HOWARD B. NEWCOMBE 



There seems to be an almost limitless variety of these subsequent changes, 

 occurring apparently in a stepwise manner ; most of these are associated 

 with still further instabilities, and so on indefinitely. The result is an 

 apparently limitless variety of mutant types which it would be very difficult 

 to account for in terms of a cytoplasmic origin. For this reason it seems 

 altogether probable that the initial changes are either genie or gross chro- 

 mosomal in nature, and it is quite possible that both kinds of change occur. 

 It should be noted that even the unstable variant lines can be subcultured 

 almost indefinitely without apparent change, provided one selects at each 

 subculture a colony of a certain appearance ; thus the alterations are clearly 

 heritable. 



DOSE-RESPONSE RELATIONSHIPS 



Our initial experiments confirmed Kelner's finding ; the X-ray dose-muta- 

 tion curve reached a peak in the vicinity of 8,000 r, and the proportion of 

 mutants fell to approximately one-half when this dose was doubled. A 

 similar peak was found for the corresponding ultraviolet curve at about 

 400 ergs per mm 2. Further, it appeared that the non-linearities must have 

 a common cause, since the mutagenic effects of ultraviolet and ionizing 

 radiation were less than additive in combined treatments (Newcombe and 

 McGregor^). 



fractionation experiments 



When it is found that a certain dose of radiation causes some change in the 

 sensitivity to a second and similar exposure, it is of interest to know whether 

 this sensitivity change is permanent or whether, when the second exposure 

 is sufficiently delayed, there is some recovery of the original capacitv to 

 respond so that the effects of the two exposures become more nearly additive. 

 Fractionation experiments of this nature were carried out, the spores being 

 incubated in medium for as long as eight hours between two exposures of 

 8,000 r each. No appreciable change was found in the mutational response 

 to the second irradiation {see Table I). 



When spores which had received the first exposure and had been incu- 

 bated for the eight hours were stained, about 2 per cent of them were 

 found to have undergone a single nuclear division. Also, when the suspen- 

 sions were plated at this stage about 2 per cent of the spores grew into 

 visible colonies. Since these proportions are very nearly the same, it seems 

 likely that the spores in which nuclear division was observed were in fact 

 the survivors. If this was the case it would appear that the capacity to 

 respond to the mutagenic effect of a second exposure must remain impaired 

 even after a nuclear division has intervened. 



A further inference, likewise relevant to the nature of the non-linear 

 response relationship, can be drawn from this experiment. It will be noted 

 that with an eight-hour incubation between the two irradiations, there was 

 almost no additional killing from the second exposure. (Two per cent of 

 the spores survived the first irradiation, while 95 per cent of these survived 

 the second.) It has been suggested that the plateau or decline in proportion 

 of mutants with increasing dose could be due to differential killing of the 

 induced mutant cells. However, in this experiment, where the second 



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