552 RADIATION BIOLOGY 



chemical mutagenesis and radiation mutagenesis can no longer be sepa- 

 rated but are becoming interlocked into one closely knit subject of inquiry, 

 even though the mode of connection of the parts is only beginning to be 

 understood. From this point of view, review of the knowledge of chem- 

 ical mutagenesis would be in order here, if more were known about the 

 processes actually at work. In the absence of this knowledge, it is suffi- 

 cient to call attention to the fact that oxidants and enhancers of oxidation 

 are conspicuous among the substances which have been reported to have 

 a mutagenic effect. This was in fact pointed out some years ago by 

 Rapoport (1943), who even at that time took the position that the 

 mutagenic action of radiation is connected with its oxidizing effects. On 

 the other hand, it must be borne in mind that, in general, oxidations are 

 under rather rigorous cellular control by varied and elaborate protective 

 and regulatory mechanisms. Thus it is not surprising that, in the experi- 

 ments of Wyss, Stone, and Clark (1947), various agents other than 

 peroxides, which promote oxidation in the more general sense, such as 

 chlorine, iodine, potassium permanganate, or oxygen itself, failed to 

 cause the medium to which they were applied to become mutagenic to 

 Staphylococcus, and in this respect differed from hydrogen peroxide. 

 Again, salts of heavy metals, although supposed to attack sulfhydryl- 

 containing compounds, proved unable, in the experiments of Clark, 

 Wyss, and Stone (1950), to take the place of menadione in mutagenesis. 



14. INFLUENCE OF BIOLOGICALLY DETERMINED CONDITIONS 

 ON HIGH-ENERGY MUTAGENESIS 



In view of the great variability in the effectiveness of ultraviolet, as 

 well as of given chemicals, in causing mutations it might be supposed 

 that mutagenesis by ionizing radiation also would be subject to modifi- 

 cation. Although advocates of the target hypothesis have attempted to 

 deny or belittle such effects, some of them, based on biologically deter- 

 mined differences, have been known since the earliest years of the study 

 of X-ray mutagenesis. In this connection it may be recalled (see Chap. 

 7) that both lethal and visible point mutations were found to be produced 

 by X rays at a considerably higher rate in spermatozoa of Drosophila than 

 in immature female germ cells (Muller, 1928b, 1930; Muller, Valencia, 

 and Valencia, 1949) or spermatogonia (Muller, 1930), and at eight times 

 the rate in soaked sprouting seeds of barley as in dry dormant ones 

 (Stadler, 1928c). 



Again, Drosophila spermatozoa, in various stages of maturity, have 

 very different induced-mutation rates, at least in so far as chromosome 

 breakages are concerned (Luning, 1952a, b, d), and, in regard to recessive 

 lethals, spermatozoa from very old males have them produced by X 

 rays at a much higher rate than those from young males (Offermann, 



