Genetic control of horizontal virus transmission in the chestnut blight fungus, Cryphonectria parasitica

Vegetative incompatibility in fungi has long been known to reduce the trans mission of viruses between individuals, but the barrier to transmission is incomplete. In replicated laboratory assays, we showed conclusively that th e transmission of viruses between individuals of the chestnut blight fungus Cryphonectria parasitica is controlled primarily by vegetative incompatibi lity (vic) genes. By replicating vic genotypes in independent fungal isolat es, we quantified the effect of heteroallelism at each of six vic loci on v irus transmission. Transmission occurs with 100% frequency when donor and r ecipient isolates have the same vic genotypes, but heteroallelism at one or more vic loci generally reduces virus transmission. Transmission was varia ble among single heteroallelic loci. At the extremes, heteroallelism at vic 4 had no effect on virus transmission, but transmission occurred in only 21 % of pairings that were heteroallelic at vic2. Intermediate frequencies of transmission were observed when vic3 and vic6 were heteroallelic (76 and 3 2%, respectively). When vic1, vic2, and vic7 were heteroallelic, the freque ncy of transmission depended on which alleles were present in the donor and the recipient. The effect of heteroallelism at two vic loci was mostly add itive, although small but statistically significant interactions (epistasis ) were observed in four pairs of vic loci. A logistic regression model was developed to predict the probability of virus transmission between vic geno types. Heteroallelism at vic loci, asymmetry, and epistasis were the domina nt factors controlling transmission, but host genetic background also was s tatistically significant, indicating that vic genes alone cannot explain al l the variation in virus transmission. Predictions from the logistic regres sion model were highly correlated to independent transmission tests with fi eld isolates. Our model can be used to estimate horizontal transmission rat es as a function of host genetics in natural populations of C. parasitica.