MANNER OF PRODUCTION OF MUTATIONS 595 



approximately reversed. Again, strain 89601, which had given a spon- 

 taneous rate of 0.002 in the microconidia, or about a tenth of the spon- 

 taneous rate of strain 37401, gave with ultraviolet only 0.05, or less than 

 a hundredth the ultraviolet rate of strain 37401. 



On the other hand, when X rays were applied to strains 37401 and 

 89601, their rates were found to be 2.75 and 1.25, respectively. Thus, 

 with X rays, the rate of 89601 had risen to 0.45 as great as that of 37401. 

 In the absence of X-ray tests on the other strains, however, it is too early 

 to conclude that ionizing radiation tends, in general, to give rates of 

 mutation which are more nearly alike (i.e., less discriminating) than those 

 arising from ultraviolet or spontaneously. It may be recalled (see p. 

 592) that in E. coli Newcombe found four times as high a ratio between 

 the rates of phage-resistant and streptomycin-resistant mutations with 

 7 rays as with ultraviolet. Thus the only generalization permissible so 

 far seems to be that different agents do differ, in favoring one rather than 

 another type of mutation, even of the same locus. 



The differences reported by Anderson (1951) between the rates of 

 mutation to streptomycin independence and purine independence induced 

 by X rays in E. coli, when different doses are used, may be considered as 

 another case of differential mutagenic action since, owing to the different 

 frequency-dosage curve for these two mutations, higher doses gave very 

 different ratios of them to one another than did lower doses. A similar 

 but less marked difference, of course, occurs in Drosophila, when pheno- 

 typic effects such as mottled eye, caused by chromosome changes requir- 

 ing more than one hit, are compared with those derived from point 

 mutations. How^ever, although it is possible that the purine-independent 

 variant represents a multihit mutation in the chromosome material, it is 

 alternatively possible that it represents a change similar to the antigen 

 changes in Paramecia, as was suggested on p. 593 for the K variant of 

 Chaetomium. If so, this would help to explain why its curve of frequency 

 with dosage had the same shape as that for killing, inasmuch as it would 

 tend to follow the production of a given amount of cytoplasmic disturb- 

 ance. All this indicates the desirability of either data on the frequency- 

 dosage relations or genetic analysis, before judgment is passed concerning 

 differences in the nature of mutational processes, based on differential 

 mutagenic effects of different agents or conditions on different types of 

 mutations. 



In addition to the possibility of changes of cytoplasmic origin of the 

 type mentioned, there is also that of mutations or losses of actual genetic 

 material which is included in the cytoplasm. Thus kappa in Paramecium 

 may be very readily destroyed by radiation and by some other treatments 

 (Sonneborn, 1951), chloroplastids can be destroyed by streptomycin 

 (Provasoli, Hutner, and Pintner, 1952), and the loss of certain self- 

 reproducing elements in yeast cytoplasm can be induced in 100 per cent 



