Bt corn flap

[Rob Knell]

On Wed, 29 Mar 00 22:34:42 GMT, Neil at nwjones.demon.co.uk (Neil Jones)
wrote:

>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.
>

I'm not sure what you're describing as the 'natural product' here. If
you're saying that the Bt preparations used by organic growers for
control have increased variability which helps control resistance,
then I must disagree with you: these products are single strains of Bt
and will induce resistance in much the same way as GM plants. If
you're saying that the population of Bt in the wild is variable, then
you're correct, but that isn't very relevant to pest control as it's
done at the moment.

>> 
>> >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.

Or why not put it another way. We don't know what this organism does,
so surely it's better to use just a single protein from it. We know
exactly what effect this protein has, and if we engineer it into our
crop plants then the control agent is targeted directly to where it's
needed most. Is this not a more environmentally friendly procedure
than just spraying the live organism around, killing non-target
organisms willy-nilly and running the risk that it will prove to do
something we hadn't predicted, or even worse using broad spectrum
chemical pesiticides?

I'm not saying that you're wrong, just that the argument can be turned
around 180 degrees and have just as much validity.


>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.
>

Yes indeed. Nonetheless, the protein is only expressed when the
bacterium sporulates, and if it only sporulates rarely then selection
will not be that great.


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

Speaking as someone who works, among other things, on the ecology of
pathogens of lepidoptera, I would disagree with you here. There is a
great deal of research done on the role of pathogens in the ecology of
lepidoptera, and the reason that most of this is done on pest insects
is because they're easy to get hold of and usually much easier to rear
in the lab than non-pests. Many of the conclusions that are reached by
working with pest leps are equally applicable to non-pest species,
it's just that a lot of non-pest species are a right pain to work
with. 

>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.
>

Couldn't find it on BIDS.


<snip>

>
>> 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.
>

Is this comment relevant to anything?

>> 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.
>

Well, this is getting into semantics here so the discussion is getting
a bit pointless, but by your argument snake venom is not venom it is
just something that the snake produces to help it feed. Calling it a
toxin does not mislead people because it is a toxin. If it isn't a
toxin how does it kill insects? I think you must be using a very
specialist definition of toxin here.

<snip>

>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. 
>
>

If most Bt lives in the soil, if most of its reproduction is carried
out in the soil, then it's a soil organism. It is also an
entomopathogen, but that doesn't stop it being a soil organism. You
implied that calling it a soil organism was a deliberate attempt to
mislead the public by the GM plant developers, but in fact they were
just calling it what it is.


>> 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?

It is sometimes a pathogen. It is sometimes a saprophyte, and the
latter more often than the former. One thing it is not, by the way, is
a predator (see any ecology textbook for a definition of predator). I
wasn't going to mention this but since you seem so concerned about the
correct use of certain words..... :-)


Cheers

Rob