Nucleotide Sequence of the Adh Gene Region of Drosophila pseudoobscura: Evolutionary Change and Evidence for an Ancient Gene Duplication

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Abstract
The alcohol dehydrogenase (Adh) locus (ADH; alcohol: NAD+ oxidoreductase, EC 1.1.1.1) of Drosophila pseudoobscura was cloned and sequenced. Forty-five percent of the "effectively silent sites" have changed between Adh in D. pseudoobscura of the obscura species group and the homologous DNA sequence in D. mauritiana, the latter representing the melanogaster species group. The untranslated leader sequence of the adult transcript of D. pseudoobscura has two deletions relative to the D. mauritiana message. The ADH protein sequences of D. pseudoobscura is missing the third and fourth amino acids at the N-terminus relative to the D. mauritiana enzyme. Of the remaining 254 amino acid positions, 27 (10.64%) differ between the two species. Amino acid replacements are randomly distributed into hydrophilic and hydrophobic domains of ADH. However, replacement substitutions are distributed nonrandomly across the three exons among D. pseudoobscura and members of the melanogaster subgroup, suggesting that functional constraints across the exons are different. Surprisingly, silent substitutions are also nonrandomly distributed with the third exon being the most divergent. This pattern suggests possible selective constraints on supposedly neutral silent substitutions and/or variation in underlying mutation rates across the gene. The presence of transcriptional and translational signals at the beginning and end of conserved sequences 3′ to Adh implies the existence of a previously undescribed gene. Codon usage and patterns of nucleotide divergence are consistent with a protein coding function for this gene. In addition, conservation of nucleotide and amino acid sequence and similarity in hydropathy plots suggests that the gene 3′ to Adh represents an ancient duplication of the Adh gene.