414 PHYSIOLOGY OF THE FUNGI 



unchanged by the same treatment. Indications are that these changes 

 represent mutations to the wild type. These adaptations may also be 

 initiated by X rays, nitrogen mustard, and radiophosphorus. 



Chemically induced mutations. Nitrogen and sulfur mustard gases 

 have been used to induce mutations in various fungi. The method of 

 treatment is simple and consists in exposing spores or mycelium to a 

 buffered solution of the chemical for 30 min. or longer. The spores or 

 mycelium are then washed and plated out. The methods used in detect- 

 ing mutants are then the same as when X rays or ultraviolet irradiations 

 are used. Treatment of young conidia or germinating conidia of Neuros- 

 pora crassa with nitrogen mustard produced more mutants than treat- 

 ment of old or ungerminated conidia (McElroy et al., 1947). The tech- 

 nique of using the vapor of mustard gas to induce mutation is described 

 by Hockenhull (1948). Mustard gas in a buffered solution (pH 6.9 to 

 7.0) was used by Hockenhull (1949) to produce mutants of Aspergillus 

 nidulans. 



The mustard gases, in common with nitrous acid, react with proteins. 

 For example, casein which has been treated with mustard gas no longer 

 supports the growth of the chick or rat. This is due to the inactivation 

 of certain essential amino acids (Kinsey and Grant, 1946). 



Mutation-inducing chemicals may be encountered by fungi under 

 natural conditions. It was shown that toxic metabolic products of 

 Bacillus mesentericus affected the production of mutants by certain strains 

 of Helminthosporium sativum (Christensen and Davis, 1940). The 

 filtrate of B. mesentericus cultures induced sectoring of H. sativum. These 

 mutants differed from the parent in morphology, pathogenicity, and other 

 physiological characters. 



Among the chemical compounds which induce mutations in fungi, the 

 action of nitrous acid has been especially studied (Thorn and Steinberg, 

 1939; Steinberg and Thom, 1940, 1942). Mutants of Aspergillus niger, 

 A. amstelodami, A. variecolor, A. fumigatus, A. fischeri, A. flavus, A. 

 alliaceus, and A. nidulans w^ere produced with ease by growing fungi on 

 mannitol-nitrite medium. Some of these mutants w^ere stable in culture 

 for over 20 years. In addition to morphological changes, these nitrous 

 acid-induced mutants were characterized by reduced ability to sporulate 

 and other physiological changes, especially ability to utilize certain amino 

 acids and a reduced rate of growth. It was postulated that nitrous acid 

 reacted with free amino groups of the proteins of the genes. Evidence 

 supporting this hypothesis was obtained when it w^as found that other 

 chemicals which also react with free amino groups (ninhydrin, chlora- 

 mine-T, potassium iodide, and hexamethylenetetramine) induced similar 

 mutations in A. niger. 



Certain amino acids, when added to the medium on which these 



