QUANTITATIVE AND QUALITATIVE COMPARISONS OF SPONTANEOUS AND RADIATION-INDUCED SPECIFIC-LOCUS MUTATION IN THE AD-3 REGION OF HETEROKARYON-12OF NEUROSPORA-CRASSA

The data from forward-mutation experiments to obtain specific-locus mu tations at two closely linked loci in the adenine-3 (ad-3) region of h eterokaryon 12 (H-12) of Neurospora crassa have been used to determine the relative frequencies and mutational spectra of ad-3 mutants occur ring spontaneously and those induced by 7 different radiation treatmen ts. Previous studies have demonstrated that specific-locus mutants at these two loci result from 5 major genotypic classes, namely two class es of gene/point mutations (ad-3A(R) and ad-3B(R)), and 3 classes of m ultilocus deletion mutations ([ad-3A](IR), [ad-3B](IR) and [ad-3A ad-3 B](IR)). Two different approaches were used to compare spontaneous mut ation in the ad-3 region with that induced by 7 different radiation tr eatments (UV, P-32, 447 MeV protons, Sr-85, 250 kVp X-rays, 39 MeV hel ium ions, and 101 MeV carbon ions). These comparisons included (2)(chi )-tests on the numbers of ad-3 mutants resulting in the following two sets of ratios: (1) gene/point mutations and multilocus deletion mutat ions; and (2) complementing and non-complementing ad-3B(R) mutants. Co mbination of the data from these two methods of comparison has demonst rated that each of the 7 radiation treatments induced a spectrum of ad -3 mutants that is statistically different from the spontaneous spectr um. In addition, these same two methods of comparison have been used t o compare the mutagenic effects of each of the 7 radiation treatments with each other. Combination of the data from these two methods of com parison demonstrated that the majority of radiation-induced specific-l ocus mutations: (90.5% (19/21 of the pairwise combinations)) are quali tatively different from each other. We conclude that the mechanisms by which various radiations modify DNA tend to exhibit fundamental diffe rences from each other and from the processes involved in spontaneous mutation.