Two Structural Analogs of Kairomones are Detected by an Odorant Receptor HvarOR28 in the Coccinellid <i>Hippodamia variegata</i>

In natural environments, general plant volatiles and herbivore-induced plant volatiles (HIPVs) serve as critical clues for predatory natural enemies in the search for prey. The insect olfactory system plays a vital role in perceiving plant volatiles including HIPVs. In this study, we found that HIPV (<i>E</i>,<i>E</i>)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT) and the plant volatile geranyl acetate (GA), two structurally similar chemicals, displayed electrophysiological activities on the antennae of the ladybird <i>Hippodamia variegata</i>, but were only attractive to adult females in behavior. Moreover, mated female ladybirds laid a significantly higher number of eggs on TMTT-treated and GA-treated cotton leaves compared to controls. Screening of female-biased odorant receptors (ORs) from the antennal transcriptomes, performing <i>Xenopus</i> oocytes expression coupled with two-electrode voltage clamp recordings, suggested that HvarOR28 specifically tuned to TMTT and GA. Molecular docking and site-directed mutagenesis revealed that the amino acid residues Tyr143 and Phe81 of HvarOR28 are the key site for binding with TMTT and GA. Furthermore, RNA interference (RNAi) assay demonstrated that <i>HvarOR28</i>-silenced individuals demonstrated a notable decrease in electrophysiological responses, even female adults almost lost behavioral preference for the two compounds. Thus, it could be concluded that HvarOR28 in <i>H. variegata</i> contributes to facilitating egg laying through the perception of TMTT and GA. These findings may help to develop new olfactory modulators based on the behaviorally active ligands of HvarOR28.