Summary: Two temperature-sensitive genes are described which affect conidia- tion in . One () decreases the length of aerial hyphae at 35° but not at 25°. Those strains (; ; and ; ; ) which have short aerial hyphae at 25° are aconidiate at 35°.

Growth on a rich conidiating medium results in the non-expression of in some but not all of the strains tested. To explain the observations on conidia- tion in the strains it is suggested that is active during a certain period in the ontogeny of the culture, this period being independent of genotype and environment.

Another genotype ; ; has a phenotype which is dependent on the incubation temperature. At 25° it is microconidiate but at 35° it is macroconidiate. Since ; strains are aconidiate, the gene determines whether any conidia shall be produced, but the type of conidia formed is determined by the ambient temperature. The gene removes this environmental control of conidial phenotype, the ; strains being macroconidiate at both temperatures. The gene may be described therefore as a canalization gene.


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