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bt resistance

Neal Stewart nstewart at UGA.CC.UGA.EDU
Tue Jan 31 09:22:05 EST 1995

Someone posted asking about controversies with transgenic organisms, and 
suggested resistance to insecticidal transgenes (such as Bacillus 
thuringiensis, Bt) as one.

 Wayne Barnes wrote:
>Your controversy No. 1. is only valid for 'weak Bt genes'.  Such 
>genes could induce a low level of sickness that the insects could survive, 
>yet possibly mutate resistance to.  

I believe this view is fallacious.  When strong selection pressure is 
applied (high levels of Bt expression) then insects will evolve at a 
*faster* rate.  We've been through this treadmill many times: antibiotics, 
herbicides, insecticides, ad nauseum.  The target organisms evolve 
resistance.  The question is not will insects evolve resistance to Bt, it is 
when.  The concensus is that we must control the deployment of Bt transgenic 
plants in space and time by creating islands of refuge for insects, or layer 
in several insecticidal transgenes, or use disruptive selection, and there 
are others; to affectively increase the time that these transgenic plants 
will be effective.  
        Another wrinkle in this story is that not all lepidopterans are 
equally susceptable to Bt, for example Heliothis virescens is susceptable 
and its relative Helicoverpa zea is fairly resistant.  Even if Wayne's 
hammer analogy was true, then you would only be hitting Helicoverpa zea with 
the comic section of a daily newspaper.
        Another interesting wrinkle in this story, and one that I'm doing 
research, is the scenario of Bt transgenic plants hybridizing/introgressing 
the transgenes into wild relatives.  We are using canola, which will cross 
with several other Brassicas as well as naturalized canola.  We are testing 
whether Bt canola has a selctive advantage under selection pressure from 
insect herbivory.  Hence, some transgene/crop combinations may increase the 
weediness potential of the crop species and related species, given that the 
gene could give its host some selective advantage.
I am interested in collaborating with a theoretician in modelling some  of 
these interactions (I am an experimentalist who is producing and testing Bt 
transgenic crops).
        I think that the upshot to this argument is that we must judiciously 
choose which plant species will contain Bt, and then deploy these plants in 
a manner that will slow resistance.

Neal Stewart
3111 Miller Plant Science
University of Georgia
Athens, GA 30602-7272
voice: (706) 542-0919 or 0924
fax:   (706) 542-0914
email nstewart at uga.cc.uga.edu

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