246 



FACTORS INFLUENCING CELL RADIOSENSITIVITY 



give two-hit curves, independently of the ionization density (Fig. 3). 

 As the yeast used in this experiment was diploid, one could wonder 

 whether the involvement of two units might not be linked to diploidy. 

 To answer this question, irradiations with x-rays were carried out com- 

 paratively on diploid strains and on haploid lines derived from them by 



LAg 

 4.15A 



20 30 

 erg/mm^ 



5 10 15 



a/mm^ x 10"* 



Fig. 3. Survival curves of diploid yeasts after irradiation with several monochro- 

 matic radiations (KCu, LAg, KAl) and alpha rays from polonium. All are two-hit 

 curves. [Frilley and Latarjet (7).] 



Ephrussi (16). The lesion used as a criterion was the inability to 

 multiply indefinitely (immediate death + delayed death). The results 

 (Fig. 4) show that each diploid line gives a classical two-hit sigmoid 

 survival curve, whereas the haploid lines give one-hit curves and, at 

 the same time, show much greater sensitivity than the diploids. This 

 phenomenon cannot yet be interpreted in terms of a precise mechanism, 

 but there is an undeniable influence of the number of repUcations of 

 each chromosome.* 



* Most of our results have recently been confirmed by Magni in Italy and by 

 Tobias and Zirkle in America (cf. Tobias' paper). 



