Circadian rhythms from multiple oscillators: lessons from diverse organisms

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Abstract
Daily rhythms in biochemical, cellular and behavioural activities are controlled by the biological clock, which consists of one or more endogenous oscillators. The clock exerts its effects on a wide variety of processes, ranging from development in fungi, cell division in the marine protist Gonyaulax polyedra, photosynthesis in plants, sleep in animals, to cognitive functions in humans. Although circadian rhythms are present in different organisms, several aspects of the clock mechanism, and its complexity, are not conserved among these organisms. In prokaryotic and eukaryotic microorganisms, the circadian clock system seems to consist of several oscillators. These oscillators might respond to different environmental signals and direct rhythms in specific genes and behaviours. The coupling of oscillators is thought to provide stability and precision to the timing mechanism. In eukaryotes with differentiated tissues, a network of cell autonomous oscillators is found not within a single cell, but among cells in different tissue types. The clock system in eukaryotes also regulates rhythms in diverse biological processes, but these rhythms can be specific to different tissue types. In mammals and birds, a circadian pacemaker in the brain responds to input from the environment and coordinates overt rhythmicity throughout the peripheral tissues. In mammals, lesions and metabolic and electrophysiological studies have provided incontrovertible evidence that the SCN of the hypothalamus serves as the master circadian pacemaker. This pacemaker can coordinate rhythmicity in downstream cells and tissues. In non-mammalian vertebrates, the circadian clock system seems to be more complex. In birds, the circadian system consists of at least three anatomically distinct circadian pacemakers; the retina, the pineal gland and an avian homologue of the mammalian SCN. In Drosophila melanogaster, the hierarchal model of a central pacemaker setting the time of peripheral oscillators does not hold; virtually all tissues harbour circadian oscillators that can be entrained directly by light. So, the need for a centralized pacemaker to entrain peripheral oscillators in organisms seems to be dependent on the ability of peripheral tissues to be directly entrained.