Mutations in the incompatibility factor of were obtained using two techniques. Both selected for the expression of -controlled functions (associated with nuclear migration during dikaryon morphogenesis) in heterokaryotic mycelia where these were blocked by having identical alleles present. The first technique selected for clamp cell fusion in a common heterokaryon, the second selected for septum dissolution in a heterokaryon following a common mating. Two mutants, one derived by each of the selection techniques, were studied in detail. In both, mutation was inseparable by recombination from the locus and resulted in self-compatibility. Mutant monokaryons had uninucleate cells and intact septa. Thus, full regulatory control of morphogenesis in the mutant monokaryon was maintained. However, both mutations promoted clamp cell fusion in dikaryons formed between the mutants and monokaryons having the progenitor allele or the mutant self allele. Therefore, mutation leads to the loss of regulation of clamp cell fusion. A combination of a mutation and an mutation () in the same nucleus gave a haploid mycelium which resembled a true dikaryon; thus, both the and morphogenetic sequences which are expressed in the dikaryon were operating. However, these dikaryons were not fertile.


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