Stable heat-resistant variants in populations of Chinese hamster cells.

Sublines with increased survival at elevated temperatures were obtained by repeated exposure of V79 Chinese hamster lung cells to 44.5 degrees C followed by regrowth of survivors to mass populations at 37 degrees C. After three cycles, the time at 44.5 degrees C required to reduce cell survival to 37% (D0 value) increased from 4.6 to 26.3 minutes. Clonal isolates retained their resistance to thermal stress for at least 3 months in culture at 37 degrees C and showed distinctive differences from precursor cells in growth rate and colonial morphology. These differences disappeared in hybrids constructed between resistant and sensitive cells. In assays of 44.5 degrees C, hybrid survival was only slightly increased over levels obtained with sensitive cells. Heat resistance and associated marker characteristics thus appear to be recessive or weakly codominant. Cytohybrids were also constructed by fusing cytoplasts from sensitive cells to nucleated heat-resistant cells. No cytoplasmic transfer of marker characteristics was detected in these combinations, which suggests that controls for heat resistance are nuclear in origin.