THE GENETIC BASIS OF X-RAY INDUCED 

 SOMATIC DAMAGE 



Irwin I. Oster* 

 Indiana University, "f Bloomington, Indiana, U.S.A. 



The exposure of individuals to ionizing radiation, Ijesides resulting in damage 

 to various organ systems such as the blood-forming tissues, the gonads, and 

 the gastro-intestinal tract, has been found to lead to an acceleration of the 

 so-called 'natural' processes of ageing^ Although it has been shown that 

 many of the former immediate effects of radiation can be traced back to 

 induced chromosomal aberrations, a genetic basis for shortening of the 

 life-span, if any does indeed exist, has not been demonstrated. A break- 

 through for an attack on this problem occurred when it was found that male 

 larvae o^ Drosophila melanogaster are more susceptible to killing by X-rays than 

 females-. As was pointed out, this difTerence in radio-sensitivity may have a 

 genetic basis in that males, having only one X chromosome and one relatively 

 inert (as considered from a genetic stand-point) Y chromosome, would 

 more often suffer the deleterious effects of loss of its one X chromosome 

 following its breakage by radiation than females, which have two X chromo- 

 somes. It should be pointed out, however, that chromosome breakage caused 

 by radiation can kill somatic cells and/or their descendent-cells by either one 

 or a combination of two main processes. One method is by the formation of 

 chromosome bridges by dicentric isochromatids resulting from the union of 

 centromere-bearing sister chromatid fragments. Such nuclei which are 

 joined together by a permanent chromosome bridge will subsequently give 

 rise to abnormal polypolar figures and giant cells. As was shown by Muller 

 and Pontecorvo in 1940^- *, this process appears to be the principal cause of 

 death in early Drosophila zygotes derived from irradiated spermatozoa. The 

 other process occurs when such dicentrics fail to form permanent bridges. 

 In this case the daughter nuclei which arise succeed in isolating themselves 

 while the parts of the broken chromosome are often left outside of them where 

 they degenerate. Such hypoploid cells are usually so deranged that they die; 

 when they are not so abnormal as to die they are likely to be seriously impaired 

 in their functioning. In diploid animals this derangement frequently results 

 from the hypoploid y/)fr se but the degree of impairment would be increased 

 in those cases in which the homologous member of the affected pair of chromo- 

 somes has inherited or has had induced in it a cell-damaging gene mutation. 

 In order to determine whether either one or both of these processes plays a 

 role in the long-term effects of X-rays, experiments involving the response to 

 X irradiation of individuals differing in their chromosomal constitution were 

 undertaken. 



* Present address: Division of Chemotherapy, The Institute for Cancer Research, 7701 

 Burholme Avenue, Philadelphia 11. Pennsylvania, U.S.A. 



t This is Zoology Dcparinient contribution No. 663. New observations here reported 

 were made with the aid of a grant to Dr. H.J. Muller and associates from the United States 

 Atomic Energy Commission Contract AT(1 1-1)-195. 



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