574 RADIATION BIOLOGY 



if only they are grown (as they were) in a medium which is "complete" 

 with regard to growth factors, and that they may even, at least under 

 certain circumstances, have some advantage over normals. As was noted 

 in connection with the findings of Swanson, McElroy, and Miller (1949) 

 on ultraviolet mutations (p. 547), biochemical mutants differed from 

 morphological ones in not falling off in their frequency with more extreme 

 treatments, and this was itself an indication that there was stronger selec- 

 tion at work against the morphological than against the biochemical 

 groups of mutants. On the other hand, it would be very hazardous to 

 postulate that the actual process of mutation, whereby mutants of the 

 two groups were produced, differed in a systematic way. It therefore 

 seems best at this time to consider the case for an influence of pressure 

 on induced mutagenesis as still unproved. 



The most readily understandable physical influence on the frequency 

 of mutations induced by high-energy radiation is that exerted by impreg- 

 nation with heavy elements which increase the amount of radiant energy 

 absorbed, thus raising the effective dose. The first experiments of this 

 kind were reported by Stadler (1928b), who found a significant rise in 

 the point-mutation rate induced by X rays in barley seeds when these 

 had been impregnated with salts of barium, lead, or uranium. Similarly, 

 Medvedev (1933, 1935a) was able to increase the frequency of lethals 

 induced by X rays in Drosophila spermatozoa by either feeding or inject- 

 ing lead acetate. That this increase in lethal-mutation rate was really 

 caused by the absorption of X-ray photons by the lead atoms present 

 was shown by Buehmann and Zimmer (1941), through their finding that 

 the effect varied with the wsive length of the radiation in the way expected 

 for absorption by lead, whereas this variation is not found in unimpreg- 

 nated material. The same finding was made by Schutze (1943) for flies 

 impregnated with an arsenic compound. 



17. OTHER CHEMICAL INFLUENCES ON HIGH-ENERGY MUTAGENESIS 



In contrast to the experiments cited above, which showed the purely 

 physical influence of heavy elements in increasing the frequency of muta- 

 tions produced by a given dose of ionizing radiation, it was found, in 

 experiments of Buehmann and co-workers (Buehmann and Hoth, 1937; 

 Buehmann and Sydow, 1940), that iron and uranium compounds fed to 

 Drosophila gave a much greater increase than could be accounted for by 

 the absorption of photons by their added metal atoms (as calculated by 

 Born and Zimmer, 1940). Moreover, the indirectness of the influence in 

 the case of iron was confirmed by its failure to vary with wave length 

 (Buehmann and Zimmer, 1941). Since iron salts in certain forms and 

 also ions of some other kinds are known to influence strongly the mode of 

 accumulation and the reactions of the active-oxygen-containing groups 



