A moth odorant receptor highly expressed in the ovipositor is involved in detecting host-plant volatiles

The olfactory system of insects plays a role in the survival and success of their offspring by helping them find suitable habitats, favorable food, and optimal mates. Although the antennae of insects are usually considered as their nostrils, some species of insects use other cephalic organs. Distributed on these organs are olfactory sensilla, hair-like structures whose nerve supply are from olfactory sensory neurons (OSNs). Odorant receptors (ORs) determine OSNs detection spectrum. They are found on the dendritic membrane of OSNs, they are atypical, 7-transmembrane domain proteins that bind selectively to volatile ligands in the environment. Recently, some studies have questioned the hypothesis that the detection of volatile compounds in insects is carried out by only cephalic organs. In addition to the antennae and maxilla, studies have shown that olfactory receptor genes are also expressed in the ovipositors of some moth species. There is also evidence that some sensilla located on moth ovipositors possess olfactory functions but the role of the olfactory receptor genes expressed in the ovipositor is yet to be known.

To determine whether any ORs are expressed in the ovipositor, Rui-Ting Li (graduate student), Ling-Qiao Huang and led by Professor Chen-Zhu Wang from the Institute of Zoology, Chinese Academy of Sciences, together with Dr. Jun-Feng Dong from Henan University of Science and Technology conducted studies to show that the ovipositor of Helicoverpa assulta expressed OR genes which assisted the females to determine where to lay their eggs. The work is published in the Journal eLife.

The research team carried out transcriptome sequencing and identified some chemosensory receptors in the pheromone gland – ovipositor (PG-OV) of H. assulta. Among the chemosensory receptor genes, HassOR31 and HassiGluR7 were found to have the highest Transcripts Per Kilobase of exon per Million mapped reads. HassORco was also identified, with a value lower than that of HassOR31. In the PG-OV, HassOR31 had the highest expression level, which was 15 and 7 times higher than that in male antennae and female antennae respectively.

Within the ovipositor, HassOR31 was observed to be mostly expressed alone in the cells beneath small hairs, with a few in the sensilla cells with a large size. In some instances, HassOR31 and HassORco were co-expressed in the cells beneath specific large sized sensilla. The Xenopus laevis oocytes showed no positive response when only HassOR31 was expressed and HassOR31 and HassiGluR7 were co-expressed in them. In the ovipositor and associated sensilla of H. assulta, four types of sensilla were found on scanning electron micrography.

The team carried out a test on the sensilla of H. assulta. They observed that some sensilla responded to the tested odorants. Specifically, a dose-dependent response curve was demonstrated by Z-3-hexenyl-butyrate, the most effective ligand of HassOR31/HassORco. It was observed that intact, mated H. assulta females had a preference for laying eggs on the areas of gauze exposed to the volatiles of host plant. Whereas, mated females with their antennae removed displayed a reduced, but still remarkable oviposition preference for volatile-treated areas. Both antennectomized and intact females showed a preference for laying eggs on Z-3-hexenyl butyrate treated fake leaves, with no notable difference on the oviposition preference index.

This study is the first of its kind, through which the authors have succeeded in functionally characterizing HassOR31, an OR expressed in H. assulta ovipositors. It is believed to assist H. assulta females find specific sites on their host plants to lay eggs. The authors plan future studies which will focus on the effect of destroying the related sensilla or knocking out HassOR31 has on insect behavior.