Identification of candidate chemosensory receptors in the antennal transcriptome of the large black chafer Holotrichia parallela Motschulsky (Coleoptera: Scarabaeidae)

Olfaction and gustation play critical roles in the daily activities of insects by mediating important behaviors, such as locating host plants, mates and oviposition sites. These behaviors are mediated by a series of chemosensory receptor proteins, including odorant receptors (ORs), gustatory receptors (GRs) and ionotropic receptors (IRs). Holotrichia parallela is a worldwide pest on many crops and particularly on peanuts in China. In the present study, we produced Illumina-based transcriptomes from male and female antennae and ultimately identified 47 candidate ORs, 15 GRs and 27 IRs. Based on phylogenetic analyses, the relatedness with other species and possible functions of chemosensory genes were predicted and analyzed, such as the putative OR co-receptor (HparOR40), IR co-receptors (HparIR1 and HparIR27), carbon dioxide receptors (HparGR5 and HparGR14) and sugar receptors (HparGR8, HparGR11 and HparGR13). The differences in expression between genes and between sexes were analyzed using FPKM values and visualized in heat maps. Furthermore, the qRT-PCR results of 26 candidate genes were assessed to validate the differential expression analyses between sexes and between olfactory and non-olfactory tissues. Thirteen genes were more highly expressed in female than in male antennae, whereas four genes had higher expression in male than in female antennae. Twelve genes were predominantly expressed in the antennae as olfactory organs compared with expression in non-olfactory tissues. Our study was the first to annotate the chemosensory receptors of H. parallela, and it will provide a strong foundation for future work on physiology and ecology at the molecular level.