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Fine-Level Genetic Structure of White Pine Blister Rust Populations

The fine-level genetic structure of the white pine blister rust agent, Cronartium ribicola, was investigated by sampling multiple monokaryotic spermogonia directly on cankers in four eastern Canadian white pine (Pinus strobus) plantations and assessing genetic variability, using random amplified polymorphic DNA (RAPD) markers. Ninety-eight percent of the cankers surveyed contained a single DNA haplotype, suggesting spermogonia within cankers are the result of clonal reproduction. A single canker contained two haplotypes that were divided between the upper and lower parts of the canker, suggesting it represented two confluent cankers. In contrast, genotypic diversity was high among cankers. Thirty-seven haplotypes were found among forty-three cankers sampled, and an analysis of molecular variance indicated that 93% (P < 0.001) of the total genetic diversity was attributable to sampling of different cankers, strongly suggesting that multiple infections do not take place in the white pine blister rust pathosystem, i.e., a canker is the result of infection by a single genotype. This result is in contrast with the high level of genetic diversity previously reported among dikaryotic aecidia within cankers and is consistent with the hypothesis that variability in the aecidial stage is the result of outcrossing between resident spermogonia and alien spermatia. The genetic structure of the spermogonial stage, which is the vegetative extension of infection by basidiospores and, therefore, the indirect result of meiosis, was consistent with random mating; the observed genotypic diversity was not significantly different (P > 0.05) from the genotypic diversity expected under the assumption of panmixis. The results indicate that monokaryotic cankers can be genotyped by sampling a single unopened spermogonia per canker.