ISIS Report 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.
Prof. Joe Cummins and
Dr. Mae-Wan Ho report
A longer,
fully referenced
version is posted on ISIS members’ website.
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 "Australia adopts GM cotton" and
"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 "Australia adopts GM cotton", this
series).
The regulation of Bollgard II has been ‘fast 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.
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