Bivalve mollusc circadian clock genes can run at tidal frequency.

Marine coastal habitats are complex cyclic environments as a result of sun and moon interactions. In contrast with the well-known circadian orchestration of the terrestrial animal rhythmicity (approx. 24 h), the mechanism responsible for the circatidal rhythm (approx. 12.4 h) remains largely elusive in marine organisms. We revealed in subtidal field conditions that the oyster Crassostrea gigas exhibits tidal rhythmicity of circadian clock genes and clock-associated genes. A free-running (FR) experiment showed an endogenous circatidal rhythm. In parallel, we showed in the field that oysters' valve behaviour exhibited a strong tidal rhythm combined with a daily rhythm. In the FR experiment, all behavioural rhythms were circatidal, and half of them were also circadian. Our results fuel the debate on endogenous circatidal mechanisms. In contrast with the current hypothesis on the existence of an independent tidal clock, we suggest that a single 'circadian/circatidal' clock in bivalves is sufficient to entrain behavioural patterns at tidal and daily frequencies.

Effects of pars intercerebralis removal on circatidal rhythm in the mangrove cricket, Apteronemobius asahinai.

The circatidal rhythm is an endogenous rhythm corresponding to the tidal cycles, and its neural mechanism remains unknown. The mangrove cricket, Apteronemobius asahinai, possesses both circatidal and circadian clocks, and simultaneously exhibits circatidal and circadian rhythms in its locomotor activity. In a previous study, we showed that surgical removal of the optic lobes, the principal circadian clock locus in crickets, disrupted their circadian rhythm, but not their circatidal rhythm. In this study, we focused on the pars intercerebralis (PI) because surgical removal of the PI disrupts the circadian rhythm and causes arrhythmic activity in some cricket species. After surgical removal of the PI, the proportion of crickets displaying circatidal rhythm decreased, and more than half of the crickets exhibited arrhythmic activity. Surgical removal of the regions around the PI also caused a similar effect on locomotor activity. Our results indicate that the PI and/or its surrounding regions are important not only for circadian but also for circatidal rhythm. This suggests the presence of a neural or hormonal pathway in the PI and/or its surrounding regions that is common to the circatidal and circadian rhythms.

A Cell-Autonomous Mammalian 12 hr Clock Coordinates Metabolic and Stress Rhythms.

Besides circadian rhythms, oscillations cycling with a 12 hr period exist. However, the prevalence, origin, regulation, and function of mammalian 12 hr rhythms remain elusive. Utilizing an unbiased mathematical approach identifying all superimposed oscillations, we uncovered prevalent 12 hr gene expression and metabolic rhythms in mouse liver, coupled with a physiological 12 hr unfolded protein response oscillation. The mammalian 12 hr rhythm is cell autonomous, driven by a dedicated 12 hr pacemaker distinct from the circadian clock, and can be entrained in vitro by metabolic and ER stress cues. Mechanistically, we identified XBP1s as a transcriptional regulator of the mammalian 12 hr clock. Downregulation of the 12 hr gene expression strongly correlates with human hepatic steatosis and steatohepatitis, implying its importance in maintaining metabolic homeostasis. The mammalian 12 hr rhythm of gene expression also is conserved in nematodes and crustaceans, indicating an ancient origin of the 12 hr clock. Our work sheds new light on how perturbed biological rhythms contribute to human disease.

Lunisolar tidal force and its relationship to chlorophyll fluorescence in Arabidopsis thaliana.

The yield of chlorophyll fluorescence Ft was measured in leaves of Arabidopsis thaliana over periods of several days under conditions of continuous illumination (LL) without the application of saturating light pulses. After linearization of the time series of the chlorophyll fluorescence yield (ΔFt), oscillations became apparent with periodicities in the circatidal range. Alignments of these linearized time series ΔFt with the lunisolar tidal acceleration revealed high degrees of synchrony and phase congruence. Similar congruence with the lunisolar tide was obtained with the linearized quantum yield of PSII (ΔФII), recorded after application of saturating light pulses. These findings strongly suggest that there is an exogenous timekeeper which is a stimulus for the oscillations detected in both the linearized yield of chlorophyll fluorescence (ΔFt) and the linearized quantum yield of PSII (ΔФII).

Silencing the circadian clock gene Clock using RNAi reveals dissociation of the circatidal clock from the circadian clock in the mangrove cricket.

Whether a clock that generates a circatidal rhythm shares the same elements as the circadian clock is not fully understood. The mangrove cricket, Apteronemobius asahinai, shows simultaneously two endogenous rhythms in its locomotor activity; the circatidal rhythm generates active and inactive phases, and the circadian rhythm modifies activity levels by suppressing the activity during subjective day. In the present study, we silenced Clock (Clk), a master gene of the circadian clock, in A. asahinai using RNAi to investigate the link between the circatidal and circadian clocks. The abundance of Clk mRNA in the crickets injected with double-stranded RNA of Clk (dsClk) was reduced to a half of that in control crickets. dsClk injection also reduced mRNA abundance of another circadian clock gene period (per) and weakened diel oscillation in per mRNA expression. Examination of the locomotor rhythms under constant conditions revealed that the circadian modification was disrupted after silencing Clk expression, but the circatidal rhythm remained unaffected. There were no significant changes in the free-running period of the circatidal rhythm between the controls and the crickets injected with dsClk. Our results reveal that Clk is essential for the circadian clock, but is not required for the circatidal clock. From these results we propose that the circatidal rhythm of A. asahinai is driven by a clock, the molecular components of which are distinct from that of the circadian clock.