THE RELATION OF POPULATION HETEROGENEITY TO THE PLATEAU PHENOMENON IN RADIATION DOSE-RESPONSE CURVES

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Abstract
X-ray mutation-dose curves of adenine- dependent mutants induced in yeast cells exhibit a plateau or peak characteristic of ultraviolet dose-response curves. The plateau reflects the presence of a few budding cells in the population which are resistant to both killing and mutation: with increasing doses of radiation the proportion of cells sensitive to killing and mutation decreases in the surviving population whereas the proportion of cells resistant to killing and mutation increases, thus producing a plateau in the mutation curve. The resistant cells are undiscernible from the mutation and killing curves at low doses. The exact shape of the plateau is deter-mined by the mutation rates, killing rates, and extent of heter-ogeneity. This finding should be considered in constructing interpretations of the plateaus of radiation dose-response curves and the effects of post radiation treatment of cells.

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