Effects of very low doses of ionizing radiation on the clonal life-span in Paramecium tetraurelia.

Effects of very low doses of ionizing radiation were investigated on life-span in Paramecium tetraurelia. After autogamy single cell cultures were place (1) in a control chamber with radiopermeable walls; (2) in a shielded device with 10 cm thick lead walls; (3) in a similar shielding device including a 60Co source giving a dose of 760 mrad per year at the culture level. Life-spans, expressed in days and in mean total number of fissions measured from autogamy to the death of all sublines, were about 200 fissions and 55 days in controls. Life-spans of shielded sublines were increased and lower life-spans were observed in both shielded and irradiated sublines. These findings provide new evidence for a biological effect of very low doses of ionizing radiations and show that fission potential of Paramecium tetraurelia can be affected by variations in the background radiation level.