Abstract
IntroductionEntrainment to light cycle is a prerequisite for circadian rhythms to set daily physiological events to occur at an appropriate time of day. In hemimetabolous insects, the photoreceptor molecule for photic entrainment is still unknown. Since the compound eyes are the only circadian photoreceptor in the cricket Gryllus bimaculatus, we have investigated the role of three opsin genes expressed there, opsin-Ultraviolet (opUV), opsin-Blue (opB), and opsin-Long Wave (opLW) encoding a green-sensitive opsin in photic entrainment.ResultsA daily rhythm was detected in mRNA expressions of opB and opLW but not of opUV gene. When photic entrainment of circadian locomotor rhythms was tested after injection of double-stranded RNA (dsRNA) of three opsin genes, no noticeable effects were found in opUV RNAi and opB RNAi crickets. In opLW RNAi crickets, however, some crickets lost photic entrainability and the remaining crickets re-entrained with significantly longer transient cycles to a phase-advanced light–dark cycle as compared to control crickets. Crickets often lost entrainability when treated doubly with dsRNAs of two opsin genes including opLW.ConclusionThese results show that green-sensitive OpLW is the major circadian photoreceptor molecule for photic entrainment of locomotor rhythms in the cricket G. bimaculatus. Our finding will lead to further investigation of the photic entrainment mechanism at molecular and cellular levels, which still remains largely unknown.Electronic supplementary materialThe online version of this article (doi:10.1186/s40851-015-0011-6) contains supplementary material, which is available to authorized users.
Highlights
Entrainment to light cycle is a prerequisite for circadian rhythms to set daily physiological events to occur at an appropriate time of day
In situ hybridization of opsin mRNA revealed that opsin-Long Wave (opLW) is expressed over almost the whole compound eye except for the dorsal rim area (DRA), opUV in a single proximally located cell in every ommatidium over almost the whole compound eye except for a ventral region, and opB expressed most abundantly in DRA and replaced with opUV in the ventral region (Additional file 1: Figure S1)
The mRNA levels of the three opsin genes showed a slight reduction at midnight followed by an increase at late night and these changes were statistically significant for opB and opLW (P < 0.05, ANOVA followed by Tukey-test)
Summary
Entrainment to light cycle is a prerequisite for circadian rhythms to set daily physiological events to occur at an appropriate time of day. Since the compound eyes are the only circadian photoreceptor in the cricket Gryllus bimaculatus, we have investigated the role of three opsin genes expressed there, opsin-Ultraviolet (opUV), opsin-Blue (opB), and opsin-Long Wave (opLW) encoding a green-sensitive opsin in photic entrainment. The most important role of the clock is to set physiological functions to peak at an appropriate time of day. In Drosophila melanogaster and the monarch butterfly (Danaus plexippus), a blue light receptor, CRYPTOCHROME (CRY), that is expressed in a subset of cerebral clock neurons, mediates the photic information, The entrainment through retinal photoreceptors resembles that known for the mammalian circadian clock that resides in the suprachiasmatic nucleus (SCN).
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