INFLUENCE OF PLOIDY IN YEAST 



247 



An analogous situation has recently been observed by Atwood and 

 A. Norman (personal communication) in Neurospora, where mono- 

 nucleate cells give one-hit curves with ultraviolet rays, whereas multi- 

 nucleate cells show greater resistance and give multihit curves whose 

 multiplicity equals that of the nuclei. 



100 



24 X 1000 r 



Fig. 4. Survival curves of x-rayed yeasts of the same family. I, Haploids; II, di- 

 ploids. [Latarjet and Ephrussi (16).] 



Another difference, perhaps more important, appears between diploid 

 and haploid cells. Figures observed under the microscope in irradiated 

 haploids (Fig. 5) are limited to normal colonies arising from uninjured 

 cells; single cells, normal in size or enlarged (immediate death); and 

 pairs of enlarged cells (delayed death after one division). These figures 

 are definitive and do not change in the course of prolonged incubation. 

 Haploid yeasts do not recover; their lesions are irreversible. 



In the diploids, on the other hand, one sees, besides the preceding 

 figures, numerous recovery figures which develop in the course of incuba-* 



