Comparative genomics of accessory gland protein genes in Drosophila melanogaster and D. pseudoobscura.

Male accessory gland protein genes (Acps) evolve rapidly in the melanogaster species subgroup of Drosophila. However, conservation of Acps in more diverged lineages is poorly understood. We used comparisons of the D. melanogaster and D. pseudoobscura genome sequences, along with empirical investigation of D. pseudoobscura transcription, to assay the D. pseudoobscura genome for orthologs of 13 D. melanogaster Acps (Acp26Aa, Acp26Ab, Acp29AB, Acp32CD, Acp33A, Acp36DE, Acp53Ea, Acp62F, Acp63F, Acp70A, Acp76A, Acp95EF, and Acp98AB). We find that Acp26Aa, Acp26Ab, Acp32CD, and Acp53Ea are present at the expected microsyntenic locations of D. pseudoobscura. Acp62F and Acp70A are also present, although they are located in nonsyntenic regions. For six of the remaining seven Acps, computational and molecular biological evidence suggests they are D. melanogaster orphans. The weighted average of interspecific amino acid identity for alignable residues across the six orthologous Acps is 35.6%. Population genetic data for D. pseudoobscura Acp26Aa show that this gene has been evolving under directional selection, as it has been in D. melanogaster/D. simulans. All four D. melanogaster Acps we analyze from chromosome arm 3L are absent from the homologous D. pseudoobscura XR chromosome arm, which was autosomal before an X chromosome-autosome fusion event in the D. pseudoobscura lineage. This observation is consistent with the hypothesis that male-advantage genes on the Drosophila X chromosome are disfavored by natural selection.