RBE-LET relationships of high-LET radiations in Drosophila mutations

The relative biological effectiveness (RBE) of Cf-252 neutrons and synchrot ron-generated high-energy charged particles for mutation induction was eval uated as a function of linear energy transfer (LET), using the loss of hete rozygosity for wing-hair mutations and the reversion of the mutant white-iv ory, eye-color in Drosophila melanogaster. Loss of heterozygosity for wing- hair mutations results predominantly from mitotic crossing over induced in wing anlage cells of larvae, while the reverse mutation of eye-color is due to an intragenic structural change (2.96 kb-DNA excision) in the white loc us on the X-chromosome. The measurements were performed in a combined mutat ion assay system so that induced mutant wing-hair clones as well as reverta nt eye-color clone can be detected simultaneously in the same individual. L arvae were irradiated at the age of 3 days post oviposition with Cf-252 neu trons, carbon beam or neon beam. For the neutron irradiation, the RBE value s for wing-hair mutations were larger than that for eye-color mutation by a bout 7 fold. The RBE of carbon ions for producing the wing-hair mutations i ncreased with increase in LET. The estimated RBE values were found to be in the range 2 to 6.5 for the wing-hair. For neon beam irradiation, the RBE v alues for wing-hair mutations peak near 150 keV/mu m and decrease with furt her increase in LET. On the other hand, the RBE values for the induction of the eye-color mutation are nearly unity in Cf-252 neutrons and both ions t hroughout the LET range irradiated. We discuss the relationships between th e initial DNA damage and LET in considering the mechanism of somatic mutati on induction.