Evolution of Olfactory Genes in Insects

The olfactory sense is under heavy selection pressure to detect a plethora of behaviorally relevant odor molecules. These pressures during evolution have led to highly diverse gene families in different animal phyla. Insects detect volatile molecules using either olfactory (OR) or ionotropic (IR) receptors. While IRs are expressed in olfactory organs across Protostomia (including e.g. nematods, molluscs and arthropods), ORs of the insect type have been speculated to be an adaptation to a terrestrial insect lifestyle. To test this hypothesis we investigated the olfactory system of the primary wingless bristletail Lepismachilis y-signata (Archaeognatha), the firebrat Thermobia domestica (Zygentoma) and the neopteran leaf insect Phyllium siccifolium using electrophysiological, immunocytochemical and molecular techniques. In Archaeognatha, the oldest sibling group within insects, ORs and their coreceptor Orco are with very high probability lacking. Already in Thermobia, however, we found evidence for the presence of the coreceptor, but not for fully formed Orco/OR pairs as present in Neoptera (e.g. P. siccifolium). Instead we found three independent Orco types, the function of which still needs to be elucidated. In summary our data suggest that Orco/OR pairs did not emerge as a result of colonizing land, but later in insect evolution.