E. L. TATUM 449 



deficiencies in all these organisms have resulted from mutation of single 

 genes. 



When the various amino acids and vitamins required by strains of 

 fungi isolated from nature and of those isolated following treatment with 

 mutagenic agents are compared, it Is seen that requirements found only 

 in mutant strains of fungi include riboflavin, pantothenic acid, p-amino- 

 benzoic acid, and choline. Nicotinic acid has recently been found to be 

 required by Blastodadia pringsheimii (i6). Folic acid and vitamin B12 

 are the only two B-vitamins which neither strains from nature nor 

 mutant strains have yet been found to require. In regard to the amino 

 acids, very few strains of fungi found in nature fail to grow on inorganic 

 nitrogen or on relatively simple organic nitrogen sources such as aspara- 

 gine (67,70). Blastodadia with a requirement for methionine seems an 

 exception (17). In contrast, mutant strains of fungi have been obtained 

 which have requirements for all of the known amino acids with the 

 exception of alanine and hydroxyproline. Experimentally induced de- 

 ficiencies for amino acids seem somewhat more frequent than for vita- 

 mins. The evidence might be taken to suggest, if mutation in nature is 

 qualitatively and quantitatively comparable to that Induced In the 

 laboratory, that most fungi In nature find themselves In environments 

 where strains with deficiencies for most amino acids and many vitamins 

 would be eliminated from the population. This is In contrast to the 

 occurrence and survival In nature of strains of bacteria which require all 

 of the known vitamins and amino acids with the exception of Inositol, 

 choline, alanine, and hydroxyproline. In general bacteria are found in 

 more varied and specialized environments than fungi, and they tend to 

 have lower quantitative requirements for vitamins and amino acids, so 

 that survival of deficient strains would be more likely. 



At present comparisons with a given microorganism of the qualitative 

 effects of different mutagenic agents can only be Incomplete. Some 

 evidence is available, however, suggesting that with Neurospora similar 

 types of mutations are produced by nitrogen mustards, by X-ray, and 

 by ultraviolet light, using a number of different strains as well as a 

 variety of Isolation techniques (81). This evidence Is consistent with the 

 view that mutation of a given gene Is related to the specific lability of 

 that gene rather than to the type of mutagenic treatment. According 

 to this concept a mutagenic agent only accelerates the normal mutation 

 frequency without affecting the quantitative relations between the dif- 



