![]() Light, one of the strongest zeitgebers, resets the states of organisms via their circadian clocks. Circadian clocks can synchronize with various zeitgebers to adjust to the environmental time. Molecular clocks govern the expression of various genes involved in metabolic, physiological and behavioral regulation, leading organisms to alter their status at certain times of day. Circadian clocks are composed of transcription-translation feedback loops of clock genes. These changes are fundamentally controlled by endogenous circadian clocks. Many organisms show changes in metabolic, physiological and behavioral states depending on the time of day. Given the functions of the genes showing diel oscillations in hydras and the similarities of those genes with the diel cycle genes of other cnidarian species with circadian clocks, it is possible that diel cycle genes play an important role across cnidarian species regardless of the presence or absence of core clock genes under light–dark cycles. Hydras show diel behavioral rhythms under light–dark cycles despite the absence of canonical core clock genes. Some of these genes shared common features with diel cycle genes of other cnidarian species with endogenous clock systems. Using three different analytic algorithms, we found that 380 genes showed robust diel oscillations in expression. To identify the genes underlying diel behavior, we performed genome-wide transcriptome analysis of hydras under light–dark cycles. However, neither total behavior nor polyp contraction frequency showed rhythmic changes under constant light and constant dark conditions. ![]() Polyp contraction frequency, one component of behavior, exhibited a clear diel rhythm. We found that the total behavior of hydras was elevated during the day and decreased at night under a 12-h light–dark cycle. In this study, we examined whether hydras exhibit diel rhythms in terms of behavior and gene expression levels without typical clock genes. Hydra species, which are freshwater cnidarians, are thought to lack the core clock genes that form transcription–translation feedback loops in clock systems. Day–night behavioral variation is observed in most organisms, and is generally controlled by circadian clocks and/or synchronization to environmental cues.
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