The Induction of Recessive Lethals in Drosophila Melanogaster by Radiations of Different Ion Density.

ARLY investigators in radiation genetics claimed that X-rays of different wave E lengths, gamma rays, and beta particles of radium were equally effective in inducing sex-linked recessive mutations in Drosophila melanogaster and that densely ionizing radiations such as recoil protons from neutrons and alpha particles were less efficient than the less densely ionizing X, gamma, and beta radiations. This early work is described in reviews by ZIMMER and TIMOF~EFF-RESSOVSKY (1942), CATCHESIDE (1948), and MULLER (1954). Later data, however, support opposite conclusions. MICKEY (1954) and IVES et al. (1954) found fast neutrons from the cyclotron and nuclear detonations to be 2-3.5 times as effective as X-rays in inducing recessive lethals in D. melanogaster, and KING (1954) reports that 90 kv X-rays are more efficient than beta particles from P32 bakelite plaques. The more recent data would, therefore, indicate that the relative biological effectiveness (RBE) of radiations of different ion densities increases with increasing ion density. In the present study, a detailed comparison was made of the effects of fast neutrons, X-rays, and Co60 gamma-rays on the induction of recessive lethals in D. melanogaster. Results of this study also show that the RBE of different radiations increases with increasing ion density, and that the frequency of recessive lethals induced by X-rays increases more rapidly with increasing dose than expected on the basis of linearity.