| Introduction of the pathogenic fungus, Cryphonectria parasitica , to North America devastated the population of American chestnut, reducing it from a dominant overstory tree into a shrub. The appearance of hypovirulent strains provided an opportunity for biological control, due to their ability to transmit dsRNA viral elements into virulent strains. Unfortunately, this program was not a success, in part, at least, because of the fungal vegetative incompatibility system that impeded transmission of the virus from infected to uninfected strains. Vegetative incompatibility in C. parasitica is associated with six unlinked vic loci ( vic 1, 2, 3, 4, 6, and 7), each with at least two alleles. Fusion of two strains that differ at one or more vic loci can result in Programmed Cell Death (PCD). The focus of this thesis was to characterize the factor(s) encoded by the viral genome that interacts with the vic-associated PCD in C. parasitica. Here, I report a significant reduction in PCD upon fusion of strains differing at vic3 is associated with hypovirus infection. Using chimeric hypoviruses of CHV1-EP713 and CHV1-Euro7, this PCD reduction effect was assigned to the virus ORF A region that encodes proteins p29 and p40. The p29-effect was studied in some detail. In addition to vic3, p29 caused a significant reduction in PCD rates triggered by differences at all other vic loci studied, including vic2, 6 and 7. p29 is a multifunctional protein that was previously documented to alter fungal pigmentation patterns, conidiation and enzyme expression, and p29 acts as suppressor of RNA silencing. My results further showed that the vic3-associated PCD reduction is associated with production of p29 RNA rather that the p29 protein. Using dicer-like mutant strains I also showed that RNA silencing is involved in vic3-associated PCD reduction. This information suggests a model in which p29 interferes with RNA silencing, that in turn reduces vic-associated PCD. Finally, microscopy analyses of strains with multiple vic differences suggested that there are interactions among vic genes involving independent pathways. |
