Bt corn flap

[Neil Jones]

In article <38e25a3b.108978 at news.freeserve.net>
           robnpam at acieed.fsnet.co.uk "Rob and Pam" writes:

> On Wed, 29 Mar 00 14:14:10 GMT, Neil at nwjones.demon.co.uk (Neil Jones)
> wrote:
> 
> >
> >The so called Toxin expressed in the BT corn is NOT identical to that
> >produced in organic control agents. 
> 
> Well, it's the delta-endotoxin gene from Bt var kurstaki, which is the
> strain used for lepidoptera control. How about this : The toxin (and
> it is a toxin, see later) expressed in Bt corn is identical to the
> major component of the toxin produced in one of the commonest organic
> control agents.

But not the entire component. The natural product contains variants. These
help to prevent resistance taking hold.

> 
> >Bacillus thurigiensis is a specialised
> >bacerial predator of invertebrates.
> 
> We don't know too much about the 'normal' ecological niche for Bt but
> it seems to be a bacterium that lives as a conventional saprophyte a
> lot of the time and only occasionally kills insects. It's certainly
> not an obligate specialised predator.

This is worrying. We don't know what this organism does but it is OK
to incorporate its ecologically active characteristics in other
organisms.
Secondly, it may well be able to survive and grow without lepidoptera but
it does not have this gene by accident.
It is an intricate and specialised molecule that fits like a key in a lock.
What is more it only fits this lock once it has been partially digested by the
caterpillar. Variants of this gene exist that do this in a variety of
other organisms. This did not happen by accident. It has obviously evolved
for this purpose. If the organism that produces this precisely formed protein
in quantity does not have a use for it then selection would soon
weed out those organisms that wasted energy producing it.

The truth is that very very little research is done on the role of pathogens
in the ecology of non-pest lepidoptera.

>  It exists is a variety of strains which
> >differ from each other. These natural predators are one of the factors which
> >influence the population dynamics of many species. For example Bacillus
> >species are a major natural cause of mortality in the endangered Jamaican
> >Swallowtail Papilio homerus.
> >
> 
> Intersting. Do you have a reference for the P. homerus stuff? I'd like
> to know more. Most of the reports I'm aware of of Bt causing much
> mortality in wild populations comes from stored product pests, which
> live in environments that are particularly good for Bt spores and
> toxin to survive in (dry, no UV), and it doesn't seem to be important
> in the population dynamics of many species in natural environments.

I don't recall a reference I read it recently whilst browsing through
a journal but I would guess a search on P. homerus would find it.


> 
> >Engineered corn contains PART of a gene from ONE strain of Bacillus
> >thurigiensis. This is an important factor to take into consideration.
> >
> >The variability of wild Bacillus thurigiensis is an important tool in its
> >armoury. The "toxin" (actually a tool for enabling it to gain access to
> >the insect's body) exists in nature in such a range of forms that it is
> >difficult for insects to develop immunity.
> 
> You seem to have some resistance with calling it a 'toxin'. Consider
> the following
> 
> 1) When an insect ingests Bt toxin, the toxin binds to the gut wall if
> the insect is of a species susceptible to that strain of Bt. It
> creates large pores in the cell membranes, disrupting their water
> balance and causing them to rupture. The gut lining of an insect
> that's ingested Bt toxin looks like someone's fired a shotgun at it if
> you look at it with an SEM.
> 
> 2) Some species of insect are killed just by the action of the toxin
> (i.e. Bombyx mori, Plodia interpunctella). Many others are killed by a
> combination of the toxin action and septicaemia from the bacterium.
 
I suppose you could describe anthrax in humans as septicaemia.

> 3) If it isn't a toxin, then why are plants expressing the gene
> protected from insect pests?
> 
> Sounds like a toxin to me...

Its toxic effects are only a side effect of its true purpose. 
Describing it as a toxin  misleads people. Arsenic is toxic but
its does not exist to kill people. This molecule that BT produces
has a specific design to open holes in the stomach wall of its prey.
This allows the Bacillus thurigiensis bacterium to breed.

> >However the ecological consequences of flooding an ecosystem with high
> >levels of one specific form of the toxin are difficult to predict.
> 
> Well not really. What will happen is:
> 
> >...  resistance will rapidly spread in the
> >populations of many species. It is also likely that this will facilitate
> >the developement of resistance to other wild strains. This could easily
> >occur once the majority of the population of a species carries a gene
> >that that works against one strain. Natural selection works just
> >like this.
> >The consequences are that we actually decrease the ability of Natural
> >predators to control pests. That is not a smart thing to do.
> 
> Quite right. Unfortunately as far as I'm aware the organic industry
> doesn't seem to be doing a great deal to manage the resistance that
> may develop from their use of 'high levels of one specific form of the
> toxin', which many of them are doing, I'm afraid.
> 
I never said that the organic industry were right to use this.


> >
> >What also worries me is that the industry has chosen repeatedly to
> >describe Bacillus thurigiensis as a "soil bacterium" this is
> >not truthful at all. I tend to distrust people who are not truthful.
> >
> 
> Well Neil, the thing is that Bt IS a soil bacterium. If you want to
> find Bt, look in the soil. Most isolates of Bt come from the soil.
> It's a damn sight easier to find it in the soil than it is in diseased
> insects, for example. It is also found elsewhere, I'll grant you that,
> but if you want to describe Bt and you only have two words, 'soil
> bacterium' is pretty good. 

Is its close relative Bacillus anthracis (which causes anthrax) also most
truthfully described as a soil bacterium ? Of course not. It is a disease
organism just like Bacillus thurigiensis. The fact is that since the
protein is being put in food, it does not sound good to say that it comes from
a bug that eats its victims from the guts outwards. 


> I would be interested to know your source for this information: maybe
> someone in the organic food industry is not being truthful.

Charles Darwin and logic :-). Seriously though this is a predatory
organism. If not why does it have a highly sophisticated, intricate,
specialised tool to get into its victims?
(And please no flames from certain Americans or Taliban types etc. on mentioning
 that name.)

 Remember,
> it is an industry, and whereas some organic growers are dedicated
> selfless protectors of the environment there are others who are in it
> for the money, and they stand to lose big wads of cash if the public
> decides that one of their most effective control agents is no longer
> acceptable.

> Best wishes
> 
> Rob Knell
> 
> 

-- 
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