| Effects of 14 MeV neutrons in T. monococcum Taro FUJII National Institute of Genetics, Misima, Japan F1 seeds from the cross between the normal type and a chlorina mutant were subjected to gamma-rays and 14 MeV neutrons, as in the previous experiment (cf. Ann. Rep. Nos. 17,18). 8,600 rad of gamma-rays and up to 900 rad of neutrons had no apparent effect on germination rate. Survival rate in the gamma-ray lot was slightly lower and was in the three neutron lots slowly decreasing with increasing dosage. A similar tendency was also observed in the average number of tillers, but it was not marked. Somatic mutation rate was calculated on spike basis. As shown in Table 1, the frequency of tillers with chlorina stripes increased with increasing neutron dosage. The frequency of mutated tillers in the highest dosage lot was 0.79%; it was twice as large as that of the lowest dosage lot, and that of 8,600 rad of gamma-rays was about one half as large as that of the 324 rad neutron lot. An almost linear relation was observed between mutation frequency and neutron dosage, but the mutation rate at higher dosages was too high for linear relation in the gamma-irradiation experiments. From the result, we may say that somatic mutation is caused, for the most part, by chromosome aberrations. Decrease of fertility was marked with increasing dosage. When the fertility was compared between mutated and non-mutated tillers within each individual, average fertility of the former was lower than that of the latter. These facts also support the view that most of somatic mutation arise from chromosome aberrations. Average number of mutated tillers per individual roughly increased with increasing dosage, as the result of repeated occurrence of the same kind of mutation within the same individual. |