RELATIVE FREQUENCIES OF SECTORED AND NONSECTORED MUTANT COLONIES IN YEAST AS A FUNCTION OF ULTRAVIOLET DOSE

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Abstract
The effect of dose of 2537 A u.-v. radiation on sectoring in variant colonies of Saccharomvces cerevisiae was examined, using hereditary changes from galactose-EMB pos. to galactose-EMB neg. In the strain of yeast used, the variant colonies are induced with frequencies approaching 10%. A proportion of these are sectored, positive sectors being present within negative variant colonies. With increasing dose, increasing frequency of variant colonies is accompanied by a progressive decrease in proportion of those which are sectored. Investigations were concerned with effects of irradiation which might contribute to this dose-sectoring phenomenon. It was concluded that degree of sectoring could be modified by failure of, or delay in, recovery of one or more cells of microcolonies in early divisions following irradiation. Such failures could result either from nongenetic effects of radiation or from delayed dominant lethal mutations. It is suggested that sector frequencies at high doses, as detd. by colony counts, are a gross underestimate of the actual amt. of sectoring among irradiated cells.

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