Comparison of Chemosensory Receptor Genes in the Antennae Transcriptomes of Sirex noctilio and Sirex nitobei (Hymenoptera: Siricidae)

The woodwasp Sirex noctilio Fabricius is a worldwide quarantine pest for forestry that mainly harms conifers, especially Pinus species. Sirex nitobei Matsumura, a native species from China and closely related to S. noctilio. Olfaction and gustation play a vital role in the life movements of woodwasps, but the molecular mechanisms of chemoperception in these species remain unclear. We performed a comparative analysis of 41 odorant receptors (ORs), 13 ionotropic receptors (IRs), and 8 gustatory receptors (GRs) of S. noctilio and 43 ORs, 16 IRs, and 10 GRs of S. nitobei. Phylogenetic analysis showed that two species-specific OR subfamilies were identified in each species. In addition to conserved “antennal IRs”, “divergent IRs”, including 7 SnocIRs and 7 SnitIRs, were found. Moreover, a sugar receptor subfamily (SnocGR5 and SnitGR11), a carbon dioxide receptor subfamily (SnocGR2/GR3 and SnitGR3), and a fructose receptor subfamily (SnitGR9) emerged, but bitter receptors were not identified. The tissue-specific expression profiles showed 36 ORs were enriched in the antennae of S. noctilio. Among them, 19 ORs were female-biased, whereas 4 ORs (SnocOR6/15/18/30) were male-biased. In addition, 34 ORs were highly expressed in S. nitobei antennae, of which 22 ORs were female-biased, whereas SnitOR2/18/30 were male-biased. Seven IRs were enriched in the antennae of both species, of which SnocIR4, 6, 10, 11, and 12 were significantly male-biased, while SnitIR4, 6, 10, 11, and 12 were significantly female-biased. Three GRs were highly expressed in the antennae of both species. SnocGR2 and SnocGR6 were also highly expressed in the head and leg, respectively. In the present study, a total 62 and 69 chemosensory receptor genes were identified in the antennal transcriptomes of S. noctilio and S. nitobei, respectively. Although most receptor genes are homologous, there are also some specific receptor genes, suggesting similarities and differences in molecular mechanisms between the two closely related species. OR genes may be involved in different physiological functions by whether they are expressed in olfactory organs, or obvious gender bias. Our results provide a foundation for further investigating the molecular mechanisms of chemoreception in these two closely related woodwasp species, and establishes a starting point for further research on molecular mechanisms of the olfactory system in symphyta woodwasps.