INDUCTION KINETICS AND GENETIC ANALYSIS OF X-RAY-INDUCED MUTATIONS IN THE AD-3 REGION OF NEUROSPORA CRASSA

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Abstract
An analysis of the induction of ad-3R and ad-3IR mutations in a balanced dikaryon of Neurospora crassa with X irradiation shows that they have different induction kinetics. Ad- 3R mutants show single-hit kinetics, while ad-3IR mutants show 2-hit kinetics. The spectrum of X-ray-induced forward mutation in the ad-3 region is dose-dependent; whereas at 1 kr, 97% of the mutants are ad-3R (and affect only the ad-3A locus or ad-3B locus), at 40 kr, 56% are ad-3IR and 26% inactivate both the ad-3A and ad-3B loci. Estimates of the forward-mutation rates for the 2 loci in the ad-3 region are 1.75 x 10-9 mutants- r-1. survivors-1 for ad-3AR mutants and 3.64 x 10-9 mutants- r-1. survivors-1 for ad-3BR mutants. A minimal estimate for the incidence of the hits that cooperate to give ad-3IR mutants is 4.01 x 10-7 hits. r-1. survivors-1. The finding that the spectrum of X-ray-induced mutations in the ad-3 region includes both intragenic and extragenic alterations makes it possible to reconcile the differences observed in previous analyses of X-ray-induced mutations in haploid and diploid organisms.