CHROMOSOME ABERRATIONS IN ANIMALS 685 



The marked reduction in frequency after 12 to 15 days has been 

 attributed to the fact that in actively copulating males the spermatozoa 

 that were mature at the time of irradiation are used up, and are replaced 

 by cells that then existed as spermatocytes or spermatogonia. Accord- 

 ing to this explanation, these cells are subject to germinal selection; that 

 is, they are eliminated from the germ line if they carry aberrations 

 that prove cell-lethal, and as a result the spermatozoa maturing after 

 irradiation are for the most part free of chromosomal rearrangements. 

 Although this theory may seem adequate to account for the decline in 

 frequency of aberrations, it does not take into consideration the fact that 

 the viable types of exchanges such as translocations and inversions, which 

 are perpetuated through cleavage and embryonic mitoses if induced in 

 mature spermatozoa, are eliminated if induced in spermatogonia or 

 spermatocytes. Histological examination of irradiated testes of Dro- 

 sophila showed some necrotic apical cells to be present, but no necrosis 

 of spermatogonia, spermatocytes, or spermatids was seen (Pontecorvo, 

 1944). On the basis of this observation the conclusion was reached that 

 no germinal selection takes place. Lea (1947b) suggested, however, that 

 germinal selection operates among spermatogonia. He attributed the 

 decline in dominant lethals in later matings to the elimination of affected 

 germ cells in spermatogonial stages. This explanation implied that 

 mature and immature germ cells are equally sensitive with regard to the 

 induction of the dominant-lethal type of aberration, and Lea stated that 

 there is little basis for assuming that chromosomes in spermatogonia are 

 less sensitive to lethal changes than those in the sperm. As regards the 

 induction of viable types of chromosome change, on the other hand, this 

 author concluded that "spermatogonia are much less sensitive than 

 mature sperm" (Lea, 1947b). 



Because of this series of conflicting interpretations, the problem of 

 germinal selection and relative sensitivity of different stages in the male 

 germ line of Drosophila requires further consideration. Clarification may 

 possibly be obtained through the study of chemical-induced rearrange- 

 ments, since it was shown by Kaufmann, Gay, and Rothberg (1949) that 

 the frequency of translocations induced by nitrogen mustard treatment 

 of Drosophila males is higher among spermatozoa transferred 13-18 days 

 after treatment than among those utilized in earlier matings. 



Differences in radiation sensitivity of cells of the testis of the grass- 

 hopper, Decticus, were reported by Cocchi and Uggeri (1944). Treat- 

 ment of males with moderate doses of X rays caused chromosomal damage 

 that could be detected after a few days, whereas high doses were required 

 to effect more rapid necrosis. A dose of 25 r affected cells in telophases 

 of the last spermatogonial division, the damage being detected by pyc- 

 nosis of these cells or of the primary spermatocytes into which they were 

 transformed. A dose of 100 r affected all secondary spermatogonia. 



