Rhythmic conidiation (spore formation) in Neurospora crassa provides a model system for investigating the molecular mechanisms of circadian rhythmicity. A feedback loop involving the frq, wc-1, and wc-2 gene products (FRQ/ WCC) is an important component of the mechanism; however, rhythmic conidiation can still be observed when these gene products are absent. The nature of the oscillator(s) that drives this FRQ-less rhythmicity (FLO) is an important question in Neurospora circadian biology. We have looked for interactions between FRQ/WCC and FLO by assaying the effects on FRQ-less rhythms of mutations known to affect the period in the presence of FRQ. We assayed 4 prd mutations (prd-1, prd-2, prd-3, and prd-4) under 2 conditions in frq(null) strains: long-period free-running rhythms in chol-1 strains grown without choline, and heat-entrainable rhythms in choline-sufficient conditions. We found effects of all 4 mutations on both types of FRQ-less rhythms. The greatest effects were seen with prd-1 and prd-2, which abolished free-running rhythms in the chol-1; frq(10) backgrounds and significantly affected entrained peak timing under heat-entrainment conditions in frq( 10) backgrounds. The prd-3 and prd-4 mutations had more subtle effects on period and stability of free-running rhythms in the chol-1; frq(10) backgrounds and had little effect on peak timing under heat-entrainment conditions in frq(10) backgrounds. These results, along with previously published evidence for effects of prd mutations on other FRQ-less rhythms, suggest that either there are common components shared between the FRQ/WCC oscillator and several FRQ-less oscillators or that there is a single oscillator driving all conidiation rhythms. We favor a model of the Neurospora circadian system in which a single FRQ-less oscillator drives conidiation and interacts with the FRQ/WCC feedback loop; the output or amplitude of the FRQ-less oscillator can be affected by many gene products and metabolic conditions that reveal FRQ-less rhythmicity. We propose that prd-1 and prd-2 are good candidates for components of the FRQ-less oscillator and that prd-3 and prd-4 act on the system mainly through effects on FRQ/WCC.
