Diversity of vegetative incompatibility (vic) genotypes for chestnut blight at seven sites in Michigan: A comparison of pathogen populations where mycovirus hyperparasites are present or absent

Background/Question/Methods

Most filamentous fungi have a self/non-self recognition system which is thought to control the spread of alien nuclei and cytoplasmic elements such as mycovirus hyperparasites during hyphal anastomosis that occurs when colonies converge. In the chestnut blight pathogen, Cryphonectria parasitica, self/non-self recognition is determined by matching alleles at 6 unlinked vegetative incompatibility (vic) loci, each possessing two alleles. Selection should maximize diversity of vegetative compatibility (VC) groups to inhibit spread of parasitic elements. A previous study investigating vegetative compatibility group diversity in seven Michigan populations of C. parasitica found a total of 29 VC groups. The presence or absence of mycovirus hyperparasites appeared to influence VC structure in pathogen populations. Populations where mycoviruses were present displayed lower diversity for VC groups and VC groups found at any given site were unique to the site. In contrast, when mycoviruses were absent VC diversity was higher and five VC groups were shared among populations. This study explores the underlying genetics of VC diversity by determining the vic genotype of the 29 VC groups. Vic genotype was determined by testing each of the 29 VC groups from Michigan against the 64 EU testers which have been previously genotyped by Cortesi and Milgroom. The EU testers allow us to determine specific alleles at the six characterized vic loci.

Results/Conclusions

Comparisons are complete for the seven most common VC groups in Michigan with six VC groups matching up with a EU tester.  The lone exception was the main VC group at the County Line site, where greater than 90% of the isolates contain mycovirus.  This VC group did not match up with any of the EU testers, suggesting that this VC group has a unique allele at a vic locus which has not yet been characterized.  Inspection of the six isolates, which matched with specific EU testers, indicates that there is allelic variability at all six of the characterized vic loci.  With six polymorphic vic loci there are 128 (2⁶) possible VC groups. Thus, the 29 VC groups detected in Michigan represent only a small fraction of the possible variation.