Analysis of prefission morphogenesis in Paramecium aurelia

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Abstract
Paramecium aurelia has been exposed to varying concentrations of actinomycin D for extended periods and to a high concentration starting at various specific times in the cell cycle. Extended exposures at 5 μg of actinomycin per 1.0 ml of culture medium are lethal as are 24 hour exposures at 20 μ/1.0 ml. Exposures initiated at selected times in the interfission period reveal transition and stabilization points, similar to those found in Tetrahymena, that is, exposure prior to the transition point almost invariably blocks a given developmental event. Exposure after the transition point may or may not result in blockage but exposure after the stabilization point typically never shows blockage. Also the normal sequence of developmental events can be upset by exposure to actinomycin. These results are interpreted as being consistent with sequential action of the genes in the cell cycle and as indicating independence of final genetic control of different developmental processes, though all processes are dependent on common prior events. The macronucleus of exposed cells shows aggregation of dark material that is neither DNA nor RNA, and is probably a protein, but whose normal function is not known.