Integrated view of resetting a circadian clock.

Abstract This report extends and systematizes the observations of Pittendrigh and his coworkers on the phase resetting of the circadian rhythm of pupal eclosion in populations of the fruitfly, Drosophila pseudoobscura, in response to a single discrete perturbation. Using a pulse of dim blue light as the resetting stimulus, the daily eclosion time is plotted as a function of the duration of the stimulus and the time at which it is given. The resulting three-dimensional graph, covering four days of eclosion, resembles four turns of a corkscrew linking tilted planes. The corkscrew axis points to a unique stimulus time and duration, small variations of which result in radically altered phase resetting. Approximations to this stimulus result in unusual broadening of the daily eclosion peaks, even to the extreme of obscuring the circadian rhythm. The resetting data published by investigators of other circadian systems suggest that this behavior in fruitflies is typical of circadian clocks in many phyla. Abolition of the steady-state rhythm following exposure to the unique stimulus might be attributed to diversification of phases within the pupal population. Further experiments are reported which seem to exclude this interpretation. The circadian clock may have been “turned off” in each individual pupa, thus returning them to the state characteristic of darkreared populations.