The Genetic Control of Conidiation in a Heterokaryon of Neurospora crassa

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Abstract
A microconidial non-colonial heterokaryon of Neurospora crassa was produced from a macroconidial colonial and a macroconidial non-colonial homokaryon. The macroconidial colonial strain was found to have the genetic constitution microconidial (m) (the pem of Barratt and Garnjobst, 1949) suppressor of microconidial (sum); col-1 and the other m+ sum+; col-1+. The dominance relationships of m, sum and col-1 with their wild type alleles and the particular nuclear ratio in which they are present determine the microconidial non-colonial phenotype of this heterokaryon. As a further consequence of the dominance relationships between col-1 and its wild-type allele, only one of the two pleiotropic effects of the mutant gene is expressed in the heterokaryon. The enormous discrepancy between the estimates of the nuclear ratio by two different plating methods is shown to be due to an almost complete suppression of one type of homokaryotic colony by the other.