgroup. Sequences were obtained from multiple strains of the Palearctic species, D. virilis and D. lummei, and the Nearctic species, D. novamexicana and two chromosomal forms of D. americana. Analyses support the recent emergence of the different chromosomal forms in North America. However, none of these chromosomally divergent forms exhibit reciprocal monophyly of their mtDNA sequences, which is the requirement for attaining genealogical species status.