Two Structural\nAnalogs of Kairomones are Detected\nby an Odorant Receptor HvarOR28 in the Coccinellid Hippodamia variegata

In natural environments, general plant volatiles and\nherbivore-induced\nplant volatiles (HIPVs) serve as critical clues for predatory natural\nenemies in the search for prey. The insect olfactory system plays\na vital role in perceiving plant volatiles including HIPVs. In this\nstudy, we found that HIPV (<i>E</i>,<i>E</i>)-4,8,12-trimethyl-1,3,7,11-tridecatetraene\n(TMTT) and the plant volatile geranyl acetate (GA), two structurally\nsimilar chemicals, displayed electrophysiological activities on the\nantennae of the ladybird Hippodamia variegata, but were only attractive to adult females in behavior. Moreover,\nmated female ladybirds laid a significantly higher number of eggs\non TMTT-treated and GA-treated cotton leaves compared to controls.\nScreening of female-biased odorant receptors (ORs) from the antennal\ntranscriptomes, performing Xenopus oocytes\nexpression coupled with two-electrode voltage clamp recordings, suggested\nthat HvarOR28 specifically tuned to TMTT and GA. Molecular docking\nand site-directed mutagenesis revealed that the amino acid residues\nTyr143 and Phe81 of HvarOR28 are the key site for binding with TMTT\nand GA. Furthermore, RNA interference (RNAi) assay demonstrated that <i>HvarOR28</i>-silenced individuals demonstrated a notable decrease\nin electrophysiological responses, even female adults almost lost\nbehavioral preference for the two compounds. Thus, it could be concluded\nthat HvarOR28 in H. variegata contributes\nto facilitating egg laying through the perception of TMTT and GA.\nThese findings may help to develop new olfactory modulators based\non the behaviorally active ligands of HvarOR28.