chromosome aberrations in Tradescantia 



743 



ness of substances responsible for actual breakage (to be discussed in detail 

 later). If, for example, the "half life" of such mutagenic substances is 

 increased at low temperature, this might permit these substances to 

 increase their relative spheres of effectiveness, within the nucleus, in 

 aberration production at low temperatures. 



Additional evidence regarding the temperature effect is provided by 

 experiments (Giles, Beatty, and Riley, 1951; and unpublished) per- 

 formed in the absence of oxygen (in helium). The relation between 



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 TEMPERATURE,°C 



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• ISOCHROMATID BREAKS ( IN HELIUM - 200 r) 



O ISOCHROMATID BREAKS ( IN OXYGEN- 150 f) 

 Fig. 10-8. Effect of temperature on yield of chromosome interchanges when X irradia- 

 tion is performed in 5 per cent oxygen (400 r at 50 r /minute), helium (900 r at 300 

 r /minute). (Data of Giles, Beatty, and Riley, unpublished.) 



temperature and chromosome aberration frequency under these circum- 

 stances is just the reverse of that found when oxygen is present, more 

 interchanges and deletions being present in helium at high than at low 

 temperatures. The same relationship holds for all types of chromatid 

 aberrations as well. Data for isochromatid aberrations are shown in 

 Fig. 10-8. How this result is to be interpreted is not yet clear. If the 

 action of X rays in the absence of oxygen is considered to be largely 

 direct, as some evidence indicates (Giles and Beatty, 1950), then the 

 temperature effect would presumably be on the recovery process, such 

 that restitution is favored at low and reunion at high temperatures. 

 However, if the production of aberrations in the absence of oxygen is 

 assumed to result largely from the production of OH radicals, which is 



