Genetic diversity between and within cankers of the white pine blister rust

The hierarchical structure of genetic diversity was studied in the monokaryotic haploid (spermatia) and the dikaryotic (aeciospores) phases of the white pine blister rust fungus, Cronartium ribicola. Multiple samples of aeciospores and spermatia were collected on cankers in three plantations of eastern white pine, Pinus strobus, situated more than 200 km apart. Of the 20 random amplified polymorphic DNA (RAPD) markers scored, 11 were either fixed or nearly fixed, and the remaining markers were polymorphic. Eighty-nine percent of the cankers sampled contained aecidia with more than one RAPD profile. Partitioning of the total genetic diversity by an analysis of molecular variance revealed that 69 to 74% (P < 0.001) of the genetic variability was attributable to the sampling of different aecidia within cankers, while genetic diversity between cankers within sites and between sites made up 22 to 24% (P < 0.001) and 2.7 to 6.7% (P = 0.027 to 0.001), respectively, of the total diversity. Surprisingly, different RAPD profiles were also found between spermatial samples within cankers. The presence of several monokaryotic haploid genotypes within a single canker raises the possibility of outcrossing within cankers and might explain previous inconclusive results concerning heterothallism in this fungus. On cankers with monomorphic spermatia, 75% of the aecidia were the result of outcrossing, i.e., at least one of the RAPD loci had the null allele in the spermatia but the marker allele in the aecidia. Because of the small number of polymorphic markers studied, this should be considered an underestimate of outcrossing.