Functional characterization of chemosensory proteins in three sympatric Tomicus bark beetles feeding on Pinus yunnanensis: Implication for the conservation and divergence of ligand-binding profiles

Abstract

Three sympatric Tomicus bark beetles, Tomicus yunnanensis, Tomicus brevipilosus and Tomicus minor that coexist in the trunks of a pine but occupy different spatiotemporal ecological niches, are the best-studied examples for addressing functional differentiation of chemosensory protein (CSP) orthologs. Here, totally 36 CSP-coding genes belonging to 12 groups of orthologs were identified from three Tomicus beetles. Tomicus CSP orthologs shared the high conservation and slight differences in sequence characteristics, phylogenetic relationships and tissue expression profiles. Binding assays revealed that 12 antenna-enriched Tomicus CSPs could respond strongly to non-host volatiles, pheromones and insecticides where each group of CSP orthologs presented diverse ligand-binding properties. CSP1 and CSP2 could interact strongly with benzaldehyde and salicylaldehyde (dissociation constant, K_i < 14 μM) whereas CSP10 and CSP11 preferred to bind 1-hexanol and acetophenone (K_i < 13 μM). All the proteins were tuned to chlorpyrifos with particularly high affinities (K_i < 10 μM). Two conserved residues (glutamine, Q64 and leucine, L72) of CSP11 orthologs contributed to the specific binding to optimal ligands, but exhibited opposite binding properties between chlorpyrifos and other four compounds. Our study sheds light on the functional conservation and divergence of Tomicus CSP orthologs in olfactory reception and insecticide resistance, implying the correlation between ecological niche differentiation and olfactory specialization in three Tomicus bar beetles.