Insect chemosensory receptor genes : expression, expansion and functional evolution

Insects depend on chemosensation - olfaction and gustation - for foraging, mating, defense against parasites etc. These vital activities are mediated primarily by three chemosensory receptor gene families: odorant receptors (ORs), ionotropic receptors (IRs), and gustatory receptors (GRs). Using gene expression profiling and ligand identification, my dissertation investigated the function and evolution of insect chemosensory receptor genes in the hawkmoth, Manduca sexta, and the locust, Locusta migratoria. In adult M. sexta, we observed diverse expression patterns of ORs, IRs, and GRs across nine putatively chemosensory organs, indicating olfactory and gustatory capabilities throughout the body. Notably, the expression of OR co-receptor gene, MsexORCo, was found in the labial palps and forelegs of M. sexta for the first time. We identified sex-dependent but not mating-state dependent differential expression of some genes. Furthermore, we studied four M. sexta OR duplicates from a lineage-specific OR expansion. Experiments revealed substantial functional divergence among these gene duplicates and suggested potential co-expression in olfactory sensory neurons projecting to a feeding-related brain region. Two of the duplicates also exhibited an increased sensitivity towards volatiles emitted by valuable nectar sources; altogether indicating an evolutionary adaptation for improved foraging on these sources. In L. migratoria, we found a much higher proportion of narrowly tuned ORs compared to other insects, likely indicating a unique odor coding mechanism in this species. Chemosensory receptor expression in locusts was dependent on age, phase (solitary or gregarious), and sex. Overall, my dissertation lays the groundwork for future studies on ecological interactions and evolutionary pressures shaping species-specific receptor repertoires.