Interaction of the Circadian Cycle with the Cell Cycle in Pyrocystis fusiformis

Abstract

Dividing pairs or single cells of the large dinoflagellate, P. fusiformis Murray, were isolated in capillary tubes and their morphology was observed over a number of days, either in a light-dark cycle or in constant darkness. Morphological stages were correlated with the 1st growth stage, G1, DNA synthesis, S, the 2nd growth stage, G2, mitosis, M, and cytokinesis, C, segments of the cell division cycle. The S phase was identified by measuring the nuclear DNA content of cells of different morphologies by the fluorescence of 4'',6-diamidino-2-phenylindole dichloride. Cells changed from 1 morphological stage to the next only during the night phase of the circadian cycle, under light-dark conditions and in continuous darkness. Cells in all segments of the cell division cycle displayed a circadian rhythm in bioluminescence. These findings are incompatible with a mechanism for circadian oscillations that invokes cycling in Gq, a hypothesized side loop from G1. All morphological stages, not only division, appear to be phased by the circadian clock.