Can antibiotic resistance genes used as
markers in transgenic crops be transferred to pathogenic bacteria? The
chain of events that would be necessary for such a transfer is quite
unlikely, and there is no evidence that it occurs. However, in response to
concerns about this remote possibility scientists are starting to use
alternative marker genes in transgenic plants, such as the GFP gene which
makes the plant fluoresce when placed under UV light. See Iamtham and Day,
2000, Zuo et al., 2001, and Daniell et al., 2001 for reports of possible
alternatives to the current practice. A detailed 1998 report by the U.S.
Food and Drug Administrations on the use of antibiotic resistance genes is
available at http://vm.cfsan.fda.gov/~dms/opa-armg.html.
See also Kaeppler 2000.
Are other parts of the inserted genetic
material a health hazard? Some people worry that the cauliflower
mosaic virus (CaMV) promoter, which has been used in many transgenic
crops, might escape the normal digestive breakdown and insert itself into
a human chromosome where it could turn on genes. The chain of events that
would be necessary for this development is unlikely, and there is no
evidence that it occurs. Research on plasmid DNA in rice by Kohli et al.
1999 is cited as evidence that the CaMV promoter can insert itself into
strands of DNA. A fuller discussion of this viewpoint, by opponents of
transgenic crops, is available at http://www.btinternet.com/~nlpwessex/Documents/camv.htm.
A discussion of the details from the Kohli paper, also by opponents of
transgenic crops, is available at http://www.netlink.de/gen/Zeitung/1999/990715.htm.
Many experts believe that the risk from eating foods containing the CaMV
promoter is extremely small. CaMV infects several common crop plants,
including cauliflower, broccoli, cabbage, bok choy, and canola. People and
animals have been eating the CaMV promoter for as many years as these
crops have been cultivated with no documented negative effects on health.
For an opinion that insertion of the CaMV promoter by itself into other
genomes is unlikely, see http://www.foodsafetynetwork.ca/gmo/camv35s/camv35s.htm.
transgenic crops a threat to other organisms in the environment? Bt
corn, which contains a bacterial gene enabling the plants to manufacture a
substance toxic to the larvae of butterflies and moths but harmless to
other organisms, has been a target of criticism since a laboratory study
showed that Bt corn pollen dusted onto milkweed leaves was harmful to
monarch butterfly larvae feeding on them (Losey et al. 1999). Follow-up
studies have shown that pollen from Bt corn rarely reaches toxic levels on
milkweed in the field even when monarch butterfly larvae are feeding on
plants adjacent to a corn field (Strickland 1999). The latest findings
from a study done in Canada are available at http://inspection.gc.ca/english/plaveg/pbo/btmone.shtml.
We review the research on this issue in our page on the Monarch butterfly.
transgenic crops reduce biodiversity? It has been argued that
transgenic crops will replace traditional crop varieties, especially in
developing countries, causing loss of biological diversity. This risk is
real, but not restricted to transgenic crops. Farmers around the world
have adopted new commercial varieties in the past and they will continue
to do so as long as it is to their advantage. What is needed is better
conservation of traditional crop varieties in danger of being lost, so
that the useful genes they contain can be preserved.
Larva of monarch butterfly
feeding on milkweed
Will pollen from transgenic
crops contaminate non-transgenic crop varieties? More information is
needed about the extent of this risk for different crops. However, a
recently completed study at the University of Maine found that
cross-pollination of conventional corn by transgenic corn grown in an
adjacent plot was 1% at a distance of 100 feet and declined to zero at a
distance of 1000 feet. This suggests that it will be quite feasible to
prevent the transfer of transgenes to non-transgenic varieties by
following recommended planting distances, just as is currently done to
maintain purity with conventional varieties. See also Barton and Dracup
2000, Treu and Emberlin, 2000.
herbicide-resistant transgenic crops create "superweeds"?
This is a real threat where transgenic crops containing a
herbicide-resistance gene grow alongside closely related weed species,
such as wild mustards in canola or jointed goatgrass in wheat. Gene
movement from crop to weed through pollen transfer has been demonstrated
for both of these crops. Proposals to reduce the risk of creating
transgenic "superweeds" include linking
Jointed goatgrass (left)
and wheat (right)
herbicide-resistance genes to other genes which are harmless to the crop
but damaging to a weed, such as genes which affect seed dormancy or
prevent flowering in the next generation. Thus if a weed did acquire a
herbicide-resistance gene from a transgenic crop, its offspring would not
survive to spread the herbicide resistance through the weed population.
Traditional Mayan farmer harvesting corn
Will insect pests become resistant to Bt
A major concern is that the widespread planting of transgenic crops
containing the insecticidal Bt gene will expose many more insect pests to
the Bt toxin, accelerating the development of Bt resistance in pest
populations. We already know this can happen: Bt resistance has been
widely documented in the diamondback moth (Plutella xylostella), a
major pest of vegetable crops, in the US and in some Asian countries. This
resulted from many years of growers using Bt as an
environmentally-friendly insecticidal spray. Decreased susceptibility to
Bt has recently been documented in soybean loopers collected from Bt
cotton, indicating that resistance can arise from exposure to transgenic
crops (see Mascarenhas et al. 1998). Strategies such as the use of
non-transgenic "refuge"areas have been proposed to prevent or
slow the development of Bt resistance in pest populations - for more
information on this topic go to the Current Transgenic Products section of
this website and click on Insect Resistance to Bt Toxins at the top
of the page. Ferre and Van Rie 2002 discuss the biochemistry and genetics
of insect resistance to Bt. See also Barton and Dracup 2000.
Will the widespread adoption of
transgenic crops lead to increased corporate control of the world's food
Research and development leading to the production of transgenic crop
varieties is expensive and requires resources unavailable to many
developing countries. Critics of the technology point out that ownership
and control of the transgenic crops currently grown is in the hands of a
small number of powerful corporations, which often impose restrictive
conditions on farmers buying their seed. This trend has been accelerated
by the widespread patenting of plant genes and germplasm in the US and
some other countries. For more detailed discussion of some of these
issues, see AgBioForum Vol.1 No.2 (Fall 1998), available at http://www.agbioforum.org/
under Archives Jordan 2000, or browse http://www.cgiar.org/biotech/rep0100/contents.htm
. An extremely critical review of consolidation in the agricultural
biotechnology industry can be found in "The Gene Giants",
available at http://www.rafi.org/