HOWARD B. NEWCOMBE 



The proportion of induced mutations was observed to increase to more 

 than double when the spores had become binucleate at the time of irradia- 

 tion (from 22 up to 79 per cent for X-rays, and from 19 up to 46 per cent 

 for ultraviolet), and to decline to 10 per cent or less as multinucleate strands 

 developed. As the two curves are based on approximately 6,000 mutants 

 scored, out of a total of approximately 30,000 colonies observed, they are 

 relatively accurate. Also, this general relationship between mutability and 

 number of nuclei has been found consistently in other similar experiments. 

 It should be noted also that when the cells are most sensitive to the mutagenic 

 effects of the irradiation, they tend in addition to be most sensitive to the 

 lethal effects. 



Table III. — Sensitivity to X-ray induced mutation 

 Effect of pre-incubation in the absence of nutrient 

 (Spores han'ested in saline ; diluted 1/10- in saline before incubation. 



from two experiments. Dose, 8,000 r) 



Combined data 



Growth during pre-incubation : at 3 days, out of 68 spores examined, 48 were still uni- 

 nucleate, 2 binucleate, and 22 were very much shrunken and appeared dead. 



Unirradiated control : 0-5 percent mutation (18/3,439) ; no apparent difference between 

 hour and 9-1 1 day incubation. 



Sur\ival : in unirradiated controls, 40-50 per cent after 9-11 days' incubation; in 

 irradiated samples, between 0-06 and 0-18 per cent throughout. 



The frequency of induced mutation eventually declines to 9-4 per cent (232/2,479) when 

 irradiated after 9-1 1 days' incubation. 



Certain similarities in the actions of X-rays and ultraviolet should be 

 noted. The peak sensitivity coincides in both cases with the binucleate 

 state ; the extent of the increase is similar ; and so also is the extent of 

 the subsequent decline in sensitivity. In addition there are minor irregu- 

 larities so that certain of the points do not fall precisely on a smooth curve ; 

 these are probably significant, and they deviate in the same directions for 

 the two agents. The X-ray and ultraviolet survivals are also similar in 

 their major features ; that is, sensitivity is high during the binucleate stage, 

 and strikingly low when many nuclei are present. 



These variations in response could be due directly to the observed nuclear 

 changes, or alternatively, the association could be fortuitous, and the result 

 of some loosely associated physiological changes. However, our early 

 attempts to separate the two failed. It was found that spores incubated in 

 saline which lacked nutrient, showed neither the nuclear divisions nor 



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