Investigation of factors associated with vegetative incompatibility and virus transmission in Cryphonectria parasitica

Cryphonectria parasitica is the agent of chestnut blight. However, a fungal RNA virus (mycovirus) can greatly reduce fungal virulence (hypovirulence), resulting in canker healing. The virus can be transmitted via hyphal anastomosis (fungal hyphal fusion) suggesting a possible mechanism of virus-based biological control of the fungus. In this thesis, we first examined structural features of the 5' untranslated regions of two viral genomes, CHV1-EP713 and CHV1-Euro7 that differ significantly in their virulence. UTR structure plays crucial role in the control of translation and genome replication in other viruses. We have used the Mfold structure prediction software as well as RNase digestion information to develop validated models of 5' untranslated region structures for both viruses. These models showed certain structural similarities between the two viruses but also reveal considerable differences. These data will provide the basis for further mechanistic studies of the functional requirements for hypovirus replication and translation.;Despite the success of biological control of chestnut blight in Europe, this strategy has not worked well in North America. One possible explanation is that virus transmission is affected by a fungal non-self recognition system, termed vegetative incompatibility. In this study, we demonstrated that the fungal gene Cpvib-1 is involved in vegetative incompatibility. Cpvib-1 shares significant similarity with VIB-1, a key regulator for several pathways leading to cell death response that functions as a suppressor of vegetative incompatibility in Neurospora crassa. Both genes were predicted to contain a NDT80/PhoG-like domain and a p53-like transcription factor DNA-binding domain. We now report that Cpvib-1 is required for regulation of sporulation and aerial hyphal growth. Cpvib-1 is also a key factor for vegetative incompatibility determined by differences at the vic loci, especially for barrage formation during the interaction of incompatible strains. Our data suggest that Cpvib-1 might be an essential trigger of cell death after fungal hyphal fusion. Additionally, the observation that deletion of Cpvib-1 can decrease canker formation indicates that DeltaCpvib-1 strain is significantly impaired in virulence and, therefore, Cpvib-1 is required for the pathogenicity of C. parasitica. These results suggest that there is a close relationship between virus-mediated fungal hypovirulence and Cpvib-1 regulated fungal pathogenicity. Better understanding of fungal pathogenicity regulation will help to clarify the mechanism of virus-mediated biological control of chestnut blight.