264 CORNELIUS A. TOBIAS, TOR BRUSTAD AND THOMAS MANNEY 



SURVEY OF RADIATION EFFECTS ON CELL DIVISION INHIBITION 



ON *'. CEREVISIAE 



Now I proj)Ose to describe briefly to you the present state of experi- 

 ments and theories concerning radiation effects on the yeast cells, 

 Saccharomyces cerevisiae. There are 10 strains of interest, ranging from 

 haploid to hexaploid. Due to work by Latarjet and EjDhrussi, Zirkle, 

 Mortimer and myself, we now have the following general picture of the 

 radiobiological events (See Zirkle and Tobias, 1953; Mortimer and 

 Tobias, 1953; Mortimer, 1955; Birge and Tobias, 1954; Tobias et al., 

 1959). 



Inhibition of colony formation occurs by one of two major pathways: 

 recessive or dominant lethal mutation. 



In a diploid cell a homozygous pair of chromosome defects will cause 

 inhibition of colony formation, whereas heterozygous, unpaired defects 

 will not. This mode of effect can be directly demonstrated by sporulat- 

 ing the radiated diploid cells and discovering haploid S23ores with 

 lethals in them. 



The dominant lethal defect of the chromosomes is, by definition, one 

 where a heterozygous lethal in one of a pair of chromosomes only, leads 

 to inhibition of cell division. This effect is demonstrated by mating an 

 irradiated haploid with an unirradiated one and demonstrating that 

 some cases the diploid zygote is not viable. 



The survival curves in presence of environmental modifiers of haploid 

 and higher ploicly cells exposed to X-rays can be mathematically 

 accounted for by the above mentioned two models for radiation effect. 

 However, it was found that the post-irradiation physical environment 

 of the cells and their physiological state modify the survival function. 

 The major findings to be accounted for are: 



1 . Budding haploid cells show a tenfold increase in radioresistance 

 and multi-hit survival curves. 



2. Post-irradiation temperature treatment and changes in the com- 

 position of the nutrient medium modify the diploid macro-colony 

 survival but not that of haploid cells (Korogodin et at., 1959; see 

 also presentation by Tarusov at the jiresent conference). 



3. Phenotypic appearance of mutations and production of lethals 

 continue for at least a few generations following irradiation. In 

 this period recovery from at least some of the genetic effects occurs. 



In attempting to give a model for radiation action we should also 

 discuss the types of genetic change that occur in irradiated yeast cells. 

 The following major classes of genetic change have been observed: 



a. Deletions and productions of auxotrophic mutants ; these are fre- 

 quently recessive; 



