Identification and characterization of three chemosensory receptor families in the cotton bollworm Helicoverpa armigera

Background: Chemosensory receptors including olfactory receptors (ORs), gustatory receptors (GRs) and ionotropic receptors (IRs) play a central role in sensing chemical signals and guiding insect behaviours, and are potential target genes in insect pest control. The cotton bollworm Helicoverpa armigera is one of the most destructive pest species that can feed on over 200 different plant species. This diversity of host plants is likely linked to a complex chemosensory system. Here we built on previous work to characterize crucial chemosensory tissues linked to environmental interactions including larval antennae, larval mouthparts and larval fat bodies, as well as male and female adult heads, male and female adult tarsi, and female abdomens. Results: Using transcriptome sequencing, Trinity RNA-seq assemblies and extensive manual curation, we identified a total of 91 candidate chemosensory receptors (60 candidate ORs, 10 GRs and 21 IRs). Thirty-five of these candidates present full-length transcripts. First, we performed in silico differential expression analysis on different sequenced tissues. Further, we created extensive expression profiles using reverse transcription (RT)-PCR on a variety of adult and larval stages. We found that the expression profile of HarmOR51 was limited to adult male antenna suggesting a role in mating that was further supported by a phylogenetic analysis clustering it into the pheromone receptor clade. HarmOR51 in calcium imaging analysis did not show responses to either of the two H. armigera sex pheromone components (Z9-16: Ald or Z11-16: Ald) inviting a future detailed study. In addition, we found four novel HarmORs (OR1, 53, 54 and 58) that appeared to be larvae-antennal specific. Finally, our expression profiling showed that four "divergent" HarmIRs (IR2, 7d. 1, 7d. 2 and 7d. 3) were expressed in both adult and larval antennae, suggesting a functional divergence from their Drosophila homologues. Conclusions: This study explored three chemoreceptor superfamily genes using a curated transcriptomic approach coupled with extensive expression profiling and a more limited functional characterization. Our results have now provided an extensive resource for investigating the chemoreceptor complement of this insect pest, and meanwhile allow for targeted experiments to identify potential molecular targets for pest control and to investigate insect-plant interactions.