Ithaca, new York
June 17, 2005
Genetically modified crops containing two
insecticidal proteins in a single plant efficiently kill
insects. But when crops engineered with just one of those toxins
grow nearby, insects may more rapidly develop resistance to all
the insect-killing plants, report
Cornell University
researchers.
|
Jian-Zhou
Zhao
|
This picture
shows the damage done to broccoli plants with,
from left, zero, one and two Bt genes, when
caged with diamondback moths resistant to one Bt
protein.
Copyright © Cornell University
|
|
A soil bacterium called Bacillus thuringiensis (Bt), whose genes
are inserted into crop plants, such as maize and cotton, creates
these toxins that are deadly to insects but harmless to humans.
Bt crops were first commercialized in 1996, and scientists,
critics and others have been concerned that widespread use of Bt
crops would create conditions for insects to evolve and develop
resistance to the toxins.
Until now, it has not been shown if neighboring plants producing
a single Bt toxic protein might play a role in insect resistance
to transgenic crops expressing two insecticidal proteins.
"Our findings suggest that concurrent use of single- and
dual-gene Bt plants can put the dual-gene plants at risk if
single-gene plants are deployed in the same area
simultaneously," said Anthony Shelton, professor of entomology
at Cornell's College of Agriculture and Life Sciences and an
author of the study, which was posted online June 6 in the
Proceedings of the National Academy of Sciences (PNAS) and is in
the June 14 print edition of the journal. "Single-gene plants
really function as a steppingstone in resistance of two-gene
plants if the single gene plants contain one of the same Bt
proteins as in the two-gene plant."
Cotton and maize are the only commercial crops engineered with
Bt genes. In 2004 these crops were grown on more than 13 million
hectares (about 32 million acres) domestically and 22.4 million
hectares (more than 55 million acres) worldwide. After eight
years of extensive use, there have been no reports of crop
failure or insect resistance in the field to genetically
modified Bt crops, Shelton said. Still, several insects have
developed resistance to Bt toxins in the lab, and recently,
cabbage loopers (a moth whose larvae feed on plants in the
cabbage family) have shown resistance to Bt sprays in commercial
greenhouses.
|
Joe
Ogrodnick/NYSAES
|
An adult
diamondback moth Copyright
© Cornell University |
|
So far, only diamondback moths, which were used in this study,
have developed resistance to Bt toxins in the field. The
resistance resulted from farmers and gardeners spraying Bt toxin
on plants for insect control, a long-standing practice. While Bt
toxin sprayed on leaves quickly degrades in sunlight and often
does not reach the insect, genetically modified (GM) Bt plants
express the bacterium in the plant tissue, which makes Bt plants
especially effective against insects that bore into stems, such
as the European corn borer, which causes more than $1 billion in
damage annually in the United States.
In greenhouses at the New York State Agriculture Experiment
Station in Geneva, N.Y., the researchers used three types of GM
broccoli plants: two types of plants each expressed a different
Bt toxin, and a third -- known as a pyramided plant -- expressed
both toxins. Elizabeth Earle and Jun Cao, co-authors of the PNAS
paper and members of the Department of Plant Breeding and
Genetics at Cornell created the plants.
For their studies, the researchers employed strains of
diamondback moth that were resistant to each of the Bt proteins.
The combination of Bt plants and Bt-resistant insects allowed
them to explore the concurrent use of single- and dual-gene Bt
plants in a way that could not be done with cotton or maize,
although their results are relevant to these widely grown
plants.
First, the researchers bred moth populations in which a low
percent of the moths were resistant to a single Bt toxin. The
insects were then released into caged growing areas with either
single-gene plants, dual-gene plants or mixed populations and
allowed to reproduce for two years.
|
Joe
Ogrodnick/NYSAES
|
Cornell researchers, from left,
Jun Cao, Elizabeth Earle, Anthony Shelton and
Jian-Zhou Zhao
Copyright © Cornell University
|
|
The researchers found that after 26 generations of the insect
living in the greenhouse with single-gene and dual-gene plants
housed together, all the plants were eventually damaged by the
insects, because over time, greater numbers of insects developed
resistance to the plants' toxins. However, in the same two-year
time frame, all or
almost all of the insects died when exposed to pyramided plants
alone.
"It's easier for an insect to develop resistance to a single
toxin," said Shelton. "If an insect gets a jump on one toxin,
then it becomes more rapidly resistant to that same toxin in a
dual-gene plant. And when one line of defense starts to fail, it
puts more pressure on the second toxin in a pyramided plant to
control the insect," Shelton added.
While single-gene Bt plants are most prevalent, industry trends
suggest that pyramided plants may be favored in the future. In
Australia, the use of single-gene Bt cotton was discontinued two
years after farmers began planting dual-gene cotton in 2002. In
the United States, companies introduced dual-gene cotton in
2003, but single-gene varieties remain on the market.
"Single-gene Bt plants have provided good economic and
environmental benefits, but from a resistance management
standpoint they are inferior to dual-gene plants. U.S.
regulatory agencies should consider discontinuing the use of
those single-gene plants as soon as dual-gene plants become
available," Shelton said. "And industries should be encouraged
to create more dual-gene plants."
Along with effective insect control, pyramided plants have an
added advantage of requiring a smaller refuge -- a part of the
field where non-Bt plants are grown. Refuges create
opportunities for Bt-resistant insects to mate with other
insects that do not have resistance. The offspring of such a
mating will be susceptible to the toxins.
"Having a refuge is a good management strategy, but it is not
suitable for small farmers in China and India," said lead author
Jian-Zhou Zhao, a senior research associate in entomology at
Cornell. "The two-gene strategy is more suitable in developing
countries like China where farmers have an average of half a
hectare (1.2 acres) of
land, much less land than American farmers, and not enough to
spare for refuges."
A U.S. Department of Agriculture Biotechnology Risk Assessment
Program grant supported the study.
Related World Wide Web sites:
New York State Agriculture Experiment Station: <http://www.nysaes.cornell.edu/>
Agricultural Biotechnology -- USDA: <http://www.usda.gov/agencies/biotech/>
Field refuges prevent moth's resistance to genetic insecticides,
Cornell scientists show in the latest Nature Biotechnology
(March 3, 2000): <http://www.news.cornell.edu/releases/March00/BrocMoth.bpf.html>
Entomologists show evidence of better insect control with 'gene
pyramiding' (Dec. 11, 2003):
<http://www.news.cornell.edu/Chronicle/03/12.11.03/gene_pyramiding.html>
Researchers take issue with recent studies on genetically
engineered crops (Sept. 16, 1999):
<http://www.news.cornell.edu/Chronicle/99/9.16.99/Shelton.html>
Toxic pollen from widely planted, genetically modified corn can
kill monarch butterflies, Cornell study shows (May 19, 1999): <http://www.news.cornell.edu/releases/May99/Butterflies.bpf.html> |