MECHANISMS OF MUTATION PRODUCTION IN MICRO-ORGANISMS 



the characteristic (X-ray) sensitivity changes, over a 48-hour period (see 

 Table III). The test was carried out using a stock saline suspension of 

 spores (which contained traces of nutrient leached from the agar on which 

 the spores had been grown) and diluting by a factor of 1 /1 00 with pure saline 

 to reduce the amount of nutrient. When a parallel experiment was carried 

 out without dilution, a proportion of the spores (between a quarter and a 

 half of them) were able to germinate and grow into short multinucleate 

 strands, while the rest remained uninucleate. Under these conditions the 

 characteristic rise and decline in sensitivity to X-rays could be observed, 

 but was less striking than under the standard conditions {see Table IV). 



It seemed important at this stage to determine whether, as we supposed, 

 the sensitivity changes were pecuHar to the growing spores, and absent in 



Table IV. — Sensitivity to X-ray induced mutation 



Effect of pre-incubation in the presence of traces of nutrient 



(Spores harvested from nutrient agar culture by washing with saline ; incubated undiluted. 



Combined data from two experiments. Dose, 8,000 r) 



Growth during pre-incubation : at 1 day 20-24 per cent germination, average approxi- 

 mately 10 nuclei per germinated spore ; at 2 days, no detectable change. 



Unirradiated control : 0-4 per cent mutation (3/857). 



Survival: 0-07-0 -27 in one experiment; 0-02-0 -04 in the other. (There was no 

 appreciable difference between the two experiments with respect to mutation.) 



The frequency of induced mutation remains at a low level, 9-9 per cent (325/3285), when 

 irradiated after 6-14 days' incubation. 



those which remained uninucleate. To confirm this, a suspension which 

 had been incubated in this manner for 28 hours, and which had become 

 relatively insensitive to the mutagenic effects of X-rays, was passed through 

 very fine filter paper (Watman No, 3) to remove the mycelial strands. 

 This filtrate, which contained mainly the ungerminated spores, showed a 

 threefold increase in sensitivity to X-ray induced mutation {see Table V). 



A similar increase would also be expected as the result of filtering the 

 suspension after it had been irradiated, and this was in fact observed {see 

 Table V). Thus the eflfect could not be due to some subtle change in the 

 condition of the suspension at the time of irradiation, and it is clear that the 

 greater part of the sensitivity changes observed during the incubation must 

 be associated with the growing cells, and that they are not a function of 

 the suspension as a whole. 



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