Pathways of Radiobiological Damage 265 



The beautiful experiments of Latarjet and Ephrussi (1949) 

 with haploid and diploid yeast have become classical in this 

 connection. Within the last few years our knowledge of the 

 radiobiology of yeast has been greatly extended by the very 

 careful investigations of the Berkeley Group led by Tobias 

 (1952). Haploid yeast cells exposed to X-rays either form a 

 double, of which only one cell contains nuclear material, or 

 form a complete colony. Irradiated diploid cells give rise to 

 all intermediate forms between the double and the complete 

 colony. Correspondingly haploid cells which had not budded 

 at the time of irradiation show a strictly exponential relation 

 between survival fraction and dose, whether exposed to X- or 

 alpha-radiation, while diploid cells show a sigmoid relation 

 for both radiations, indicating that more than one particle is 

 responsible for the lethal damage. Budding cells, in which 

 mitosis is in progress, are much more resistant than the 

 interphase cells and even in haploid yeast do not show an 

 exponential relation between survival and dose. 



An elaborate analysis of the radiosensitivity of haploid, 

 diploid, triploid and tetraploid cells, and of the sensitivity of 

 clones grown from survivors from a previous irradiation, led 

 the authors at one time to conclude that the lethality was 

 entirely explicable in terms of recessive lethal mutations. 

 This proved to be too simple an interpretation. At present, 

 death of haploid cells is ascribed to the induction of recessive 

 lethal mutations — some of which could be chromosomal — 

 and that of other ploidies to a mixture of recessive and 

 dominant lethality. Contributions from cytoplasmic damage 

 cannot be excluded in the case of the higher ploidies. 



Finally, we may consider the very interesting experiments of 

 Atwood (1955, and personal communication), who studied the 

 radiosensitivity of binucleate and multinucleate Neurospora 

 conidia. Binucleate conidia were formed in which the two 

 nuclei carried, as markers, genes for nutritional deficiencies. 

 Such binucleate cells will grow on minimal medium. When 

 irradiated and grown on complete medium, sigmoid survival 

 curves were observed similar to those for diploid yeast, the 



