Analysis of population structure of the chestnut blight fungus based on vegetative incompatibility genotypes

Vegetative incompatibility is a self/nonself-recognition system in fungi th at has often been used for describing phenotypic diversity in fungal popula tions. A common hypothesis is that vegetative incompatibility polymorphisms are maintained by balancing selection. However, understanding the evolutio nary significance of vegetative incompatibility and the factors that mainta in these polymorphisms has been limited by a lack of knowledge of the under lying genetics of vegetative compatibility (vc) types. Genotypes of 64 vc t ypes, controlled by six unlinked vegetative incompatibility (vic) loci, hav e been identified in the chestnut blight fungus, Cryphonectria parasitica. By interpreting ve type survey data in terms of vic genotypes, we estimated vic-allele frequencies and analyzed the multilocus genetic structure of 13 populations in Europe and 3 populations in the U.S. European populations h ave less vc type diversity than the US populations because of a combination of lower vic-allele diversity and limited recombination. Genotypic diversi ty of 10 populations in italy correlated to the abundance of sexual structu res; however, significant deviations from random mating suggest that either sexual reproduction may not contribute many offspring in these populations or that vie genes (or vic genotypes) are under selection. Most vic-allele frequencies deviated from 0.5, the equilibrium frequency predicted under fr equency-dependent selection, providing no evidence for selection acting on these loci.