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

Area of Science:

• * Evolutionary biology
• * Chronobiology
• * Molecular genetics

Background:

• * Circadian rhythms, essential for anticipating daily cycles, are conserved across animals.
• * The transcription-translation feedback loop (TTFL) involving CLOCK is a key pacemaker mechanism.
• * The evolutionary origin and function of CLOCK in basal animals remain unclear.

Purpose of the Study:

• * To investigate the evolutionary origin and function of the CLOCK gene in the cnidarian *Nematostella vectensis*.
• * To understand the role of *NvClk* in generating circadian rhythms and its interaction with light cues.

Main Methods:

• * Generation of *NvClk* mutant using CRISPR/Cas9 gene editing.
• * Behavioral analysis under constant dark (DD), constant light (LL), and light-dark (LD) conditions.
• * Transcriptomic analysis to assess rhythmic gene expression patterns.

Main Results:

• * *NvClk* mutant polyps lost circadian behavior in DD and LL but maintained a 24-hour rhythm in LD.
• * Transcriptomic analysis showed altered rhythmic gene expression in the mutant, particularly under DD.
• * A light-responsive pathway partially compensated for the loss of circadian rhythm in the mutant.

Conclusions:

• * The *Clock* gene has evolved in cnidarians to synchronize physiological and behavioral rhythms with the diel cycle.
• * Light-dependent pathways can partially rescue circadian clock disruption, indicating functional redundancy.
• * *Nematostella vectensis* provides a model for studying the evolution of circadian clocks in early animal lineages.