Fig. 12. A surface-dispersed plate of a black mono- 



sporidial line of Ustilago maydis grown in a shake culture 



in potato-dextrose agar. The black line has given rise to 

 several kinds of white mutants (Lu, 203). 



in factors for compatibility, virulence, and physiology 

 without obvious modification of cultural characters. 

 Naturally, genetic changes for certain morphological 

 characters such as size and echinulation of chlamydo- 

 spores and changes in parasitism cannot be ascer- 

 tained without inoculating the host. 



Frequency of mutation. — Although mutation some- 

 times appears to be extremely common in U. maydis, 

 one should bear in mind that in a single sporidial 

 colony, 25-30 mm diam, there may be many million 

 individual sporidia. Therefore, even if there were 

 25-30 mutants/colony, the rate of mutation would 

 be relatively low. The mutation rate, however, may 

 be much higher than is readily apparent for there is 

 good evidence that many mutants are overlooked when 

 the organism is grown on a solid substrate. Lu (203) 

 and Rowell (280) have demonstrated that mutants 

 are often carried along for many cultural generations 

 as cohabitants with their parental line. Certain bio- 

 types in a culture may be slow growers, others fast; 

 some may tolerate a substance and others may be 

 inhibited by it. Therefore, such biotypes may be 

 suppressed during their sojourn on a particular medium. 

 Even the cultures that appear most constant may 

 harbor mutants that may be unobserved in many suc- 

 cessive cultural transfers in nutrient media and 

 become visible only when dilution plates are made 

 from shake-cultures. Obviously, this has many im- 

 portant implications in both genetic and physiological 

 studies and in practical application. 



The most accurate method for detecting the fre- 

 quency and kind of mutation is by a surface-dilution 

 method on solid agar medium. This is particularly 

 true if the organism is increased by the shake-culture 

 method. By this technique, Lu (203 ) demonstrated 

 that the rate of mutation of a black monosporidial 

 line to white was about 1-1.000 (Fig. 12). 



Stability of mutants. — There are marked differences 



in the tendency among monosporidial lines to mutate 

 (55, 314). Certain lines of U. maydis apparently 

 seldom mutate under a normal range of growing con- 

 ditions on artificial media, whereas other lines mutate 

 frequently (54, 314, 319). Moreover, Stakman, et al. 

 (319) found that the tendency to mutate in U. maydis 

 was due to genetic factors; and that in certain crosses, 

 there were clear-cut segregations for mutability and 

 constancy. In general, there is a tendency for sporidial 

 lines to mutate more frequently than mycelial lines, 

 but this is not necessarily true (55, 319). Some 

 sporidial lines have been remarkably stable; this was 

 true of large populations of sporidial lines isolated 

 from one particular gall (55). Some mutants remain 

 relatively constant through many successive cultural 

 generations, whereas others produce new mutants and 

 these in turn may produce still other mutants and so 

 on (Fig. 13). Mutants with specific characters can be 

 asexually propagated for many cultural generations 

 without any apparent change. Stakman, Tyler, and 

 Hafstad (323) showed that cultural characters of 

 several mutants retained their distinctive characters 

 for several years, although transferred many times 

 and grown under different conditions. The virulence 

 of these mutants which differed greatly from their 

 parent also remained constant for more than 3 years. 

 There is considerable evidence which indicates that 

 environmental factors affect the frequency of mutation 

 as some monosporidial lines mutate readily on one 

 medium, but seldom if ever on another (314). It is 

 regrettable that in many cases experimental data on 

 the effect of environmental factors influencing the 

 frequency of mutation were based on small samples 

 and on material that lacked genetic diversity. There- 

 fore, many of the results may be indicative rather than 

 conclusive. Stakman. et al. (316) grew 4 monosporidial 



Fig. 13. A mutant of Ustilago maydis which originated 

 through 6 successive mutations and still producing mu- 

 tants (Stakman, et al., 314). 



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