Bt resistance

Neal Stewart nstewart at UGA.CC.UGA.EDU
Mon Feb 6 14:27:26 EST 1995


In article <55874.barnes at biodec.wustl.edu> barnes at BIODEC.WUSTL.EDU ("Wayne M. Barnes, Ph.D.") writes:
>From: barnes at BIODEC.WUSTL.EDU ("Wayne M. Barnes, Ph.D.")
>Subject: Bt resistance
>Date: 4 Feb 1995 09:59:01 -0800

>On Fri, 3 Feb 1995 23:20:26 -0600, 
>Neal Stewart (nstewart at uga.cc.uga.edu) wrote:

>>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.

>    My point is that the insects have to survive to evolve, and the 
>companies' trangenic Bt genes are so strong, they don't allow enough 
>survival for evolution.  

This is the same old story that we've heard before about _____ (you name it) 
resistance.  Organisms evolve!  All it takes is one Bt resistant insect to 
pass the resistance gene to the next generation and here we go.  Evolution 
happens!  In ecological time, probably, and in evolutionary time, certainly, 
there *will* be Bt resistant leps.  Tabashnick in Hawaii already has field 
collected diamondback moths resistant to Bt.     

>>        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.

>      'Fairly resistant' is not enough.  It just so happens that you 
>mentioned 2 of the 3 insects that I have tested my weak 'academic strength' 
>Bt gene against, and it did fine:

>Hoffmann, M., Zalom, F.G., Wilson, L.T., Smilanick, J.M. Malyj, L.D.,
>Kiser, J., Hilder, V.A., and Barnes, W.M. (1992)  Field evaluation of
>transgenic tobacco containing genes encoding Bacillus thuringiensis
>delta-endotoxin or cowpea trypsin inhibitor:  Efficacy against Helicoverpa
>zea (Lepidoptera: Noctuidae), J. Econ. Entomol. 85, 2516-2522.  

>      The 'industrial strength' genes of i.e. Monsanto, being composed of 
>synthetic, plant-preferred codons, are 100 times stronger.  The bacterial 
>codons, using usually A,U in the 3d position, are expressed poorly in plant 
>cytoplasm, worse in some plants then in others.

>    Actually, 50-100 times stronger is their conclusion.  When I read their 
>paper and did my own math from their data, I came up with 1000 times better!

We have seen incomplete kills in H. zea using transgenic plants with a 
synthetic Bt gene.  Different stages of the larvae have different 
susceptibilities within a species.  Mathmatical models by Hugh Comins and 
others have shown that insects will evolve, its just a matter of time.  The 
time to resistance may be  increased by certain management 
strategies, but evolution will occur.    Even Monsanto, who is releasing Bt 
cotton is putting in two "industrial strength" genes and is recommending 
refugia in space and time as part of their IPM.  They know that insects will 
evolve resistance, they just want to slow it down so that all the big $$$ 
they've poured into the technology won't immediately go down the proverbial 
hole.  However, once farmers are growing Bt cotton, corn, canola, wheat, 
soybean, you name it,  in the same place at the same time  insects (target and 
non-target) will evolve resistance quite rapidly.

The brute fact is that in the multifactorial ecological systems 
where Bt plants will be deployed, no one knows what will actually occur.  To 
think that strong expression *by itself* is a panacea, I think, is naive.  If 
we have not learned anything by the plethora of data on antibiotic resistance, 
insecticide resistance, etc, then we sprint fool-hardily into biotechno-hell. 
 We must remember that evolution cannot be easily studied by biochemistry 
alone.   The ramifications of the release of transgenic plants must be 
assessed in an integrative manner.  I don't believe, as many do, that 
transgenic plants are evil.  I think that they will play a strong role in the 
future of agriculture, and they are *part* of the way agriculturalist will 
deal with pest problems.  We must try to understand how they will affect 
natural and agricutural ecosystems.

BTW, is it really so that as you get closer to Monsanto's HQ in St. Louis, 
then the more you believe that transgenic plants are the silver bullet?

>Barnes at biodec.wustl.edu    -------------------------
>Wayne M. Barnes, Ph.D.             fax: 314/362-7183
>Dept. Biochemistry 8231, Washington Univ. Med.School
>4566 Scott Avenue
>St. Louis, Missouri 63110 USA       ph: 314/362-3351


Cheers,

C. Neal Stewart, Ph.D
PostDoc, Molecular Ecology
CSS Dept.
The University of Georgia
Athens GA 306020-7272





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