Conservation biologists have taken up many of these tools. They learned
how to sequence DNA, for example, so that they could map populations of
endangered species and track the flow of genes between them. They’ve
used advanced reproductive technology to raise their success rate with
captive breeding programs. The
San Diego Zoo has
frozen stem cells and tissues from thousands of species of animals to investigate for new ways to conserve them in the wild.
But conservation biologists have also seen some risks to
biotechnology. If a synthetic organism could establish itself in the
wild, for example, it could become an invasive species, putting native
species at risk. (It’s important to point out that there’s no evidence
that such an invasion has happened yet.) If we think of biodiversity as
the world’s storehouse of genetic variation, then biotechnology has the
potential to drive it down. Genetically engineered plants or animals
might interbreed with wild relatives and spread their modified genes
into the environment, reducing genetic variation in the wild.
Here’s a passage from the meeting’s framing statement:
“Critics have focused on the threats posed by novel life
forms released into the environment, but little attention is paid to
potential opportunities–to reconstruct extinct species or create
customized ecological communities designed to produce ecosystem
services. They may change the public perception of what is “natural” and
certainly challenge the notion of evolution as a process beyond human
construction.”
 |
One of the few surviving American chestnuts,
located in Maine.
Photo courtesy of William Powell |
To me, there’s no better example of the ambiguous future of conservation biology than the story of the American chestnut.
When Europeans arrived in North America, they found forests filled
with American chestnut trees. These mighty plants, which could grow to
be 100 feet tall, were the most abundant trees in the forests, making up
25 percent of the standing timber of the eastern United States. In the
summer, the peaks of Appalachian mountains appeared to be capped with
snow, thanks to the explosion of white chestnut flowers. Chestnut trees
anchored the ecosystems of eastern American forests, providing food and
shelter to bears, Carolina parakeets, and a vast number of other
species. They were also a mainstay of loggers, who could fill an entire
train car with boards cut from a single tree.
In 1904, a scientist observed that a chestnut tree at the Bronx Zoo
was dying. It turned out to be infected with a fungus that came to be
known as chestnut blight. No one is quite sure how it got to the United
States, but all the evidence we have indicates it hitch-hiked its way in
the 1870s on chestnut trees imported from Japan.
Chestnut blight, while harmless to Asian trees, proved devastating to
the American ones. The fungi released a toxic substance called oxalic
acid that killed off the tissue, allowing them to feed on it. An
infected tree developed cankers on its trunk, and once they spread
around the full circumference of a tree, it could no longer carry water
and nutrients from its roots to its branches.
 |
| A stand of blight-infected chestnuts in New York, 1915. Courtesy of William Powell |
Over the course of about eighty years, the chestnut blight spread
across almost the entire range of the American chestnut, from Maine to
Missippi. It conquered nine million acres and infected three billion
trees. A few lone trees still survive unharmed here and there, but no
one under the age of sixty has ever seen the forests of the eastern
United States as they once were.
In the pantheon of extinction, American chestnuts are poised
awkwardly at the door. Chestnut blight doesn’t kill the trees outright;
as it spreads down to the roots, it encounters other microbes that
outcompete it. As a result, infected trees become stumps. Sometimes they
send up a new shoot, but once it reaches a few feet in height, the
fungus attacks it again, and the shoot dies back.
“It’s basically functionally dead,” William Powell of SUNY College of
Environmental Science and Forestry in Syracuse, New York, told me.
“They sprout up, they get the blight again, and they are killed down to
the ground. You know the story of Sisyphus? The guy who rolled the rock
up the hill and it just kept rolling back down? Well, that’s kind of
like what’s happening with the chestnut.”
It’s been a century since American foresters started trying to save
the tree. They sprayed the trees with fungicial chemicals, to no avail.
They infected the blight with fungus-invading viruses, but resistant
strains continued to kill trees. They tried burning down chestnut trees
to create a fungal firebreak, only to discover that the blight could
silently infect oak trees, too.
They did what conservationists have always done–try to remove the threat–but nothing worked.
In the 1980s, a group of scientists embarked on a different approach,
one that is now showing signs of success. If they couldn’t stop the
blight, they would help the trees defend themselves.
The reason that chestnut blight was able to come to America in the
first place was that Asian chestnuts can fight the fungus. They have
genes that allow them to hold the cankers in check and scar them over.
The trees can continue to grow and produce pollen and seeds. American
chestnuts, evolving thousands of miles across the Pacific, never got the
opportunity to evolve defenses against the blight. So the American
Chestnut Foundation, a non-profit established to save the tree, decided
to start breeding the two trees together, to see if they could provide
the American chestnuts with Asian defenses.
When the foundation’s scientists interbred the American and Asian
trees, the plants mixed together their genes in different combinations
in their hybrid seeds. The scientists grew the seeds into saplings, and
after a few years, it became clear that some of the hybird chestnuts had
inherited some of the Asian defense genes. The cankers grew more slowly
on them than on their American ancestors.
But the trees were no longer recognizable as American chestnuts,
since half of their DNA came from Asian chestnuts. Asian chestnuts are
small, orchard-like trees, and so the hybrids were far smaller than
their towering American ancestors. These hybrids were not the solution
to the chestnut blight, in other words. Their defenses were still weak,
and they would not survive in American forests in the shadow of oaks and
other big trees.
So the scientists kept breeding the trees. They used another
tried-and-true method, known as backcrossing. They bred the
American-Asian hybrids with American chestnuts, producing trees with
only a quarter of their DNA coming from Asian chestnuts. Again, some of
the new trees could resist the blight, while the others couldn’t. That
was because the quarter of their DNA from the Asian trees contained the
genes essential for fighting the disease. At the same time, the trees
more closely resembled American chestnuts, because they inherited more
of their DNA.
From this generation, the scientists picked the best-defended trees
and back-crossed them again. They also mated hybrids with one another,
shuffling the genes into new combinations, and selectively breeding the
chestnuts that were both more resistant and bigger. They’ve now got
thousands of trees that are 15 parts American and one part Asian growing
on their experimental farm in Virginia.
 |
| A diagram of backcrossing experiments. American Chestnut Foundation |
That one-sixteenth of Asian chestnut DNA may not sound like a lot, but it is. “There are thousands of genes in there,” says Powell. For all we know, some of those genes may impair the success of chestnuts in American forests. “It’s better to be precise about the genes you put in,” Powell argues. Working with the American Chestnut Foundation, he and his colleagues have developed a surgical approach to breeding resistant chestnut trees.
In 1990, Powell and some colleagues started investigating how to move
single genes into American chestnuts. It took years to get the project
off the ground. You can’t insert genes into a tree simply by sticking a
needle into a trunk. Genes can only be inserted into individual cells.
So Powell and his colleagues had to figure out how to rear chestnuts in
their lab.
Some plants can survive as cells in a lab forever. But chestnuts are
not one of those plants. Powell and his colleagues found that they had
to combine pollen and ovules to produce embryos. With just the right
concentration of hormones, the embryos bud off more embryos, which bud
off embryos in turn. The scientists can then pick off individual
embryonic cells, insert genes into the, and then grow the cells into
full-blown chestnut trees.
After figuring all of this out, the scientists began to search for
genes to insert into the chestnut cells. At the time, no one had mapped
Chinese chestnut genes, so Powell and his colleagues turned to better
studied plants. Plant scientists had figured out how wheat fights fungi,
making enzymes that chop up the oxyalic acid into harmless byproducts.
Powell and his colleagues inserted the wheat gene for the enzyme into
chestnut cells and then grew the cells into trees.
At first the enzyme wasn’t much help, so the scientists fine-tuned
the genes so that the chestnuts made more of it. The more oxalic acid
they made, the better they fought the chestnut blight. The scientists
eventually produced trees that could limit the cankers and heal them
over.
 |
| William Powell inspects American chestnuts with blight-inhibting enzymes. Photo: Syracuse University |
Last spring, the
New York Botanic Gardens planted a few of the
chestnuts for public display. (You can see the video of the ceremony
here.)
You can go to the gardens now and can see for yourself that the trees
are growing and thriving, despite being exposed to chestnut blight
spores wafting by. “
We want to do everything transparently,” says
Powell. “
We don’t want people to think we’re hiding anything here.”
It may be five years or longer before these trees start growing in
the wild. Powell and his colleagues need to spend a couple more years
collecting data before submitting an application to the U.S. Department
of Agriculture, and then the Environmental Protection Agency has to sign
off on the project. Even the Food and Drug Administration will have to
get in on the act, because the trees will produce nuts that people might
eat.
But the trees growing in the Bronx are not the final version Powell
hopes to see reviving America’s forests. It’s now finally possible for
him and his colleagues to explore the Chinese chestnut tree genome, and
so they’ve started hunting for blight resistance genes. One gene for
chopping up oxalic acid won’t be enough to provide full resistance,
Powell suspects. He’s pretty sure that Chinese chestnut trees have
evolved a number of genes that together render the blight harmless.
Adding in extra genes is essential, Powell believes, because the
chestnut blight is not a fixed target. It is evolving, and it will
probably be easy for it to evolve its way around just one line of
defense. Each tree will need to be equipped for many attacks from
evolved pathogens over the course of its lifetime, which can be as long
as a century. Powell suspects a few genes will provide a durable
defense, but he can’t say for sure which genes those are. So far, he and
his colleagues have identified a list of candidate genes in Chinese
chestnuts. “We’ve narrowed it down to about 900 now,” Powell told me
with a laugh.
If, a century from now, Powell’s chestnuts tower once again over the
eastern United States, how will we think of those forests? Will we think
of them as nature restored to its former glory, ecosystems thriving
once more? Or will we think of them as unnatural, the product of human
tinkering? Or both? Given the past century of struggle to save the
chestnut, the choice here is not natural versus unnatural. It’s
chestnuts versus no chestnuts. “It’s not going to fix itself,” says
Powell.
____________
(Update, 3/12: I got a good question on Twitter when I pointed readers to this post:
There are indeed companies developing patented genetically modified trees.
This article in the
Guardian
in November describes a company that has produced a
eucalyptus tree
that can grow faster and produce more wood, which could be raised in
plantations. Environmental groups like the
Sierra Club have
criticized this research because of the potential environmental damage it might lead to and called for a moratorium.
Powell and his colleagues have not patented their chestnut trees,
however, nor do they have any plans to do so. As I wrote above, they’re
searching for additional genes for resistance, and they’ve avoided
patented ones as much as possible. Once they have figured out which
genes they need to use, they will do a complete patent search. If the
genes do turn out to be patented, they’ll ask the patent holders for a
license for free use. “I view this as a not-for-profit endeavor,” says
Powell.)
____________
Bosques, servicios ecosistémicos y cambio climático
