Washington– Billions of American chestnut trees Once it covered the eastern United States. It rose in elevation and produced so many nuts that vendors had to transport it by train car. Every Christmas, the holiday lyric “Chestnuts Roasting on an Open Fire” comes to mind.
But by the 1950s, this venerable tree was functionally extinct, having been culled by a deadly airborne fungal blight and deadly root rot. A new study published Thursday in the journal Science offers hope of revitalizing it, finding that genetic testing of individual trees can reveal which ones are most likely to resist disease and grow tall, thus shortening the time it takes to grow the next, more robust generation.
Reducing the generation gap means a faster path to many disease-resistant trees that will once again be able to compete for space in eastern forests. The authors hope this will happen in the coming decades.
“What’s new here is the engine we’re building for restoration,” said Jared Westbrook, lead author and science director of the American Chestnut Foundation, which wants to return the tree to its original habitat that once extended from Maine to the Mississippi.
The American chestnut, sometimes called the “redwood of the East,” can grow rapidly and reach more than 100 feet (30 m), producing huge quantities of nutritious chestnuts and providing a preferred wood for its straight grain and durability.
But it had little defense against blight and root rot introduced outside. But another type of chestnut developed alongside those diseases. Chinese chestnut was introduced for its valuable nuts and ability to resist disease. But they are not as tall or competitive in U.S. forests, nor do they play the same critical role in supporting other species.
So, the authors wanted a tree with the characteristics of the American chestnut and the disease resistance of the Chinese chestnut.
This goal is not new; scientists have been pursuing it for decades, and have made some progress.
But it was difficult because the desired traits of the American chestnut are spread across multiple regions along its genome, the strand of DNA that tells the tree how to develop and perform.
“It’s a very complex trait, and in this case, you can’t just pick one thing because you’ll be picking things that are negatively correlated,” said John Lovell, senior author and researcher at the HudsonAlpha Genome Sequencing Center.
They reproduce for disease resistance alone and the trees become shorter and less competitive.
To deal with this problem, researchers sequenced the genomes of multiple chestnut species, and found several loci that were associated with the desired traits. They can then use this information to breed trees that are more likely to possess desirable traits while maintaining significant amounts of American chestnut DNA—roughly 70% to 85%.
Genetic testing allows the process to move faster, detecting the best offspring years before they show their traits through normal growth and combat disease. The closer the generation gap, the faster the gains accumulate.
The research has identified some promising genes, said Stephen Strauss, a professor of forest biotechnology at Oregon State University who was not involved in the study. He wants scientists to be able to do that Gene editing themselves, perhaps a faster and more accurate route to a better tree. In an accompanying commentary in Science, he says regulations could stymie these ideas for years.
“People won’t think about biotechnology because it’s on the other side of that social and legal barrier,” he said, and that’s short-sighted.
For people who have studied the American chestnut closely, the work poses 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 and 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 kind of the gold standard.”
He said the tree was a keystone species, beneficial to humans and vital to larger populations of squirrels, chipmunks and black bears — and hybrids may not be as imposing or efficient. He was pleased that the authors included some surviving American chestnut trees in their proposal, but preferred an approach that relied more on them.
“It’s not that the hybrid approach in and of itself is bad, but why don’t we try to put wild American trees back into the forest, back into the ecosystem, and exhaust every possibility of doing that before we move on to some of these other approaches?” He said.
Reviving the species will require introducing genetic diversity from outside the traditional group of American chestnut trees, Lovells said. The study authors’ goal is tall, resilient trees, and they are optimistic.
“I think if we only selected the American chestnut (the tree’s genes), there would be a very small pool, and we would end up with a genetic bottleneck that would lead to extinction in the future,” Lovell said.
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