Print PageVirulence of the chestnut blight pathogen, Cryphonectria parasitica can be altered by the invasion of hypoviruses. Hypovirus infection reduces pathogen growth rate, which can allow the chestnut host, Castanea dentata, to recover. However, spread and persistence of hypoviruses is in part limited by vegetative compatibility (vc) within the pathogen population. In C. parasitica, vegetative compatibility is governed by at least six vegetative incompatibility (vic) loci, each locus having two alleles. Mismatches of alleles at any vic locus causes apoptosis at the point of hyphal fusion, which inhibits the spread of hypoviruses that reside in the fungal cytoplasm. We characterize vc diversity within seven C. parasitica populations in Michigan in 1996 and 2009. Four populations are causing major epidemics and hypoviruses are absent. Two of the epidemic populations have been resident from the mid-1980s, while epidemics at the two remaining sites started in the late 1990s. Three other populations have been invaded by hypoviruses leading to recovery of the chestnut host. Single-spore isolates from each population were paired on petri dishes containing chestnut pieces and scored for the presence or absence barrage lines at the point of hyphal fusion. Barrage formation is evidence of vegetative incompatibility.
Results/Conclusions
Two of the C. parasitica populations at recovering sites were dominated by a single vc group. These dominant vc groups were stable over time. The population at the third site, Frankfort, was more variable, and its vc structure increased in diversity from 1996 to 2009. Significantly, all vc groups associated with recovering chestnut populations were unique to the site in which they were found. Pathogen populations at epidemic sites were more variable for vc diversity, regardless of whether the epidemic was initiated in the 1980s or late 1990s. vc groups were shared among epidemic sites, with one vc group being found at all four sites. However, vc diversity did not display a significant spatial pattern among sites. The low diversity and stability at recovering sites is probably due to the fact that hypoviruses prevent sexual reproduction in C. parasitica. Epidemic sites may be more variable both spatially and temporally because the highly mobile sexual spores, ascospores, are more likely to migrate among populations and may cause existing genotypes at a site to be recombined regularly.
