MECHANISMS OF MUTATION PRODUCTION IN MICRO-ORGANISMS 



noted in Figure 1. In addition the effects of various treatments which might 

 influence the physiological state were tested using both multinucleate strands 

 and uninucleate spores. Three such tests will be described. 



In the first, spore suspensions containing 6-5x10' spores per ml of 

 medium were grown under the following sets of alternative conditions : 

 {a) with 5 ml of air, and with 20 ml of air, in a 30 ml tube ; {b) incubating 

 continuously for 32 hours, and incubating for four 8-hour periods with 

 storage in the cold after each ; and (c) sealing the tubes lightly with plastic 

 screw caps, and covering with loose aluminium caps ; these being applied 

 in all combinations to make a total of eight different treatments. The 

 amount of growth was very similar in all cases, the resulting strands being 



Table VI. — Effect on frequency of spontaneous mutant colonies of amount of air available 



during preliminary growth 



* C=continuous incubation (32 hours). 



I=intermittent incubation (4 eight-hour periods, at 24-hour intervals). 

 T=tube tightly capped (plastic screw cap). 

 L=tube loosely capped (aluminium cap). 



Inoculum 6-5 X 10' spores per ml of medium. 



Growth : CT5 and CL5, strands 4-10 diameters in length ; in other suspensions 6-14 

 diameters in length. Nuclei stained clearly where the incubation was intermittent ; where 

 the incubation was continuous, the nuclei were obscured by stainable material in the form 

 of much larger cylindrical bodies which nearly filled the strands. 



for the most part more than four times their own diameter in length, and 

 less than sixteen times. No difference was observed as the result of the 

 two different kinds of caps used, and these treatments can be considered as 

 replicates. 



Samples from all eight treatm.ents were plated without irradiation, and 

 irradiated with standard doses of X-rays and of ultraviolet. The greater 

 quantity of air resulted throughout in a considerably higher spontaneous 

 mutant frequency (5 to 12 per cent, as compared with 1 to 3 per cent ; 

 see Table VI) ; the numbers of ultraviolet induced mutants was not noticeably 

 affected, but the X-ray induced mutants were reduced to about one half. 

 Intermittent incubation on the other hand reduced the numbers of ultra- 

 violet induced changes to half, but had little effect on the X-ray changes. 

 There was also a consistent effect of intermittent incubation on the staining 



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