240 H. J. MULLER 



embryo, has been sbown for Drosophila in experiments initiated by Oster 

 (Oster and Cicak, 1958; Oster, 1959a, b, 1960, 1961b) at Indiana University. 

 They have been carried further by Ostertag (Ostertag and Muller, 1959; 

 Muller, 1960, 1961; Ostertag 1961, Ph.D. thesis and in press), and more 

 recently by Meyer, in conjunction with the present writer. Oster and Cicak's 

 finding that irradiation of larvae increased the pupal mortality of the males 

 more than that of the females, was followed by the discovery that this was 

 not basically a sex difference since females with attached X-chromosomes also 

 have a high mortality, as expected on the chromosome-loss interpretation. 

 Equally striking was the exceptionally high induced mortality of males with 

 rino'-shaped X-chromosomes. For this result is in harmony with the fact that 

 even when rings are restituted the frequent cases where the chromosome thread 

 is subject to an axial twist of one or more turns before union of its broken ends 

 occurs would result in interlocked chromatids, forming an effectively di- 

 centric chromatin structure which failed to reach the daughter-nuclei. 



In mice the X-chromosome forms a much smaller portion of the total 

 chromatin than in Drosophila. Correspondingly, Lindop and Rotblat (1961) 

 have found that the difference in X-ray-induced life-shortening between the 

 sexes is not statistically significant, but that there is an mdication of the 

 male's life-span being reduced more. 



Ostertag showed that the increased mortality caused by irradiation of 

 Drosophila larvae affected not only the pupae but continued unabated 

 throughout life (that is, over some 3 months), thus constituting a type of 

 premature ageing. Moreover, if one member of any major pair of homologous 

 chromosomes (X, 2nd or 3rd) contains a small deficiency, the mortality is 

 raised as though only the normal member had been present to begin with. 

 Thus, females with a deficiency in one of their X-chromosomes are as much 

 damaged as males, and females with a deficiency in a large autosome are 

 much more damaged, since the autosome, being longer, is correspondingly 

 more subject to breakage and loss. Moreover, as expected, females with one 

 ring X-chromosome and a deficiency in their other, non-ring X-chromosome, 

 are affected as much as males with a ring. 



Among other things, this finding shows that in this material (unlike the 

 zygote or "the individual as a whole) one member of any pair of chromosomes 

 is enough for cell viability. Secondly, the result shows that it is not the 

 chromatin bridge but the chromatin loss that kills these cells. The qualifica- 

 tion should be made, however, that special types of Drosophila chromosomes 

 namely, those with what are called strong centromeres— and the chromo- 

 somes in special types of cells— namely, those in early embryonic stages— do 

 give evidence of forming lethal bridges, as has also been foimd for the early 

 embryos of Hnhrohrocon by Clark and Mitchell (1952). Thirdly, and most 

 important of all, it shows that it is not the induction of pomt mutations or 



