Identification of Specificity Determinants and Generation of Alleles with Novel Specificity at the het-c Heterokaryon Incompatibility Locus of Neurospora crassa

Abstract

ABSTRACT The capacity for nonself recognition is a ubiquitous and essential aspect of biology. In filamentous fungi, nonself recognition during vegetative growth is believed to be mediated by genetic differences at heterokaryon incompatibility ( het ) loci. Filamentous fungi are capable of undergoing hyphal fusion to form mycelial networks and with other individuals to form vegetative heterokaryons, in which genetically distinct nuclei occupy a common cytoplasm. In Neurospora crassa , 11 het loci have been identified that affect the viability of such vegetative heterokaryons. The het-c locus has at least three mutually incompatible alleles, termed het-c OR , het-c PA , and het-c GR . Hyphal fusion between strains that are of alternative het-c specificity results in vegetative heterokaryons that are aconidial and which show growth inhibition and hyphal compartmentation and death. A 34- to 48-amino-acid variable domain, which is dissimilar in HET-C OR , HET-C PA , and HET-C GR , confers allelic specificity. To assess requirements for allelic specificity, we constructed chimeras between the het-c variable domain from 24 different isolates that displayed amino acid and insertion or deletion variations and determined their het-c specificity by introduction into N. crassa . We also constructed a number of artificial alleles that contained novel het-c specificity domains. By this method, we identified four additional and novel het-c specificities. Our results indicate that amino acid and length variations within the insertion or deletion motif are the primary determinants for conferring het-c allelic specificity. These results provide a molecular model for nonself recognition in multicellular eucaryotes.