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基
因
作
物
的
现
状
与
未
来
系别：信工
姓名：周旭
学号：
082
2008020204
The world seems
increasingly divided into those who favor
genetically
modified (GM) foods and
those who fear them. Advocates assert that growing
genetically altered crops can be kinder
to the environment and that eating foods
from those plants is perfectly safe.
And, they say, genetic
engineering
－
which
can induce plants to grow in poor soils
or to produce more nutritious
foods
－
will
soon
become an essential tool for helpi the world's
burgeoning population.
Skeptics contend
that GM crops could pose unique risks to the
environment and
to
health
－
risks too troubling
to accept placidly. Taking that view, many
European countries are restricting the
planting and importation of GM
agricultural products. Much of the
debate hinges on perceptions of safety. But
what exactly does recent scientific
research say about the hazards? The
answers, too often lost in reports on
the controversy, are served up in the pages
that to feed
Two years ago
in Edinburgh, Scotland, eco-vandals stormed a
field,
crushing canola plants. Last
year in Maine, midnight raiders hacked down more
than 3,000 experimental poplar trees.
And in San Diego, protesters smashed
sorghum and sprayed paint over
greenhouse walls.
This far-flung outrage took aim at
genetically modified crops. But the
protests backfired: all the destroyed
plants were conventionally bred. In each
case, activists mistook ordinary plants
for GM varieties.
It's easy to understand
why. In a way, GM crops
－
now
on some 109 million
acres of farmland
worldwide
－
are invisible. You
can't see, taste or touch a gene
inserted into a plant or sense its
effects on the environment. You can't tell, just
by looking, whether pollen containing a
foreign gene can poison butterflies or
fertilize plants miles away. That
invisibility is precisely what worries people.
How, exactly, will GM crops affect the
environment
－
and when will we
notice?
Advocates of GM, or transgenic, crops
say the plants will benefit the
environment by requiring fewer toxic
pesticides than conventional crops. But
critics fear the potential risks and
wonder how big the benefits really are. “We
have so many questions about these
plants,” remarks Guenther Stotz
ky, a
soil
microbiologist at New York
University. “There's a lot we don't know and need
to
find out.”
As GM crops multiply in the
landscape, unprecedented numbers of
researchers have started fanning into
the fields to get the missing information.
Some of their recent findings are
reassuring; others suggest a need for
vigilance.
Fewer Poisons in
the Soil?
Every year u.s. growers shower crops
with an estimated 971 million pounds
of
pesticides, mostly to kill insects, weeds and
fungi. But pesticide residues
linger on
crops and the surrounding soil, leaching into
groundwater
running into streams and
getting gobbled up by wildlife. The constant
chemical
trickle is an old worry for
environmentalists.
In
the mid-1990s agribusinesses began advertising GM
seeds that promised
to reduce a
farmer's use of toxic pesticides. Today most GM
crops
－
mainly
soybean, corn, cotton and
canola
－
contain genes
enabling them to either resist
insect
pests or tolerate weed-killing herbicides. The
insect-resistant varieties
make their
own insecticide, a property meant to reduce the
need for chemical
sprays. The
herbicidetolerant types survive when exposed to
broad-spectrum
weed killers,
potentially allowing farmers to forgo more
poisonous chemicals
that target
specific weed species. Farmers like to limit the
use of more
hazardous pesticides when
they can, but GM crops also hold appeal because
they simplify operations (reducing the
frequency and complexity of pesticide
applications) and, in some cases,
increase yields.
But confirming
environmental benefit is tricky. Virtually no
peer-reviewed
papers have addressed
such advantages, which would be expected to vary
from
plant to plant and place to place.
Some information is available, however.
According to the U.S. Department of
Agriculture, farmers who plant
herbicidetolerant crops do not
necessarily use fewer sprays, but they do apply a
more benign mix of chemicals. For
instance, those who grow herbicide-tolerant
soybeans typically avoid the most
noxious weed killer, turning instead to
glyphosate herbicides, which are less
toxic and degrade more quickly.
Insect-resistant crops also
bring mixed benefits. To date, insect resistance
has been provided by a gene from the
soil bacterium Bacillus thuringiensis (Bt).
This gene directs cells to manufacture
a crystalline protein that is toxic to
certain
insects
－
especially
caterpillars and beetles that gnaw on
crops
－
but
does
not harm other organisms. The toxin gene in
different strains of B.
thuringiensis
can affect different mixes of insects, so seed
makers can select the
version that
seems best suited to a particular crop.
Of all the
crops carrying Bt genes, cotton has brought the
biggest drop in
pesticide use.
According to the Environmental Protection Agency,
in 1999
growers in states using high
amounts of Bt cotton sprayed 21 percent less
insecticide than usual
on
the crop. That's a “dramatic and impressive”
reduction,
says Stephen Johnson, an
administrator in the EPA's Office of Pesticide
Programs. Typically, Johnson says, a
farmer might spray insecticides on a
cotton field seven to 14 times during a
single growing
season. “If you choose a
Bt cotton product, you may have little or no use
for
these pretty harsh chemicals,” he
notes. Growers of Bt corn and potatoes report
less of a pesticide reduction, partly
because those plants normally require fewer
pesticides and face fluctuating numbers
of pests.
Defining the environmental risks of GM
crops seems even harder than
calculating their benefits. At the
moment, public attention is most trained on Bt
crops, thanks to several negative
studies. Regulators, too, are surveying the
risks intensely. This spring or summer
the EPA is expected to issue major new
guidelines for Bt crops, ordering seed
producers to show more thoroughly that
the crops can be planted safely and
monitored in farm fields.
In the face of mounting
consumer concern, scientists are stepping up
research into
the consequences of Bt and other GM crops. Among
their
questions: How do Bt crops affect
“nontarget” organisms
－
the
innocent bugs,
birds, worms and other
creatures that happen to pass by the modified
plants?
Will GM crops pollinate nearby
plants, casting their genes into the wild to
create
superweeds that grow unchecked?
What are the odds that the genetically
engineered traits will lose their
ability to protect against insects and invasive
weeds, leaving GM plants suddenly
vulnerable?
At What Cost to Wildlife?
In 1998 a swiss study provoked
widespread worry that Bt plants can
inadvertently harm unlucky creatures.
In this laboratory experiment, green
lacewing caterpillars proved more
likely to die after eating European corn-borer
caterpillars that had fed on Bt corn
instead of regular corn. The flames of fear
erupted again a year later, when
Cornell University entomologist John Losey
and his colleagues reported that they
had fed milkweed leaves dusted with Bt
corn pollen to monarch butterfly larvae
in the lab and that those larvae, too, had
died.
“That was the straw that broke the
camel's back,” says David Pimentel, also
an entomologist at Cornell. Suddenly,
all eyes turned to the organisms
munching GM plant leaves, nipping
modified pollen or wriggling around in the
soil below the
plants
－
organisms that play
vital roles in sustaining plant
populations. Another alarming study
relating to monarch butterflies appeared
last August.
But the lab bench is not a
farm field, and many scientists question the
usefulness of these early experiments.
The lab insects, they note, consumed far
higher doses of Bt toxin than they
would outside, in the real world. So
researchers have headed into nature
themselves, measuring the toxin in pollen
from plots of GM corn, estimating how
much of it drifts onto plants such as
milkweed and, finally, determining the
exposure of butterfly and moth larvae to
the protein. Much of that work, done
during the 2000 growing season, is slated
to be reported to the EPA shortly.
According to the agency, however,
preliminary studies evaluating the two
most common Bt corn plants (from
Novartis and Monsanto) already indicate
that monarch larvae encounter Bt corn
pollen on milkweed
plants
－
but at levels
too low to be toxic. What is toxic? The
EPA estimates that the insects face no
observable harm when consuming milkweed
leaves laden with up to 150 corn
pollen
grains per square centimeter of leaf surface.
Recent studies of milkweed
plants in
and around the cornfields of Maryland, Nebraska
and Ontario report far
lower levels of
Bt pollen, ranging from just six to 78 grains of
Bt corn pollen per
square centimeter of
milkweed leaf surface. “The weight of the evidence
suggests Bt corn pollen in the field
does not pose a hazard to monarch
la
rvae,”
concludes EPA
scientist Zigfridas Vaituzis, who heads the
agency's team
studying the ecological
effects of Bt crops.
But the jury is still out.
“There's not much evidence to weigh,” notes Jane
Rissler of the Union of Concerned
Scientists. “This i
ssue of nontarget
effects is