Correlation between hypovirus transmission and the number of vegetative incompatibility (vic) genes different among isolates from a natural population of Cryphonectria parasitica

Correlation between hypovirus transmission and the number of different vegetative incompatibility (vic) genes among isolates of Cryphonectria parasitica was estimated using isolates sampled from a natural population. We tested the hypothesis that transmission of hypoviruses among isolates is negatively correlated to the number of vic genes that are different between vegetative compatibility (vc) groups. In a sample of 58 isolates collected from a population in Finzel, MD, eight isolates, each in a different vc group, were randomly selected and infected with each of the Cryphonectria hypoviruses CHV1-EP43 and CHV2-NB58. The frequency and time required for virus transmission were estimated by pairing infected donor isolates with uninfected recipient isolates in vitro. The number of vic genes different between vc groups was estimated by crossing the donor and recipient isolates and determining the proportion of ascospore progeny that were vegetatively compatible with either parent. Hypovirus transmission occurred between all pairs of isolates that were vegetatively compatible. The frequencies of transmission between vc groups that differed by one vic gene were 0.50 and 0.48 when the donor isolates were infected with CHV1-EP43 or CHV2-NB58, respectively. Transmission frequencies decreased to 0.13 for CHV1-EP43 and 0.14 for CHV2-NB58 when vc groups different by two vic genes. When vc groups were different by more than two vic genes, transmission of hypoviruses occurred in only one out of 37 pairs (3%) and one out of 25 pairs (4%) with CHV1-EP43 and CHV2-NB58 in the donor isolates, respectively. The transmission frequency was negatively correlated to the number of vic genes different between isolates (P < 0.01). In contrast, the time taken for transmission to occur was only weakly correlated (r = 0.40, P = 0.05) to the number of vic genes different between vc groups. Unidirectional transmission of hypoviruses was observed between six pairs of vc groups. This study provides evidence for a significant negative correlation between the frequency of hypovirus transmission and the number of vic genes different between isolates of C. parasitica from a natural population.