Hypoviruses and chestnut blight: Exploiting viruses to understand and modulate fungal pathogenesis

Fungal viruses are considered unconventional because they lack an extracellular route of infection and persistently infect their hosts, often in the absence of apparent symptoms. Because mycoviruses are limited to intracellular modes of transmission, they can be considered as intrinsic fungal genetic elements. Such long-term genetic interactions, even involving apparently asymptomatic mycoviruses, are likely to have an impact on fungal ecology and evolution. One of the clearest examples supporting this view is the phenomenon of hypovirulence (virulence attenuation) observed for strains of the chestnut blight fungus, Cryphonectria parasitica, harboring members of the virus family Hypoviridae. The goal of this chapter is to document recent advances in hypovirus molecular genetics and to provide examples of how that progress is leading to the identification of virus-encoded determinants responsible for altering fungal host phenotype, insights into essential and dispensable elements of hypovirus replication, revelations concerning the role of G-protein signaling in fungal pathogenesis, and new avenues for enhancing biological control potential.