NUCLEOTIDE-SEQUENCE DIVERGENCE IN THE A-RICH REGION OF MITOCHONDRIAL-DNA IN DROSOPHILA-SINULANS AND DROSOPHILA-MAURITIANA(T)

We have determined the nucleotide sequences of two regions within the A+T-rich region of mitochondrial DNA (mtDNA) in the siIII type of Dros ophila simulans and the maI type of D. mauritiana. The sequences of th e two regions in siIII and maI are almost identical. The sequences inc lude elements corresponding to the type I and type II repeat elements and the T-stretches as reported in D. melanogaster, an approximately 3 40-bp region (A region) adjacent to the tRNA(Ile) gene includes a part of the type II repeat element, and an approximately 440-bp region (B region) includes a central portion of the AS-T-rich region between the type I and type II repeat arrays. Each sequence of the two species wa s compared with those of D. melanogaster and D. yakuba. The sequences of the A region are relatively well conserved among the four species. The alignment of the two sequences of the B region with those of D. me lanogaster and D. yakuba requires numerous insertions/deletions. For b oth legions, nucleotide differences between D. simulans or D. mauritia na and D. melanogaster are similar to those between the two and D. yak uba. The tendency is obvious in a subregion within the type II repeat element in the A region. These findings suggest that the rate of nucle otide substitution in the subregion is accelerated in the lineage lead ing to D. melanogaster. Loss of functional constraint in the stem-loop -forming sequence is proposed for this acceleration.