586 RADIATION BIOLOGY 



siderable selection between these isotopes of sulfur, they must have had 

 more total sulfur, to keep the S^^ constant. 



In the presence of varying amounts of total sulfur there are three con- 

 ceivable ways in which the ionizing radiation itself, from the same amount 

 of S^^, might act to give fewer mutations with more total sulfur. First, 

 as these investigators recognize, some of the quantitatively varying total 

 sulfur might be in combinations, such as cysteine and other group- 1 sub- 

 stances, in which it exerted a protective action against the mutagenic 

 effect of ionizing radiation. Hence, with less sulfur there would be less 

 protection. Second, the cultures with less sulfur were found to have 

 retarded growth and therefore were suffering from an actual dearth of 

 sulfur. Since it was shown by Doring and Stubbe (1938) that the dis- 

 proportional undernutrition of Antirrhinum, in respect to given elements 

 (one of which was sulfur), resulted in a higher susceptibility to the pro- 

 duction of mutations by externally applied ionizing radiation, this may 

 have been true in Neurospora also. This possibility as well as the first 

 was refuted by the finding that the mutagenic efficiency of externally 

 applied ionizing radiation was unimpaired by the addition of stable sulfur. 



Third, in the cells with less total sulfur but with the same amount of 

 S*^, there must have been a larger amount of S^^ in the genetic material 

 of the chromosomes (since this material presumably would vary little if 

 any in its total sulfur content). This greater concentration of S^^ in the 

 chromosomes (even though not in the cells) might result not only in more 

 transmutation within this material but also in its receiving somewhat 

 more ionizing radiation. For the jS rays given off in this case happen to 

 have rather short tracks, and so the number of these tracks traversing the 

 chromosomes might be significantly greater when more of the tracks 

 actually arose within these chromosomes. However, calculations have 

 shown that the difference thereby caused would be insignificant. Thus 

 it may be concluded, all in all, that the interpretation that the higher 

 mutagenesis in the cultures with less S^^ had been caused by transmuta- 

 tion is well founded. 



The question of the possible role of transmutation has also arisen in 

 connection with Powers' (1947) finding of the mutagenic effect of P^^ in 

 Paramecium. His results show that, for a given amount of ionization 

 induced in the composite mass of organisms plus medium, P^- is much 

 more efficient in producing mutations than are the radioactive isotopes of 

 strontium (Sr^^ and Sr^o) or yttrium (Y^o). This effect is in part 

 explained by the greater concentration of the element absorbed in the 

 cells, as compared with that left in the medium, in the case of phosphorus 

 than in the case of strontium or yttrium. However, Rubin (1948) has 

 stated that his calculations show the mutagenic effect to have been 

 greater for the P^- than could be explained solely on the basis of the 

 greater amount of activation and ionization produced in the cell by this 



