Academic journal article Genetics

Rhythmic Conidiation in Constant Light in Vivid Mutants of Neurospora Crassa

Academic journal article Genetics

Rhythmic Conidiation in Constant Light in Vivid Mutants of Neurospora Crassa

Article excerpt

ABSTRACT

In Neurospora crassa, a circadian rhythm of conidiation (asexual spore formation) can be seen on the surface of agar media. This rhythm has a period of 22 hr in constant darkness (D/D). Under constant illumination (L/L), no rhythm is visible and cultures show constant conidiation. However, here we report that strains with a mutation in the vivid (vvd) gene, previously shown to code for the photoreceptor involved in photo-adaptation, exhibit conidiation rhythms in L/L as well as in D/D. The period of the rhythm of vvd strains ranges between 6 and 21 hr in L/L, depending upon the intensity of the light, the carbon source, and the presence of other mutations. Temperature compensation of the period also depends on light intensity. Dark pulses given in L/L shift the phase of the rhythm. Shifts from L/L to D/D show unexpected after effects; i.e., the short period of a vvd strain in L/L gradually lengthens over 2-3 days in D/D. The rhythm in L/L requires the white collar (wc-1) gene, but not the frequency (frq) gene. FRQ protein shows no rhythm in L/L in a vvd strain. The conidiation rhythm in L/L in vvd is therefore driven by a FRQ-less oscillator (FLO).

CIRCADIAN rhythms are biological rhythms with a period of about a day (Sweeney 1976). These rhythms and their regulation play important roles in nearly all organisms and control numerous biological processes ranging from sexual and asexual reproduction to complex behavioral changes. A typical circadian clock possesses several characteristics, such as having a period close to that of the geophysical day, is endogenous and self-sustaining, and is entrained by environmental cues such as light and temperature (Edmunds 1988; Pittendrigh 1993).

Neurospora crassa, a model organism for the study of circadian rhythms (Lakin-Thomas et al. 1990; Dunlap and Loros 2006; Liu and Bell-Pedersen 2006), expresses its rhythm by alternating areas of asexual spore formation (conidiation) with areas of thinner hyphal growth without spore formation when growing on solid agar medium. Areas of spore formation are known as ''bands'' and thinner areas without spore formation are known as ''interbands.'' The period and phase of a culture can be easily determined by the position of the bands on the agar surface and can be shifted and entrained by pulses or cycles of light or temperature (Sargent and Briggs 1967; Francis and Sargent 1979).

The frequency (frq) and white-collar (wc-1, wc-2) genes play important roles in the expression of the circadian rhythm. Point mutations in the frq gene can increase or decrease the period of the conidiation rhythm(Feldman and Hoyle 1973; Aronson et al. 1994). Null mutations at the frq locus, either a point mutation (Loros and Feldman 1986) or a complete deletion of the coding sequence (Aronson et al. 1994), show a residual rhythm that lacks temperature compensation. Wc-1 and wc-2 mutants were originally isolated as ''blind'' mutants and do not display conidiation banding under ordinary laboratory conditions (Degli-Innocenti and Russo 1984; Crosthwaite et al. 1997). It is generally held that the conidiation cycle in Neurospora is partially the product of a negative molecular feedback loop, the frq/ wc oscillator (FWC), involving the rhythmic levels and activities of products of the frq, wc-1, and wc-2 genes. There are also positive feedback mechanisms involving the influence of frq expression on wc-1 and wc-2 activity and an important role played by post-translational modifications and protein degradation. Details may be found in recent reviews (Lakin-Thomas and Brody 2004; Liu 2005; Brunner and Schafmeier 2006; Dunlap 2006; Liu and Bell-Pedersen 2006).

Oscillators other than the FWC oscillator, sometimes referred to as frq-less oscillators, or FLOs (Iwasaki and Dunlap 2000), have been proposed on the basis of the observation of residual rhythmicity in the frq null strains frq^sup 9^ (Loros and Feldman 1986) and frq^sup 10^ (Aronson et al. …

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