Genetic analysis could speed up restoration of iconic American chestnut: Scientists

WASHINGTON– Billions of American chestnut trees once covered the eastern United States. They rose high, producing so many nuts that sellers transported them by wagon. Every Christmas they are remembered by the holiday sayings “chestnuts roasting on an open fire.”

But by the 1950s, this venerable tree was functionally extinct, wiped out by deadly airborne fungal blight and deadly root rot. A new study published Thursday in the journal Science offers hope for its revitalization, revealing that genetic testing of individual trees can reveal which ones are most likely to resist disease and grow tall, shortening the time needed to plant the next, hardier generation.

A smaller gap between generations means a faster path to large numbers of disease-resistant trees that can once again compete for space in Eastern forests. The authors hope this can happen in the coming decades.

“What’s new here is the engine we’re creating for restoration,” said Jared Westbrook, lead author and science director of the American Chestnut Foundation, which wants to return the tree to its original range that once stretched from Maine to Mississippi.

The American chestnut tree, sometimes called the “Eastern redwood,” can grow rapidly to more than 100 feet, produce prodigious quantities of nutritious chestnuts, and provide lumber valued for its straight grain and durability.

But it had little defense against blight and root rot of foreign origin. However, another type of chestnut evolved alongside these diseases. The Chinese chestnut tree had been introduced for its valuable nuts and its ability to resist disease. But it is not as large or competitive in American forests, nor has it played the same critical role in supporting other species.

So, the authors want 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 achieving it for decades and have made some progress.

But it’s been difficult because the American chestnut tree’s desirable traits are scattered in several places along its genome, the string of DNA that tells the tree how to grow and function.

“It’s a very complex trait, and in this case you can’t just select one thing because you’ll select related things that are negative,” said John Lovell, lead author and researcher at the HudsonAlpha Genome Sequencing Center.

Select only for disease resistance and the trees become shorter and less competitive.

To solve this problem, the authors sequenced the genome of several types of chestnuts and found the many locations that correlated with the desired traits. They can then use this information to select trees that are more likely to exhibit desirable traits while still retaining large amounts of American chestnut DNA – around 70 to 85 percent.

And genetic testing allows the process to move more quickly, revealing the best offspring years before their traits are demonstrated by natural growth and disease. The smaller the gap between generations, the faster the gains accumulate.

Steven Strauss, a professor of forest biotechnology at Oregon State University who was not involved in the study, said the paper identified some promising genes. He wants scientists to be able to edit genes themselves, which could result in a better tree faster and more precisely. In a commentary in Science, he says regulations can bog down these ideas for years.

“People just don’t consider biotechnology because it’s on the other side of this social and legal barrier,” and that’s short-sighted, he said.

For those who have closely studied the American chestnut, the work raises an almost existential question: To what extent can the American chestnut be modified and still remain an American chestnut?

“The American chestnut has a unique evolutionary history, it occupies a specific place in the North American ecosystem,” said Donald Edward Davis, author of The American Chestnut, an Environmental History. “Having this tree and no other tree would kind of be the benchmark.”

He said the tree was a keystone species, useful to humans and vital to larger populations of squirrels, chipmunks and black bears – hybrids might not be as majestic or effective. He was glad that the authors included some surviving American chestnuts in their proposal, but he favored an approach that relied more on them.

“Not that the hybrid approach is inherently bad, it’s just why not try to bring American wild trees back into the forest, into the ecosystem, and exhaust all the possibilities before moving 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 to create tall, resilient trees, and they are optimistic.

“I think if we only select for American chestnut genes (tree genes), period, the pool is going to be too small and we’re going to end up with a genetic bottleneck that will lead to extinction in the future,” Lovell said.

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