The fungus Neurospora crassa is a model system for investigating the mechanism of circadian rhythmicity, and the core of its circadian oscillator is thought to be a transcription/translation feedback loop involving the products of the frq (frequency), wc-1 (white-collar-1) and wc-2 (white-collar-2) genes. Several reports of rhythmicity in frq and wc null mutants have raised questions about how central the FRQ/WC loop is to the circadian system of Neurospora. Several research groups have attempted to answer this question by looking for entrainment of the conidiation banding rhythm in frq null mutants. Because the frq mutants are blind to light and cannot be entrained to light/dark cycles, these groups have used symmetric temperature cycles of equal-duration cool and warm phases to entrain the rhythm. Under these conditions, the direct effects of temperature on conidiation (masking effects) can compromise observations of the endogenous rhythm. I have reexamined this question by using short heat pulses to clearly separate masking from endogenous rhythms, and I have assayed entrainment in both frq and wc-1 null mutants. I found similar patterns of entrainment in the wild type and both mutant strains. Strong masking effects were found in the frq mutant but not in the wc-1 mutant. I conclude that a rapidly damping temperature-entrainable oscillator is present in the null mutants. A single temperature-entrainable oscillator may drive the conidiation rhythm in all strains, and additional properties such as light sensitivity and temperature compensation may be conferred by the intact FRQ/WC loop in the WT strain.
