OpsinLW2 serves as a circadian photoreceptor in the entrainment of circadian locomotor rhythm of a firebrat

• Published in: Journal of insect physiology 2024-06, Vol.155, p.104636, Article 104636
• Main Authors: Takeuchi, Kazuki, Tomioka, Kenji
• Format: Article
• Language: English
• Subjects: Animals; Circadian Clocks - genetics; Circadian Rhythm; Cryptochrome 2; Entrainment; Firebrat; Insect Proteins - genetics; Insect Proteins - metabolism; Light; Locomotion; Opsin; Opsins - genetics; Opsins - metabolism; Photoreceptor; Photoreceptor Cells, Invertebrate - metabolism; Photoreceptor Cells, Invertebrate - physiology; Tephritidae - genetics; Tephritidae - physiology
• ISSN: 0022-1910; 1879-1611; 1879-1611
• DOI: 10.1016/j.jinsphys.2024.104636

[Display omitted] •Firebrats use green light in photic entrainment of locomotor rhythms.•Opsin Long Wavelength 2 is involved in their photic entrainment.•Firebrats have only cryptochrome 2, which is not involved in photic entrainment. Photic entrainment is an essential function of the circadian clock, which enables organisms to set the appropriate timing of daily behavioral and physiological events. Recent studies have shown that the mechanisms of the circadian clock and photic entrainment vary among insect species. This study aimed to elucidate the circadian photoreceptors necessary for photic entrainment in firebrats Thermobia domestica, one of the most primitive apterygote insects. A homology search of publicly available RNA sequence (RNA-seq) data from T. domestica exhibited a cryptochrome 2 (cry2) gene and three opsin genes, opsin long wavelength 1 (opLW1), opLW2, and opUV, as candidate circadian photoreceptors. We examined the possible involvement of these genes in photic entrainment of firebrat locomotor rhythms. Firebrats had the highest entrainability to the light–dark cycle of green light. Treatment with dsRNA of the candidate genes strongly downregulated the respective targeted genes, and in the case of opsin genes, other untargeted genes were occasionally downregulated to various degrees. Under constant light, most control firebrats became arrhythmic, whereas a fraction of those treated with double RNAi of the two opLWs remained rhythmic. Behavioral experiments revealed that the transient cycles necessary for re-entrainment to shifted light cycles were lengthened when opLW2 expression was reduced. These results suggest that opLW2 is involved in the photic entrainment of circadian rhythm in firebrats.