Nonself recognition in Neurospora crassa and Cryphonectria parasitica

Intraspecific nonself recognition is a widespread phenomenon that occurs during both the sexual and asexual life cycle phases of fungi. During vegetative growth nonself recognition is a result of a genetic-mediated process called vegetative incompatibility. Vegetative incompatibility is recognized macroscopically as a demarcation zone (barrage) where genetically different individuals meet on the same substratum. Vegetative incompatibility is also represented by heterokaryon incompatibility, a system that restricts the ability of genetically different nuclei to coexist in the same cytoplasm. This thesis focuses on un-24 in Neurospora crassa and vic4 in Cryphonectria parasitica. The un-24 gene of N. crassa has heterokaryon incompatibility function and encodes the large subunit of a type I ribonucleotide reductase (RNR), an evolutionarily conserved enzyme that is essential for de novo DNA synthesis. I examined un-24-derived transcript levels in strains before and after escape from self-incompatibility due to heteroallelic duplications of un-24. In addition to documented mechanisms including deletions within het genes and spontaneous suppressor mutations, my results suggest that escape from incompatibility may involve a change in the balance of transcript levels. Surprisingly, transcript levels of both un-24PA and un-24OR increased, but to different extents, in stable self-incompatible strains grown on a low nitrogen medium that promotes the sexual cycle. Therefore, localized inactivation of un-24-mediated incompatibility in the perithecium may be through differential transcript regulation of the two allelic forms. The vic4 gene of C. parasitica has barrage function but does not restrict heterokaryon formation. Heterokaryons with vic4 differences were found to have growth rates, morphology and protein profiles similar to heterokaryons with no vic differences. My results suggest that the compatibility of vic4 heterokaryons may function through skewed nuclear proportions of the two parental nuclear types.