Selective pressures on Drosophila chemosensory receptor genes

The evolution and patterns of selection of genes encoding 10 Drosophila odorant receptors (Or) and the sex pheromone receptor Gr68a were investigated by comparing orthologous sequences across five to eight ecologically diverse species of Drosophila. Using maximum likelihood estimates of dN/dS ratios we show that all 11 genes sampled are under purifying selection, indicating functional constraint. Four of these genes (Or33c, Or42a, Or85e, and Gr68a) may be under positive selection, and if so, there is good evidence that 12 specific amino acid sites may be under positive selection. All of these sites are predicted to be located either in loop regions or just inside membrane spanning regions, and interestingly one of the two sites in Gr68a is in a similar position to a previously described polymorphism in Gr5a that causes a shift in sensitivity to its ligand trehalose. For three Ors, possible evidence for positive selection was detected along a lineage. These include Or22a in the lineage leading to D. mauritiana and Or22b in the lineage leading to D. simulans. This is of interest in light of previous data showing a change in ligand response profile for these species in the sensory neuron (ab3A) which expresses both Or22a and Or22b in D. melanogaster. In summary, while the main chemosensory function and/or structural integrity of these 10 Or genes and Gr68a are evolutionarily preserved, positive selection appears to be acting on some of these genes, at specific sites and along certain lineages, and provides testable hypotheses for further functional experimentation.