Circabidian rhythm of sex pheromone reception in a scarab beetle

Abstract

Animals have endogenous clocks that regulate their behavior and physiology. These clocks rely on environmental cues (time givers) that appear approximately every 24h due to the Earth's rotation; thus, most insects exhibit a circadian rhythm. One notable exception is the scarab beetle, Holotrichia parallela, a severe agricultural pest in China, Japan, South Korea, and India. Females emerge from the soil every other night, reach the canopy of host plants, evert an abdominal gland, and release a pheromone bouquet comprising l-isoleucine methyl ester (LIME) and l-linalool. To determine whether this circa'bi'dian rhythm affects the olfactory system, we aimed to identify H.parallela sex pheromone receptor(s) and study their expression patterns. We cloned 14 odorant receptors (ORs) and attempted de-orphanizing them in the Xenopus oocyte recording system. HparOR14 gave robust responses to LIME and smaller responses to l-linalool. Structural modeling, tissue expression profile, and RNAi treatment followed by physiological and behavioral studies support that HparOR14 is a sex pheromone receptor-the first of its kind discovered in Coleoptera. Examination of the HparOR14 transcript levels throughout the adult's life showed that on sexually active days, gene expression was significantly higher in the scotophase than in the photophase. Additionally, the HparOR14 expression profile showed a circabidian rhythm synchronized with the previously identified pattern of sex pheromone emission. 48h of electroantennogram recordings showed that responses to LIME were abolished on non-calling nights. In contrast, responses to the green leaf volatile (Z)-3-henexyl acetate remained almost constant throughout the recording period.