Evolution of the noncoding regions in Drosophila mitochondrial DNA: two intergenic regions.

The sequences of the mitochondrial DNA (mtDNA) segment containing the two intergenic regions were determined for six species belonging to the Drosophila immigrans species group and compared to the corresponding segments of Drosophila species which had been studied previously. We found remarkable differences in the evolutionary rates of the two intergenic regions. The Intergenic I region, which lies between the tRNA(gln) and the tRNA(ile) genes, was found to be highly conserved in terms of both size (30 ntp) and nucleotide sequence among the species studied. In contrast, the sequences of the Intergenic II region, which lies between the tRNA(f-met) and the tRNA(ile) genes, showed considerable variation. The size of the Intergenic II region ranged from 0 to 88 ntp, and accurate alignment was possible only among sequences from geographical strains or very closely related species in the nasuta species subgroup. The observed differences in conservation of the two mtDNA intergenic regions are discussed in light of functional constraints on mtDNA sequences.