Hyperparasite influence on pathogen population structure: Mycoviruses and the chestnut blight pathogen, cryphonectria parasitica

Background/Question/Methods

Previous work with vegetative compatibility groups (VCGs) in seven Michigan Cryphonectria parasitica populations indicated that populations of the blight pathogen were structured according hyperparasitic mycovirus presence or absence. In populations with mycovirus presence, fewer VCGs were present (in two of three mycovirus infected populations) and VCGs found at any mycovirus infected site were unique within Michigan.  Where mycoviruses were absent, VCG diversity was higher and at least five VCGs were shared among mycovirus-free populations.  It has been hypothesized that VCGs are under selection for increased diversity to escape debilitating mycovirus infection.  Here we have characterized seven chestnut blight pathogen populations using neutral microsatellite markers recently developed for European isolates of the pathogen by Breuillin et al. Populations tested from Michigan were of two types: three populations were infected with mycoviruses and four populations were mycovirus-free. Thirty samples from each population were used and analyzed at eleven microsatellite regions of two types: five genomic and six EST markers for each isolate.  Our goal was to determine if neutral microsatellite regions mimic the diversity patterns seen in VCG data and to further understand blight pathogen diversity and its relationship to mycovirus presence and absence. 

Results/Conclusions

Preliminary results for three microsatellite regions CPE-1, CPE-8, and CPG-3 suggest that the pattern of microsatellite variation and VCG data are not concordant. CPE-8 appears to be monomorphic across the 42 samples characterized to date while CPE-1 and CPG-3 were quite variable.  A total of thirteen haplotypes were found across populations; for populations with mycovirus we found eight haplotypes in 12 samples, while in populations without mycovirus we found 12 haplotypes in 28 samples.  Seven haplotypes were shared across population types and the only haplotypes that were not shared were those found only once. We expected that blight populations where mycoviruses were present to have unique microsatellite haplotypes this are not the case.  Although mycoviruses may have changed phenotypic structure for VCGs within chestnut blight populations at the genomic level there is much similarity.