Background
The olfactory system is the foundation of insect behavior. Odorant binding proteins (OBPs) are key components of the insect olfactory system. The woodwasp Sirex noctilio Fabricius is a major quarantine pest worldwide that was first discovered in China in 2013 and mainly harms members of the Pinus genus. S. nitobei Matsumura is a native species in China and is closely related to S. noctilio. To gain insights into the olfactory mechanisms of these two woodwasp species, olfactory genes were identified using antennal transcriptome analysis. We also analyzed the expression profiles of OBPs with RT-qPCR.
Results
From our transcriptome analysis, 16 OBPs, 7 chemosensory proteins (CSPs), 41 odorant receptors (ORs), 8 gustatory receptors (GRs), 13 ionotropic receptors (IRs), and one sensory neuron membrane protein (SNMP) were identified in S. noctilio, while 15 OBPs, 6 CSPs, 43 ORs, 10 GRs, 16 IRs, and 1 SNMP were identified in S. nitobei. Most of the olfactory genes identified in two species were homologous. However, some species-specific olfactory genes were identified in the antennal transcriptomes, including SnocOBP13, SnocCSP6, SnocOR26, SnitGR9, and SnitIR17. In total, 14 OBPs (7 in S. noctilio and 7 in S. nitobei) were expressed primarily in the antennae of the two woodwasps. SnocOBP11 and SnitOBP11 were highly expressed in antennae and were also clearly expressed in the external genitalia. SnocOBP3 is highly expressed in the genitalia of females, and SnocOBP7 and SnitOBP7 are highly expressed in the genitalia of males. Meanwhile, SnocOBP10 was specifically expressed in male heads.
Conclusion
In total, 86 olfactory proteins were identified in S. noctilio, and 91 were identified in S. nitobei. Most SnocOBPs and SnitOBPs displayed enriched expression in the antennae, which are involved in odor recognition. A few OBPs were mainly expressed in the external genitals or heads and exhibited an obvious sex bias, which may indicate that the external genitals and heads are able to recognize sex pheromones or plant volatile compounds as a part of normal behaviors such as feeding, mating, or spawning. Our study provides key insights regarding the mechanism of interactions between the insect olfactory system and specific odor molecules.