A Highly Expressed Odorant Receptor Detects the Aggregation Pheromone Rhynchophorol in the Invasive American Palm Weevil.

Communication via aggregation pheromones is responsible for the behavioural and ecological characteristics of many species of weevils (Coleoptera). In insects, pheromones are detected by specialized odorant receptors (ORs), called pheromone receptors (PRs), which are usually highly expressed in olfactory sensory neurons localized in the antennae. Yet, PRs in Coleoptera remain understudied, which limits our understanding of their response characteristics and potential multiple evolutionary origins. In this study, we search for PRs in the American palm weevil, Rhynchophorus palmarum, a pest species that poses a major threat to oil palm and coconut production in the Americas, for which no PR responding to its aggregation pheromone (2E)-6-methyl-2-hepten-4-ol (rhynchophorol) has been characterized. Combining published RNA-seq data with new data generated in this study, we identified two ORs highly expressed in the weevil antennae that represent strong candidate aggregation PRs. Sequence-based binding pocket prediction revealed overall structural similarity between these two ORs, but amino acid differences were identified, suggesting functional divergence. Using the Xenopus oocyte heterologous expression system, we demonstrated that RpalOR32 displayed specificity and strong sensitivity to the aggregation pheromone rhynchophorol, with minor responses to several structurally related compounds, while RpalOR25 showed no responses to pheromones or host plant volatiles. The newly identified PR in R. palmarum appeared phylogenetically distant from the aggregation PR previously identified in the related species R. ferrugineus, confirming the independent origin of weevil PRs, even at the genus level.