Resetting the amplitude ofDrosophila's circadian chronometer

SummaryEven in constant temperature darkness, mature flies emerge from populations ofDrosophila pupae only in a 6-hour-wide pulse once every 24 hours. The “circadian clock” governing this behavior can be rephased by exposing the pupae to even a few seconds of dim light in otherwise constant darkness. But there exists a critical exposure which, if and only if administered at a critical hour,turns off the circadian clock instead of rephasing it.This paper describes measurements in which a rephasing light pulse is followed by a second pulse so that the once-reset clock can be examined for normal rhythmicity of its sensitivity to light. The latter is found to be rephased in step with the emergence rhythm, but additionally attenuated in a way which strongly suggests that not only phase, but also the amplitude, intensity, or vigor of circadian oscillation is lastingly reset by a light pulse. Amplitude is apparently reset to zero by the critical pulse, but to other subnormal values by nearby non-critical pulses, even those which leave phase unaffected. The results of two thousand such rephasings are fitted by a simple theoretical model to within the reproducibility of measurements.