VARIATION AND INHERITANCE 415 



mutants were cultured, induced partial or complete reversion to the 

 morphology of the parent type. Of the single amino acids tested, only 

 lysine, cystine, /3-phenyl-/3-alanine, threonine, and vahne induced com- 

 plete reversion with the mutant of A. nigcr studied. Nicotinic acid, 

 lysine, and valine in combination gave the best results. Complete 

 reversion of a mutant of A. amstelodami was obtained only with a mixture 

 of lysine and threonine. 



It should be pointed out that, although the mutants of Aspergillus 

 have been apparently stable for a long period of time, proof of the inherit- 

 ance of these characters by crossing the "mutants" with the parent type 

 is not available. 



SUMMARY 



Some knowledge of physiological variation and of the inheritance of 

 the underlying factors is of great importance to the investigator who 

 cultures fungi. The plant pathologist is greatly interested in knowing 

 the stability of the pathogenicity of the plant pathogenic fungi and in 

 learning whether the variations which he finds are apt to be permanent 

 or only temporary. Considerable effort has been made toward an under- 

 standing of the genetics of the smuts, the rusts, and the yeasts. A few 

 other fungi, such as Neurospora, Glomerella, and Phycomyces, have received 

 attention because they are particularly adapted to genetic studies. 

 However, the work up to the present leaves much to be desired regarding 

 the relation between genetics and physiology. 



The genetics of sex and compatibility has been most frequently studied. 

 It is only natural that such is the case, for much of the understanding 

 of a fungus depends upon the completion of its sexual life cycle and a 

 clear knowledge of the various stages. Studies on the inheritance of 

 morphological characters are represented by numerous isolated reports 

 on characters more or less clearly defined. The reasons for the limited 

 number of investigations on the genetics of physiological characters are 

 numerous. Perhaps the principal reason is our meager knovvledge of the 

 intricate physiological processes of the fungi. The difficulty in obtaining 

 strains of the different sexes of suitable fungi which have clear-cut physio- 

 logical differences is great. The actual difficulty in carrying out the 

 physiological tests has no doubt discouraged work along this line. 



Studies on the inheritance of induced deficiencies for a number of 

 vitamins and amino acids have indicated that such deficiencies may be 

 inherited in a simple Mendelian fashion. On the other hand, the inherit- 

 ance of partial thiamine deficiency in Lenzites trabea is complex and does 

 not follow the simple Mendelian pattern. Studies of the yeasts have 

 indicated that some physiological characters are inherited through, or 

 influenced by, the cytoplasm. This possibility should not be overlooked 

 in the filamentous fungi, where anastomoses of hyphae are common. 



