The optic lobe-pars intercerebralis axis is involved in circa'bi'dian rhythm of the large black chafer Holotrichia parallela

Circabidian rhythm of sex pheromone reception in a scarab beetle

Animals have endogenous clocks that regulate their behavior and physiology. These clocks rely on environmental cues (time givers) that appear approximately every 24 h 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. 48 h 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.

Phylogenetic Separation of Holotrichia Species (Insecta, Coleoptera, Scarabaeidae) Exhibiting Circadian Rhythm and Circa'bi'dian Rhythm

A unique two-day rhythm, circabidian rhythm, has been reported in the black chafer, Holotrichia parallela. However, it remains unknown how widely the circabidian rhythm appears in related species. We examined the activity rhythm and phylogeny of congeneric species inhabiting Japan to investigate the appearance of circabidian rhythms in a few subgenera of the genus Holotrichia. We found that Holotrichia picea also exhibited circabidian rhythm. In addition to the regular circabidian pattern, circabidian rhythms with day-switching or with a circadian activity component were also observed. In the day-switching pattern, H. picea switched appearance from odd to even days, or vice versa. In the circadian-like activity patterns, a major night activity and a minor dusk activity appeared alternately. Holotrichia kiotonensis, Holotrichia convexopyga, and Holotrichia loochooana loochooana exhibited a circadian rhythm. Two distinct clades, A and B, were recognized in the histone H3, cytochrome c oxidase subunit 1, and 16S ribosomal RNA phylogenetic trees. This phylogenetic separation was in accordance with the subgeneric classification based on external morphology in a previous study and with behavioral rhythm in the present study: clade A included Nigrotrichia group members, H. kiotonensis, H. convexopyga, H. loochooana loochooana, and H. loochooana okinawana, while clade B included Pedinotrichia group members, H. paralella and H. picea. We suggest that after separation into Nigrotrichia and Pedinotrichia, the behavioral trait of circabidian rhythm probably appeared once in an ancestral species of the Pedinotrichia group, including H. parallela and H. picea.