How to Identify and Score Genes That Confer Vegetative (heterokaryon) Incompatibility

The heterokaryotic condition, first described by Dodge (1927) in N. tetrasperma, provided a ready means for determining the dominance of traits expressed in the haploid vegetative phase. Heterokaryons also seemed to offer a novel genetic system, in contrast to diploidy. The advent of recessive auxotrophic mutations in 1941 provided a means of selecting heterokaryons composed of strains with complementing requirements. Early attempts to exploit heterokaryons in N. crassa were frustrated by heterokaryon incompatibility (Beadle and Coonradt 1944). Stable heterokaryons could not be obtained if two strains differed in alleles at any of the numerous genes at what came to be called heterokaryon incompatibility (het) loci. The mating type genes of N. crassa were also found to act as het genes: stable heterokaryons could not be formed between strains of opposite mating type. het genes at several loci were polymorphic in laboratory stocks (Holloway 1955), and the different genes were all alike in being recognized because they blocked heterokaryon formation. This made it very difficult to enumerate the het genes and to map and characterize them individually. Nevertheless, Garnjobst (1953) and Wilson and Garnjobst (1966) succeeded in identifying and mapping het-c, het-d, and het-e.