564 RADIATION BIOLOGY 



The effect of oxygen applied during X irradiation in increasing the fre- 

 quency of mutations has been extended to microorganisms by Kimball 

 and Gaither (1951), working on Paramecium aurclia; by Stapleton and 

 Hollaender (1952), on Aspergillus; and by Anderson (1951), on E. coli. 

 In the work on Aspergillus, already mentioned for its bearing on the influ- 

 ence of water content, both wet spores (suspended in buffer) and dry 

 spores were irradiated with and without oxygen, respectively, forming 

 four groups. The frequency-dose curve for morphological mutants was 

 for all groups linear for the lower effective doses, an indication of the 

 gene-mutational nature of these mutants, but convex for the higher 

 doses, where, in the reviewer's opinion, an appreciable proportion 

 probably represented multihit chromosome aberrations. Both types of 

 mutations were obviously greatly furthered by the presence of oxygen, 

 in the wet as well as in the dry spores. However, in the absence of 

 oxygen the wet spores had only a slightly higher mutation rate than the 

 dry ones. 



In E. coli one of the two types of mutations studied, the back mutation 

 to streptomycin independence, showed a linear frequency-dose relation 

 to radiation, indicating a one-hit effect, as is usual for gene mutations, 

 and the effective X-ray dose was increased about 20 per cent for each 

 dose when oxygen was substituted for nitrogen. For the killing effect, 

 on the other hand, the frequency-dose curve was highly concave, as 

 though resulting from a multihit effect; moreover, in harmony with the 

 multihit conception if oxygen influences the effectiveness of each hit 

 separately, oxygen as contrasted with nitrogen gave a 23^^- to 3-fold 

 increase in the effective dose. 



Strangely, the other type of mutation studied, the back mutation to 

 purine independence, showed the same highly concave frequency-dose 

 curve as the killing effect and also the same large increase in effective 

 dose when oxygen was substituted for nitrogen, so that the frequencies 

 of this type of mutation and of killing were throughout proportional to 

 each other. It therefore appeared as though the second type of mutation 

 constituted a less direct effect than the first and might have resulted from 

 the same kind of protoplasmic change which also led to killing. If this 

 interpretation should be correct and if this type of mutation is repre- 

 sentative of any considerable class common to many kinds of organisms, 

 a considerable modification of the current view that most mutations are 

 single-hit phenomena would be required and perhaps also of the view, 

 proposed on pp. 505 50(5, that the concavity of the frequency-dose curve 

 of Aspergillus and of some other organisms at higher doses results from 

 the addition of the position effects of chromosome aberrations to the 

 single-hit gene-mutation curve. A further investigation of this matter, 

 in organisms in which the mutations can be analyzed genetically or cyto- 

 logically, becomes of fvmdamental importance. 



