Abstract
Comparative investigation of the effectiveness was carried out between 14MeV neutrons and 137Cs γ-rays in the induction of visible recessive mutations in early and in late gonial cells of the silkworm.
(1) Dose-frequency curve for each of these two radiation experiments was obtained. The curves are qualitatively similar, each showing an initial linear part followed by an exponential rise with increasing dose. This was explained by assuming that mutations scored at lower doses and constituting the linear part of the curves are due to single hits, whereas those recovered at higher doses and forming the exponential part of the curve are caused by multi-hit events.
(2) Dose-frequency curves for 14MeV neutrons showed that the drop in mutation appears at doses beyond about 2, 000rad for early and late spermatogonia and late oögonia. The data on the mutation drop fit to the selective killing hypothesis.
(3) Mutation frequencies recovered after exposure to 14MeV neutrons were more or less similar between the two, early and late groups, for both sexes in the dose range below 1, 000rad, but in the dose range above 1, 000rad they were much higher for the late than for the early group.
(4) The mutation frequencies in late gonia were higher for 14MeV neutrons than γ-rays, while in early gonia they were almost equal for both.
(5) The RBE of neutrons to γ-rays was found to depend sensitively on the germ cell stage; it was about 1 for early gonia and 2 to 3 for late gonia, when comparisons were made at the level of 10-3 mutation frequency. The variation in RBE is due to the variation with stage in frequencies of acute γ-ray-induced mutations.
