Modeling a circadian surface.

Experiments that systematically varied T, τ, and photoperiod in Neurospora crassa revealed that the traditional nonparametric and parametric approaches could not explain entrainment for all of the tested conditions. The authors have developed a new approach to understanding entrainment that incorporates several features of the old paradigms but allows exploration of the underlying mechanisms in synchronized clocks, making extrapolations from constant conditions to entrained state unnecessary. It is based on a circadian integrated response characteristic (CIRC) that makes no assumptions about how entrainment occurs (by phase shifts or velocity changes). All it presumes is that, during entrainment, the clock's cycle length must match that of the zeitgeber. With the help of the CIRC, entrainment to all zeitgeber conditions can be modeled by changing 3 parameters: the CIRC's shape and asymmetry and an assumed internal cycle length (τ under entrainment: τ(E)) that the clock adopts under stable entrainment to produce a specific phase relationship to the zeitgeber (τ(E) is reflected in a period aftereffect when clocks are released to constant conditions). The few parameters of the CIRC make it highly amenable to modeling. Here, the authors describe the results of modeling Neurospora's circadian surface and show that the new approach can explain and unify all results of the circadian surface. The qualities of the CIRC are highly systematic for the respective entrainment condition and show that τ(E) is an important variable in the entrainment process. The results also show that the wild-type strain is excellently tuned for entrainment under the natural 24-h cycle despite its shorter period (~22 h) in constant darkness. Experiments measuring aftereffects support the prediction that τ(E) plays an important role in entrainment.