EVIDENCE THAT NATURAL RADIOACTIVITY IS INADEQUATE TO EXPLAIN THE FREQUENCY OF “NATURAL” MUTATIONS

Abstract

It has been shown by Hanson that the number of mutations caused by radiations is directly proportional to the ionization which these radiations produce. In attempting to discover whether natural radioactivity is sufficient to account for the "natural" rate of mutation in an animal like Drosophila, the authors have, therefore, attempted to measure the ratio of ionization from natural radioactivity to which these animals are exposed to the ionization to which they are exposed in irradiation experiments. This ratio is then compared with that of the number of "natural" mutations in this organism to the number produced by artificial irradiation. The intensity of the artificial rays was measured by a "dosimeter" reading in "r" units per min., and the number of ions produced during a specific treatment was calculated from this. This result was checked by calculating the intensity of radiation from a tungsten target Coolidge tube by the data of Terrill, and figuring the ionization from this. On the basis of evidence from various observers as to the amount of ionization produced by natural radioactivity in the vicinity of the flies, it was found to be in about the ratio of 1:200,000 as compared with that to which the animals were subjected in artificial treatments. Yet the ratio of the number of "natural" mutations to those produced artificially in a given time is about 1: 150. Hence it is concluded that the amount of natural radioactivity from the outer environment is far too small to account for the "natural" mutation rate. The radioactivity of substances such as K or U which may occur in or near the organisms was then considered, and it was calculated that the concentrations in which these substances occur in such situations were likewise far too low to produce enough ionization to account for "natural" mutation. In this connection measurements were made of the radioactivity of the bodies of 5000 dead Drosophila, and also of that of their food. The possibility remains that the germ cells or the tissues in their immediate vicinity store radioactive substances in far greater concentrations than occur in the organism as a whole. The mature germ cells, however, certainly do not contain any such concentration, since they form so large a proportion of the whole animal that radiation from this source would have been detected in the above mentioned measurements. Also the investigators think it highly improbable that sufficient concentration exists in other tissues. Hence they conclude that some cause or causes other than natural radioactivity must be responsible for the vast majority of "natural" mutations in this animal.