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What is "gene flow?"
How does "gene flow" happen?
What happens when genes move?
Gene flow from conventional crops
In a nutshell

One concern associated with genetic engineering is "gene flow"--that is, the movement of genes from one organism to another. As a part of their normal reproductive cycle, plants transmit their DNA to other compatible plants via pollen. Genes from fields of crop plants can be transmitted by pollination to plants in the same or other fields, or in some cases even to other closely-related non-crop plants. This phenomenon is common among many crop species, and must be considered when a new genetically engineered crop is developed. Like all of the other genes in a crop plant, transgenes (genes engineered into a GE plant) can potentially be transmitted to other nearby plants, whose offspring will then acquire the new trait of the GE plant. In the popular press, gene flow is also referred to variously as outcrossing, gene escape, horizontal gene transfer, introgression, or even "pollen drift" (although this term is more associated with the movement of pollen itself, rather than the genes it contains).
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Pollen is the "sperm" of the plant world-- that is, the DNA-carrying male part of the equation needed to make the next generation (the female part is the egg contained in flower ovaries). Because plants are stationary, they have evolved creative ways to spread their pollen from one plant to another in order to procreate-- pollen dispersed by wind, insects, or a variety of mechanical means. Many plants get around the problem by fertilizing their own flowers-- "self-pollinating"-- a feat rare in the animal kingdom.

A plant seed contains the embryo of the new generation, the result of fertilization by two parents. Like animals (including humans), plants get half of their genes from their "fathers" (the pollen donor) and half from their "mothers" (the egg donor). If the pollen-donor father happens to be genetically engineered, then the young plant growing from the new seed will carry the genetically engineered genes ("transgenes") in additional to all of the other genes it inherited from its father (and will thus be half genetically engineered).

Pollen from one plant may fertilize either other plants of the same species, or in some cases, plants of other closely-related species:

• Gene flow to others of same species

In the case of plants that are wind or insect pollinated, some small fraction of their pollen may travel considerable distances before fertilizing a flower. Examples of these among genetically engineered crop plants include corn, canola, squash, and sugarbeets. Pollen from a field of GE corn may fertilize some of the plants in an adjacent field of non-GE corn. Thus, some seeds harvested from the non-GE field will be half-genetically engineered. This will likely prove to be a problem for farmers trying to market non-GE crops if the crop is wind or insect pollinated and their neighbors are growing a GE variety of the same crop. Some crops, such as soybeans and tomato, are self-pollinated and do not pose the same level of risk.

• Gene flow to other species: crossing the "species barrier"

In some cases, crop plants are able to fertilize the flowers of other closely-related plant species. These close relatives are usually considered "weeds," and are often found growing near agricultural fields (increasing the probability that they will be fertilized by the crop plants). However, these interfertile weedy relatives are normally limited in distribution to the geographical area in which the crop species was originally domesticated. Because most crops grown in the U.S. are not native to the United States, most do not have wild relatives here that they could pollinate. The two most widely-grown GE crops in the U.S., corn (native to Central America) and soybeans (native to Southeast Asia), have no wild relatives in the United States. Rapeseed (aka canola), widely grown in Canada, does have common wild relatives in North America with which it is interfertile-- a few members of the mustard family.

Follow this link to a table summarizing the various gene-flow risks of the GE crops currently grown in the U.S.
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If a non-GE plant acquires a transgene from a GE crop plant (same species or different), then the seeds resulting from that pollination-- and the plant growing from that seed-- will also express the genetically engineered trait. For example, if a weedy relative of rapeseed is pollinated by a rapeseed plant engineered to be resistant to an herbicide, then the offspring of that will also be resistant to the herbicide. In that case, farmers might no longer be able to use that particular herbicide to control the weed. Likewise, herbicide-resistant volunteers of crop plants might also prove more difficult to control.

Some speculate that this would cause farmers to adopt tougher, more toxic herbicides to control the weeds, but this is not necessarily true. When growers adopt GE herbicide tolerant crops, they must switch from one set of herbicides to the single herbicide to which the crop is resistant (usually Roundup). If weeds become resistant to that herbicide, the grower has the option of switching back to the old herbicides without necessarily having to seek new "more toxic" herbicides. The result of herbicide resistant weeds might be reduced adoption of the genetically engineered crop!

GE traits such as insect or virus resistance could hypothetically increase the fitness of weed populations if they happen to have been significantly limited by those particular pests. Also, if the GE trait substantially alters the growth habit of the GE plant, then weeds acquiring the trait could potentially become more weedy. Although regulators believe that these scenarios are very unlikely- most GE crops with insect or virus resistance do not have weedy relatives in the US-- few studies have been conducted to evaluate the possibility.
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Genes are as likely to move from non-GE crops as they are from GE crops. Likewise, all of the other naturally-occurring genes in a GE plant can also be transmitted to other plants. Using conventional plant breeding, varieties of crop plants have been developed which are resistant to insects, viruses, herbicides, are drought-tolerant, have slow-ripening fruit, and a host of other traits- many traits similar to those added via genetic. The genes controlling these traits in conventionally-bred plants may also move to other related plants engineering-- thus presenting similar risks-- and these gene movements are not uncommon.
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Genes engineered into GE plants can be transmitted to other, non-GE plants by pollination (along with all of the other non-GE genes from the pollen donor). In this event, the progeny plants of the fertilization will also be genetically engineered, acquiring the same GE trait as its parent. Pollen is more likely to move longer distances from wind or insect pollinated crops (like corn) than from self-pollinated crops (like soybeans). Additionally, if there are weedy interfertile relatives growing in the area, it is possible that they too could acquire the genetically engineered trait-- although the most widely-grown GE crops in the US do not have wild relatives here.
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