Abstract
Background: Entrainment to the environmental light cycle is an essential property of the circadian clock. Although the compound eye is known to be the major photoreceptor necessary for entrainment in many insects, the molecular mechanisms of photic entrainment remain to be explored. Results: We found that cryptochromes (crys) and c-fos mediate photic entrainment of the circadian clock in a hemimetabolous insect, the cricket Gryllus bimaculatus. We examined the effects of RNA interference (RNAi)-mediated knockdown of the cry genes, Gb'cry1 and Gb'cry2, on photic entrainment, and light-induced resetting of the circadian locomotor rhythm. Gb'cry2 RNAi accelerated entrainment for delay shifts, while Gb'cry1/ Gb'cry2 double RNAi resulted in significant lengthening of transient cycles in both advance and delay shifts, and even in entrainment failure in some crickets. Double RNAi also strongly suppressed light induced resetting. The Gb'cry-mediated phase shift or resetting of the rhythm was preceded by light-induced Gb'c-fosB expression. We also found that Gb'c-fosB, Gb'cry2 and Gb'period (Gb'per) were likely co-expressed in some optic lobe neurons. Conclusion: Based on these results, we propose a novel model for photic entrainment of the insect circadian clock, which relies on the light information perceived by the compound eye.