Circadian rhythms in Neurospora crassa:: Lipid deficiencies restore robust rhythmicity to null frequency and white-collar mutants

The conidiation rhythm in the fungus Neurospora crassa is a model system for investigating the genetics of circadian clocks. Null mutants at the fro (frequency) locus (frq(9) and frq(10)) make no functional fro gene products and are arrhythmic under standard conditions. The white-collar strains (wc-l and wc-2) are insensitive to most effects of light, and are also arrhythmic. All three genes are proposed to be central components of the circadian oscillator. We have been investigating two mutants, eel (chain-elongation) and chol-1 (choline-requirer), which are defective in lipid synthesis and affect the period and temperature compensation of the rhythm. We have constructed the double mutant strains chol-1 frq(9), chol-1 frq(10) chol-1 wc-1, chol-1 wc-2, cel frq(9) cel frq(10), and cel wc-2, We find that these double mutant strains are robustly rhythmic when assayed under lipid-deficient conditions, indicating that free-running rhythmicity does not require the fro, wc-1, or wc-2 gene products. The rhythms in the double mutant strains are similar to the eel and chol-1 parents, except that they are less sensitive to light. This suggests that the fro, wc-1, and wc-2 gene products may be components of a pathway that normally supplies input to a core oscillator to transduce light signals and sustain rhythmicity. This pathway can be bypassed when lipid metabolism is altered.