412 PHYSIOLOGY OF THE FUNGI 



In a later paper, Hansen and Snyder (1943) state that the change from 

 the C to the M type is a true mutation. In Hypomyces solani f . cucurhitae 

 this change also involved the change from the hermaphroditic phase to 

 the unisexual male phase. They conclude that the M and C genes are 

 alleles and are inherited independently of the factors for compatibility. 

 On the contrary, Robbins and McVeigh (1949) have presented evidence 

 that variants of Trichophyton mentagrophytes arise as mutations and that 

 the dual phenomenon does not exist in this fungus. 



Nutritional adaptations. Most of our present knowledge regarding 

 nutritional adaptations has come from experiments with yeasts, and to a 

 lesser extent with Neurospora and bacteria. Leonian and Lilly (1942) 

 were able to "train" eight strains of Saccharomyces cerevisiae so that they 

 grew without the addition of one or more of the vitamins which they 

 formerly required. The techniciue employed consisted in increasing the 

 amount of initial inoculum, prolongation of the incubation period, and 

 repeated subculturing on media deficient for one of the necessary vita- 

 mins. Reversions occurred in most of the strains after being cultured 

 continuously on media containing all the vitamins. Such adaptations 

 as these may or may not involve gene changes. 



Lindegren and his associates have written numerous articles on the 

 genetics and adaptations of yeasts. From over 400 isolates of Sac- 

 charomyces cerevisiae, Skoog and Lindegren (1947) found 12 which could 

 not utilize glucose. Eleven of these isolates reverted to glucose utiliza- 

 tion within a period of a few days. One isolate remained glucose-negative 

 for a period of 3 months when grown on lactate medium. They believe 

 that the reversion to glucose utilization involves more than a single-step 

 change. 



Spiegelman (1950) points out that "a basic assumption of modern 

 biology is that genes function by controlling enzyme synthesis. From 

 this point of view it is obvious that enzymatic adaptation has profound 

 implications for one of the central themes of biological thinking." This 

 does not mean that the presence of the gene is always accompanied by 

 the presence of the enzyme in the cell, but merely that the potentialities 

 for the production of the enzyme are present. The synthesis of the 

 specific enzyme, as well as its subsequent activity, depends upon other 

 factors, a major one being the type of substrate. From numerous experi- 

 ments it is evident that the specific enzyme either is produced, or becomes 

 detectable, only when its corresponding substrate is present in the 

 medium. It appears, however, that the specific enzyme may be formed 

 even when the corresponding substrate is not present but that, under 

 such conditions, the adaptive enzymes are usually not detected. In the 

 course of a "long-term adaptation," there is not only synthesis of the 

 specific enzyme but also an increase in the rate of enzyme formation. In 



