Pests could overcome GM cotton toxins

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Pests could overcome GM cotton toxins

Caterpillars reveal a chink in the armour of transgenic crops.

Pink bollwormPink bollworms developed resistance to two Bt cotton
toxins.USDA

Laboratory studies suggest that it may be possible for insects to
overcome two disparate toxins produced by genetically modified cotton.
The results strike a cautionary note at a time when developers are
racing to create crops that produce many different pesticides.

Insects can become resistant to individual insecticides in much the
same way as bacteria develop resistance to antibiotics. One way to
reduce this threat is to adopt a 'pyramid' approach and create crops
that produce multiple toxins that target the same pest.

"This is the current trend of all the companies," says Juan Ferré, a
geneticist at the University of Valencia in Spain. "They are all
combining more than one gene to have better control and to delay
resistance." For example, next year, Monsanto, a US agricultural
products company based in St Louis, Missouri, intends to launch a line
of maize (corn) that contains eight different genes that make the crop
resistant to herbicides and to attack by insects.

“Evolution by insects is not something that scientists are going
to stop.”

Bruce Tabashnik
University of Arizona

One of the most common 'pyramided' crops on the market is cotton that
produces two different 'Bt' toxins made naturally by the bacterium
Bacillus thuringensis. The two toxic proteins, Cry1Ac and Cry2Ab, have
very different amino-acid sequences and bind to different target
sites.

As a result, mutations that confer resistance to both toxins were
thought to be unlikely, says Bruce Tabashnik, an entomologist at the
University of Arizona in Tucson. "The main way that insects become
resistant is by altering the binding site of the toxin," he says.
"These two toxins don't bind to the same site — if the insects altered
the Cry1Ac binding site, it's not going to give cross resistance to
Cry2Ab."

But when Tabashnik and his colleagues tried to selectively breed
insects that were resistant to Cry2Ab, they found that that some were
also resistant to Cry1Ac. The results are reported this week in
Proceedings of the National Academy of Sciences1.
Arms race

The researchers were studying pink bollworm (Pectinophora gossypiella)
— a particular nuisance in the cotton fields of the southern United
States. Crops expressing Cry1Ac have thus far largely held the pest at
bay, and there has been no sign of Cry1Ac resistance emerging in the
insects.

Tabashnik wanted to learn more about how insects may become resistant
to the less-studied Cry2Ab protein, so the team raised a number of
different laboratory strains of pink bollworms on a diet that
contained the toxin. To their surprise, they generated a strain of
pink bollworm that was not only resistant to 240-times higher levels
of Cry2Ab than normal, but also to 420-times higher concentrations of
Cry1Ac.

Although the binding sites of the two toxins differ, both toxins are
activated via the same pathway in the insect. A change in the protease
responsible for activating the toxins could provide an avenue to cross-
resistance, Tabashnik says. Other changes in the insect's ability to
cope with damaged cells could also play a part, says Ferré, who was
not involved with the study.

The results show that cross-resistance between the two toxins is
possible. But "this does not pose a threat for control by the current
pyramided Bt cotton of this insect", Tabashnik says. The resistant
pink bollworms were able to withstand high concentrations of both
toxins in their diets, but they were not able to survive the higher
concentrations of Cry2Ab found on cotton bolls produced by the
pyramided transgenic cotton.

Ferré urges caution on extrapolating laboratory results to the field.
"This is a special condition in the laboratory," he says. "The
important thing is to find out whether that resistance can be obtained
in the field."

Nevertheless, the results do highlight the continued threat of
resistance, adds Tabashnik. "Pyramids are not a panacea," he says.
"Evolution by insects is not something that scientists are going to
stop."