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Evolutionary Time Missing from Equation

Zucchini crop

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The key to organic farming is not substituting biological for chemical controls, but adopting an holistic system design which captures ecological synergies within a strategically chosen mix of enterprises says Dr E Ann Clark, associate professor of Plant Agriculture at Guelph University, Ontario.

Dr Clark says genetic engineering to produce pesticidal crops has been portrayed as a blessing to environmentally conscious farmers and to organic farmers in particular. Many growers have responded to this premise with an intuitive or ethical aversion. However, the notion of replacing synthetic pesticides with "plant-produced" pesticides has achieved some resonance with society in general and perhaps with some organic growers as well.

Proponents are quick to point out - and rightly so - that plant-produced anti-herbivory compounds did not originate with genetic engineering, says Dr Clark.

A truly staggering range of self-defense compounds evolved in many higher and lower order species and it is fair to say that chemicals have been and continue to be prominent in mediating relationships among organisms in nature, irrespective of human intervention, she says.

"With commendable dexterity, the same argument on the naturalness of chemical deterrents has since been reformulated to rationalize the safety of transgenic insertion of plant pesticidal compounds such as Bt or lectin, into crop plants."

Because chemical deterrence is natural, it is reasoned that engineered deterrence is just mimicking nature. However what distinguishes natural vs. synthetic deterrents to herbivory is evolutionary time, she says.

"Biological life has evolved in a kind of intricate, long-term dance of many competing partners. Each newly evolving screen or defense mechanism - often involving chemical deterrents - engendered a corresponding new resistance or response strategy, which in turn stimulated yet another defense strategy."

The partners "knew" each other, and recognized each other's co-evolving chemistry over evolutionary time.

"But not so the synthetic interlopers. There hasn't been time."

Endocrine disruptors - chemicals which through a variety of means are able to affect hormone balance in the bloodstream are an illustrative case, she says. Both natural and synthetic chemicals have now been identified as acting in a wide range of animals, including humans. Hormone imbalance before and during pregnancy has been associated with a wide range of reproductive, immune system, and growth abnormalities in offspring.

Natural estrogens operate at extremely low concentrations - as in parts per trillion in the blood. However, synthetic estrogen mimics can occur in parts per billion or million, because they are not recognized and removed by the natural regulatory processes. Synthetic mimics cannot be dealt with properly because, in the words of Claude Hughes, a specialist in reproductive endocrinology at Wake Forest University, humans lack evolutionary history with them.

As a result, while natural estrogens are regularly broken down and excreted to maintain very low and physiologically appropriate concentrations, synthetic estrogens can accumulate, leading to chronic, low-level exposures. Chronic human exposure to blood estrogen levels thousands or millions of times higher than normal is without precedent in human evolution. The tragic implications of such exposure may be manifested inter-generationally Dr Clark says.

"The premise that engineering synthetic pesticides into plants is 'natural,' because chemical deterrents to herbivory evolved naturally, is overly-simplistic."


In the case of Bt, the ecological advantage of Bt crops, if any, remains to be seen.

"Bt" in genetically engineered crops is different from naturally occurring Bt. In effect, genetic engineering has transformed a highly selective, very short-lived foliar biocontrol agent imposing very little risk of resistance into a weakly selective, persistent, ecologically ramifying, bio-accumulating pesticide.

The endotoxin in Bt crops consists of a crystal protein toxin ("Cry" toxin) coded for by genes which have been isolated from Bacillus thuringiensis, a soil organism. It was believed that one could insert the gene(s) coding for specific toxins into crop plants, and act selectively against the target group.

However, the selectivity of foliar-applied Bt arises from at least two critical steps which are bypassed entirely in Bt-crops says Dr Clark. The Bt in soil microbes exists as a protoxin, a precursor which is not insecticidal. It becomes activated (and insecticidal) only when a) ingested by an insect with the proper, alkaline intestinal pH, and b) specific enzymes are present to cleave the precursor into the active form, which then c) binds with receptor sites in the gut, leading to the death of the insect.

"In GMO applications, it is active endotoxin - not the precursor molecule - which is synthesized in the plant cells. Thus, the first two screening steps are absent, and the potential for non-target effects is increased."

Selectivity/Ecological Ramifications

Researchers at the Swiss Federal Research Station for Agroecology and Agriculture demonstrated that the Bt in GMO crops behaves entirely differently from what would be expected from foliar Bt.

Dr Clark says that, in essence, if validated over a wider range of organisms and conditions, the Swiss findings suggest a loss of selectivity in transgenic vs. natural Bt applications. The study further suggests the potential for adverse ramification out into the wider ecosystem - analogous to what happens with bio-accumulative pesticides.

"The process of engineering insecticidal traits into crop plants has taken a product that was short-lived and selective in its native state and turned it into a product that mirrors the persistent, bio-accumulative, ramifying harms associated with chemical insecticides."

Healthier Option

That pesticidal plants displace insecticide use and hence reduce risks to human health, is, at least on the face of it, a plausible and compelling argument.

But a diet where every bite of sweet corn or mashed potatoes contains active Bt endotoxin is without precedent in human evolution, she says.

"Does it matter that every kilo of grain or litre of milk from fed cattle is derived from grain with the same raw endotoxins? And what of the soil, to which their manure is applied, year after year? Have we even begun to ask these really critical questions?"

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