IRWIN I. OSTER 



a separation of broken chromosomes favouring the disarrangement of 

 pieces during spermiogenesis. If this had been the case, it would have been 

 expected that the recovered rate of recessive lethals from irradiated spermatids 

 would have been lower in the case of a ring-shaped chromosome due to the 

 loss by t\vistcd restitutions, of those mutations associated with structural 

 changes by the shuffling of the chromosomes during the elongation of the 

 spermatid to form the spermatozoon". We shall reconsider the probable 

 mechanism (s) involved in the differential sensitivity of the various stages of 

 spermiogenesis in the section on the effects of variation of oxygen tension on 

 the radio-sensitivity of the male and female germ lines. 



Radio-sensitivity of the female germ Ime 



As was mentioned previously, although it was already noted as early as 

 1928^'^' ^*'' ^^ that in the female of Drosophila melanogaster mature germ cells 

 are more sensitive to X-rays than immature germ cells, King's extensive 

 histological studies of oogenesis, which indicated that the development of an 

 egg in an ovariole can be subdivided into at least 14 distinct stages, raised the 

 question whether these diverse stages also exhibit different sensitivities^^. 

 Soon afterwards he showed that susceptibility to radiation-induced chromo- 

 some breakage as measured by dominant lethality, X chromosome loss, and 

 induced recessive lethal mutations does indeed vary strikingly during 

 oogenesis''^. In view of this it was decided to try to obtain additional 

 cjuantitative data on the sensitivity of three stages of oogenesis which differ 

 markedly from each other with regard to their morphological characteristics. 

 Thes6 included oogonia, stage 7 oocytes which show a Feulgen-positive 

 nucleolus or karyosome lying in a reticulate nucleoplasm surrounded by a 

 nuclear membrane, and stage 14 oocytes which show a Feulgen-positive 

 karyosphere lying free in the ooplasm, the nuclear membrane having dis- 

 appeared. The chromosomes of stage 14 oocytes are in first meioticmetaphase 

 and hence are in a condensed condition'^. In addition, since it had been 

 shown by Muller in 1946^^ and later by Hildreth and Carson^^ and Schalet-* 

 that an inordinately high percentage of all spontaneous mutations occur dur- 

 ing the early cleavage stages of the developing egg in Drosophila it was decided 

 to investigate the response of such early embryos to the mutagenic action of 

 X-rays. 



Non-treated females serving as controls yielded the offspring for the 



Table 2. Results of X irradiation of different cell stages in the female Drosophila melanogaster 

 Cell stage Dose Per cent lethal mutations 



Untreated female germ cells — 49/ 1 1 ,630 =0-4 



Early cleavage stages 800 r 2 1 /345 =6-1 



Oogonia 2400 r 33/2,056 =1-6 



Stage 7 oocytes 1600 r 28/1056 =2-7 



Stage 14 oocytes 1600 r 8/149 =5-4 



determination of the spontaneous mutation rate of our stocks. Table 2 gives 

 the results obtained by us. 



Thus in the female germ line, the chromosomes are most susceptible to the 



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