The Adh genomic region of Drosophila ambigua: evolutionary trends in different species.

The study of individual genes is essential to a comprehensive understanding of genome evolution. The wealth of information on alcohol dehydrogenase (Adh) in Drosophila makes this gene particularly suitable for such analysis. We have characterized more than 4 kb of the genomic Adh region in Drosophila ambigua and compared this region to Drosophila mauritiana and Drosophila pseudoobscura. The presence of two genes, Adh and 3'ORF (open reading frame), has been confirmed and some of their essential features have been inferred from primary structural analysis. Inter- and intraspecific comparisons have led us to support that both genes may have diverged from an ancient precursor. They appear to be evolving independently, and show a species-specific pattern. The Adh in the obscura group species lacks amino acids three and four when compared to the species of the melanogaster group and has accumulated most of its amino acid replacements in the third exon. Neither characteristic is observed when any other group species are compared, which suggests that these may be particular features of the evolution of the obscura group. The 3'ORF is highly conserved among the three species analyzed, although variability in the length of the third exon and the nucleotide substitution rate, which is much higher than in Adh, are worth noting. According to our data, both mutation/fixation rates and the distribution of mutations vary over time, which makes it difficult to predict the evolutionary dynamics of specific genome regions.