CLOCK evolved in cnidaria to synchronize internal rhythms with circadian environmental cues

Abstract

AbstractThe circadian clock enables anticipation of the day/night cycle in animals ranging from cnidarians to mammals. Circadian rhythms are generated through a transcription-translation feedback loop (TTFL) involving negative and positive factors. CLOCK is a central conserved positive factor in the animal kingdom. However, the functional evolutionary origin and mechanism of action of CLOCK in basal animals are unknown. In cnidarians, the transcription of putative core clock genes, includingClock, is arrhythmic under constant conditions, and it is unclear how the TTFL model can regulate physiological rhythms. Here, we used the CRISPR/Cas9 system to generate aClockmutant (Clock-/-) inNematostella vectensis. High-throughput video tracking of locomotor activity inClock-/-adults revealed that light regulates rhythmic behavior, while CLOCK maintains a 24-hour circadian rhythm under constant light conditions, and even under a 6-hour cycle, reminiscent of the tidal rhythm. Transcriptome profiling identified hundreds of differentially expressed genes inClock-/-adults, particularly genes involved in cell cycle and neural development. In accordance with the rhythmic behavioral profile, computational analysis revealed rhythmic and arrhythmic gene expression in wild-type andClock-/-adults, respectively, under constant darkconditions. Notably, hierarchal light-andClock-signaling is required to drive rhythmic transcription of the core clock genes. These findings demonstrate that CLOCK has evolved in cnidarians to maintain synchronized 24-hour rhythmic physiology and behavior despite the chaotic inputs of environmental cues.