SENSITI\rrV OF l')ROSOPHILA HERM CELL STACF.S TO X IRRADIATION 



induction of lethal mutations by X-rays in early cleavage stages, less suscep- 

 tible in statue 14 oocytes, still less suscejitible in stage 7 oocytes, and even still 

 less suscejjtible in oogonia. These results indicate that the high radio- 

 sensiti\ity of the early mitotic stages parallels the high frecjuency with which 

 spontaneous changes occur in them, the induced rate during these early 

 cleavage stages being similar to that of the most sensitive stage hitherto 

 known, i.e. spermatids. Whether this is due to the fact that these cells have 

 one of the highest rates of division known (10 minutes per cycle as compared 

 to 20 minutes for bacteria and 5 minutes for some viruses) is not yet quite 

 certain. This is so because although it may very well be that mutations can 

 occur most readily either spontaneously or after treatment with a mutagen 

 during the process of chromosome synthesis by an error in gene duplication, 

 the fact that X-rays can induce whole-body mutations with an extremely high 

 frequency {i.e. in both strands of a single chromosome) after treatment of 

 spermatids and spermatozoa indicates that mistakes in gene-copying are not 

 solely responsible for the origination of mutations. In this connection it 

 should also be mentioned that unlike the evidence found for the chromo- 

 somes of some plants-^, increasing the oxygen tension above that present in 

 air did not enhance the spontaneous mutation rate during the highly mutable 

 early cleavage stages oi Drosophila in experiments carried out by us. 



Considering the germ cells in both sexes of Drosophila on the basis of their 

 induced rates of heritable changes we have the following order of decreasing 

 sensitivity to X-rays: (1) spermatids and early cleavage stages, (2) spermato- 

 zoa in the female, (3) spermatozoa released one day after treatment, sperma- 

 tozoa released two days after treatment, and stage 14 oocytes, (4) stage 7 

 oocytes, and (5) spermatogonia and oogonia. 



The data also indicate that the most sensitive stages, spermatids and early 

 cleavage stages, are 12 times more susceptible than the least sensitive stage, 

 oogonia, to having lethal mutations induced in them. The rates obtained 

 from spermatogonia have not been used for this comparison since the fre- 

 quency of lethals detected in these cells is even lower than their relative 

 degree of sensitivity would lead us to suspect. This is due to the fact that some 

 of the mutations produced in spermatogonia are weeded out during meiosis 

 {i.e. by germinal selection) because the X chromosome in the male is only 

 present in one dose. Thus induced changes in it are not covered by their 

 unchanged counterparts in the homologous chromosome as occurs in the 

 female of Drosophila which has t\vo X chromosomes. 



Effects of variation of oxygen tension on the radio-sensitivity of the male and female 

 germ lines 



In view of the fact that the work of Read on growth inhibition of Viciafaba 

 roots, Giles and Beatty'^^ on chromosome interchanges in Tradescantia micro- 

 spores, HoUaender, Baker and Anderson" on sex-linked mutations and 

 translocations in Drosophila melanogaster and D. virilis, respectively, and 

 Baker and Edington-^ on translocations in Drosophila virilis, had given a 

 rather similar pattern of results when X-rays were applied in combination 

 with different oxygen tensions, it has been generally assumed that the amount 

 of X-ray induced damage rises steeply as the oxygen supply is increased from 

 per cent to 2 1 per cent of an atmosphere {i.e. the amount present in air) 



260 



