Cornell University, Ithaca, New York 14853, USA

Background and objectives
A fundamental question in the study of host-pathogen relationships is, how do pathogens evolve virulence towards their hosts? A recurring theme in theories on the evolution of virulence is that a pathogen will evolve virulence directly in proportion to the rate at which it is transmitted to new, healthy hosts (the horizontal transmission rate). If the horizontal transmission rate is low, a highly virulent pathogen may debilitate its host too much, thereby preventing its own transmission to new hosts. This theory, applied to the viruses infecting fungi, predicts that horizontal transmission is a key factor in the evolution of virulence in fungal viruses. Horizontal transmission of fungal viruses is regulated by vegetative (or mycelial) incompatibility; transmission is inhibited between vegetative compatibility (vc) types. Therefore, in populations with high vc type diversity, there should be selection against virus virulence. Conversely, selection should favour higher virulence in populations with low vc type diversity.

In accord with these predictions, viruses virulent to Cryphonectria parasitica, the chestnut blight fungus, occur primarily in populations that have low vc type diversity. Hypoviruses, which cause hypovirulence, have become established naturally in Europe and Michigan, USA, where vc type diversity is low. Hypoviruses have failed to become established in populations of C. parasitica in eastern North America where vc type diversity is relatively high, despite repeated introductions. The objective of this work is to explore the relationships between vc type diversity, virus transmission and the virulence of hypoviruses to C. ;parasitica.

Results and discussion
Recent research in our lab has identified the vegetative incompatibility (vic) genotypes for all vc types of C. ;parasitica found in Europe (and many from the USA also). In addition, we determined the effect of each vic gene on virus transmission between vc types. By combining these findings with surveys of vc types, we estimated the expected probability of virus transmission for many populations. As expected, the probability of transmission was highly negatively correlated with vc type diversity. Studies are underway to estimate gene flow of viruses between vc types in two populations. Preliminary data from Teano, Italy agree with the expected transmission rate; high levels of virus gene flow occur between the dominant vc types in this population. Based on theory for the evolution of virulence, we predicted that, because of high vc type diversity, virulent viruses that were released into North American populations of C. ;parasitica for biological control either would not persist, or would mutate to less virulent strains. In surveys of C. ;parasitica from West Virginia, USA, we found that virus strains from Europe did not persist, even after intensive release efforts. Only viruses from Michigan were recovered in West Virginia release sites. These recovered virus strains, however, no longer have any apparent effect on host phenotype. We also predicted from theory that virulence of hypoviruses in east Asia, where C. ;parasitica is native, should be low because vc type diversity is high in these populations. Some published reports support this claim, but more data are needed. Continuing efforts are being made to test the virulence of viruses recovered in West Virginia, and those from Asia.

The question remains: Why are viruses virulent in Europe if they are not virulent in ancestral populations? We are currently testing several hypotheses: (1) virulence increased because of selection after introduction into Europe (where vc type diversity is low and virus transmission rates are high); (2) Rare virulent strains were introduced by chance from Asia (founder effects); or (3) Virulence is high because C. ;parasitica populations introduced from Japan did not coevolve with the viruses that were introduced into Europe. Phylogenetic studies are underway to determine the origin of European hypoviruses and the number of introductions that occurred.