. Corn smut caused by Ustilago maydis. Corn; Smut diseases. 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.—Alt
. Corn smut caused by Ustilago maydis. Corn; Smut diseases. 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.
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