Mexican Jumping Genes

by Howard Fienberg
March 18, 2002

A common nightmare for opponents of genetically modified (GM) (also known as transgene) crops: What would happen if the genes got loose? Would the herbicide resistance in some GM crops spread to regular corn, resulting in a mutant super-weed immune to all herbicides? What if GM seedlings destroyed the regular ones? The list of theoretical fright-fests seems endless.


So the news media and the environmentally concerned stood at attention in November, when the respected scientific journal Nature reported that genes from GM corn had been discovered in native Mexican corn and were destabilizing the Mexican corns genome. As one of the researchers told the Washington Post (Dec. 3), “Whatever its source, its clear that genes are somehow moving from bioengineered corn to native corn.”


In 1998, the Mexican government had declared a moratorium on the planting of GM corn, primarily as a means to protect the crop diversity found in the region where the genetic interlopers were discovered, Oaxaca. In the wake of the Nature article, activists called for the banning of all GM crops in Mexico and even used it as a platform to ban GM crops worldwide. An activist for Greenpeace, after the Nature article was published in November, called it “a worse attack on our culture than if they had torn down the Cathedral of Oaxaca and built a McDonalds over it.” Accurate information was getting lost in a sea of rumors and accusations. A clerk at a government store told Newsweek International (Jan. 28) that the corn could “cause a disease called cancer.”


But while politics raged, the scientific community seethed: fundamental flaws called into question all of the conclusions of the Nature study.


The authors of the study, David Quist and Ignacio H. Chapela, investigated corn native to Oaxaca (meaning corn varieties regularly grown there, since no variety actually qualifies as “native”), called landraces. Using highly sensitive polymerase chain reaction (PCR) and inverse PCR (IPCR) approaches, they tested for the presence of elements common to commercial GM crops.


Quist and Chapela reportedly discovered traces of the 35S promoter from the cauliflower mosaic virus (CaMV) in local landraces. CaMV35S is regularly incorporated into GM crops in order to activate the implanted genes. The researchers concluded that these GM corn genes had “introgressed” (shifted from one pool of genes to another) with landraces and implied that such gene flow was widespread from presumed illegal plantations of GM corn within Mexico.


The researchers also claimed the introgressed genes were unstable, having “become reassorted and introduced into different genomic backgrounds.” In plain English, the mixing of the two corns put the DNA chains in the genome into strange orders. This could lead to unknown and unpredicted effects, since a “gene’s behavior depends on its place in the genome.” (Science, Mar. 1)


When the Mexican government was informed of the corn findings last year, Mexico’s Secretary for Environment and Natural Resources ordered further tests. The government later confirmed that the “contamination” of native strains had indeed occurred.


However, their tests simply duplicated those of the authors of the Nature study. And the scientific community was not convinced that those methods were any good.


The Center for the Improvement of Maize and Wheat in Mexico checked out its own extensive stocks of corn as well as samples collected fresh from the fields and found no evidence of the genes Quist and Chapela reportedly discovered. A molecular biologist at the Center, Marilyn Warburton, expressed concern to Science on the reliability of the PCR tests. “If you get a positive result, you have to check it repeatedly... And even then you need to confirm it by another method to be completely sure you’re not fooling yourself.” IPCR false positives occur often because samples can become contaminated easily with the substance for which the IPCR is testing. Both PCR and IPCR are prone to false positives because they are so sensitive. Unfortunately, Quist and Chapela did not report conducting any extra tests.


An editorial in Transgenic Research pointed out many problems in the tests that Quist and Chapela did report. For instance, only one of the gene sequences, the CaMV35S, was analyzed with both IPCR and PCR. The rest were run only through the PCR. Transgenic Research concluded that the results probably resulted from “minute contamination of the ground sample powders.” Transgenic Research’s editors also complained that the IPCR results were “problematic, internally inconsistent and not what is expected from cross-pollination” by GM crops.


Like Warburton, the Transgenic Research editors were baffled by the lack of extra testing. In particular, they wondered why the Nature authors had not conducted “the easy and incontrovertible experiment of growing out the suspected contaminated lines” which would make any hybridization plainly obvious.


But the most disturbing aspect of the Nature study, the reassortment and unpredictable instability of the resulting hybrid genome, elicited the harshest criticism. “Cross pollination and introgression would not produce these results.” Since the authors did not show “the presence of intact inserts, which are more likely to be present than fragments of unknown origin,” Transgenic Research concluded that the Nature study was most likely a “testimony to technical failure” and “common artifacts.” Transgenic Research claimed to be disappointed “that the editors of Nature did not insist on a level of scientific evidence that should have been easily accessible if the interpretations were true. Consequently, no evidence is presented to justify any of the conclusions.”


Concerned about such criticisms, anti-GM activists released a “Joint Statement” condemning the scientific challenges as “academic intimidation” and a “highly unethical mudslinging campaign.” A bevy of researchers and biotechnology advocates fired back with a “Joint Statement” of their own in support of “vigorous scientific discourse.”


Public debates aside, Transgenic Research is not alone in its concerns. Three other different academic teams have officially challenged Nature to withdraw Quist and Chapelas article, but have been under a “blackout” imposed by the journal while it considers their input. Editor Philip Campbell has been ducking questions about them and the article, saying, “Our policy in general is to consider criticisms received after publication as promptly as possible.” Serious scientific questions have been raised about a published article with major public policy implications. Sensible policy debate requires as much sensible data as possible. Hopefully, Nature will realize that and publish the critical rejoinders soon.  

See the original:

return to Howard Fienberg's page