Do mycoviruses alter population structure of the chestnut blight pathogen?

Background/Question/Methods

Michigan populations of the chestnut blight pathogen, Cryphonectria parasitica, are structured according to mycovirus presence or absence. In populations where mycoviruses are present, fewer vegetative compatibility groups (VCGs) are present compared to sites where mycoviruses are absent.  Mycoviruses inhibit sexual reproduction, thus C. parasitica populations infected with mycoviruses are asexual and clonal. Previous work found that blight populations where mycoviruses are present had low VCG diversity and VCGs at a site were unique to the site.  In contrast, sites where mycoviruses were absent had higher VCG diversity and five VCGs were shared among sites. This suggests that selection changes in blight populations infected with mycoviruses, favoring the most-fit mycovirus-blight pathogen genotype. Microsatellite markers were utilized to characterize seven C. parasitica populations in Michigan and the data were used to address two objectives: 1) determine if microsatellite diversity is concordant with VCG data, and 2) further understand the genetic diversity in populations of C. parasiticaand its relationship to mycovirus presence. To test these objectives we used the same 210 isolates previously characterized for VCG diversity.

Results/Conclusions

Preliminary results for ten microsatellite markers suggest that nine of the ten are polymorphic.  For the polymorphic loci, four have two alleles, three markers have three alleles and two loci have four alleles. Overall, five of seventeen previously characterized VCGs were found to have only a single microsatellite genotype, while the remaining twelve VCGs were variable and contained between two to four microsatellite genotypes. Blight populations infected with mycoviruses have a unique genetic signature, with two microsatellite loci being fixed for an allele not found in populations where the mycovirus is absent. For four other microsatellite loci, C. parasitica populations infected with mycovirus contained only a subset of the alleles found in mycovirus-free C. parasitica populations.  Mycovirus infected C. parasitica populations also had lower genetic diversity, which is similar to the lower VCG diversity found earlier.  Although the presence of mycoviruses may have allowed selection to change population structure with respect to VCG types within chestnut blight populations, at the genomic level there is still some genetic overlap across populations.