VARIATION AND INHERITANCE 413 



this respect, Spiegelman states, "In particular, the rate of formation 

 of a given enzyme is an autocatalytic function of the amount of that 

 particular enzyme present in the cytoplasm." 



Ryan (1946) found that certain adaptations of the " prolineless " and 

 " thiamineless " mutants of Neurospora are not inherited. For example, 

 the "thiamineless" mutant may not grow for several days after being 

 placed on a thiamine-free agar medium and may finally begin to grow. 

 This is explained on the basis of adaptive enzyme formation. For further 

 discussion of adaptive enzymes, see Chap. 4. For a more complete 

 discussion of cytoplasmic inheritance and adaptive enzymes in yeast, the 

 reader is referred to Lindegren (1945, 1949) and Spiegelman (1950). 



Back mutations. The mutations studied by Beadle and others were 

 those involving deficiencies for growth factors which the wild type was 

 able to synthesize. It has been shown that, under certain conditions, 

 there may be a reversion from the deficient type to the wild type. This 

 may be clue to a noninherited condition (an adaptation), such as that 

 described above, or it may involve a gene change (a back mutation). 

 Ryan (1946) has discussed at some length the topic of back mutation and 

 adaptation in certain organisms. Only a few of his ideas will be presented 

 here, omitting the detailed results of his experiments. He points out 

 that the change from a deficient to a self-sufficient habit for growth fac- 

 tors may be induced experimentally in both the fungi and the bacteria. 

 In some cases these nutritional changes are inherited, indicating gene 

 changes. In the case of the "leucineless" mutant of Neurospora the 

 adaptation back to the autotrophic habit was determined to be due to a 

 reverse mutation of the leucineless factor to the wild type. Ryan believes 

 that the ultraviolet rays, in causing the original "leucineless" mutation, 

 caused a change in the wild-type gene so that it still retained the ability 

 to reproduce but was unable to act in the synthesis of leucine. He found 

 that the "adaptation frequency" varied inversely with the amount of 

 leucine present in the medium. This theory assumes merely the inactiva- 

 tion and reactivation rather than the destruction of a gene by the ultra- 

 violet rays. 



Lindegren (1949) found that a mutation from pantothenate deficiency 

 to pantothenate independence was at a different locus, and that the 

 synthesis of pantothenate by the mutant was by a different route than 

 in the original wild type. 



Giles and Lederberg (1948) have recently studied the effects of various 

 mutagenic agents in inducing adaptations (reversions) of deficient 

 mutants of Neurospora crassa. They found that the frequency of adapta- 

 tion of certain mutants vaay be greatly increased by ultraviolet radiation. 

 This was true with the "inositolless," "cholineless," "methionineless," 

 and ' ' ribofla vinless ' ' mutants. The ' ' pantothenicless ' ' mutants remained 



