. Corn smut caused by Ustilago maydis. Corn; Smut diseases. 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 mutatio
. Corn smut caused by Ustilago maydis. Corn; Smut diseases. 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- a
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