EVIDENCE FOR A MULTI-ALLELIC HETEROKARYON INCOMPATIBILITY (HET) LOCUSDETECTED BY HYBRIDIZATION AMONG 3 HETEROKARYON-COMPATIBILITY (H-C) GROUPS OF ASPERGILLUS-NIDULANS

A strain of heterokaryon-compatibility (h-c) group A was crossed sexua lly to strains of h-cB and h-cGL of Aspergillus nidulans. A back-cross ing programme established that there were seven hetero-allelic heterok aryon compatibility (het) genes controlling somatic incompatibility be tween strains of h-cA and h-cB. A similar back-crossing programme betw een strains of h-cA and h-cGL confirmed that these two groups differ a t six het loci. Previous work has shown that h-cB differs from h-cGL a t 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 com patibility relationship. One het locus is multi-allelic and this locus must be either hetA or hetB.