In the natural habitat, it is difficult to distinguish between the effects of many variables such as temperature, light, aeration and humidity on growth and reproduction but sufficient experiments with pure cultures under controlled conditions have demonstrated that light has a strong influence, chiefly in the timing of the production and discharge of spores—the most efficient means of dispersal and migration of species. As the given examples become widely known, more investigators will be prompted to study the effect of light in fungal development and more such fungi are bound to be discovered in the future. The significance of these responses is not obvious but, fungi being largely terrestrial organisms, these events must in some way be linked to daily cycles of light and darkness for optimal fitness and survival. A surprising number of homologues to blue and red light-sensing genes have been discovered in a terrestrial fungus such as Neurospora (Borkovich et al., 2004). At present, little is known of the mechanism of the perception of light into reproduction. In N. crassa, rhythmic response can be quantitated, it is amenable for genetic analysis and it is extremely well-suited for the analysis of complex phenomenon of biological rhythm. As the Earth rotates and revolves round the sun, the daily cycle of light and dark in different regions varies annually with latitude. The studies of conidiation rhythm in Neurospora collected from different latitudes could provide clues into the role of circadian sporulation rhythm. Which part of the mycelium perceives light, and what is the molecular identity of the photoreceptor and its cellular localization, are among other questions awaiting answers. A breakthrough has come from the genetic approach that has identified certain genes underlying the timekeeping mechanism. The product of the frequency locus contributes to a molecular oscillator whose rate of degradation is a major determining factor for the period length of the circadian clock. At present, the model of circadian rhythm in this fungus envisages transcription of frq gene(s), followed by production of FRQ pro-tein(s), their feedback on self-transcription, degradation of FRQ protein(s) releasing the negative feedback, allowing a new round of transcription and resulting in molecular oscillations of RNA and protein. Among important research goals is the identification of genes regulated by frq and the signaling pathways from the environment through which the cellular clock is synchronized to the external world. The "race tube" system of easily visualizing a rhythm in conidiation in Neurospora crassa and screening genetic mutants affected in periodicities makes Neurospora one of the most attractive models for approaching studies of rhythmic activities in plants and animals showing higher complexity of cellular organization.

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