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  • Picture a forest full of gigantic trees soaring 30 meters into the sky with five-meter wide trunks.

  • You probably envisioned something like the giant sequoias and redwoods that grow on the

  • western coast of the United States.

  • But a little over a century ago, the east coast of America was also home to giant trees.

  • Though somewhat smaller than their western counterparts, American chestnuts were huge,

  • and they were all over the eastern US at the dawn of the 20th century.

  • Then, within a few decades, they were almost extinct.

  • The culprit: a fungus that strangled the trees from within, brought by accident from Asia.

  • Since their demise, scientists have been trying to figure out if there's a way to bring

  • the American chestnut back.

  • And thanks to technological advances, they may finally have a solutionif they can

  • convince the government to let them plant genetically modified trees.

  • To understand what happened to the American chestnut, we have to go back in time to the end of the 19th century.

  • Back then, American chestnut trees were known as the 'Sequoias of the East' because

  • they had huge trunks and were tall like the West Coast giants.

  • And they were all over.

  • In 1900, around a quarter of the hardwood trees east of the Mississippi were American

  • chestnutsin some places, they made up as much as 40% of the forest.

  • But by the 1940s, they were all but gone.

  • The first signs of trouble were seen in the Bronx Zoo in 1904, when sores called cankers

  • were discovered on a stand of dying trees.

  • Scientists soon realized the disease was widespread, and by 1912, botanists had managed to identify

  • both the fungus responsible and its point of origin.

  • The chestnut blight fungus gets under tree bark by hitching a ride on insects.

  • The fungus then attacks and feeds off of the trees water-transmitting cambium tissues, essentially choking the tree.

  • The blight fungus probably arrived in New England in the 1870s, when Japanese chestnut

  • trees became popular ornamental plants.

  • The imports are resistant to the blight, so it's likely they carried it to America where

  • the chestnut trees were totally susceptible.

  • And by the 1940s, it's estimated that nearly 4 billion trees had died.

  • But they didn't go extinct entirely.

  • A few scattered populations still exist, mostly trees that people planted outside of their original range.

  • There are also smaller specimens along the east coast that were isolated enough from their kin to avoid infection.

  • And it turns out that, like the Dread Pirate Roberts, even thedeadtrees are only mostly dead.

  • While the blight destroyed their trunks, their root systems remained.

  • And even decades later, theseliving stumpsoccasionally eke out a shoot of new growth.

  • But it's usually in vain because the blight is still around.

  • Although doesn't do much damage to them, it's lurking in those oaks that took over

  • after the chestnuts were wiped out.

  • So before any chestnut shoots can reach reproductive maturity, they catch the blight.

  • But where there's growth, there's hope, so scientists have been trying to figure out

  • a way to bring American chestnuts back to their former glory.

  • Since the 1980s, forestry specialists and geneticists have tried all sorts of things

  • make blight-resistant trees.

  • They attempted a technique called backcrossing, for example, where surviving specimens and

  • their offspring were carefully bred together to select for natural resistance genes.

  • But, while this method seems to work for European chestnuts, it hasn't worked as well with

  • American onesprobably because the European ones were more resistant to begin with.

  • Researchers have also tried hybridizing American chestnuts with blight-resistant Chinese chestnuts,

  • but so far, they haven't been able to get the resistance traits to reliably pass down

  • from generation to generation.

  • But one method that does seem to work is genetically modifying the trees.

  • It turns out that wheat rust, a fungal disease of wheat, has a similar mechanism of infection to chestnut blight.

  • Both use a compound called oxalic acid to soften up important structural tissues, while

  • also attacking their hosts' cambium by stimulating the growth of calcium oxalate crystals, blocking the flow of nutrients.

  • Resistant forms of wheat produce an enzyme called oxalate oxidase, which breaks down

  • the acid, thereby blocking the dispersal of the disease and preventing the growth of those crystals.

  • Scientists have introduced this wheat gene into American chestnuts.

  • And in 2014, they revealed that they'd produced a 100% resistant tree that passed the trait

  • onto its offspringsuccess!

  • Butthe trees haven't been planted.

  • Yet.

  • The researchers have conducted some preliminary studies to show the trees don't cause any

  • unexpected harm to the organisms that live in the environments they once inhabited.

  • And then, they requested permission from the US Department of Agriculture to release the transgenic trees into the wild.

  • But they're still waiting for the green light.

  • And that could take a while, if it's ever granted at all.

  • Aside from the general anxiety that accompanies the development of any GMO, some ecologists

  • worry that a return of the American chestnut would disrupt a century-old ecosystem that's developed without it.

  • On the other hand, if successfully put in action, this method could also work for restoring

  • other wild tree populations beleaguered by fungal invasives, like elm trees.

  • I guess only time will tell if the Sequoia of the East will once again stand tall.

  • Thanks for watching this episode of SciShow!

  • If you liked learning about the efforts to revive the American chestnut, you might like

  • our episode on how scientists could bring extinct animals back.

  • And if you like what you see in general, click that subscribe button to catch every episode!

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失われたアメリカ栗の木を持ち帰る (Bringing Back the Lost American Chestnut Tree)

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    林宜悉 に公開 2021 年 01 月 14 日