Allozyme genetic diversity among<i>Fagus grandifolia</i>trees resistant or susceptible to beech bark disease in natural populations

Abstract

American beech (Fagus grandifolia Ehrh.) trees resistant (R = 760) and susceptible (S = 681) to beech bark disease were located and mapped in nine natural stands in West Virginia (WV), Massachusetts (MA), Maine (ME), Nova Scotia (NS), and Prince Edward Island (PEI). Dormant bud tissue collected from each tree was examined by isozyme analysis to characterize the population genetic structure of R and S subpopulations mapped in each of four intensively sampled (R/S) stands, and five additional populations in which only R trees were sampled. Seventeen enzymes (with 9 polymorphic and 14 monomorphic loci) were analyzed to estimate variation across the spatial range of disease occurrence. All populations possessed significant levels of inter- and intra-population diversity; several parameters appeared to increase in magnitude from southwest to northeast across the range. In the R/S stands, percent polymorphic loci averaged 38%, average number of alleles/locus was 1.8 (3.0 for polymorphic loci), and mean expected heterozygosity was 0.165. Most genetic variation resided within populations (97%; G ST = 0.03). Analyses of pooled R and S subpopulations demonstrated that observed heterozygosities were up to 26% higher in S trees, i.e., R tree populations exhibited consistent heterozygote deficits.