This 1964 study by Klein investigated how temperature and time influence pleomorphism—the spontaneous appearance of mutant patches with altered morphology—in Trichophyton mentagrophytes, a dermatophytic fungus. Granular (normal) strains were cultured at 26°C and 36°C on glucose-containing agar media. At 26°C, pleomorphic patches rarely appeared (spontaneous rate ~1 per 4-5 plates). However, at 36°C, multiple patches developed consistently. A minimum of 8-9 days at elevated temperature was required to induce pleomorphism; longer exposure increased both patch number and size. Notably, the timing of temperature exposure did not matter—applying 36°C early or late in the incubation period yielded similar results. Isolated mutants were classified using three phenotypic characters: surface mycelium type (fluffy vs. granular), pigmentation on colony reverse, and methionine sensitivity. These traits changed independently, suggesting pleomorphism results from alterations at multiple genetic loci rather than a single gene. The data indicate temperature is the key environmental factor inducing pleomorphic mutations, provided the medium contains adequate carbohydrate.

Key findings

• A minimum of 8-9 days of incubation at 36°C was required to induce pleomorphic patch formation in Trichophyton mentagrophytes, whereas patches were rarely seen at 26°C over 35 days
• The three phenotypic characters defining pleomorphism (surface mycelium type, pigmentation, and methionine sensitivity) changed independently, indicating alterations in multiple closely-linked chromosome-borne loci rather than a single pleiotropic gene
• Elevated temperature acts as the key inducing factor for pleomorphism development, with the timing of temperature exposure not affecting the final outcome
• Visual scoring of patches was reasonably accurate for identifying pleomorphic mutants, as most visually scored patches yielded typical or atypical pleomorphic isolates upon culture