4 - Circadian rhythms in filamentous fungi

Summary

Time is nature's way of keeping everything from happening at once

Richard Feynman (National Geographic Magazine, 1991)

Introduction

This review focuses upon circadian rhythmicity in the fungi; in particular circadian rhythmicity in one of the best-studied model systems, the Neurospora clock. Consideration will also be given to the limited number of true circadian rhythms that have been detected in other filamentous fungi. Many fungi display non-circadian rhythms (In gold, 1971; Lysek, 1978, 1984), which are not discussed in detail. Suffice to say that the out put rhythms of most fungi probably reflect direct changes in the environment, triggering immediate developmental or physiological responses. Reactions to light are the most prevalent, although temperature also causes rhythmicity. Non-circadian rhythms may also arise from self-sustaining metabolic cycles. Examples of the latter may include the formation of concentric zones or archimedean spirals of conidia by Nectria cinnabarina (Bourret et al., 1969), the sporulation of Leptosphaeria controlled by oscillations in the asparagine-pyruvate path-way (Jerebzoff & Jerebzoff-Quintin, 1982) and the ‘hormonal’ triggering of hyphal growth rhythms in Ascobolus immerses and Podospora anuerina (Chevnugeon & Nguyen Van, 1969).

Fungi are useful model systems for the study of circadian rhythmicity for a number of reasons. They often have fast generation times, a long history of classical genetics, and now also molecular genetics. Many developmental and biochemical mutants are available that allow interactions between the clock, and input/output pathways, to be dissected.