SENSITIVITY OF DROSOPHII.A CiKRM C:KI.I. STACKS TO X IRRADIATION 



In view of this consideration il may \ery well be that some other factor(s) 

 is operating" during the spermatid stage causing it to be more susceptible to 

 X-rays. It has been suggested ^^'^^, on the basis of electron-microscope 

 studies by Ris*^ which seem to indicate that the chromosomes during the 

 speiniatid stages show a decrease in the amount of the ])rotein surrounding 

 them, that the lack of this 'protective' protein coat increases the radio- 

 sensitivity of the nucleic acid of spermatid chromosomes. In addition, 

 histochemical studies have indicated that spermatids have mainly histone- 

 type proteins, while in spermatozoa the histones have been replaced by 

 protamines'^. These latter compounds, by virtue of their greater number of 

 basic NH2 groups which can neutralize some of the mutagenic radicals 

 produced by X-rays, could afford protection for spermatozoa during 

 irradiation thereby making them less sensitive to X-rays than spermatids. 



As a matter of fact this phenomenon may also be involved in the extreme 

 radio-sensitivity of the early cleavage stages in that during such a period of 

 extremely rapid duplication of the genetic material (i.e. nucleic acid), there 

 is insufficient time for the building up of the normal cjuantities of the protein 

 components of the chromosomes, hence the genes are more likely to be 

 injured by X irradiation. Further histochemical and electron-microscope 

 work will be needed to verify this suggestion. 



Effect of variations of other conditions on the radio-sensitivity of the male germ line 



Abrahamson and Telfer^' had shown that when spermatozoa obtained 

 under a variety of conditions were irradiated in the seminal receptacles and 

 spermathecae of females no significant differences in sex-linked lethal 

 mutation and translocation rates among the different series were obtained. 

 These experiments included sperm which had been aged in the female prior 

 to irradiation, sperm not aged, sperm aged following irradiation, sperm 

 from aged males, and sperm from non-aged males. Although it is con- 

 ceivable that the age of the sperm might have an influence on its sensitivity 

 to X-rays, Abrahamson's results are not altogether surprising in view of the 

 fact that ageing of sperm in Drosophila per se has no effect on its viability, 

 thus indicating its relative stability. On the other hand, Dubovsky^^ and 

 Stromnaes'^ had claimed that sperm from different wild-type stocks often 

 yielded different rates of sex-linked and dominant lethals, respectively, 

 following an exposure to X-rays. Such results can be interpreted on the 

 basis of either a heterogeneity of germ cells sampled caused by different 

 degrees of sperm utilization amongst the different stocks or the pi-e-existence 

 of different frecjuencies of detrimentals amongst the different stocks. The 

 latter together with induced detrimentals would act synergistically giving 

 rise to completely lethal chromosomes with different frequencies amongst 

 the stocks. That either or both of these interpretations are probably correct 

 was borne out by our results which indicated that sperm obtained from males 

 of different origin gave sensibly equal frequencies of autosomal translocations 

 when they were irradiated in the female. These results are shown in Table 4. 

 Translocations were used to demonstrate this since their rate of spontaneous 

 origin is negligible, hence induced frequencies are not easily confused with 

 variable numbers of pre-existing cases such as may exist for lethals amongst 

 different stocks. 



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