Plating efficiency as a function of time postirradiation: evidence for the delayed expression of lethal mutations.

The plating efficiency (PE) of a gamma-irradiated (7 Gy) human cell hybrid line (HeLa X skin fibroblast, designated as CGL1) has been measured as a function of time postirradiation and compared to that of unirradiated cells at similar cell densities and under the same growth conditions. The results indicate that following irradiation, the PE of the irradiated cells initially increases but never returns to that of unirradiated cells during the experimental period that we have examined. Furthermore, after a period of 9 to 10 days (equivalent to at least 10 cell doublings) postirradiation and plating, the PE of the irradiated cells begins to decrease and continues to do so over the next 5 days. A decrease does not occur in unirradiated cells until much later (i.e., Day 15) corresponding to at least 5 additional cell doublings. The data are discussed in terms of a delayed expression of lethal mutations. The possible impact of these observations on the estimation of radiation-induced transformation frequencies is also considered.