A size filtration method to synchronize cultures of the dinoflagellate Gonyaulax polyedra to the beginning of the G1 phase has been developed. This technique selects newly born cells by two sequential filtrations, based on the fact that cell division is restricted to the beginning of the day, so that a decrease in cell volume occurs at this time. The fraction of synchronized cells immediately after the second filtration is about 90%; the procedures do not alter the free-running period or phase of glow rhythm, and the selected cells divide again in a few days. Applying this method, we have found that the generation times of this species in a light-dark cycle (LD 12:12) are indeed quantized to multiples of 24 hr, but are variable from generation to generation.
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