On nonself recognition in Neurospora crassa: Macroscopic and genetic studies

Nonself recognition is a universal biological process that entails the detection of genetically different individuals. In filamentous fungi, nonself recognition limits cellular interactions such as hyphal fusions, to individuals that are genetically similar. The nonself recognition response is apparent at the macroscopic scale, by the presence of lines of demarcation (barrages) at the interface between genetically different individuals. Nonself recognition is also characterized in fungi by heterokaryon incompatibility as governed by het genes. Fusions between cells with genetic differences at any het locus produce unstable fusion products (heterokaryons) that disintegrate into their component homokaryons, display inhibited growth rates or die.; This thesis explores nonself recognition phenomena in the filamentous fungus Neurospora crassa by two different approaches. First, in Chapter 1, nonself recognition at the macroscopic scale was analyzed by studying the appearance and genetic basis of barrage formation. Phenotypically diverse barrages were observed to form between genetically different isolates under limiting nutrient growth conditions. The appearance of barrages in N. crassa is similar to barrages observed in other fungal species. I provide evidence suggesting that barrage formation is under complex genetic control, that is independent from that of heterokaryon incompatibility.; The genetic basis of heterokaryon incompatibility in N. crassa was explored in Chapters 2 and 3 of the thesis. Genetic differences at un-24 and het-6 trigger strong incompatibility reactions leading to cell death. The genes exist as two allelic variants Oak Ridge (OR) and Panama (PA) and are found in linkage disequilibrium in the world population. Only OR/OR and PA/PA combinations at un-24 and het-6 are found in nature. The thesis explores structural and functional factors responsible for linkage disequilibrium at un-24 and het-6. I provide evidence that the two loci are situated close to the centromere-proximal breakpoint of a novel paracentric inversion In(IIL)1131 het-6. The decreased occurrence of recombination events near the inversion breakpoints may be responsible for linkage disequilibrium at un-24 and het-6. Inversion-associated polymorphisms appear to be maintained by balancing selection acting at un-24 and het-6 .; Whether linkage disequilibrium at un-24 and het-6 is maintained because OR/OR and PAPA combinations at the two loci operate optimally together (the supergene hypothesis) was explored. The isolation and characterization of functional PA alleles at un-24 and het-6 revealed that incompatibility reactions associated with the transformation of PA alleles into OR cells are weaker than when the OR alleles are introduced into PA cells. However, when combined, the effects of PA alleles at un-24 and het-6 result in a strong incompatibility reaction leading to cell death. Selection for strong, rather than weak incompatibility reactions may have led to selection for inversion polymorphisms at In(IIL) 1131 het-6 and ultimately to linkage disequilibrium at un-24 and het-6.