Clonal heterogeneity in delayed decrease of plating efficiency of irradiated HeLa cells.

The clonogenic potential of progeny of irradiated HeLa cells was studied at different times after single doses of 4-12 Gy. The dose-dependent decrease in plating efficiency that was observed resembled the effect termed "delayed lethal mutation" by Seymour et al. (1986). The effect decreased with time after irradiation. Individual clones of irradiated and non-irradiated cells were isolated, expanded and replated 5 weeks after irradiation, i.e., after between 200,000 and 1,000,000 progeny had formed from the individual parent cell. The plating efficiency of progeny of unirradiated cells did not vary much, whereas clonal progeny of irradiated cells had plating efficiencies ranging from 3% to 76%. The plating efficiency was not related to the cell number in the original clone.