Diversity of vegetative incompatibility (vic) genotypes in epidemic and recovering populations of Cryphonectria parasitica in Michigan

Background/Question/Methods

Ascomycetes have a self / non-self recognition system that is thought to inhibit spread of alien nuclei and parasitic cytoplasmic elements such as double-stranded RNA mycovirus hyperparasites.  In the chestnut blight pathogen, Cryphonectria parasitica, this recognition system is controlled by the multilocus genotype at 6 or more unlinked vegetative incompatibility (vic) loci, each having two alleles. A previous study examined vegetative compatibility (VC) group diversity across seven Michigan populations of C. parasitica and identified of 22 VC groups across the state, and a pattern of diversity that appeared to be affected by the presence or absence of mycovirus. Populations where mycoviruses were present (recovering populations) displayed lower diversity for VC groups, and most VC groups were unique to the site. Sites without mycovirus (epidemic populations) displayed higher VC group diversity and VC groups were frequently shared among populations. The underlying genetics of VC group diversity is being determined for the 22 VC groups isolated in Michigan.  Sixty-four EU testers previously genotyped by Cortesi and Milgroom were paired with each Michigan VC type to determine its multilocus genotype. These data are being used to determine variability of vic loci within and among C. parasitica populations. 

Results/Conclusions

Sixteen of 22 Michigan VC groups matched one of the EU testers, while three VC groups remain to be characterized. The three remaining VC groups had no match to EU testers, suggesting that these VC groups may have undetected variation at one or more uncharacterized vic locus. Collectively, the Michigan VC groups were variable at all six of the characterized vic loci.  All six loci were variable across the four epidemic populations, whereas only four vic loci (1, 2, 3 and 4) were variable across the three recovering populations.  The pattern of variability also differed at a population level. One recovering site, Roscommon, was monomorphic for vic genotype.   The County Line recovering site contained only two VC groups, and one VC group remains to be characterized. Four VC groups have been characterized at the Frankfort recovering site with VC diversity due to polymorphisms at vic loci 1 and 4.  In contrast, epidemic populations had between four and 11 VC groups total and between 2 and 5 variable vic loci within each site.  With six polymorphic vic loci there are 64 (2⁶ haploid multilocus genotypes) possible VC groups. The 22 VC groups detected in Michigan represent only a fraction of the potential genotypic variation.