THE LIFl-:-SPA.\ ()!• Ml" l.ViK.NS l'R(MJLC;Kl) IN Cl.l.LS HV IRRADIA 1 1().\ 



Giles and his co-workers*'' assumed that the simplest and most direct 

 explanations of the oxygen efTect on chromosome breakage was by indirect 

 increase to the primary breakage along the lines of the above argument. 

 As evidence against theories that oxygen might alter the radio-sensitivity of 

 the cells, or that it might modify the chromosomes to make them more 

 easily broken, or that the ox\gen might alter the rate of restitution as opposed 

 to visible rcimions, they point out that with these theories the oxygen effect 

 would not l)e limited to low ion density irradiation. They also draw atten- 

 tion to the experiments of Conger and Fairchild^* and Gerschman et al^^ 

 from which it is evident that vmder suita))le conditions oxygen alone can 

 cause chromosome breakage and other efTects indistinguishable from those 

 produced by irradiation. As direct evidence they quote the experiments of 

 Riley, Giles and Beatty^" with microspores of Tradescantia which indicate that 

 the restitution times in oxygen and nitrogen are substantially similar. 



The evidence favouring the theory that oxygen influences the restitution 

 of breaks has been summarized by Swanson^. Experiments using the pollen 

 grain division in Tradescantia where both chromosome and chromatid breaks 

 are observed show the following results with irradiation of increasing ion 

 density : 



(7) total breakage increases and the oxygen effect decreases, 

 {2) chromosome breakage increases and the oxygen effect for these breaks 

 increases, 

 {3) chromatid breakage decreases and the oxygen effect for these breaks 



decreases, 



[4) exchanges increase and the oxygen effect for these decreases slightly. 



The fact that oxygen concentration alters the relative numbers as well as 

 the absolute numbers of these different aberrations and the fact that numbers 

 of chromosome and chromatid breaks seemed to compensate one another, 

 lead Swanson to suppose that some of the chromosome (iso-chromatid) 

 breaks are converted by incomplete restitution to chromatid breaks. This 

 incomplete restitution was thought to occur with increased frequency at 

 low oxygen tension explaining the relationship between ion density, oxygen 

 effect and the ratio of chromosome and chromatid breaks. Further evidence 

 for this hypothesis has come from the study of the loss of ring X chromosomes 

 in Drosophild^'^' *'. 



As Swanson^ points out, there is some contradictory evidence in experi- 

 ments with high ion density irradiation. The results for a-rays obtained by 

 Thoday and Read^ and the results of Giles, Beatty and Riley ■» for neutron 

 irradiation do not show the expected shifts in aberration frequencies. An 

 analysis of the degree of sister union in chromosome breaks and the non-sister 

 union in chromatid breaks by Conger*^ shows that the differential yield of 

 chromosome breaks with different oxygen concentrations cannot be ex- 

 plained by an alteration in the fusibility of broken ends. Exchange breaks 

 show a marked influence of oxygen but in the opposite direction to that 

 expected if oxygen increases breakage by affecting fusion. 



A view which offered some compromise between the breakage hypothesis 

 and the reunion hypothesis for the oxygen enhancement of radiation damage 

 was put forward by Thoday'^ and Gray^^. They suggested that the initial 

 radiation damage to chromosomes varied in character and was modified 



202 



