E. L. TATUM 457 



described. In mutant strains of Neurospora one type of behavior typical 

 of what has been termed an "absolute deficiency" is characterized by 

 the failure of the mutant strain to grow in the absence of the required 

 supplement, or by its being capable of a very slight amount of growth 

 which does not continue after the stored material in the inoculum has 

 been exhausted. Most of the mutant strains used in biochemical studies 

 appear to belong in this group. A second type of behavior is that shown 

 by strains with so-called "partial deficiencies" which are capable of 

 growing on minimal medium at a more or less constant rate which is 

 less than that of the wild-type or of the mutant on supplemented 

 medium. The behavior of these mutants has been attributed to a modi- 

 fication of the enzyme systems such that the limiting reaction can proceed 

 to a certain extent but at a rate insufficient to permit optimal growth 

 (53). A third type of behavior is typified by a strain which on inoculation 

 into minimal medium grows very slowly for a considerable period of 

 time. Finally, after a lag period of variable duration, its growth begins 

 to improve and may approach or even reach the wild-type rate. In some 

 cases this growth in minimal has been shown to be due to gene reversion, 

 as in the inositoless mutants investigated by Giles and Zimmer (30). 

 In other cases the phenomenon seems not to be due to gene reversion 

 and has been termed "adaptation" (14). In these instances the ability 

 to grow in the absence of the specific supplement is lost on passing 

 through the asexual spores, that is the conidia, or through the sexual 

 spores, the ascospores. Reasonable interpretations of this phenomenon 

 of adaptation are those of the development of an alternative route of 

 synthesis by-passing the genetically blocked reaction, or of the recon- 

 struction of the genetically blocked reaction through the production of 

 an alternative adaptative enzyme system. Similar behavior patterns 

 have been described in other fungi, for example in pleomorphic cultures 

 of the imperfect fungus Trichophyton (66), in which the genetic basis 

 of the phenomenon cannot be directly examined. 



The phenomenon of adaptation in Neurospora has been further ex- 

 amined recently in a few strains in which gene reversion has been 

 rigorously excluded by genetic methods. Regnery (65) has examined the 

 adaptive behavior of leucineless 47313, and Tatum and Garnjobst (83), 

 the adaptive behavior of tyrosineless Y-6994. With these mutants it 

 has been found that mycelium which has reached a wild-type rate of 

 growth, either after a prolonged lag period on minimal medium or even 



