American Chestnut’s Enduring Genetic Diversity (1)

by Sophia Stoltz

(1) Stoltz, Sophia. 2023. American Chestnut’s Enduring Genetic Diversity. Article and photos with permission of Sophia Stoltz.
See the research paper at: https://www.researchgate.net/publication/365298674_High_genetic_diversity_in_
American_chestnut_Castanea_dentata_despite_a_century_of_decline (Accessed Thurs., Dec.7, 2023).

The history between the American chestnut and humans is one of bountiful harvest, followed by tragedy and more recently, a glimmer of hope for recovery.
American chestnut was once a charismatic species. It was an emblem of the eastern deciduous forest and known for its lumber and nut production, with a range stretching from Mississippi and Georgia all the way north into Maine and Ontario. As we move through the winter months, the famous “chestnuts roasting on an open fire” lyric may come to mind, or perhaps you are reminded of them while passing the produce shelves adorned with imported Asian and European chestnuts at the grocery store. Today, in Ontario, American chestnuts are considered endangered under the provincial Endangered Species Act, 2007.

American Chestnut leaves (Photo: John F. Foster)

American Chestnut infected by
Cryphonectria parasitica (Photo: John F. Foster)

Since the 1904 introduction of the fungal pathogen, Cryphonectria parasitica, that causes chestnut blight, populations of American chestnut (Castanea dentata) have been reduced to 1–10% of their original size. The trees are now dubbed as “frozen in time”, where rootstock shoots re-sprout but often succumb to blight before they can flower and produce nuts. Without the ability to reproduce, chestnut trees are unable to produce new genetic variation that may lead them to adapt to the blight. So far, recovery is minimal. However, surveys of chestnuts in southern Ontario report that some of our existing trees seem to be more blight-tolerant and can survive long enough to flower and produce nuts.

It was unknown whether chestnut populations were suffering from consequences of low genetic diversity and inbreeding or whether the chestnuts in Ontario had become isolated from other populations in the United-States. We were also interested in explaining if the seemingly blight-tolerant chestnut trees were hybrid trees from American chestnut reproducing with more blight-tolerant species, such as Chinese chestnut, which are sometimes planted as ornamental trees in Ontario.

As a PhD candidate in Dr. Brian Husband’s research lab at the University of Guelph, I was determined to find out if genetic diversity was limiting the recovery of American chestnut. I was also interested in how genetic diversity was organized within and among chestnut populations and how often hybridization occurs with other chestnut species. To address these unknowns, we collected chestnut leaf samples from seven populations in southern Ontario and six populations in the United-States (U.S.) extending southward from the Niagara-Buffalo border to Virginia. Then, DNA was extracted from the leaf samples to obtain genetic information on each tree. Using this data and a variety of computer programs, we can determine how closely related and diverse these trees are within and among populations.

Collecting leaf samples (Photo: Sophia Stoltz)

American Chestnut burrs
(Photo: Sophia Stoltz)

Unexpectedly, we found that chestnut populations have high genetic diversity! In fact, their levels of genetic diversity were as high as species that had not endured such a population decline, such as European and Chinese chestnut. This is good news, as genetic diversity is essential for species to adapt to environmental change. We also found that American chestnuts in Ontario were quite genetically distinct from the populations in the United-States. In fact, a previous Master’s student from our lab, John Gerrath, noted differences in leaf morphology when comparing Ontario American chestnut trees with those from the U.S.

Although these findings were striking, it is important to note that the differences are not necessarily related to aspects of tree health, but more so related to their evolutionary history. These patterns are likely due to the colonization routes of American chestnuts into Ontario from the U.S., along the narrow stretches of land surrounding Lake Erie. With pollen unable to travel over Lake Erie and reach trees flowering across the border, this may have caused Ontario’s American chestnuts to become more like one another and more differentiated from those in the U.S. Only four (1.24%) of the 324 trees we tested were detected as hybrid trees between American chestnut and another chestnut species. These trees are few and scattered around southern Ontario, therefore, it is unlikely that they pose a significant threat to the American chestnut.

So, what does this mean for the future of American chestnut in Ontario?

While the trees in Ontario have retained genetic diversity following the drastic population decline caused by chestnut blight, they remain limited by their lack of reproduction and their vulnerability to the blight. Therefore, it is vital that we continue to conserve habitat and the trees that remain and learn more from the trees in Ontario that seem to be healthier than others. Research programs in Canada and the U.S., along with restoration initiatives underway through the Canadian Chestnut Council, are ongoing strategies supporting the recovery of the endangered American chestnut in Ontario.

More on this research and ongoing restoration initiatives can be found in our recently published article with the journal, Conservation Genetics, and through the Canadian Chestnut Council, respectively.

American Chestnut
(Photo: Sophia Stoltz)

Current American Chestnut range in Southwestern Ontario (Map: Courtesy of Tom Purdy - Tallgrass Ontario)
 

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