Abstract: To determine the molecular characteristics and tissue expression profiles of the odorant receptor co-receptor (Orco) gene in Arma chinensis, a predatory species successfully used for the biocontrol of lepidopteran and coleopteran pests. The Orco gene was cloned using reverse transcription-PCR (RT-PCR) techniques. Structural and phylogenetic analysis was performed on the Orco protein, and the expression of the Orco gene in different adult tissues was determined using qPCR． The cloned Orco gene of A. chinensis was named AchiOrco (GenBank acc. no. PP965794). The open reading frame of this gene is 1425 bp and encodes 474 amino acids. The secondary structure of AchiOrco protein mainly consists of α helices and irregular coiled-coil folds; The AchiOrco protein is an non-secretory protein without a signal peptide, with a molecular weight and isoelectric point of 53.41 kD and 6.10, respectively. It has seven transmembrane domains and was thus identified as a hydrophobic transmembrane protein. Phylogenetic analysis suggests that AchiOrco is closely related to the Orco gene of Halyomorpha halys, a hemipteran pest. qPCR results indicate that AchiOrco is mainly expressed in the antennae of adult A. chinensis, with relatively low expression in the proboscis, head, thorax, abdomen, legs and wings. Expression of AchiOrco in adult female antennae was 1.33-fold higher than in adult male antennae. Expression in the abdomen was also significantly higher in females than males. Analysis of the AchiOrco protein structure indicates that it is structurally atypical compared to other insect odorant receptors. The qPCR results indicate that the AchiOrco gene is mainly expressed in the adult antennae, which is the main insect olfactory organ. It is possible, therefore, that the Orco gene plays an important role in olfactory recognition and signaling in insects.

Key words: Arma chinensis, odorant receptor co-receptor, gene cloning, transmembrane domain, tissue expression profile