450 PLANT GROWTH SUBSTANCES 



ferent gene mutations produced. The types of deficiency mutations found 

 in different fungi under mutational treatment, although similar, have 

 been found to vary somewhat from one organism to another. For 

 example, in yeast the most frequent deficiencies yet reported are for 

 adenine, methionine, leucine, and lysine (62,64). Deficiencies for adenine 

 or hypoxanthine, thiamin, nicotinic acid, and reduced sulfur are most 

 frequent in Ustilago maydis (59). Deficiencies for arginine, lysine, and 

 methionine have been most frequently found in Penicillium (11). In 

 Ophiostoma the most common requirements of mutant strains appear to 

 be for arginine, purines, and pyrimidines (25). In Neurospora with the 

 mutagenic agents, strains, and techniques so far used, the most frequent 

 deficiencies have been for methionine, lysine, arginine, and adenine. 

 With Absidia glauca the most frequent mutation seems to be for histidine 

 (29). If the specific effects of the various treatments and techniques 

 involved in the isolation of mutants in these fungi has not resulted in 

 selection of particular types of deficiencies, these results would suggest 

 that genes controlling different steps in growth-factor biosynthesis in a 

 given fungus differ in their stability to mutation, and that those genes 

 concerned in particular biosynthetic steps may have entirely different 

 labilities to mutation in different organisms. 



Another tentative conclusion regarding the nature of gene control of 

 growth-factor synthesis may well be made at this time. Any technique 

 of production and isolation of mutant strains of a microorganism which 

 does not require carrying the organism through a sexual stage before 

 detection and isolation of a mutant stock, would in theory permit the 

 detection of nutritional deficiencies controlled by extra-nuclear factors, 

 such as those involved in Paramecium (77), and in yeast with adaptive 

 sugar utilization (46) and with respiratory systems (20), This condition 

 is met with the fungi listed above, including Neurospora in which mutant 

 strains derived from asexual uninucleate microconidia (4) have been 

 isolated (81), Genetic examination of mutants in the fungi so far obtained 

 has failed to indicate the existence of extra-nuclear control of growth- 

 factor or metabolite synthesis. Even in Saccharomyces cerevisiae, in which 

 extra-nuclear control of sugar utilization has been suggested, recent 

 work by Lindegren and Lindegren (47) and by Pomper (62) has shown 

 that growth-factor deficiency characters are inherited as if they were 

 characterized by single gene changes. We may, therefore, conclude that 

 in the fungi so far examined, growth-factor deficiencies in mutant 



