HOWARD B. NEWCOMBE 



division was suppressed so that few of the cells became binucleate, and a 

 negligible proportion had more than two nuclei. These limited divisions 

 might have been expected to increase the sensitivity to radiation induced 

 mutation and killing, but instead the net effect of the treatment was to reduce 

 both the ultraviolet and the X-ray mutability to about half, and to increase 

 survival by 2- to 3-fold for X-rays and by 8- to 1 0-fold for ultraviolet {see 

 Table X). The effects could not have been due to any differences in dis- 

 soK^ed oxygen at the time of irradiation since all suspensions were diluted 

 with the same chilled saline before being exposed to X-rays or ultraviolet. 



Table X. — Physiological control of sensitivity in the uninucleate state 

 Effect of bubbling with nitrogen during 24-hour incubation in medium 



Note : Some of the spores were clearly uninuncleate (8 out of 49) ; some were uninucleate 

 but the nuclei appeared 'dumb-bell' shaped as if incompletely divided (32/49) ; and a very 

 few were binucleate (6/49) ; three out of 49 had 3-8 nuclei. 



Thus it is clear that various changes in the physiological state of a cell 

 can influence the likelihood that it will produce a mutant colony as a result 

 of irradiation. In the case of irradiated multinucleate cells it is altogether 

 likely that mutant and non-mutant nuclei compete, and differences in the 

 physiological state at the time of irradiation might persist long enough to 

 influence the outcome of such internal competition. It is therefore difficult 

 to show conclusively, using multinucleate cells, that tlie physiological state 

 can influence the susceptibility of individual nuclei to induced mutation ; 

 although certain of the present results could be most simply interpreted on 

 this assumption. 



337 z 



