The "Circadian Paradigm": A Test of Involvement of the Circadian System in the Photoperiodic Clock

A long-standing and yet open question is whether the circadian system is involved in photoperiodism. A limited set of protocols were credited with the ability of answering this question. The most widely used are the so-called "resonance experiments". Recent models, however, showed how the working of a non-circadian photoperiodic clock (hourglass) could go with a positive result in resonance experiments, thus giving a strong indication of the unreliability of this test. This paper showed how inferring a series of logical consequences from two properties of circadian system made it possible to describe the behaviour of an ideal circadian photoperiodic clock when submitted to regimes derived from a resonance protocol. This design was dubbed "Circadian paradigm" (CP). Comparing the predicted circadian behaviour with that of a concrete photoperiodic system provided a test of circadian involvement. Application of the CP to an insect species, Pieris brassicae, which gave a positive response to classical resonance experiments, yielded evidence that the inset clock was an hourglass and that the observed circadian modulation could readily be attributed to an inhibitory action of the circadian system on the output of the clock, as proposed by recent models. Another issue was that short- and long-night responses were likely to result from different mechanisms. The CP thus appeared as a theoretical tool fitted to decide whether the circadian system is part of a photoperiodic clock working. The results, which reinforced the grounds for discarding resonance as a circadian test, besides supported some of the theoretical properties of recent models.