Genome characterization and population genetic structure of white pine blister rust, Cronartium ribicola

Rust fungi cause some of the most severe pine diseases. Cronartium ribicola (J. C. Fisch.), the causal agent of white pine blister rust, was introduced accidentally to North America from Europe in the late 1800s. Since then, it has devastated a large number of native, commercially valuable white pines, and is threatening alpine ecosystem stability by endangering high elevation white pines. In order to better understand the global epidemiology of this pathogen, we conducted a genome scan of a global collection of C. ribicola using Genotyping-by-Sequencing (GBS) to: 1) ascertain the origin and the routes of introduction of C. ribicola, and 2) uncover cryptic population structure of C. ribicola in western North America, in relation to different pine hosts, climates and landscapes. More than eight thousand single nucleotide polymorphism markers were genotyped on 192 samples of C. ribicola from three continents. The highest genetic and nucleotidic diversity were observed in Siberian samples, supporting the hypothesis that central Russia is the center of origin of C. ribicola. Diversity was reduced in all other populations and was lowest in western North America. Genetic and nucleotidic diversity were two to five times lower in western than in eastern North America. This result supports the observation of multiple introductions of the pathogen in eastern North America and contrasts with the single reported introduction in western North America. However, western populations had a larger number of rare alleles. This could represent the signature of population expansion following a bottleneck or a selective sweep. A cryptic Coast/Interior split was detected within the western cluster, most likely maintained by the scarcity of white pines in central British Columbia acting like a barrier to gene flow. Finally, western individuals with a high level of eastern admixture were discovered in two populations east of the Continental Divide. This could indicate that the eastern-western barrier to gene flow is leaky. Such information is of significance to white pine resistance breeding programs and to the monitoring of this disease.Forestry, Faculty ofGraduat