IRWIN I. OSTER 



but tends to level off after that. However, it should be pointed out that this 

 may not necessarily be a universal principle because not only was the work 

 mentioned above carried out on samples of cells heterogeneous with regard 

 to meiotic and mitotic stage but other work, such as that of Glass^^ on 

 Drosophila, Hornsey^" on chicken fibroblasts, and TrowelP^ on rat lympho- 

 cytes, had yielded ciuantitatively different results. It should be particularly 

 emphasized that the above cited work on Drosophila was carried out at a 

 time when the latest techniques were unavailable; thus account could not 

 be taken of those differences in the developmental stages of sperm treated 

 and subsequently analysed which have been shown to exert such a great 

 influence on the magnitude of the response elicited by different treatments^ 2. 

 This enhancement of X-ray mutagenesis by oxygen is presumably brought 

 about by the foi'mation of highly mutagenic radicals resulting from the 

 ionization of water molecules and their subsequent reactions with oxygen. 

 Reinvestigating this effect on homogeneous samples of germ cells we obtained 

 the results shown in Table 3. 



Table 3. Results of X irradiation of homogeneous samples of Drosophila melanogaster germ 



cells in different atmospheres 



Cell stage Dose Per cent lethal mutations in 



Nj Air O2 



127 105 91 



Spermatids (48-hour-old pupae) 1200 r 4421=2-9 1087^^'^ 788^^^'^ 



Spermatozoa (96-hour-old pupae) 1 440 r 009^ =3-0 noon =4-9 ocZc = ^ " ^ 

 Stage 7 oocytes 1600 r y^ =1-1 Yp)^ ^'^ 



These data demonstrate that the response of particular stages of germ cell 

 development to variations of the oxygen tension differs considerably. 

 Decreasing the oxygen tension below that present in air has a relatively greater 

 effect on the radio-sensitivity of spermatids as compared to spermatozoa in 

 the male and stage 7 oocytes, while increasing it above that present in air has 

 a relatively lesser effect on spermatids as compared with spermatozoa. 

 Spermatozoa X-rayed in inseminated females respond to variations in 

 oxygen tension in the same manner as spermatozoa in 96-hour-old pupae. 

 These findings originally led us to speculate that the high sensitivity of 

 spermatids to X-rays under normal aerobic conditions may be due to more 

 intracellular and/or intercellular oxygen being normally present (or avail- 

 able) in these cells^-^. On this interpi-etation decreasing their supply of 

 oxygen by placing them in nitrogen would considerably reduce their suscep- 

 tibility to X-ray induced damage while increasing their oxygen supply 

 would have comparatively little effect since a large amount is already present. 

 However, it was noted that this is probably not the only factor responsible 

 for the differential radio-sensitivity and that other mechanisms may also be 

 involved here since it was found that spermatozoa treated in pure oxygen 

 are not as sensitive to the same dose of X-rays as spermatids treated in air. 



18 261 



