THE PRODUCTION OF CHROMOSOME BREAKAGE BY IONIZING RADIATIONS 



is a compensatory one, for the total frequency of deletions does not vary 

 appreciably with quality of radiation. In view of the results of Kirby- 

 Smith and Daniels^" (1953), who have demonstrated a decrease in total 

 frequency with decreasing ion density of the radiation, these findings are 

 somewhat surprising. However, since four rather extensive and essentially 

 identical experiments have yielded almost identical results, it appears that 

 the compensatory shift in type of deletion is a real one. 



The relation of chromatid exchanges to quality of radiation in air is 

 similar to that found for isochromatid deletions, and thus agrees with the 

 earlier data of Kirby-Smith and Daniels. 



Irradiation vmder anoxia yielded isochromatid deletions which, although 

 greatly reduced in frequency, appeared to show no relation to quality of 

 radiation. This, of course, leads to air/nitrogen ratios which are large with 

 50-kvp X-rays, and which become progressively smaller as the ion density 

 of the radiation decreases. The chromatid deletions, on the other hand, 

 now exhibit an opposite trend to that found after irradiation in air. They 

 increase rather than deciease as the ion density of the radiation increases, 

 and this leads to air /nitrogen ratios which are largest in value with gamma 

 rays and smallest with 50-kvp X-rays. With 50- and 100-kvp X-rays, 

 there is, in fact, an actual increase in the absolute frequency of chromatid 

 deletions obtained in nitrogen as compared to air, and the air/nitrogen 

 ratios, as a consequence, have values less than 1 . The chromatid exchanges, 

 like the isochromatid deletions, are much reduced in nitrogen, but the air/ 

 nitrogen ratios follow a trend comparable to that for chromatid deletions, 

 as might be expected. 



As pointed out in an earlier account of this work (Swanson^), it is believed 

 that the differential air /nitrogen ratios for the several types of chromatid 

 aberrations can be accounted for by assuming that there is a shift, in nitrogen, 

 of one type of aberration into another. Thus, if we consider that the forma- 

 tion of isochromatid deletions and exchanges involves two broken chro- 

 matids, the repair or restitution of one of these but not of the other would 

 essentially transform potential isochromatid deletions and exchanges into 

 chromatid deletions. It is diflficult to account in any other way for the 

 increases in chromatid deletions at 50 and lOOkvp in nitrogen. 



If we now convert the aberrations obtained in air and in nitrogen into 

 terms of total breakage, i.e. by scoring chromatid and isochromatid deletions 

 as single events, and exchanges as resulting from two independent breaks — • 

 it can be demonstrated that the inverse relation of ion density of the radia- 

 tion to magnitude of the oxygen effect holds very well {Table II), as Thoday 

 and Read' (1949) and Giles, Beatty and Riley^ (1952) pointed out 

 earlier. 



An interpretation 



Two hypotheses have been advanced to explain the reduction in frequency 

 of aberrations obtained when irradiation is carried out under conditions of 

 anoxia. Giles and his co-workers^ proposed a hypothesis in which the 

 principal effect of oxygen during irradiation is considered to be on the 

 breakage mechanism, but they do not specify in detail how this is accom- 

 plished. Schwartz ^^ (1952) considers that oxygen affects the processes 



256 



