MECHANISMS OF MUTATION PRODUCTION IN MICRO-ORGANISMS 



exposure caused almost no additional killing, the proportion of mutants still 

 failed to rise and in fact declined appreciably. 



Differential killing of those cells which would otherwise have developed 

 into mutant colonies can be ruled out with an even greater degree of 

 certainty, as the explanation of a similar non-linear response found in a 

 different type of cell. As a part of the above experiment, spore suspensions 

 were also incubated without any preliminary irradiation, and were then 



Table I. — Effect of fractionation on the consequences of doubling the dose of X-rays 



A non-linear mutagenic response to the second irradiation, in the absence of altered survival 



(Dose : x — 8000 r. Incubation in medium) 



Growth of 'x-inc' suspensions : 4 hours, no germination, no nuclear division ; 6 hours, 

 7-8 per cent germination, 4 binucleate cells out of 380 ; 8 hours, 8-0 per cent germination, 

 4 binucleate cells out of 187. 



Controls : untreated, 9 mutation colonies out of 814=1 • 1 per cent ; o-inc-o with 4 hours 

 incubation 45/1,225 = 3-7 per cent, with 6 hours 24/732 = 3-3 per cent, and with 8 hours 

 16/541 = 3-0 percent 



Note : All incubations carried out in closed metal boxes with little or no air-space above 

 suspension. 



exposed to the single and the double dose of X-rays {see Table II). Where 

 this incubation was for an eight-hour period, the spores grew into short 

 mycelial strands containing an average of about eight nuclei apiece. These 

 strands, as might be expected, were very resistant to the lethal action of 

 X-rays, the single and the double exposures both resulting in a net survival 

 of 50 per cent. The proportion of cells which developed into mutant 

 colonies was likewise similar at the two doses (34 and 39 per cent respectively). 

 We cannot in this case suppose that the second exposure induced as many 

 mutations as the first fbut killed a disproportionate number of the mutant 



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