Abstract
The nucleotide sequence of a segment of the mitochondrial DNA from three Drosophila species (D. erecta, D. eugracilis, and D. takahashii), belonging to different subgroups of the melanogaster group has been determined. The segment encompasses three complete tRNA genes (tRNAtrp, tRNAcys, and tRNAtyr) and portions of two protein-coding genes: the subunit 2 of the NADH dehydrogenase (ND2) and the subunit 1 of the cytochrome oxidase (COI). Comparisons also involve homologous sequences already known for four other Drosophila species of the melanogaster group. Length differences were confined in the intergenic region where a long stretch of AT repeats was observed in one of the species analyzed. The three tRNA genes exhibit very different evolutionary rates, the most slowly evolving one, tRNAtyr, is adjacent to the 5' end of COI; tRNAs in similar positions have been previously shown to evolve slowly because they are probably involved in transcript processing. Although the rate of synonymous substitutions was very similar between ND2 and COI genes there were strong discrepancies between them in terms of the number of nonsynonymous substitutions. Differences have also been found in G + C content of the genes, which are likely to be linked to different selective pressures. There is a reduction in G + C content in the region where selective constraints are reduced. This suggests the existence of different levels of constraints along the sequenced segment. An overall analysis of the types of substitutions showed a decrease in A + T content during the course of evolution of the species.
