Genetics, Vol. 172, 1545-1555, March 2006, Copyright © 2006
doi:10.1534/genetics.105.051490

Nonallelic Interactions Between het-c and a Polymorphic Locus, pin-c, Are Essential for Nonself Recognition and Programmed Cell Death in Neurospora crassa

Department of Plant and Microbial Biology, University of California, Berkeley, California 94720

3 Corresponding author: Department of Plant and Microbial Biology, 111 Koshland Hall, 3102, University of California, Berkeley, CA 94720-3102.
E-mail: lglass{at}nature.berkeley.edu

Nonself recognition in filamentous fungi is conferred by genetic differences at het (heterokaryon incompatibility) loci. When individuals that differ in het specificity undergo hyphal fusion, the heterokaryon undergoes a programmed cell death reaction or is highly unstable. In Neurospora crassa, three allelic specificities at the het-c locus are conferred by a highly polymorphic domain. This domain shows trans-species polymorphisms indicative of balancing selection, consistent with the role of het loci in nonself recognition. We determined that a locus closely linked to het-c, called pin-c (partner for incompatibility with het-c) was required for het-c nonself recognition and heterokaryon incompatibility (HI). The pin-c alleles in isolates that differ in het-c specificity were extremely polymorphic. Heterokaryon and transformation tests showed that nonself recognition was mediated by synergistic nonallelic interactions between het-c and pin-c, while allelic interactions at het-c increased the severity of the HI phenotype. The pin-c locus encodes a protein containing a HET domain; predicted proteins containing HET domains are frequent in filamentous ascomycete genomes. These data suggest that nonallelic interactions may be important in nonself recognition in filamentous fungi and that proteins containing a HET domain may be a key factor in these interactions.




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