By means of micromanipulation the fate of the descendants of X-irradiated Saccharomyces cells was investigated. Within the first four generations after irradiation with doses below 12 krads, all descendants of surviving haploid and diploid cells can form colonies. These results are supplementary to data for higher X-ray doses at which, at least for diploid Saccharomyces, an inactivation of some descendants was observed ["lethal sectoring"; K. Haefner, Intern. J. Radiation Biol. 9, 545 (1965); A. P. James, and M. M. Werner, Radiation Res. 29, 523-536 (1966)]. The observed lack of lethal sectoring in haploid Saccharomyces, for doses that inactivate up to 95% of the population, indicate that lethal sectoring must not generally appear in the survivors after irradiation. Based on the results presented, a rediscussion of several models proposed to explain lethal sectoring was undertaken. The following two explanations were compatible with our results: (1) The appearance of inactive descendants in the progeny of cells surviving irradiation is the result of a random distribution of radiation-damaged self-reproducing cytoplasmatic particles in the progeny of the irradiated cell. Descendants that do not get enough intact particles become inactive. (2) Cells lethally damaged by the irradiation can still undergo residual divisions, and repair processes act in some of their descendants. Recovered descendants form a colony, whereas the others remain unable to do so.

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