Abstract
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.
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Acknowledgements
We thank Doug Fagan (The Canadian Chestnut Council) for support and funding assistance. Thanks to all providers of plant material; Stephen Van Drunen and Kerry Schutten (University of Guelph); Kendra Collins and Sara Fitzsimmons (American Chestnut Foundation); Sandra Anagnostakis (The Connecticut Agricultural Experiment Station); Russ MacFarlane, Gerardo Paez and Bruce Baraniak (U.S. Forest Service- George Washington and Jefferson National Forests); Melissa Thomas-Van Gundy (U.S. Forest Service- Monongahela National Forest), Scott Campbell and Sean Nolan (Maryland Department of Natural Resources- Savage River State Forest). Thanks to Paul Kron for support with microsatellite scoring, to Mason Stothart and Alyson Van Natto for assistance with early stages of microsatellite testing and to the AAC Genomics Facility at the University of Guelph.
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Funding for this project was from a Natural Science and Engineering Research Council (NSERC) discovery grant (RGPIN-2020-05652) to Brian C. Husband.
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S.S.S. designed and performed the research, analyzed the data, and wrote the manuscript. B.C.H. assisted with the conception and design of the project, writing of the manuscript, and secured funding.
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Stoltz, S.S., Husband, B.C. High genetic diversity in American chestnut (Castanea dentata) despite a century of decline. Conserv Genet 24, 25–39 (2023). https://doi.org/10.1007/s10592-022-01473-3
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DOI: https://doi.org/10.1007/s10592-022-01473-3