Transcriptional oscillation of peripheral circadian clocks during a maturation cycle in Atlantic cod

Synchronization of almost all biological processes is governed by the cyclic pattern of gene expression in response to the dynamics of the external environment, including photoperiod. The circadian system is a coordinated feedback loop that is composed of brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like (Bmal), Circadian Locomotor Output Cycles Kaput (Clock), Period (Per) and Cryptochrome (Cry). In Atlantic cod, photoperiod manipulation is being practiced in delaying precocious sexual maturation which is a major farming bottleneck. Though promising results have been obtained, the molecular basis of this phenomenon is largely unknown. This study was conducted to clone the circadian genes in cod and evaluate their transcriptional oscillation in peripheral tissues during a maturation cycle, since identifying and characterizing these molecules is necessary to understand this photic plasticity. Partial sequences of 8 clock genes in cod have been isolated, namely, bmal1a, bmal1b, clock, clock3, per1, per2, cry1 and cry2. Except for clock, clock3, cry1and cry2, all genes could only be detected at 50% epiboly, suggesting that most are zygotic transcripts. Their constitutive expression in almost all tissues indicates that peripheral circadian oscillators are present in cod. Their expression in two peripheral tissues, testis and gonads, was quantified and rhythmic oscillation on the transcription was observed during a maturation cycle. This study identified the molecular components of the circadian system of Atlantic cod and revealed how seasonal variation like maturation cycle served as an environmental cue in their transcription. Further, these environmentally-driven changes provide practical information in cod aquaculture where photoperiod manipulation is considered as important strategy in delaying sexual maturation.