594 RADIATION BIOLOGY 



types of nutritional-deficiency mutations in Neurospora, occasioned by 

 X rays, ultraviolet, and various mustards. The results do not seem very 

 suitable for an accurate quantitative comparison, being rather variable 

 even with the same agent and stage of treatment. This is in part because 

 of the use of different methods of detection and the fact that the numbers 

 of mutants were rather small in most cases in which complete analyses 

 and records were made. Making allowance for all this, however, these 

 results, in so far as they go, have failed to show clear differences in the 

 mutational spectrum obtained by the use of different agents or by appli- 

 cation of the same agent to different kinds of cells such as macroconidia, 

 microconidia, and young perithecia. This is true despite the facts that 

 (1) different types of deficiencies do arise with significantly different fre- 

 quencies and (2) the different kinds of cells used for treatment have very 

 different total mutation rates, in response to a given dose of a muta- 

 genic agent. Previously published results on Penicillium (Bonner, 1946) 

 and on bacteria (Tatum, 1946) are regarded by Tatum as having given 

 results of essentially the same kind, i.e., a similarity in the mutational 

 spectrum with different agents, although, of course, it did differ markedly 

 from one kind of organism to another. 



More recently, however, Giles (1952), in genetically analytical and very 

 precise and large-scale work on point mutations of a particular locus group 

 in Neurospora, that mutating to and from inositol deficiency, has obtained 

 definite evidence of differences in the relative frequencies of mutations of 

 different alleles (or pseudo-alleles) of this locus group, in untreated mate- 

 rial and in that subjected to ultraviolet and to X rays, respectively. 

 Eight different strains of inositoUess, all much alike in growth responses 

 but representing independently arisen alleles or pseudo-alleles (some pro- 

 duced by ultraviolet and some by mustard) of the same multiple locus, 

 were extensively tested to determine their frequencies of reverse muta- 

 tion, at that locus, to the inositol-independent type. They were found 

 to dilYer widely from one another in their spontaneous rate of reverse 

 mutation, falling into at least five (and probably eight) classes that were 

 significantly different in this respect. One strain gave no spontaneous 

 reverses at all, one less than 0.01 per million, others different rates from 

 0.01-0.1, and one 15 per million surviving conidia. It was shown by 

 genetic tests that these differences depended on the locus for inositoUess 

 itself that was present in the given case, not on differences in "modifying 

 genes." When ultraviolet was applied, the rates thus induced also varied 

 greatly, but they showed relative frequencies significantly different from 

 the spontaneous ones. 



Thus, with ultraviolet, although the completely nonmutating strain 

 still gave no mutations, two strains, Nos. 37401 and 64001. which had 

 given, in the microconidia, 0.02 and 0.1 spontaneous mutants per million, 

 respectively, now gave 10.0 and 1.0, their relative frequencies being 



