Adaptation of the circadian rhythm of mating‐reactivity to abnormal light‐dark cycles inparamecium bursaria

Circadian and ultradian clock-controlled rhythms in unicellular microorganisms

The time structure of a biological system is at least as intricate as its spatial structure. Whereas we have detailed information about the latter, our understanding of the former is still rudimentary. As techniques for monitoring intracellular processes continuously in single cells become more refined, it becomes increasingly evident that periodic behaviour abounds in all time domains. Circadian timekeeping dominates in natural environments. Here the free-running period is about 24 h. Circadian rhythms in eukaryotes and prokaryotes allow predictive matching of intracellular states with environmental changes during the daily cycles. Unicellular organisms provide excellent systems for the study of these phenomena, which pervade all higher life forms. Intracellular timekeeping is essential. The presence of a temperature-compensated oscillator provides such a timer. The coupled outputs (epigenetic oscillations) of this ultradian clock constitute a special class of ultradian rhythm. These are undamped and endogenously driven by a device which shows biochemical properties characteristic of transcriptional and translational elements. Energy-yielding processes, protein turnover, motility and the timing of the cell-division cycle processes are all controlled by the ultradian clock. Different periods characterize different species, and this indicates a genetic determinant. Periods range from 30 min to 4 h. Mechanisms of clock control are being elucidated; it is becoming evident that many different control circuits can provide these functions.

Circadian rhythmicity within single cells of Paramecium bursaria

Cell populations of Paramecium bursaria show mating reactivity in the light period, but not in the dark period, when exposed to a light-dark cycle (LD 12:12). After they are transferred to constant-light (LL) conditions (1,000 lux), they continue to show a circadian rhythm of mating reactivity. The rhythm gradually dampens in LL so that mating reactivity in populations becomes arrhythmic in LL within 2 weeks. We wanted to know whether the arrhythmicity of this population was due to the absence of circadian rhythmicity within each individual cell, or merely due to asynchrony of a population of individually rhythmic cells. Therefore, single cells were isolated randomly from an arrhythmic population that had been in LL for a long time. Then the mating reactivity of these single cells was individually tested every 3 hr for 2 days. Each single cell showed a circadian mating rhythm in LL. This shows that the disappearance of the mating rhythm in cell populations under LL is not caused by disappearance of circadian rhythmicity within individual cells, but is due to desynchronization among cells in a population. When an arrhythmic population in LL is darkened for 9 hr, the mating reactivity rhythm of the cell population reappears. This occurs by resynchronization of the rhythms among individual cells, as can be shown by exposing single cells to pulses of 9 hr of darkness. This dark treatment causes phase shifts of single-cell rhythms, and a phase response curve is obtained for this stimulus. This phase-shifting behavior explains the efficacy of 9-hr dark pulses in restoring the population's rhythm.