VARIATION AND INHERITANCE 401 



Different isolates of Ustilago striiformis have shown strikingly different 

 responses to sources of carbon and nitrogen (Cheo, 1949). The isolates 

 from bluegrass segregated into two groups based on mycelial type, ''frag- 

 menting" and "mycelial." The "fragmenting" type grew well only 

 on media containing sucrose and organic nitrogen, while the "mycelial" 

 type could utilize a number of sugars and nitrate nitrogen. Single-spore 

 (haploid) isolates from the same fruit body of Lenzites trdbea collected in 

 nature varied nearly fourfold in their ability to synthesize thiamine 

 (Lilly and Barnett, 1948). 



Induced deficiencies for a number of vitamins and amino acids have 

 been demonstrated by Beadle (1946) in mutants of Neurospora and by 

 Bonner (1946) in mutants of Penicilliiim. The mutations were induced 

 by exposure of spores of these fungi to ultraviolet and X-ray radiation. 

 Mutants that showed deficiencies for thiamine and differences in nitrogen 

 requirements were also reported for Aspergillus terreus (Thorn and Raper, 

 1945). One mutant differed from most species of Aspergillus in its 

 inability to utilize nitrate nitrogen. Fries (1948) describes spontaneous 

 mutations of Ophiostoma which yield the same strains and in the same 

 proportion as those induced by X rays. These results lead us to conclude 

 that similar mutations are the principal cause of variation in the isolates 

 obtained from nature. 



Response to environment. Isolates of the same species frequently 

 vary in their physiological responses to some environmental factors, 

 among which are temperature and light. For example, isolates of 

 Phytophthora infestans were found to vary in their resistance to high 

 temperature (Martin, 1949). Of the eight isolates studied, four from 

 Louisiana withstood exposure to 36°C. for 6 days, while three isolates 

 from Minnesota were killed after 4 days and one isolate from New York 

 was killed in less than 6 hr. at the same temperature. The presence of 

 the high-temperature strain is believed to be responsible for the prevalence 

 of late blight in Louisiana during the past few years. Houston (1945) 

 found that, for one group of isolates of Corticiiim solani, the optimum 

 temperature for growth was 24 to 25°C. and the maximum was 33°C. 

 For two other groups the optimum and maximum temperatures were 28 

 to 29°C. and 40°C., respectively. The three groups also varied in growth 

 rates. 



Variation in response to light is illustrated by Choanephora cucur- 

 hitarum. This was indicated first by Wolf (1917) for two isolates. The 

 isolate used by Christenberry (1938) produced conidia in continuous 

 total darkness, while two isolates used in our laboratory failed to produce 

 conidia in continuous darkness (Barnett and Lilly, 1950). 



Metabolic products. Both qualitative and quantitative variations in 

 the metabolic products of different isolates of the same species are com- 



