Trouble on the horizon for GM crops?
Now for the first time, the international team gathered genetic evidence from pests in the field, enabling them to directly compare the genes involved in the resistance of wild and lab-reared populations.
They found some resistance-conferring mutations in the field were the same as in lab-reared pests, but some others were strikingly different.
"We found exactly the same mutation in the field that was detected in the lab," Tabashnik said. "But we also found lots of other mutations, most of them in the same gene and one in a completely different gene."
A major surprise came when the team identified two unrelated, dominant mutations in the field populations. "Dominant" means that one copy of the genetic variant is enough to confer resistance to Bt toxin. In contrast, resistance mutations characterized before from lab selection are recessive—meaning it takes two copies of the mutation, one provided by each parent, to make an insect resistant to Bt toxin.
"Dominant resistance is more difficult to manage and cannot be readily slowed with refuges, which are especially useful when resistance is recessive," Tabashnik said.
Refuges consist of plants that do not have a Bt toxin gene and thus allow survival of insects that are susceptible to the toxin. Refuges are planted near Bt crops with the goal of producing enough susceptible insects to dilute the population of resistant insects, by making it unlikely two resistant insects will mate and produce resistant offspring.
According to Tabashnik, the refuge strategy worked brilliantly against the pink bollworm in Arizona, where this pest had plagued cotton farmers for a century, but is now scarce.
The dominant mutations discovered in China throw a wrench in the refuge strategy because resistant offspring arise from matings between susceptible and resistant insects.
He added that the study will enable regulators and growers to better manage emerging resistance to Bt crops.
"We have been speculating and using indirect methods to try and predict what would happen in the field. Only now that resistance is starting to pop up in many places is it possible to actually examine resistance in the field. I think the techniques from this study will be applied to many other situations around the world, and we'll begin to develop a general understanding of the genetic basis of resistance in the field."
The current study is part of a collaboration funded by the Chinese government, involving a dozen scientists at four institutions in China and the U.S. Yidong Wu at Nanjing Agricultural University designed the study and led the Chinese effort. He emphasized the importance of the ongoing collaboration for addressing resistance to Bt crops, which is a major issue in China. He also pointed out that the discovery of dominant resistance will encourage the scientific community to rethink the refuge strategy.