Introgression is the process by which one species, usually a plant (although animal models clearly exist), hybridizes with another. Then, the resulting second generation mixed species then hybridizes again with one or the other of the initial parent plants. If that second generation hybrid is itself able to reproduce and thrive, its existence is said to represent a stable introgression.
Linnaeus , the great classifier of plants and animals, won a prize from the St. Petersburg Academy of Sciences in Russia in the 1700’s when he successfully hybridized two related but nonetheless distinct plants. The point he was trying to prove at that time was that plants can reproduce sexually (which of course, many or most can), but he also suggested that this hybridization in nature might well be the source of many (or even most ) new species.
Linnaeus was right up to a point. But as orchid and rose breeders will tell you, the creation of new hybrids, to say nothing of new species, rarely goes as easily or as planned as that.
A great many hybrids are sterile, that is, they cannot reproduce, even if they can thrive for a single generation.
Other hybrids unravel in terms of their hybrid appearance or in terms of their desirable properties and go right back to being more like their native ancestors. (They revert to being wild.)
A fairly common occurrence among hybrids appears to be that they have seeds that are too small for successful soil penetration, or which will sprout only in well-tilled soil and only under gentle conditions.
Other hybrids are unable to compete with either parent plant once they sprout, and simply go out of existence themselves.
All these points about introgression in plants would be almost purely academic save for a new worry: the effect of genetically modified (hereafter, GM) crops and their nearby wild plant relatives, and both human and wild creature life.
The most commonly encountered GM crops are soybeans, cotton, corn, canola (rapeseed oil), squash, alfalfa, rice and now, potatoes.
The most common traits which are introduced by GM are increased resistance to harmful insects, decreased susceptibility to weed killers (i.e. enabling weed killers to be used without harming the main crop), the ability to thrive or utilize better what water and fertilizer is available so as to increase yield, and in food crops , the ability to synthesize or maximize other missing or low levels of nutrients for humans.
Will their GM genes introgress into the native species and somehow wipe them out?
Or will there be other and more insidious unintended consequences than can only be imagined at this time.
Current opinion on GM introgression is more politically driven, than experience based, and therefore working assumptions are unsurprisingly diametrically opposed.
A recent editorial in the June 2, 2008 issue of America’s conservative flagship, The National Review , says that holding GM back retards the ability of Third World countries to feed themselves, and cites Robert Mugabe and Hugo Chavez, as prime examples of GM opponents who are consumed by conspiracy theories of GM, rather than by the existing science concerning GM crops.
But that journal’s equally prestigious liberal American counterpart, The Nation, warns in its May 12, 2008 issue, that efforts by agribusiness to promote GM crops are deliberately designed to maximize profits, not solve the world food crisis. In fact, the journal suggests that getting poor farmers hooked on GM seeds will eventually impoverish them, and starve their families and produce customers.
The European Union on the whole agrees with the The Nation, and has either bans, or high regulatory barriers, on many GM foods including plants, that are driven by fears of what some commentators refer to as the unpredictable consequences of having Europeans eat “Frankenfood.”
But perhaps the most balanced scientific assessment, and one that merits wider distribution among politicians is found in the May 2008 issue of BioScience (v.58, no.5, p.379). Its essential message is:
"So far, careful scrutiny has found no evidence of health dangers from growing or eating genetically engineered crops. Certainly any ecological or health consequences of these products need to be monitored and prudence observed.”
Have there been unintended outbreaks of GM hybridization?
The most common reasons reported for these outbreaks are planting GM crops too close to stands of non-GM or wild varieties so that there is pollen interchange, and random dispersals of GM seed along truck routes through tire treads or through adherence of seed or pollen to workers clothing.
One example is the hybridization of one GM member of the turnip family (the oil-seed rape plant Brassica napa) with Brassica olearacea or with Brassica rapa (two turnip top greens favoring varieties ) which thrive better in the wild.
In another case, a ten year study indicated that some GM sunflowers had in fact, hybridized with wild sunflowers.
In yet another, a GM variety of rice had indeed introgressed into a wild rice variety.
In still another, wild lettuce had co-mingled with GM lettuce.
Have those consequences been of any great significance?
Sampling of the Brassica indicated that at one point in 2002, 85 out of 200 plants sampled near the zone of hybridization, had in fact, shown some evidence of introgression, but by 2005 it had gone down to 5 out of 200.
The GM introgression yielded wild sunflowers that turned out to have greater hardiness, including the ability to withstand better harsher environmental conditions, particularly less water and poorer soil.
The rice study showed that even with close co-planting, the risk for transgenic hybridization was much less than 1%, and that the farther away from the experimental field, the risk diminished to a virtual zero point. Nearby wild grasses that had the theoretical capacity for introgression, were complete unaffected.
The lettuce collision showed that the wild type favoring hybrid was actually more likely to survive than the cultivated one , so the chance of the GM favoring hybrid running amuck seemed unlikely, because it was in fact, less viable outside its intended area of cultivation.
Probably the most serious wildlife consequences any of these hybridizations is alleged to have is on insects and insect eaters.
Butterflies and moths dependent on one type of flower may not do as well on the GM introgressed hybrid variety.
Streamside plants that have hybridized with GM plants that are insect resistant, may support fewer insects to fall into in the water for fish to eat.
The environmental upsets that have been detected have not been documented as yet to be persistent. And the odds are they will not in fact be so.
There are some rare but somewhat more realistic worries about human health in that genes for certain proteins -----most commonly mentioned are nut proteins-----to which some people are allergic may unintentionally migrate through GM to a plant which was previously not known to cause this particular allergic reaction, so that the consumer was taken unaware. Certain soybeans have been found to contains proteins from brazil nuts, for example.
But conversely, there is already progress on creating allergy medicines using transgenic rice, and vaccines have been made using transgenic potatoes.
In any case, the clearest recommendation about GM and introgression, and its benefits and risks, that can be drawn for biomedical librarians for now, is a very reliable one: Encourage your clientele to read more of the science.
The very thought alone of GM crops should not make your readers fact-resistant.
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Beardsley TM. 2008. The right fight for biologists. BioScience 58 (5): 379.
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Choo V. 1996. Allergens can be transferred to transgenic products. The Lancet 347 (9004): 819.
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Tony Stankus [email protected] Life Sciences Librarian & Professor
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