Lost in the maize

Fears about the safety of genetically modified maize have been ridiculed by scientists - but they refuse to go away.

Toby Andrew

Topics Science & Tech

After yet another round of scare stories about genetically modified Mexican maize, it seems that media campaigners are more interested in promoting worst-case scenarios than pursuing the truth.

The original furore kicked off at the end of 2001, with the publication of a paper in Nature magazine by David Quist and Ignacio Chapela, who claimed that GM corn in Mexico was contaminating indigenous ‘landraces’ (1).

Prior to that, in September 2001, there was a stir when Dr Chapela, an environmental campaigner and assistant professor of microbial ecology at the University of California, notified the Mexican authorities that his studies revealed the presence of scattered plots of illegally grown transgenic maize in the states of Oaxaca and Puebla. Although Mexico annually imports more than five million tonnes of transgenic corn from the USA for consumption purposes, a ban has been imposed on planting GM crops following a government moratorium in 1998 (2).

After a series of increasingly bitter accusations and counter-accusations over claims that indigenous Mexican maize is being ‘genetically polluted’ by genetically modified maize and is ‘in danger of extinction’ (3), the row, dubbed the ‘maize scandal’, has been flaring for almost a year now. ‘I’ve never seen anything like it’, says Peggy Lemaux, a US molecular biologist and one of the most public critics of the Quist and Chapela paper that appeared in Nature. ‘There’s been a lot of fighting about transgenics, but this is something else.’ (4)

In the Nature paper, Quist and Chapela claimed that their laboratory tests confirmed worst fears about GM maize having been found growing alongside traditional maize varieties in Mexico. Specifically, they made three claims – each with potentially serious implications.

The first was that cross-fertilisation or hybridisation between the two types of maize varieties had already taken place. Secondly, that the transgenic DNA from the GM corn had introgressed into traditional maize – meaning that the transgene had become thoroughly bred into a number of traditional varieties through a process of repeated breeding between hybrid and original varieties. Thirdly, the authors claimed that the transgene, rather than remaining at the same locus in the GM maize, seemed to have appeared at a number of new locations in the genome of Mexican varieties.

And this, argued Quist and Chapela, suggested either the possibility of multiple introgression events or that the transgene had fragmented or even relocated in the genome. It was this third claim that got alarm bells ringing. As one microbiology research group put it, ‘the discovery of transgenes fragmenting and promiscuously scattering throughout genomes would be unprecedented’ (5).

But all three of the claims, including evidence that cross-pollination had occurred, have been widely condemned by microbiologists. They claim that Quist and Chapela’s findings were the result of inadequately conducted laboratory experiments (6). Subsequently, a more rigorous series of tests have failed to find any transgenic DNA in any of the 152 Mexican varieties screened (7).

Critics claim the study is a poor piece of work that should not have been published in the first place. Indeed, in April 2002, Nature issued an editorial, unprecedented in its 133-year history, stating that ‘the evidence available is not sufficient to justify the publication of the original paper’ (8).

Despite the lack of evidence, the ‘no smoke without fire’ sentiment seems to persist. In June 2002, BBC2’s current affairs programme Newsnight accused the editor of Nature, Phillip Campbell, of conspiratorially overriding the advice of two of the three critical reviewers in order to censor the paper with a statement of retraction (9), after coming under ‘strong pressure from sections of the scientific-business community’ (10).

The decision to withdraw the article was not a conspiratorial act of censorship. It was a decision based on science. The original paper was flawed on many counts – but the central problem was that the transgenic traces detected in the maize samples were so tiny that the authors had to conduct two rounds of polymerase chain reaction (PCR), greatly enhancing the possibility of low levels of contamination introduced during the handling of samples causing a positive result (11).

Key control data results were withheld with the woolly statement that control tests were ‘invariably negative’, raising the suspicion that control samples (by definition containing no transgene) were also yielding positive results. Indeed, the research was so poor that some scientists suspected that editor Campbell’s environmentalist sympathies might be one reason why such speculative research was published (12).

Although there is currently no evidence that cross-pollination, introgression or transgene relocation has occurred, it is widely accepted by scientists that transgenes will sooner or later move into other maize varieties. After all, corn is a wind-pollinated plant. So the question then becomes: would this matter?

Environmentalist claims that transgenic transfer would lead to a loss of biodiversity are disingenuous (13). Despite the use of the term ‘landraces’, portraying an image of diverse wild strains, traditional Mexican varieties are in fact all closely related. They have all been highly cultivated and, like GM corn, they are all thought to have descended from the wild grass Teosintes. And in this case, fears over potential super weeds being created are misplaced – since hybrid maize would not be a weed, but merely a GM crop with increased productivity.

Environmentalists’ biggest fear in the Mexican maize story is an underlying concern about (uncontrolled) horizontal gene transfer (HGT) – the direct uptake and incorporation of foreign DNA into cells, by mechanisms other than natural breeding. The potential danger posed by ‘jumping genes’ was first illustrated in the 1950s with the fruit-fly Drosophilia, in which a DNA fragment, known as the P element, somehow (perhaps via a blood-sucking mite) managed to transfer between a laboratory and a wild fruitfully species in South America (14). No harm occurred, since the DNA fragment simply remained as a harmless passenger.

This is partly why Quist and Chapela’s claim of transgenic fragmentation and relocation in the maize genome caused such widespread concern. Genes introduced into a genome by genetic engineering are invariably introduced by researchers using a modified, harmless virus. For example, the viral promoter used to deliver the gene that expresses the insecticide Bt is the commonly used cauliflower mosaic virus (CaMV). Current delivery methods are hit and miss, but once successfully delivered and the host plant subsequently bred, the transgene tends to stay put at the same location in the genome.

One possibility is that the very cellular mechanism that allows a transgene to be inserted into a genome may also facilitate it to ‘jump out’ again, since the process is naturally reversible. What happens then is subject to speculation. Insects with sharp mouthparts might visit the transgenic maize and landraces in succession and deliver the transgene to the new host genome – which then may or may not be passed on to succeeding generations. Once introgressed, the transgene might then move around the genome causing other genes to be read incorrectly.

Or the introduced viral promoter might ‘wake’ previously dormant proviruses known to exist in the ‘junk’ DNA of all plants and animals, which might then go on to malignantly infect other organisms – including humans. Veteran anti-GM campaigner Mae-Wan Ho asserts that any genetic engineering technology is inherently dangerous, as it increases the potential for horizontal gene transfer. She claims that HGT inevitably leads to new viral and bacterial pathogens, new diseases, cancer and ecological destruction.

Indeed, Ho even blames genetic engineering technologies for already causing the recent rise of anti-bacterial resistance, based upon the laughable observation that both phenomena have been on the increase since the 1980s (15).

Scientists do not dismiss the potential problem of horizontal gene transfer lightly. But although there is a potential risk that HGT might occur, the dangers put forward by campaigners can only be realised by piling one worst-case scenario upon another. So while, in principle, each event might conceivably be plausible, as a chain of consecutive events it is a practical impossibility. So science fiction tends to be promoted as science fact (16).

There are three lessons from the Mexican maize row. Firstly, it is wrong to always assume the worst. Secondly, critics that object to biotechnology per se should be criticised for their refusal even to contemplate whether the potential benefits might outweigh the potential costs. Thirdly, wise policy decisions can only be based upon the best available evidence.

Continually anticipating the worst, as Nature discovered to its cost, yields unanticipated hazards of its own.

Toby Andrew is a genetic statistician at the Twin and Genetic
Epidemiology Research Unit, St Thomas’ Hospital, London (toby.andrew@kcl.ac.uk).

Read on:

spiked-issue: Genetics

(1) ‘Transgenic DNA introgressed into traditional maize landraces in Oaxaca, Mexico’, D Quist, IH Chapela, Nature, 414:541-3, 2001

(2) ‘Transgenic found growing in Mexico’, R Dalton, Nature, 413:337, 2001

(3) ‘Serious Genetic Contamination Revealed in Mexican Maize’, Greenpeace News, September 27, 2001; ‘Preserving the integrity of Indian corn – Part I’, Indian Country News (USA), 8 February 2002

(4) ‘Has GM Corn “Invaded” Mexico?’, Robert Mann Science Magazine, Volume 295, Number 5560, p1617-1619, 1 March 2002

(5) ‘Suspect evidence of transgenic contamination’, M Metz, J Fütterer, Nature, 416:600-1, 11 April 2002

(6) ‘Suspect evidence of transgenic contamination’, M Metz, J Fütterer, Nature, 416:600-1, 11 April 2002; ‘No credible scientific evidence is presented to support claims that transgenic DNA was introgressed into traditional maize landraces in Oaxaca, Mexico’, P Christou, Transgenic Research, 11:iii-v, 2002

(7) Greenpeace and Biotech: Truth or Deliberate Scare, Biotech Website, 6 February 2002; Further tests at CIMMYT find no presence of promoter associated with transgenes in Mexican landraces in gene bank, Centro Internacional de Mejoramiento de Maíz y Trigo, 7 February 2002

(8) Editorial note, Nature, 416:600, 2002

(9) BBC Newsnight, 7 June 2002

(10) Science journal accused over GM article, Guardian, 8 June 2002

(11) ‘No credible scientific evidence is presented to support claims that transgenic DNA was introgressed into traditional maize landraces in Oaxaca, Mexico’, P Christou, Transgenic Research, 11:iii-v, 2002

(12) Corn Goes Pop, Then Kaboom, Barry A Palevitz, The Scientist 16[9]:18, 29 April 2002

(13) ‘Serious Genetic Contamination Revealed in Mexican Maize’, Greenpeace News, 27 September 2001

(14) Genome. The autobiography of a species in 23 chapters, Matt Ridley, Fourth Estate, 1999, p129. Buy this book from Amazon (UK) or Amazon (USA)

(15) Report on horizontal gene transfer – Department of Public Prosecution versus Gavin Harte and others, New Ross, Ireland, Mae-Wan Ho, Institute of Science in Society, 22 March 1999

(16) ‘Science Fiction’, Fiona Fox, Independent Review, 18 June 2002

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Topics Science & Tech


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