RADIATION STUDIES ON FUNGI 



445 



Tablk 11-7. Types ok Survival Curves Obtained with X Rays for a 



Few Fuxfii 



2 



reported for yeast exposed to X rays have been sigmoidal with diploid 

 S. cerevisiae (Wyckoff and Luyet, 1931 ; Latarjet and Ephrussi, 1949) but 

 exponential with a haploid strain (Fig. 

 11-6). Independent confirmatory re- 

 sults have also been obtained, showing 

 that, with ionizing radiations, a hap- 

 loid yeast is killed logarithmically, a 

 diploid sigmoidally, and a tetraploid 

 also sigmoidally (Pomper, unpub- 

 lished; Tobias, 1952; Lucke and 

 Sarachek, 1953). An exponential 

 survival curve has been reported for 

 presumably diploid S. cerevisiae ex- 

 posed to X rays (Henshaw and Turko- 

 witz, 1940), and here a genetic analy- 

 sis of the presumed diploid would be a 

 helpful datum in appraisal. Tobias 

 (1952) has reported that certain 

 stocks obtained from originally diploid 

 cells surviving X irradiation were 

 found to have survival curves differ- 

 ing from those of their diploid pro- 

 genitors in being more nearly expo- 

 nential. One is tempted to interpret 

 this finding as indicative of the loss of a large portion of the genetic appa 



8 16 24 



DOSE, rx 10^ 



Fig. 11-6. Survival of haploid and 

 diploid S. cerevisiae exposed to X rays. 

 The inactivation of yeast cells by 

 X rays. Curve I, haploid cells; curve 

 II, diploid cells. (The data, from 

 Latarjet and Ephrussi, 1949, were 

 adapted by Norman.) 



