Differential expression of circadian clock genes in two strains of beetles reveals candidates related to photoperiodic induction of summer diapause.

Diapause (also known as dormancy) is a state of arrested development induced by photoperiod or temperature that allows insects to survive adverse environmental conditions. By regulating diapause induction, the circadian clock is involved in short-day-induced winter diapause but whether this is also the case in long-day (LD)-induced summer diapause remains unknown. The cabbage beetle Colaphellus bowringi could enter summer diapause under LD conditions. However, a non-photoperiodic-diapause (NPD) strain of this species, which was developed in our laboratory by artificial selection, could not enter diapause under LD photoperiod. Therefore, we identified circadian clock genes in this species and measured differences in their expression between a high diapause (HD) strain and the NPD strain to investigate the potential relationship between circadian clock genes and summer diapause induction in C. bowringi. We successfully cloned eight circadian clock genes and obtained intact ORFs of four; cryptochrome2, double-time, shaggy and vrille. Phylogenetic trees and sequence alignment analyses indicated that these circadian clock genes were conserved across insect taxa. The quantitative real-time PCR indicated that clock, cycle, period, timeless, cryptochrome2, and vrille were differentially expressed between HD and NPD strains reared under LD photoperiod during the diapause induction phase. These findings suggest the potential relationship between circadian clock genes and LD-regulated summer diapause induction in C. bowringi.