Bt in corn: Only Monarchs

Neil Jones Neil at nwjones.demon.co.uk
Sun May 23 15:39:15 EDT 1999


In article <990522103719.ZM3766 at Gochfeld>
           gochfeld at EOHSI.RUTGERS.EDU "Michael Gochfeld" writes:

> I may have missed part of this BT thread, but why is it only Monarchs. 
> The corn pollen containing BT (if it has significant amounts of BT) 
> would fall on all sorts of leaves and kill all sorts of caterpillars. 

It isn't only Monarchs. It is very very clear that the recent 
paper is a first findings publication that indicates that more research
is needed.  Monarchs were used presumably because that lab has a reliable
stock. This has become evident in discussions on the Monarch list
dplex-l.
 
> It's difficult to sort the vested wheat from the vested chaff. Cynics 
> have said that a major outcome of the bioengineered BT, is to speed up 
> the resistance of insects to BT, thereby wiping out "organic" farming 
> which relies on BT---in case we don't have enough to worry about.

I don't know but Monsanto has come in for a lot of criticism of the
handling of the soya bean issue. As to cynics I am reminded of a quotation
by George Bernard Shaw,"The power of accurate observation is commonly
called cynicism by those who have not got it."
As ever a corporation's motive is difficult to prove. The best route is
to understand the science and make a judgement based on fact.

> It 
> would be useful to hear from some of our more knowledgeable colleagues 
> about how resistance to BT evoltes. 

Well first of all Bacillus thurigiensis is not a uniform organism.
There are many strains and these attack different invertebrates.
In ecological terms it is a lepidopteran predator which invades the body
and consumes it. The so called toxin is just a protein which facilitates
this process. On its own the toxin causes the larva severe problems
with its gut which leads to death.
B. thurigiensis produces a protein which is hightly variable in nature.
It will only disolve if the conditions inside the gut of the prey species
are correct.

This protein is not toxic but is converted by the digestive enzymes in the 
lepidopteran gut into a toxic form. This process of digestion has
according to research up to 7 different steps.
After digestion the toxic molecule attaches itself to specific receptor
molecules on the gut wall where is makes holes.
This is the toxins purpose. These holes are then used by the bacteria
to enter the body cavity of the larva. The bacteria then proceed to 
eat the bacteria.
 
Now the so called BT gene in the corn isn't
it is part of the gene. It bypasses sone of not all of the digestion steps.
The problem is that natural BT toxin is extraordinarly variable.
There are often deveral different plasmids with in the bacterium which
contain genes for expressing the toxin. These may be different from 
each other. Also B. thurigiensis posesses several transposons
or jumping genes which can add to the variability of the toxin.

As to the evolution of resistance. In the natural state it is difficult
for resistance to B thurigiensis to evolve. There are several means
by which this can happen.

1. Changes to the gut conditions (most =notably the PH) to prevent
solubility of the pre-toxin protein.

2. Changes to the gut digestive enzymes so that any one of the 7
digestive steps no longer works.

3. Changes to the gut wall receptors to prevent the attachment of the
toxin molecule.

Since there are so many different varieties of Bt it is more difficult for
a gene for total resistance to evolve. It is worth remembering that changes
like this often have a reduction in fitness. It is easy to see that
changes to gut enzymes for example can make an organism less fit in
the absence of the toxin. They will therefore be selected against
where the toxin is absent. This is rather like the oft quoted case
of Sickle Cell Anaemia in humans. One gene you don't die from malaria
2 genes you die from anaemia. The gene only persists where there
is a high level of malarial infection. 

Now the problems with the genetically engineered Bt are that

1. It is very uniform in its make up.

2. It is present in a large amount of the crop.

These conditions are good for the development of resistance. 
If a gene is present which confers resistance. It will have higher
fitness in most if the situations where the pest species is feeding.

It has also been found that the Diamondback moth can gain resistance
to 4 different strains of B. thurigiensis by the possesion of a single
gene.



-- 
Neil Jones- Neil at nwjones.demon.co.uk http://www.nwjones.demon.co.uk/
"At some point I had to stand up and be counted. Who speaks for the
butterflies?" Andrew Lees - The quotation on his memorial at Crymlyn Bog
National Nature Reserve


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