High genetic diversity in American chestnut (Castanea dentata) despite a century of decline

Knowledge of the magnitude and geographic patterns of genetic diversity is instrumental for recovery of endangered tree species whose persistence is limited by genetic variation. One such species is American chestnut (Castanea dentata), which has experienced a dramatic reduction in population size in North America in association with the spread of the parasitic fungus, Cryphonectria parasitica, causing chestnut blight. To examine the impact of the bottleneck and role of genetic diversity on population dynamics and recovery, we conducted a population genetic assessment of native American chestnut populations in the understudied northern range in Canada and along a transect towards the center of the U.S. range. Leaf tissue from 13 natural populations in Canada (N = 7) and northern U.S. (N = 6) were genetically characterized using 16 microsatellite loci and compared to a sample of reference Castanea species. Genetic diversity and population structure were assessed within and among populations to determine population connectivity and the presence of admixture with other Castanea spp. Populations throughout the range displayed high genetic diversity and significant inbreeding, with no significant difference in diversity between those at the center and edge of the range. We found evidence of infrequent interspecific hybridization in some Canadian populations but no relationship between admixture and tree health, assessed in a previous demographic survey. Unexpectedly, Canadian populations clustered separately from U.S. populations. American chestnut appears to have retained substantial genetic diversity following the population bottleneck, which is at odds with the limited incidence of blight resistance/tolerance in extant populations.