Evolution of expression patterns of two odorant-binding protein genes, Obp57d and Obp57e, in Drosophila.

Odorant-binding proteins (OBPs) function in the perception of chemical signals together with odorant and taste receptors. Genes encoding OBPs form a large family in insect genomes. In Drosophila, the evolution of OBP gene repertoire has been well studied by comparisons of the whole genome sequences from 12 closely related species. In contrast, their expression patterns are known only in Drosophila melanogaster. Two OBP genes, Obp57d and Obp57e, arose by gene duplication at the early stage of D. melanogaster species group evolution, followed by the divergence of open reading frame (ORF) sequences from each other. While most species in the melanogaster group maintain both Obp57d and Obp57e, some species have lost either gene, suggesting that the birth-and-death process is a dominating pattern of evolution at the Obp57d/e locus. However, it has not been explored whether the expression patterns of these two OBP genes are diverged or conserved among species. Here, we examined the expression patterns of Obp57d and Obp57e in the selected species from the melanogaster group using a combination of reporter analysis, RNA in situ hybridization, and quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) analysis. As previously reported for D. melanogaster, expression in the chemosensilla on the legs was observed in all the species examined. Unlike in D. melnanogaster, however, additional expression in the chemosensilla on the mouthparts was observed in some species including Drosophila pseudoobscura, which maintains an ancestral OBP gene at the Obp57d/e locus. This result shows that, as well as their ORF sequences, the expression patterns of Obp57d and Obp57e have diverged substantially between closely related Drosophila species.