Further Evidence that the Circadian Clock in Drosophila is a Population of Coupled Ultradian Oscillators

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Abstract
We hypothesize that ultradian oscillators are coupled to yield a composite circadian clock in Drosophila. In such a system, period would be a function of the tightness of coupling of these oscillators, increasing as coupling loosens. Ultradian oscillations would become appar ent under weak coupling or in the absence of coupling. A new technique for calculating signal- to-noise ratio (SNR) for biological rhythms to characterize their precision has yielded support for this hypothesis. SNR of rhythms of the allelic series of mutations at the period ( per) locus of Drosophila melanogaster were compared. per0 was the noisiest, grading through perL, per+ , and pers, the least noisy. SNR decreases significantly with increasing period in pers, per+, and perL; pero typically has multiple ultradian oscillations and the lowest SNR. At least 70% of perL individuals also exhibit ultradian periodicities.