Dose Fractionation on Chromosome Aberration Frequencies I K Tradescantia Microspores. Neutrons

HE previous paper (DE SERRES and GILES 1953) has described the results T of experiments investigating the effects of fractionated doses of X-rays on the frequency of chromosome aberrations induced in Tradescantia microspores. The results of these studies demonstrate that there is a marked effect of X-ray dosage fractionation in reducing the yield of 2-hit aberration types and, further, that no recovery from this reduced yield occurs when the period between dose fractions is greater than four hours. These conclusions are thus in agreement with those obtained in earlier experiments of SAX (1939, 1940) and more recent experiments of SAX and LUIPPOLD (1952), and do not agree with the observations of LANE (1951). In the discussion and interpretation of his experimental results with X-rays, LANE also discussed some of the results of fast neutrons experiments. These experiments (GILES 1940, 1943; TNODAY 1942) indicated that with fast neutrons, in contrast to the situation with X-rays, the relationship between radiation dose and interchange frequency is linear, suggesting that both breaks are the result of a single hit, both being produced by a single proton path. This interpretation was confirmed by experiments in which it was shown that there was no effect of the intensity of neutron radiation on either chromosome or chromatid interchange yields, and also that fractionation of a neutron dose had no effect on the frequency of chromatid interchanges. Since the intensity and dose fractionation results with neutrons are difficult to reconcile with LANE’S interpretations of his X-ray results, LANE questions the validity of these results, although he presents no new data based on neutron experiments. Consequently, when experiments were being planned to reinvestigate the effects of fractionated doses of X-rays, it seemed desirable to perform additional complementary studies with neutrons. In the original experiments, most of the observations dealt with chromatid rather than chromosome effects and the time intervals between dose fractions did not exceed 32 minutes. I t thus seemed desirable to determine experimentally whether fractionation would have an