Time-Lapse Analysis of the Circadian Rhythms of Conidiation and Growth Rate in Neurospora

Nov 4, 2004Journal of biological rhythms

Daily patterns of spore production and growth rate in Neurospora over time

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Abstract

Conidiation rate peaks sharply and is influenced by nonlinear growth, varying by a factor of about 2 with each circadian cycle.

Conidia form simultaneously in a band region rather than after a lag time, leading to a distinct peak in conidiation rate over time.
Growth rate of Neurospora crassa is nonlinear, contradicting the common assumption of constant growth.
Errors in timing conidiation peaks can reach 1 to 2 hours if linear growth is incorrectly assumed.
Conidiation and growth rate rhythms are not detectable under conditions that disrupt spatial banding.
In light/dark cycles, the conidiation and growth rate rhythms remain synchronized across different temperature cycles.
These observations suggest that the rhythm of growth rate may arise from the rhythm of conidiation.

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The filamentous fungus Neurospora crassa has frequently served as a model organism for the study of circadian rhythms through its ability to form conidial spores on a daily basis. This phenomenon leaves a spatial pattern of conidiation bands along a solid surface of agar after several days of growth. Using time-lapse video, the authors have quantified the rate of conidiation. They have found that conidia do not form at a specified lag time after the growth front is laid down, but rather the band region tends to simultaneously develop over a short time frame. This produces a sharp peak when the conidiation rate is plotted against time. In addition, the authors used time-lapse video to assay growth rate with greater accuracy than previously reported. It is usually assumed that Neurospora's rate of growth is constant, and this assumption of linear growth has been used extensively to determine period and phase of the conidiation circadian rhythm. The authors have confirmed an earlier report of nonlinear growth rate and have shown that the growth rate varies by a factor of about 2 with each circadian cycle. They have demonstrated that the errors in calculating times of conidiation peaks are maximally 1 to 2 h if linearity is assumed. The conidiation rate and growth rate rhythms are not apparent under conditions (using mutants or high or low temperatures) where the spatial banding rhythm is not observed. In light/dark entraining conditions, the conidiation rate and growth rate rhythms maintain the same phase relationship in different T-cycles. These data are consistent with the hypothesis that the growth rate rhythm is a consequence of the conidiation rate rhythm.

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Time-Lapse Analysis of the Circadian Rhythms of Conidiation and Growth Rate in Neurospora

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