Mendel in the Kitchen A Scientist's View of Genetically Modified Foods (2004) / Chapter Skim
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11 Pollen Has Always Flown
11 Pollen Has Always Flown
Pages 223-244
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From page 223... ...
Quist was his graduate student. They collected the corncobs while working at the Mycological Facility in Oaxaca, described by Berkeley's public relations office as "a locally run biological laboratory." Chapela was trained in mycology, the study of fungi.Another press account identified them as advisors for a program that helps indigenous farmers.
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From page 224... ...
A month before the paper came out, the two authors appeared at a press conference with Mexican government officials. The New York Times trumpeted,"In a finding that has taken researchers by surprise and alarmed environmentalists, the Mexican government has discovered that some of the country's native corn varieties have been contaminated with genetically engineered DNA." Rather than sounding surprised, however, one knowledgeable critic said, "It is probably inevitable that eventually engineered genes will be found in Mexican corn, as gene flow is a normal and natural phenomenon with maize." Another said,"I'd be shocked if they didn't find it there." What puzzled the scientists -- and inspired activists-were two additional claims in the Nature paper: that the DNA was "introgressed" into the landrace, and that it was "attached" to different sequences in different samples, even to different sequences in a single sample.
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From page 225... ...
They tested the DNA using PCR to see if it contained three sequences commonly found in genetically modified corn: the CaMV 35S promoter, the NOS terminator from Agrobacterium tumefasciens, and the Cry1Ab gene from Bacillus thuringiensis. To see the results of the PCR test, the fragments of DNA were sorted by gel electrophoresis.
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From page 226... ...
Introgression means that a gene (in this case the CaMV promoter sequence) has not only been introduced into the plant by crossbreeding a landrace with a genetically modified variety, but that the hybrid plants were then repeatedly backcrossed to the landrace.
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From page 227... ...
There is still a third possible explanation for the weak bands they saw on the gels -- an explanation that does not require introgression or several seasons of backcrossing and yet is not a false positive. The plants from which the cobs were collected could be ordinary landrace plants onto whose silks a few pollen grains from a genetically modified plant growing nearby had landed that same season.
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From page 228... ...
Mexican farmers often plant corn kernels -- increasingly shipped in from the United States, where genetically modified corn is widely grown-meant as food, not seed. What puzzled the critics most was the claim that the CaMV promoter popped into various places in the genome, that the transgene was "out of control." The New York Times quoted a fellow scientist: "If real, that would have been a huge finding." It was this claim that precipitated Nature's announcement that it should not have published the paper.
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From page 229... ...
In Bt corn it is more than 1,000 base pairs long. So when Quist and Chapela ran inverse PCR, looking for the bits of DNA just next to the ends of that 200-base-pair fragment, the first thing they should have found was more of the CaMV promoter sequence.
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From page 230... ...
Studies by the Mexican government, although not yet published, have also detected transgenes in landraces, as reported at a news conference in Mexico City in February 2002. Yet far from being surprised or alarmed, as the New York Times's original report had it, the researchers and environmentalists associated with CIMMYT have been working since 1995 -- when the first genetically modified maize reached the market -- to understand and contain, if necessary, the effects of these new varieties on Mexican corn.
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From page 231... ...
corn, Iwanaga said, "and, instead of eating it, planted it, just to see what might happen." Gene flow is popularly considered a hazard of genetically modified food plants. Said Klaus Ammann, curator of the Botanical Garden at the University of Bern in Switzerland, "The debate on genetic engineering `forces' us to focus in an unfortunate way on gene flow as a basically negative effect, as if pollen would have learned to fly with the transgenes." But gene flow, he said, "has always occurred between different old landraces and between different new varieties of crops.
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From page 232... ...
But when a corn silk traps corn pollen -- and each long silk traps many pollen grains-then the chemistry is just right. The pollen grain swells and germinates, sending out a pollen tube that grows faster than any other known plant cell -- as much as a foot in just a few hours -- carrying along the
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From page 233... ...
Guided by chemical signals inside the silk, the pollen tubes grow in parallel tracks down the silk, aiming steadily for the embryo sac nestled in a little bump on the tiny, unfertilized ear. The pollen grain whose pollen tube grows fastest wins the race, delivering its two sperm cells to the waiting egg and central cell.
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From page 234... ...
But the most important point is that those hybrid plants that receive the maize genes that prevent seed dispersal are sure to die out without humans to pick them, plant them, and tend them. Long known as cross-hybridization, this problem was only recently dubbed "gene flow." It is not just a problem of crops with weedy relatives.
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From page 235... ...
Anyone hoping to grow the 2000 All-America Selections Winner sweet corn variety named Indian Summer, with its festive mix of yellow, white, red, and purple kernels, needs to be sure to read the fine print: "Requires isolation from other corn pollen." Whether gene flow between crop plants and their wild relatives is a problem depends on several things: the crop plant, where it is grown, and how it is used; whether or not the crop has weedy relatives nearby with which it can crossbreed; and how those weeds are managed. This long list of issues cannot be lumped -- as people have tried to do -- into one category.
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From page 236... ...
The lone corn plants towering over the soybean field, called volunteers, present certain weed-control problems, but they're not invasive weeds like Johnson grass, a sorghum relative. Many crop plants are not native to the places where they are cultivated and their wild relatives are not found in the same geographical areas.
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From page 237... ...
, but rather when, where, and how it would happen and what would be the consequences. Because genes that confer resistance to herbicides were among the first to be introduced into crops by molecular techniques, herbicide resistance was one of the first problems associated with gene flow to be brought to the attention of the public.
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From page 238... ...
. Some are genetically modified in the contemporary sense that genes from other plants or from bacteria have been introduced into them by molecular techniques.
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From page 239... ...
The fact that such gene flow to weeds hasn't been detected might be due to the genetic distance between the crop and its relatives. Experimental crosses between these weedy species and canola produced plants that were sterile or didn't grow well.
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From page 240... ...
When it does, they say,"you need new technology." Three months before Nature abandoned the Quist and Chapela paper, its sister journal Nature Biotechnology summarized the Mexican maize scandal. Rather than zeroing in on the technical difficulties, the journal stated that "the major point of divergence in the current discussion" about gene flow was not whether a transgene from a genetically modified variety had moved into the Mexican landraces, but how the so-called contamination would affect biodiversity.
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From page 241... ...
Yet Chapela's fear -- that genetically modified varieties of agricultural crops could push landraces to extinction -- is a valid one, though not in the way he is concerned about. According to a United Nations report, the success of commercial varieties of all kinds -- not just those created using molecular techniques -- has led to the disappearance from farms of more than 80 percent of old-fashioned apples, maize, tomatoes, wheat, and cabbages worldwide.
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From page 242... ...
Despite the availability of improved corn varieties since the 1930s and the intensive maize breeding that has been conducted in Mexico, Goodman said, there has been little impact on the indigenous landraces grown by 80 percent of Mexican farmers. The new varieties designed for the U.S.
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From page 243... ...
that inhabit the earth today. In his closing remarks at the Mexico City conference on gene flow, Peter Raven spoke of both meanings of the term.
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From page 244... ...
Far from being a dire threat, the introgression of selected genes that enhance insect and disease resistance might ensure the survival of maize landraces and preserve biodiversity -- in the larger sense -- by improving small-scale farmers' yields and lives.
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