sparrow: cytogenetic effects of ionizing radiations 91 



and confirmed later (134), that acute X- and gamma radiations are 

 equally mutagenic to spermatagonia and that both are more effective 

 than chronic gamma irradiation. No adequate explanation lor this 

 difference has been given, but it seems possible that such an effect 

 would be expected if some of the "mutations" from the acute exposure 

 were two-hit deletions and not true point mutations. Evidence of a 

 dose-rate effect as well as a nonlinear response has been reported for 

 marker losses in several different plant species. Such an effect for 

 chronically irradiated Lilium testaceum is shown in Figure 12. The 

 explanation given is that some of the marker losses result from two-hit 

 deletions and thus the dosage-squared component contributes sig- 

 nificantly to nonlinear response (32, 162, 163). 



Classically, it has been considered that so-called point mutations 

 or intragenic mutations are a valid category of genetic change. This 

 may be true for Drosophila and for many extensively studied 

 microorganisms. However, it was the view of Stadler (173, 174), who 

 did early outstanding work on the induction of mutations in maize 

 with ionizing radiations, that all the so-called point mutations in 

 maize were actually minute chromosomal aberrations or deficiencies 

 and that point mutations as defined above did not actually exist in 

 this material. As far as this author is aware, it is questionable whether 

 anyone has since analyzed mutations in higher plants with sufficient 

 detail to disprove Stadler's conclusion. It is further of considerable 

 interest that recent work of Demerec (33) has shown that most 

 mutations induced by ionizing radiation in E. coli appear to be 

 deficiencies, although point mutations apparently can be produced by 

 ultraviolet radiation or arise spontaneously in this organism. In actual 

 practise, the distinction between deletions and true mutations often 

 becomes one of resolution, since only the most critical kind of test 

 could reveal any distinction. Such tests are rarely applicable in higher 

 plants. 



If radiation-induced point mutations actually exist, one would 

 expect that they could revert back to the original form by back 

 mutation. For example, Giles (50) has shown in Neurospora that 

 reverse mutation does occur at certain loci. These mutations would 

 appear to meet the criterion for point mutation, but an equally 

 clear case has not yet been found in higher plants. While the exact 

 nature or even the actual existence of radiation-induced point muta- 



