Apparent Absence of a Separate B-Oscillator in Phasing the Circadian Rhythm of Eclosion in Drosophila Pseudoobscura
A well known 2-oscillator model proposed by Pittendrigh and Bruce (1957) postulates that (1) a light sensitive and temperature compensated A-oscillator (master) and (2) a temperature sensitive and light refractory B-oscillator (slave) underlie, govern and gate the circadian rhythm in the eclosion of Drosophila pseudoobscura. The principal merit of this coupled oscillator model is that it seemed to “explain” the phenomenon of “transients” and at least one of its tenets has found experimental proof (Chandrashekaran, 1967). There has not been any proof for nor any evidence against the existence of a separate B-oscillator. Experiments carried out by me to evoke the responses of the B-oscillator to the exclusion of the A-oscillator in Drosophila pseudoobscura have failed. Temperature pulses and light pulses (regardless of the lighting conditions of the cultures and experiments) always shifted or reset what has been considered the A-oscillator leaving no evidence or clue for the existence or involvement of a separate B-oscillator.
Much before genetic mutants (Konopka and Benzer, 1971) of the Drosophila clock were reported or transplantation of circadian oscillators in Drosophila were made (Handler and Konopka, 1979), Pittendrigh and Bruce (1957) proposed an explicit formal model to account for the response features of the circadian rhythm in the eclosion of Drosophila pseudoobscura flies. The model envisaged a coupled oscillator set up with one of the oscillators: the A-oscillator being light sensitive and the master/pacemaker; and the B-oscillator presumed to be insensitive to light but sensitive to temperature and the slave to the A-oscillator. The A-oscillator was temperature-compensated (thus qualifying for chronometry) and instantaneously phase shifted by light perturbations (Chandrashekaran, 1967) and B-oscillator gradually caught up and re-established phase with the A-oscillator after a few cycles. The efforts of the B-oscillator to regain phase with the A-oscillator were reflected in the “transients” that follow light and temperature perturbations. The principal merit of this coupled oscillator model was that it seemed to “explain” the phenomenon of transients rather picturesquely even though other models could do the same (Bunning and Zimmer, 1962) implicating only a single oscillator. One of the important tenets of the coupled oscillator model, the instantaneous resettability of the light sensitive basic oscillation could even be experimentally proved (Chandrashekaran, 1967). More recent work on Drosophila (Engelmann and Mack, 1978) and humans (Weyer, 1979) implicate a multiplicity of oscillators governing a heirarchy of circadian functions. The Pittendrigh and Bruce (1957) model and its specific postulate of a separate temperature sensitive, light refractory, slave B-oscillator have, however, lingered on with neither proof for nor evidence against the latter. I have experimentally reexamined the hypothesis and report here my failure to find any evidence or even cues for the existence of a separate B-oscillator.
KeywordsCircadian Rhythm Light Pulse Temperature Perturbation Temperature Pulse Phase Response Curve
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