Science Society Sustainability
http://www.i-sis.org.uk
ISIS Press Release 20/01/05
GM Cotton that People Forgot
GM cotton has aroused relatively little resistance outside the Third
World for the simple reason that it is wrongly perceived to be a
non-food crop. <mailto:[EMAIL PROTECTED]>Prof. Joe Cummins and
<mailto:[EMAIL PROTECTED]>Dr. Mae-Wan Ho report
A longer, <http://www.i-
sis.org.uk/full/GMCTPFFull.php>fully referenced version is posted on
ISIS members' website. <http://www.i-
sis.org.uk/membership.php>Details here.
GM cotton a triple-threat
Cotton is a triple-treat (or threat) crop because it produces fibre,
food and feed. Fibre is recovered from the flower bolls, while the
seeds are pressed to yield oil for the kitchen and cake for animal
feed. Monsanto Corporation has been a major source of genetically
modified (GM) cotton lines.
Bollgard cotton
A line called Bollgard was first marketed in the United States in
1995, followed in later years by Canada, Australia, China, Argentina,
Japan, Mexico, South Africa, India and the Philippines. In 2002, an
enhanced line called Bollgard II was approved in the United States,
Canada, Australia, Japan and the Philippines.
Bollgard II was made from Bollgard simply by inserting into the plant
cells a gene cassette containing a Bacillus thuringiensis (Bt) toxin,
Cry2Ab, different from the one in the original Bollgard, Cry1Ac. From
the transformed cells, a line containing the two different Bt toxin
genes were selected. Two toxin genes were more than twice as
effective in pest control than the original Bollgard and
theoretically, far less likely to allow insect resistant mutants to
evolve.
The Bt toxin genes, unlinked, are reported to be driven by different
versions of the cauliflower mosaic virus (CaMV) 35S promoter: that of
crylAc has a duplicated enhancer, while that of cry2Ab has the
enhancer and also the leader sequence from petunia heat shock 70 gene
as an extra booster. CrylAc is accompanied by the kanamycin
resistance marker gene, nptII, while cry2Ab is accompanied by the
marker gene uidA that produces a staining reaction. CrylAc confers
resistance to lepidopteran-insects in general, and cotton bollworm,
tobacco budworm, and pink bollworm, in particular. Upon ingestion of
this protein by susceptible insects, feeding is inhibited, eventually
resulting in death.
The Bt toxin genes are both synthetic versions of the natural genes
in the soil bacterium, Bacillus thuringiensis var. kurstaki, with
coding sequences modified to improve expression in plants. The
synthetic genes have not been subject to evolution and their
recombinational and other properties relevant to safety are unknown
and untested.
Thus, Bolgard II has two separate transgene insertions with some
regions of DNA homology (similarity). Such regions could act as
recombination signals for somatic or meiotic recombination, leading
to drastic chromosome rearrangements. The claim to genetic stability
reported in the governmental reviews is simply the finding that the
insertions segregate according to Mendelian ratios in a few crosses
and does not consider molecular and chromosomal instability
associated with inter- and intra-chromosomal recombination at sites
of DNA homology. Signs of instability and other failures have been
observed in the field (see "<http://www.i-
sis.org.uk/AAGMC.php>Australia adopts GM cotton" and
"<http://www.i-sis.org.uk/GMCFATW.php>GM cotton fiascos around the
world", this series).
Seed distribution is controlled by the licenses of the patentee, and
seed lines can, and should be screened at that point for
translations, duplications or deficiencies resulting from intra- and
inter chromosomal recombination.
Furthermore, in evaluating safety to humans and the environment, the
toxin proteins are frequently isolated from liquid culture of the
bacteria to avoid having to carry out the more expensive isolation of
the toxins from cotton plants. As the toxin transgenes are synthetic
approximations of the natural genes and the toxin proteins are not
identical, the test results with bacterial proteins do not truly
represent the impact of the toxins from the transgenic cotton plants.
Some feeding studies indicated that Bollgard II cotton controlled
insect pests more effectively. One research group predicted that the
need for supplemental insecticides would be reduced or eliminated for
lepidopteran pests. Another research group indicated, however, that
insect-resistance to Bollgard II could best be controlled with an
overspray of chemical insecticide. Further studies showed that
resistance to the two Cry toxins seemed to evolve simultaneously,
raising considerable doubt over the efficacy of gene stacking in
delaying insect resistance. Studies reported by researchers from
Monsanto Corporation showed that the Cry1Ac toxin and the Cry2Ab
toxin were produced in equivalent amounts in Bollgard II, but that
Cry2Ab was the larger contributor to insect toxicity, and they
suggested a relatively simple resistance monitoring policy. It seems
likely that chemical pesticides will be needed to combat insect
resistance arising in Bollgard II after all (see "<http://www.i-
sis.org.uk/AAGMC.php>Australia adopts GM cotton", this series).
The regulation of Bollgard II has been Îfast and loose'. Bollgard II
was supposed to address the major concern of resistance management,
but research is already indicating that gene stacking is not a
panacea and that chemical pesticide overspray will be required to
cope with developing resistance.
Round up Ready Cotton
Roundup Ready cotton, like Bollgard I and II, is also used for fibre,
food and feed. Roundup Ready (rr cotton) was first marketed in the
United States in 1995, and in later years, in Canada, Japan,
Argentina, South Africa, Australia, the Philippines and in 2004, in
China.
The herbicide tolerant cotton marketed as rr cotton was originally
derived from two different transformation events of a cotton line
called Coker 312. These events, designated 1445 and 1698, differed in
both gene sequences inserted and insertion sites in the cotton
genome. Currently, event 1445 is the primary rr cotton marketed.
Event 1445 was obtained by transformation with a plasmid containing a
synthetic version of the glyphosate oxidase (gox) gene driven by a
modified figwort mosaic virus promoter and terminated by the nos
terminator tnos from Agrobacterium, plus a synthetic CP4 epsps gene
derived from Agrobacterium strain CP4 (encoding the enzyme 5-
enolpyruvylshikimate-3-phosphate synthase) preceded by a chloroplast
targeting sequence from Arabidopsis, also driven by the figwort
mosaic virus promoter, and terminated by a terminator from the pea
plant.
In addition, two antibiotic resistance marker genes were present: aad
from a bacterial transposon,Tn5, conferring resistance to
streptomycin and spectomycin, inserted after the epsps gene cassette;
followed by kanamycin resistance gene, also from Tn5 driven by the
CaMV promoter and terminated with the tnos.
In the marketed crop, event 1445 appeared to have lost the gox gene
but retained aad, which the company claims, is inactive in the cotton
plant. However, the rr cotton failed to gain approval from the
European Commission in 1999 on account of serious concerns over the
aad antibiotic resistance marker. The fact that it is inactive in
cotton plants is irrelevant, because it is surely active in bacteria,
to which it could be transferred.
Event 1698 is similar except that it has an additional epsps gene.
The events were described as being "stably inherited", with no
molecular genetic evidence.
Monsanto and the regulators seem to agree that direct human exposure
to the transgenes and their products will be very limited because
cottonseed oil contains very little protein and DNA. Nevertheless,
farm animals consume a great deal of seed cake.
Monsanto's safety assessment of rr cotton dismissed the possibility
that the epsps gene and the antibiotic resistance marker genes could
participate in horizontal gene transfer with soil bacteria. However,
the bacterial marker gene for kanamycin reistance in transgenic sugar
beet was found to readily transform soil Pseudomonas, while
transgenic potato marker gene readily transformed soil Actinobacter
through homologous recombination. In both cases, the marker persisted
for long periods in the soil bacteria and such bacteria are capable
of exchanging genes with animal pathogens. It is very likely that the
streptomycin resistance marker gene will transform soil bacteria.
Monsanto's claim, that to effectively transform bacteria the marker
genes require co-transformation with a bacterial promoter, is not
realistic; operator fusions are commonplace in bacteria, suggesting
that the marker genes can easily become activated. There are also
special mobile genetic elements called integrons containing sites
with ready-made promoters for insertion of antibiotic resistance
coding sequences so they can be expressed.
Glyphosate applications can be used to control weeds prior to
flowering, but glyphosate application after initiation of flowering
in rr cotton reduced pollen viability and seed set, resulting in
reduced yield; while glyphosate application to rr cotton combined
with water stress resulted the young cotton bolls dropping off. Use
of rr cotton seems to require irrigation technology and considerable
technical savvy.
An additional concern related to using glyphosate on cotton is that
the herbicide has been shown to move from cotton fabric into and
through human skin.
GM cotton not safe
Regulators seem to have taken a relaxed attitude towards many safety
issues including antibiotic resistance markers going into GM crops.
The potential toxicities of the synthetic genes, their ability to
recombine and stability have yet to be documented. Already, all the
transgene products, Cry1Ac, Cry2Ab, CP4 EPSPS, as well as the marker
gene product, UidA, show stretches of amino-acid sequence identities
to known allergens, and are hence suspected allergens; at least,
until proven otherwise by further studies.
This article can be found on the I-SIS website at
http://www.i-sis.org.uk/GMCTPF.php
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