WASHINGTON — Billions of American chestnut trees once covered the eastern United States. They soared in height, producing so many nuts that sellers moved them by train car. Every Christmas, they're called to mind by the holiday lyric “chestnuts roasting on an open fire.”
But by the 1950s, this venerable tree went functionally extinct, culled by a deadly airborne fungal blight and lethal root rot. A new study out Thursday in the journal Science provides hope for its revitalization, finding that the genetic testing of individual trees can reveal which are most likely to resist disease and grow tall, thus shortening how long it takes to plant the next, more robust, generation.
A smaller gap between generations means a faster path to lots of disease-resistant trees that will once again be able to compete for space in Eastern forests. The authors hope that can occur in the coming decades.
The study shows that modern genomic tools can dramatically accelerate restoration while preserving the species’ ecological identity.
Breeding lines, experimental protocols, and data critical to the analyses included in this paper were generated at Georgia's Berry College by biology professor emeritus Martin Cipollini — who is the current president and science coordinator for the Georgia Chapter of The American Chestnut Foundation and has been involved in chestnut restoration work at Berry for about 20 years.
He was assisted by associate biology Professor Caitlin Conn, and many Berry undergraduate students as well as Georgia American Chestnut Foundation volunteers.
“With genome-enabled breeding, we expect the next generation of trees to have roughly twice the average blight resistance of our current population, with about 75% American chestnut ancestry,” said study lead author Jared Westbrook, The American Chestnut Foundation’s director of science. “These trees are expected to begin producing large quantities of seed for restoration within the next decade.”
“What’s new here is the engine that we’re creating for restoration,” Westbrook said. The American Chestnut Foundation wants to return the tree to its native range that once stretched from Maine to Mississippi.
The American chestnut, sometimes called the “redwood of the East,” can grow quickly and reach more than 100 feet (30 meters), produce prodigious amounts of nutritious chestnuts and supply lumber favored for its straight grain and durability.
But it had little defense against foreign-introduced blight and root rot. Another type of chestnut, however, had evolved alongside those diseases. The Chinese chestnut had been introduced for its valuable nuts and it could resist diseases. But it isn’t as tall or competitive in U.S. forests, nor has it served the same critical role supporting other species.
So, the authors want a tree with the characteristics of the American chestnut and the disease resistance of the Chinese chestnut.
That goal is not new — scientists have been reaching for it for decades and made some progress.
But it has been difficult because the American chestnut's desirable traits are scattered across multiple spots along its genome, the DNA string that tells the tree how to develop and function.
“It’s a very complex trait, and in that case, you can’t just select on one thing because you’ll select on linked things that are negative,” said John Lovell, senior author and researcher at the HudsonAlpha Genome Sequencing Center.
Breed for disease resistance alone and the trees get shorter, less competitive.
To deal with this, the authors sequenced the genome of multiple types of chestnuts and found the many places that correlated with the desired traits. They can then use that information to breed trees that are more likely to have desirable traits while maintaining high amounts of American chestnut DNA — roughly 70% to 85%.
And genetic testing allows the process to move faster, revealing the best offspring years before their traits would be demonstrated by natural growth and encountering disease. The closer the gap between generations, the faster gains accumulate.
Steven Strauss, a professor of forest biotechnology at Oregon State University who wasn't involved in the study, said the paper identified some promising genes. He wants scientists to be able to edit the genes themselves, a possibly faster, more precise path to a better tree. In an accompanying commentary piece in Science, he says regulations can bog down these ideas for years.
“People just won’t consider biotech because it is on the other side of this social, legal barrier" and that's shortsighted, he said.
For people who have closely studied the American chestnut, the work begs an almost existential question: How much can the American chestnut be changed and still be an American chestnut?
“The American chestnut has a unique evolutionary history, it has a specific place in the North American ecosystem,” said Donald Edward Davis, author of the American chestnut, an environmental history. “Having that tree and no other trees would be sort of the gold standard.”
He said the tree was a keystone species, useful to humans and vital to bigger populations of squirrels, chipmunks and black bears — hybrids might not be as majestic or effective. He was pleased that the authors included some surviving American chestnuts in their proposal, but favored an approach that relied on them more heavily.
“Not that the hybrid approach is itself bad, it is just that why not try to get the wild American trees back in the forest, back in the ecosystem, and exhaust all possibilities from doing that before we move on to some of these other methods?” he said.
Lovells said resurrecting the species requires introducing genetic diversity from outside the traditional pool of American chestnut trees. The study authors’ goal is tall, resilient trees and they are optimistic.
“I think if we only select American chestnut (tree genes), period, there’s going to be too small of a pool and we’re going to end up with a genetic bottleneck that will lead to extinction in the future,” said Lovell.
