Microsurgical analysis of the clonal age and the cell-cycle stage required for the onset of autogamy in Paramecium tetraurelia.

When autogamy was induced in competent cells of Paramecium tetraurelia by depriving them of food, the onset of autogamy was preceded by a critical fission which occurred in the starvation medium. When the cells were fed again immediately after the fission, they did not undergo autogamy. However, they did undergo autogamy when they were fed later than 1 hr after the critical fission. The irreversible differentiation for autogamy seems to be at about 1 hr after the critical fission. This procedure thus provides the opportunity to induce autogamy synchronously. The result of macronuclear transplantation demonstrated that autogamy was under the control of macronucleus. Moreover, the clonal age required for autogamy was found to be shortened by repetitive elimination of a part of the macronucleus. The result can be explained by the hypothesis that clonal age is measured in rounds of chromosome replication or DNA synthesis rather than cell divisions.