Summary: A temperature-sensitive mutant was isolated that conditionally affected the vegetative growth phase of the asexual developmental cycle of The formation of aerial hyphae, conidiation and the initial steps of conidial germination were not temperature-sensitive in this strain. However, germ tube elongation was blocked at the restrictive temperature. The growth rate of the vegetative hyphae was greatly reduced and their morphology was altered when the hyphae grew on the surface of agar medium at 34 °C. These properties were not temperature-sensitive when the hyphae grew within the agar medium. In liquid medium, vegetative growth was temperature-sensitive when low concentrations of conidia (less than 10 ml) were used to inoculate the cultures. After a prolonged incubation at 34 °C, the cells died. The requirement for high cell densities for survival and growth in liquid medium at 34 °C could be overcome by adding histidine to the medium, but on agar medium containing histidine, the mutant strain still grew colonially at 34 °C. In addition to these temperature-sensitive properties, the mutant strain expressed a very strong circadian conidiation rhythm when grown on agar medium at 22 °C. The temperature-sensitive growth properties of the mutant strain and its ability to conidiate rhythmically reverted simultaneously to the wild-type phenotype. In forced heterokaryons, the mutant allele was recessive to the wild-type allele. The mutant allele was mapped on the right arm of linkage group IV.


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