Phenotypic variability and mutation constitute a 

 formidable barrier to precise studies on physiology and 

 inheritance studies of U. maydis. 



Mutation. — Mass cultures of U. maydis fre- 

 quently become nonpathogenic after many transfers 

 on a solid artificial substrate. Such changes were re- 

 ported before it was realized that U. maydis was 

 heterothallic (59. 312). We know now from ex- 

 periments that the variants could have been due to 

 selecting unisexual lines from a mixed population 

 rather than due to mutants. Since 1927, studies 

 on mutation in U. maydis have been based chiefly 

 on monosporidial isolates which are uninucleate and 

 haploid or diploid. Even then, it still possible that 

 some variants that arise in the diploid lines might 

 be due to somatic segregation (58, 111. 145). 



Nature and type of mutation. — Mutation is one of 

 the primary causes of genetic variation in smut organ- 

 isms and U. maydis appears to be the most mutable 

 species of smut studied so far (310. 314). Mutations in 

 U. maydis can occur with respect to virtually any 

 physiological and cultural character selected (Fig. 10). 

 The nature and frequency of mutation in U. maydis 

 have been studied at Minnesota continuously for 30 

 years. During this period, many thousands of mutants 

 have been isolated and studied; and a great many more 

 observed. The types of mutations most commonly 

 observed are the following: 



1) Cultural characters, including color, topography, 

 consistency of colonies, direction of growth, nature 

 of margin, zonation. rate of growth, and type of 

 growth (sporidial or mycelial). 



2) Physiological characters, including enzyme pro- 

 duction, reaction to chemical ( including dyes') and 

 toxic substances, response to temperatures, and tend- 

 ency to mutate. 



3) Morphological characters, including size, shape, 

 and color of sporidia; and size of chlamydospores. 



4) Sexual compatibility and virulence. 



The magnitude of the differences in cultural char- 

 acters among mutants and their parents was often 

 striking, but sometimes almost imperceptible. For 

 instance, the mutants from a single brown mono- 

 sporidial line of U. maydis varied from near black in 

 color to pure white with an indefinite number of 

 colors between these 2 extremes. Even the degree or 

 shade of "whiteness" may vary tremendously (314. 

 319. 320). Perkins (254). in 1949. isolated biochemical 

 mutants that required special growth substances. Such 

 mutants should serve as an excellent tool in studying 

 biochemical genetics. 



Mutations in shape and size of sporidia are not 

 uncommon in cultures growing on nutrient media (55. 

 314). One mutant line formed irregular sporidia of 

 diverse shapes, another produced dumbbell-shaped 

 sporidia, and still another produced sporidia which 

 were usually blunt at the ends. Some mutants had a 

 tendency to develop dark pigment in their sporidia. 

 Actually, when certain lines were grown on a special 

 substrate, it was difficult to recognize the sporidia as 

 those of U. maydis (55). 



There may be pronounced changes in compatibility 

 among haploid lines (314). Although mutation in 

 factors for sex and virulence occasionally occurs, 



Fig. 11. A monosporidial haploid line of Ustilago may- 

 dis growing on potato-dextrose agar and mutating. Mu- 

 tants arising within a sector (mutant) (Stakinan, et al., 

 314). 



mutation for complete sex reversion has never been 

 reported. In U. maydis, most mutations of genes for 

 virulence have involved loss of factors for patho- 

 genicity. As in other mutant characters, there may be 

 many changes in degrees of virulence (54. 111. 314). 

 Diploid lines occasionally produced mutants that 

 failed to cause infection when injected singly into 

 corn plants by means of the hypodermic syringe 

 (55. 111). Obviously, it is harder to determine muta- 

 tion for virulence than it is for certain cultural char- 

 acters, as the host must be inoculated. Of IS mutants 

 that Gattani (111) obtained from a single diploid 

 line, 2 were more pathogenic than the parental line, 

 7 were less pathogenic, and 6 were about equal to the 

 parent in pathogenicity. 



Mutation in U. maydis is readily detected when the 

 organisms are grown on a solid nutrient substrate. 

 On such a medium, the mutants most frequently 

 appear as a wedge or fan-shaped sector in the colony; 

 more rarely, they occur as irregular isolated patches 

 (Fig. 11). A large percentage of the sectors usually 

 develop near the edge of a colony although they 

 also may arise at or near the center. The patch mutants 

 may occur any place on the colony, but are generally 

 more difficult to detect than sector mutants. Sectors 

 vary greatly in size, shape, color, and cultural char- 

 acters; and also in sharpness of outline (314). Some- 

 times, small distinct sectors may occur within primary 

 sectors. Many mutants that are not favored by a 

 particular medium are apt to be overrun by parental 

 lines or a vigorous-growing mutant and thus never 

 observed. Obviously, many mutants are not observed 

 because they may appear similar to their parents on 

 the cultural medium on which they arose, but some- 

 times they are quite different when grown in a 

 different environment. In addition, changes may occur 



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