RADIATION STUDIES ON FUNGI 



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(Hollaeiider, Raper, and Coghill, 1945), and Penicillium notatum (Hol- 

 laender and Zimmer, 1945) exhibit more or less similar behavior, in that 

 mutation increases to a maximum and then decreases rather erratically 

 (Fig. 11-4). Decrease in the mutation f requeue}^ at higher doses was not 

 observed with Streptotnyccs flaveolns (Kelner, 1948), although rather 

 erratic behavior was noted. A series of inositolless mutants of .V. crassa 

 have been used (Giles and Lederberg, 1948; Giles, 1948, 1951) to study the 

 frequency of induced reversions to 

 inositol independence. Similar ex- 

 periments have been carried out 

 with mutants of S. cerevisiae requir- 

 ing adenine and uracil (Pomper, 

 unpublished). Figure 11-5 shows 

 a mutation frequency curve ob- 

 tained with inositolless Nciirospora 

 (Giles, 1951). The mutation fre- 

 quency in measurements of rever- 

 sions of this sort is usually fairlj^ 

 low, so that accurate determina- 

 tions cannot be made when the sur- 

 vivors fall below a certain level. 

 On the other hand, the experiments 

 described at the beginning of this 

 paragraph measured occurrence of 

 morphological mutations, which in 

 some instances reached as high as 

 30-80 per cent. Hence, the curves 

 could be followed over greater dose 

 ranges using small cell populations. 

 Also in these studies, many different 

 "mutations" of uncertain genetic 

 status were obser^'ed as compared 

 to the more restricted assay when 

 using reversions under conditions of 

 controlled genetics. These differences may be part of the explanation for 

 the difference in shape of the mutation curves. An anomalous relation 

 between the ultraviolet dose-effect curves for survival and mutation pro- 

 duction is particularly apparent in Neurospora. Although uninucleate 

 microconidia of Neurospora give exponential survival curves, suggesting a 

 single-hit or single-target event, they do not give linear mutation curves. 

 It would seem logical to expect that the frequency of the mutational 

 event should be simply proportional to dose, since deviations from such 

 proportionality would, in the case of lethal mutation, produce corre- 

 sponding de\'iations from the exponential survival curve. It has been 



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 5 



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 X 10"' ERGS/SPORE 



Fig. 11-4. Curve showing mutation pro- 

 duction and killing. Top curve: change 

 of survival ratio of fungus spores with in- 

 creasing energy using 2650 A radiation. 

 Lower curve: variation of percentage 

 mutations of surviving spores. Both 

 curves were obtained from the same 

 material. [Adapted from Hollaender and 

 Emmons, 1941.) 



