A Comparison of the Relative Effectiveness of Radiations of Different Average Linear Energy Transfer on the Induction of Dominant and Recessive Lethals in Drosophila.

INCE 1927 numerous investigators have subjected various plants and animals to radiations of different ion density to determine the relative biological effectiveness (RBE) of the different radiations. Usually it was found that the RBE increased as the linear energy transfer (LET-energy in electron volts dissipated per micron of tissue traversed by ionizing particles) or ion density of the radiation increased. In a few cases the reverse was observed and in others the biological effect was independent of the LET (for a review of these data, see ZIRKLE 1954). Data from early Drosophila work produced conflicting opinions as to the biological effectiveness of radiations of different LET for the induction of the various genetic effects studied. It was reported that the less densely ionizing beta, gamma, and X-rays were equally effective in producing sex-linked recessive lethals (ZIMMER and TIMOFLEFF-F~ESSOVSKY 1942; CATCHESIDE 1948) and translocations (MULLER 1940), and more effective than the more densely ionizing fast neutrons for the induction of recessive lethals (DEMPSTER 1941; GILES 1943; TIMOF~EFF-RESSOVSKY and ZIMMER 1938; FANO 1944) and translocations ( CATSCH et al. 1944). On the other hand, it was reported that fast neutrons were more effective than the less densely ionizing radiations in the production of translocations (DEMPSTER 1941 ) and dominant lethals (DEMPSTER 1941 ; FANO 1944). ZIRKLE ( 1954), reporting the data of STAPLETON and ZIRKLE (1946) and ZIRKLE and PARRISH ( 1950), however, states that the RBE for the survival of Drosophila eggs after exposure to radiations that varied in average LET increased with increasing LET except that the most densely ionizing fast neutrons from uranium fission were less effective than cyclotron neutrons yet more effective than other radiations of lower average LET. Results of recently published data indicate that the RBE of different radiations for the induction of all genetic effects increases with increasing LET (BAKER and VON HALLE 1954; MICKEY 1954; MULLER 1954; LEWIS 1954; STONE et al. 1954; IVES et al. 1955; EDINGTON 1956). Since these differences in the RBE were explained to be the result of differences in the LET of the radia-