ALLELIC SPECIFICITY AT THE HET-C HETEROKARYON INCOMPATIBILITY LOCUS OF NEUROSPORA-CRASSA IS DETERMINED BY A HIGHLY VARIABLE DOMAIN

In filamentous fungi, the ability to form a productive heterokaryon wi th a genetically dissimilar individual is controlled by specific loci termed het loci. Only strains homozygous for all het loci can establis h a heterokaryon. In Neurospora crassa, 11 loci, including the mating- type locus, regulate the capacity to form heterokaryons. An allele of the het-e locus (het-c(OR)) of N. crassa has been previously character ized and encodes a nonessential 966 amino acid glycine-rich protein. H erein, we describe the genetic and molecular characterization of two h et-c alleles, het-c(PA) and het-c(GR), that have a different specifici ty from that of het-c(OR), showing that vegetative incompatibility is mediated by multiple alleles at het-c. By constructing chimeric allele s, we show that het-c specificity is determined by a highly variable d omain of 34-48 amino acids in length. In this regard, het-c is similar to loci that regulate recognition in other species, such as the (S) s elf-incompatibility locus in plants, the sexual compatibility locus in basidiomycetes and the major histocompatibility complex (MHC) genes i n vertebrates.