SUMMARY: The frequency of appearance of mutant pleomorphic patches on the surface growth of granular strains of was markedly increased by incubation at 36° as compared to the number seen at 26°. A minimum of 8–9 days of incubation at the higher temperature was required to induce the higher frequency of pleomorphic mutants and, as the number of days at 36° increased, the number and relative size of the pleomorphic patches also increased. Typical mutants could be isolated whether the granular culture was incubated for 8–9 or 14 days at 36°. Visual scoring was relatively accurate since most of those patches visually scored as pleomorphic were found on isolation to be typical or atypical mutants. Three phenotypic characters were used here to define pleomorphism: type of surface mycelium, pigmentation on reverse side of colony, and effect of methionine. These characters could change independently of one another. Thus, pleomorphism may be the result of alterations in more than one closely linked chromosome-borne locus.


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