From sequence to function : Comparative studies of insect olfactory receptors
Abstract: In insects airborne odors are detected by the odorant receptors (ORs) and ionotropic receptors (IRs), located on the dendritic membrane of the olfactory sensory neurons (OSNs) housed in olfactory sensilla. The OR and IR gene families show high plasticity in lineage expansion and contraction following the birth-and-death evolutionary model. Gene duplication is an important mechanism underlying the evolution of diversity and novel function. In this thesis, I address the questions whether the receptors within the same lineage detect structurally similar chemicals and how the specificities evolve after gene duplication. Using oocyte recordings as the major tool, I functionally characterized moth ORs from the clade with receptors for Type 0 pheromone compounds and the Cluster III clade among the pheromone receptors (PRs), bark beetle ORs from a species-specific clade, as well as moth IRs from the IR75p/q clade.Using the OSNs identified by previous single sensillum recording (SSR) studies as references, I compared the heterologous response profiles of ORs obtained from Xenopus oocyte and HEK cell systems and found that in most cases the results from the oocyte system are more consistent with the in vivo data recorded form the OSNs. In moths, most PRs identified so far are grouped in the so-called PR clade, which can be divided into four orthologous clusters. The Cluster III is conserved across noctuid moths, however most receptors within this cluster have remained ‘ligand unknown’ except for one responding to polyenes. I found that three Cluster III receptors from different noctuid subfamilies respond selectively to polyene hydrocarbons from conspecific male hairpencils, suggesting that the noctuid moths recruited receptors in the existing PR subfamily to sense the hairpencil signals from males. Moreover, the hairpencil polyene receptors are co-localized with the receptors for the major pheromone component released by conspecific females, implying that the simultaneous detection of the two signals is important for sexual communication in noctuid moths.In the bark beetle Ips typographus, we functionally characterized all seven ORs in a species-specific clade and found that receptors responding to bark beetle pheromones and volatiles released by host plants and Ips-associated fungi fell into the same clade, supporting the idea that pheromone receptors in beetles do not cluster in specific clades like they do in Lepidoptera. The common denominator of the seven ORs is that they all detect compounds with a monoterpene skeleton, which is in line with the ‘Sensory Drive’ hypothesis predicting that evolutionarily related ORs are likely to detect structurally similar compounds. In the turnip moth Agrotis segetum, I investigated all five receptors located in the IR75p and IR75q expansions and successfully characterized two receptors responding to medium-chain fatty acids; AsegIR75p.1 responded to several common plant volatiles but with hexanoic acid as the primary ligand, while AsegIR75q.2 responded primarily to octanoic acid, a common insect repellent. The response of the IR75 clade to medium-chain fatty acids is conserved across different insect orders, and a subfunctionalization may have occurred in Lepidoptera after gene duplication in the IR75p/q clade. By fluorescence in situ hybridization I showed that the acid-sensing IRs in A. segetum are not located in coeloconic sensilla as in Drosophila but in basiconic sensilla.My work on olfactory receptors has demonstrated that: 1) the response profiles from oocytes are generally consistent with the results obtained from OSNs; 2) the receptors within the same lineage detect structurally similar chemicals; 3) both neofunctionalization and subfunctionalization contribute to the olfactory specialization after gene duplications; 4) the expression of IRs in specific sensillum types differs among insect taxa.
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