THE INFLUENCE OF LARGE DELETIONS ON THE MUTATION FREQUENCY INDUCED BYTRITIATED-WATER AND X-RADIATION IN MALE DROSOPHILA-MELANOGASTER POSTMEIOTIC GERM-CELLS

Tritium beta radiation (H-3 beta-radiation) in the form of tritiated w ater was used to induce mutations at the alcohol dehydrogenase (Adh) l ocus in male Drosophila melanogaster post-meiotic germ cells. All 23 A dh null mutations were large deletions (> 20 kb), determined by geneti c complementation and Southern blot analyses. 27 Adh null mutations ha ve been induced by 100-kVp X-rays (Aaron, 1979) and have been genetica lly and molecularly characterized (Ashburner et al., 1982; Chia et al. , 1985; LoMonaco et al., 1987; Mahmoud et al., 1991). In contrast to H -3 beta-radiation, 100-kVp X-rays induced a bimodal distribution of Ad h null mutations, intragenic mutations, g 250 bp, and large deletions, > 100 kb. A statistically significant difference was observed between the frequency of large deletions (23/23 or 1.0) induced by H-3 beta-r adiation and the frequency of large deletions (19/27 or 0.7) induced b y 100-kVp X-rays. However, a statistical difference was not observed b etween the size distribution of the large deletions induced by H-3 bet a-radiation and X-rays. The relative deletion frequency (RDF) induced by H-3 beta-radiation and 100-kVp X-rays was (1.0/0.7 = 1.4). The rela tive biological effectiveness (RBE) of these two radiation sources was 1.4, determined from the ratio of the regression coefficients of the respective H-3 beta-radiation and X-ray sex-linked recessive lethal (S LRL) dose-response data. The large difference in size between the two classes of X-ray-induced Adh null mutations and the increase in mutati on frequency and deletion frequency for H-3 beta-radiation with respec t to X-rays may indicate that the relative deletion frequency (RDF) is the molecular biological basis for the increase in the RBE for radiat ion sources with a mean LET value less than or equal to 10 keV/mu m.