Persisting Circadian Rhythm of Cell Division in a Photosynthetic Mutant of Euglena

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Abstract
A persisting, free-running, circadian rhythm of cell division in a heterotrophically grown mutant of Euglena gracilis var. bacillaris having impaired photosynthesis is obtained upon placing a culture that has been previously synchronized by a 10,14 light-dark cycle into continuous darkness at 19°C (but not at 25°C). A similar persisting rhythm is initiated in exponentially increasing cultures (growing in darkness at 19°C) by a single "switch-up" in irradiance to continuous bright illumination. The results implicate an endogenous biological clock which "gates" the specific event of cell division in the cell developmental cycle.