Changes in Genetic Diversity of Whitebark Pine (Pinus albicaulis Engelm.) Associated with Inbreeding and White Pine Blister Rust Infection

We investigated the association of inbreeding and infection by the introduced disease white pine blister rust (caused by the fungus Cronartium ribicola J. C. Fisch) with genetic diversity of whitebark pine (Pinus albicaulis Engelm.) by genetically comparing cohorts of different ages in natural stands. Isozyme analysis of bud tissue was used to estimate expected and observed heterozygosity (H_e and H_o), and Wright’s fixation index (F_is) for three age cohorts (seedling, young, and mature), sampled from 14 sites in British Columbia, Oregon, Idaho, and Montana. Comparison of genetic diversity parameters among cohorts within a site was used to assess the extent and persistence of inbreeding with age, while comparisons of parameters among sites within a cohort were used to assess the impact of the disease on genetic diversity. Significant evidence of inbreeding (F_is >0) was found in all age cohorts. When sites were stratified by level of blister rust infection, differences in F_is and H_o among cohorts were only significant when level of infection was low. A significant negative association was found between level of blister rust infection and H_o in the mature cohort. This suggests that when differential selection due to blister rust is weak, more heterozygous individuals may be favored; however, more homozygous individuals may have higher fitness under higher blister rust levels