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No Bt Resistance?
Prof. Joe Cummins questions the recent
report that there has been no Bt resistance outbreaks
Worldwide, over 62 million hectares have been planted with Bt crops
GM crops engineered with Bt toxins from soil bacterium Bacillus
thuringiensis - and proponents have expressed pleased surprise that Bt
resistant insects do not seem to have evolved [1,2]. But theres more than
meets the eye.
Bt toxins do not represent a single gene product but are products of
different genes, variants of which are present in different strains of the
bacterium. Bacillus thuringiensis is fairly common in soil and creek
beds, but the varieties capable of strong insect control are rare and valuable.
Strains containing multiple unique toxins are designated israelensis (Bti),
kurstaki (Btk), azaiwai (Bta), tenbrionis (Btt) sotto (Bts), and entomocidus
(Bte), etc. The strains are differently specific for insects of the Order
Lepidoptera, Diptera or Coleoptera. The individual isolated toxin proteins are
designated CryI, CryII , CryIII or CryIV, but each of these may require further
identification related to small sequence differences. For example, .a toxin may
be designated CryIA(b), CryIIIA, CryIVD, etc. . Genes for the toxins
introduced into a crop plant are usually altered to enhance their activity.
Some codons are modified to those preferred by plants in contrast to bacteria.
Usually, an intron is introduced into the bacterial gene to enhance rapid
translocation from the plant nucleus to the cytoplasm. Examples of alterations
of Cry genes to enhance activity are included in patents [4,5].
Insects evolve resistant to individual Cry toxins and cross-resistance
appears to be limited. Resistance is most frequently due to nuclear genes,
rather than cytoplasmic genes encoded by chloroplasts and mitochondria.
Resistance can be recessive, requiring two copies of the resistance allele
(variant of a gene) to give protection against Bt toxin; dominant, requiring
only one copy of the resistance allele to give full protection against Bt
toxin; or incompletely recessive, where one copy of the allele gives partial
protection. Incompletely recessive alleles are recessive at high toxin levels
but become dominant as the toxin level decreases [6,7].
The specificity of resistance to Bt toxin is demonstrated in laboratory
experiments with the cotton bollworm. Resistant bollworm thrives on a diet
containing Cry as well as on cotton modified with a gene for Cry1Ac. But this
resistant bollworm was susceptible to commercial Bt spore formulations Dipel
and XenTari, which contains multiple toxins. The bollworm was resistant to
Cry1Ab but not to Cry2Aa or Cry 2Ab .
Thus, although millions of hectares have been planted with Bt crops, the
target for developing Bt resistance is much smaller because there are many
different Bt toxins to which the insects must develop resistance independently.
In theory, a devastating resistance to all Bt toxins could evolve, but this has
not yet been observed in laboratory or field experiments. A non-recessive
Cry1Ac-resistant mutant of tobacco budworm showed cross-resistance to a wide
array of Bt toxins , but such mutations are infrequent.
As mentioned above [1,2], there have been no outbreaks of resistant
pests in Bt crops, although such outbreaks have been observed in sprayed
populations of diamond back moth, and many laboratory experiments produced
Bt-resistant insects .
Bt toxins kill by binding to target sites in cell membranes of the
mid-gut and disrupt the membranes. One prominent mutation in resistant bollworm
involves cadherin, an adhesion protein that binds together cells in solid
tissue, thereby preventing disruption of the gut cells . Recently,
incomplete recessive alleles of Cry1Ac and Cry2Aa have been identified in
bollworm during screening of Bt-cotton crops . Apparently, the finding was
not considered an "outbreak", even though it could be the start of one.
To stave off the impending threat of resistance outbreaks, regulators
have introduced the refuge strategy, the planting of non-Bt crops
to prevent or slow the evolution of resistance. The refuge strategy is based on
the assumption that resistance will be recessive, so sensitive heterozygotes
will die from consuming Bt crop. If the mutation is dominant or or incompletely
recessive, resistance will spread despite the refuge.
Greenhouse tests showed that the refuge could prevent the spread of
resistant mutants if it was maintained as a block of non-Bt crop, rather than
as a mixed crop of Bt and non Bt plants . Regulators in North America have
set a minimum of 20% non-Bt crop in block-planting.
The introduction of the refuge has meant that farmers would have to
deal with the potential of 20% of their crops becoming infested, so regulators
allowed the refuge to be sprayed with pesticide. In a position paper produced
by the Environment Protection Agency and the United States Department of
Agriculture, it states that , "In corn growing areas (no cotton), growers
should plant a minimum of 20% non-Bt corn to serve as a refuge. In areas where
European corn borer (ECB), southwestern corn borer (SWCB), corn earworm (CEW),
or other target lepidopteran pests have historically been high, insecticide
treatment of the refuge is anticipated."
Academics have lent their authority to affirm that spraying the refuge
with pesticide was necessary to control emerging resistance [14,15]. It seems
clear, however, that regulators not only permit, but positively encourage
pesticide spraying over not just the refuge but the entire crop . The
refuge strategy is really a "double whammy" strategy! And yet, the pesticide
spray is hardly mentioned in government documents promoting refuges , or in
numerous academic publications on resistance management or in association with
the "miraculous" absence of Bt-resistance outbreaks.
That there have been no reported major outbreaks of Bt resistant
insects in the millions of hectares of Bt crop planted may be due to two major
factors that have been overlooked. The first is the numerous unique Bt alleles
used in Bt-crops, and the second is the simultaneous deployment of chemical
pesticide sprays in the non-Bt refuge as well as on Bt-crops.
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Fox J. Resistance to Bt toxin surprisingly absent from pests. Nature
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