Sites of evolutionary divergence differ between olfactory and gustatory receptors of Drosophila

Drosophila olfactory (ORs) and gustatory (GRs) receptors are evolutionarily unrelated to vertebrate ORs or nematode chemosensory receptors. Insect ORs display a reverse membrane topology compared with conventional G-protein-coupled receptors, suggesting that the mammalian scheme of chemosensory signal transduction cannot directly apply to insects. Experimental studies of GR membrane topology are lacking. We analysed the distribution of amino acid sites in GRs and ORs that show evidence for divergence under either positive selection or relaxed purifying constraints, in the genomes of 12 Drosophila species and found significant differences between these two receptor types. This suggests that insect ORs and GRs have distinct molecular properties and mechanisms of ligand recognition and/or signal transduction.