Evidence for a multi-allelic heterokaryon incompatibility (het) locus detected by hybridization among three heterokaryon-compatibility (h-c) groups of Aspergillus nidulans.

A strain of heterokaryon-compatibility (h-c) group A was crossed sexually to strains of h-cB and h-cGL of Aspergillus nidulans. A back-crossing programme established that there were seven hetero-allelic heterokaryon compatibility (het) genes controlling somatic incompatibility between strains of h-cA and h-cB. A similar back-crossing programme between strains of h-cA and h-cGL confirmed that these two groups differ at six het loci. Previous work has shown that h-cB differs from h-cGL at two het loci, hetA on linkage group V and hetB on linkage group VI. As an allelic difference at a single het locus is enough to cause two strains to be heterokaryon incompatible, 15 alleles spread over seven het loci are necessary to explain the h-cA, h-cB, h-cGL triangular compatibility relationship. One het locus is multi-allelic and this locus must be either hetA or hetB.