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. ResultsWe 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.ConclusionBased 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.Electronic supplementary materialThe online version of this article (10.1186/s40851-018-0109-8) contains supplementary material, which is available to authorized users.

Most insects show daily activity rhythms that are controlled by endogenous circadian clocks. A basic property of the clock is entrainment to daily environmental cycles to run with an exact 24-h period. The entrainment is achieved mainly through resetting by light. The present study analyses the light resetting mechanism of the clock in first-instar nymphal and adult crickets Gryllus bimaculatus (De Geer). A 3-h light pulse given at early subjective night and late subjective night causes a phase delay and an advance, respectively, of adult locomotor rhythms. The magnitude of the shift caused by the light pulse at circadian time 12 h in constant darkness is significantly smaller than that caused by a 3-h extension of light phase. Measurement of mRNA levels during the light-induced phase shifts yielded basically similar results for nymphs and adults. When a 3-h light pulse is given at early night by light phase extension, an increase of Pdp1 occurs, which is followed by upregulation of the genes Clk and subsequently by per and tim. These changes are eliminated by RNA interference of opsin-long wavelength, which is expressed in the compound eye and encodes a green-sensitive opsin, the major photoreceptor for photic entrainment of the clock. No clear changes in mRNA levels are observed for light pulses given at late subjective night or at early subjective night after 24 h of constant darkness. These results suggest that the photic entrainment mechanism of the clock may be shared by nymphal and adult crickets, including transcriptional and nontranscriptional events.