Suppressor mutants of Neurospora crassa that tolerate allelic differences at single or at multiple heterokaryon incompatibility loci.

Allelic differences at any one of at least 11 heterokaryon incompatibility (het) loci in Neurospora crassa trigger an incompatibility response: localized cell death at sites of hyphal anastomosis. We have isolated spontaneous and insertional suppressor mutants that are heterokaryon-compatible in spite of allelic differences at one or at several het loci. Some intra- and extragenic mutants tolerated allelic differences only at single het loci. Multi-tolerant spontaneous mutants were isolated by selecting simultaneously for tolerance of differences at het-c, -d and -e, or at each of these plus mating-type. Some suppressor mutants were specific for only one allele at the affected het locus; others suppressed both alleles. Insertional mutations were isolated from banks of transformants, each having a plasmid integrated into a random position in the chromosome. One mutant tolerated allelic differences at het-d. A homologous cosmid from a Neurospora genomic bank complemented the mutant phenotype. A second insertional inactivation mutant was tolerant of het-c differences. Inactivation of the wild-type locus corresponding to the integration site was accomplished by repeat-induced point mutation (RIP). The RIP progeny, like the original mutant, were tolerant of differences at het-c. It may be possible to use such suppressor mutants as universal donors of hypovirulence in pathogenic fungi.