Home

Phenotypic expression of the Neurospora clock.

Neurospora undergoes a daily cycle of differentiation with the clock controlling the developmental switch. Clock assays are typically carried out in "race tubes" (hollow glass tubes about 40 cm long and 16 mm in diameter, bent up at both ends in order to hold an agar growth medium) shown schematically in the figure. After inoculation and growth for a day in constant light, the position of the growth front is marked and the culture transferred to constant darkness (LD transfer); the position of the growing front is marked at 24 hour intervals thereafter. The LD transfer sets the clock running from CT 12 and sets a developmental switch such that mycelia, as they are laid down, are determined not to differentiate. Sometime later, at a time corresponding to late subjective night, the clock-controlled switch is thrown the other way so that mycelia as they are laid down are determined to differentiate, and thus elaborate aerial hyphae which eventually (during subsequent days) differentiate to produce asexual spores called conidia. After some hours this developmental switch is reversed and the mycelia that are laid down no longer have the capacity to differentiate. This cycle recurs approximately every 21.5 hours (the duration of one circadian cycle), and once each region is laid down the hyphae are developmentally set. Thus, following a week of growth in constant darkness, a surface is covered by fluffy yellow-orange conidiating bands alternating with undifferentiated surface growth. Since growth rate is more-or-less constant for any strain, distance grown (as determined by the daily growth marks) = time elapsed since the LD transfer, and the period length and phase of the rhythm are simply read from the pattern of growth. The Figure shows race tubes of a representative sample of the frq mutants. While differences in clock period lengths are genetically regulated, growth rates are neither genetically regulated nor clock-controlled and can vary from tube to tube. Such differences in growth rate are irrelevant in the context of rhythms since the banding pattern recurs in intervals determined by the number of hours elapsed, not the number of centimeters of growth traversed within that time.

This figure and text are adapted from several publications including but not limited to these. It should not be copied or used without permission.

Dunlap, Jay C. 1990. Closely Watched Clocks: Molecular Analysis of Circadian Rhythms in Neurospora and Drosophila. Trends in Genetics 6, 159 - 165

Jennifer J. Loros. The Molecular Basis of the Neurospora Clock. Seminars in the Neurosciences. 7 (1) pp.3-13 (1995)

Bell-Pedersen, D., N. Y. Garceau, and J.J. Loros. Circadian Rhythms in Fungi. J. of Genetics, 37 (3):387-402(1996)

Dunlap. Jay C. 1998. Biological Clocks, in The McGraw Hill Yearbook of Science and Technology, ed. S. Parker, McGraw Hill, New York, pp. 21 - 24.

Dunlap, Jay C. Molecular Bases of Circadian Oscillators. 1999. Cell 96: 271 - 290