The white pine blister rust pathosystem: Assessing Cronartium ribicola genetic structure among different hosts and environments

Cronartium ribicola, the causal agent of white pine blister rust, presents characteristics that may favor rapid evolution. Since its introduction to western North America in 1910 near Vancouver, British Columbia, it has spread across most of the distribution of five-needle pines in this region. Disease incidence has varied, due in part to host resistance. To examine potential evolutionary influences on genetic structure and diversity of C. ribicola in the western USA, population genetic analyses were conducted on four populations using amplified fragment length polymorphisms (AFLPs) as molecular markers. Two populations, Happy Camp, CA and Merry Creek, ID, were collected from plantings of resistant sugar and western white pines displaying vertical or horizontal resistance, respectively. Two other populations were collected from natural stands. These included Roman Nose Lakes, ID (which consisted of a mixed stand of western white pine and whitebark pine) and Standley Springs, OR (with whitebark pine).;Polymorphism at 441 AFLP loci was 10.4% and heterozygosity among populations ranged from 0.41 to 0.28, for Merry Creek and Happy Camp, respectively. Pairwise genetic differentiation was highest (GST = 0.166) between these two populations. Assignment of isolates to groups using Bayesian analysis placed 21 of 25 Happy Camp isolates into a discernable group, while other isolates showed no clustering with regards to population or host species. Individual AFLP loci were assessed for neutrality by comparisons with a simulated FST distribution. Seven loci (15%) deviated above neutral expectations (p < 0.05), suggesting that these loci may be under natural selection. Most loci that deviated from neutrality were due to lack of polymorphism from isolates at Happy Camp. Further AFLP analysis of two aecial and three telial hosts at one population showed no differences in heterozygosity or genetic structure among hosts.;These results indicate that genetic differentiation has occurred among C. ribicola populations. The majority of differentiation is apparently due to selection and genetic drift at Happy Camp that occurred as this population evolved to overcome vertical resistance. The influence of host species and forms of resistance on C. ribicola diversity and structure will be discussed.