Unexpected Resistance to X-Irradiation in a Strain of Hybrid Mammalian Cells

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Abstract
The radiosensitivities of a strain of mouse fibroblasts (Cl-1D), of rat pituitary cells (GH 1 2C 1 ), and of a hybrid between the two (α-RST) have been studied. Their mean chromosome numbers were 50, 70, and 111, respectively. The hybrid cells were much more resistent to radiation than either of the parent strains. The range of the D 0 (reciprocal of the slope, and therefore a measure of radiosensitivity) for the linear portion of the survival curves for each cell line was: Cl-1D, 134-142 R; GH 1 2C 1 , 154-170 R; and α-RST, 248-274 R. There were no significant differences in the magnitude of the shoulder or extrapolation number of the survival curves, nor in the ability of the three cell strains to accumulate and repair sublethal radiation damage. It appears unlikely that the unusual resistance of the hybrid strain is simply related to the increase in chromosome number; more likely, it involves some interaction between the two genomes. The study of somatic cell hybrids may offer further insight into the factors controlling the radiosensitivity of mammalian cells.