Science in Society Archive

GM Crops Face Potential Genetic Meltdown

I-SIS has drawn attention to the instability of GMOs and GM constructs all along. Prof. Joe Cummins offers this latest verdict: all GM crops may be unstable, and there is a possibility of genetic meltdown.

It is repeatedly claimed that genetically modified (GM) crops are altered with single genes that are stable and equivalent to the genes that have been selected and bred into the crops. In every case the GM crops originated from cell or tissue cultures vexed with a phenomenon called somaclonal variation - genetic instability associated with gene mutation and chromosome re-arrangement. Somaclonal variation occurs in cell and tissue culture even without genetic modification, but genetic modification often makes it worse [1]. Somaclonal variation is associated with genetic elements called retrotransposons that replicate in the plant cell nucleus and insert into structural genes, causing mutation and chromosome rearrangement [2]. Somaclonal variation has been encountered in genetic transformation using both biolistic ('gene gun') and Agrobacterium, followed by cell/tissue culture to isolate individual clones. Furthermore, the transgenes introduced into the modified crop are recognised as invaders by the host and the invading genes are silenced by mechanisms including DNA methylation or gene inactivation during or after transcription.

The evidence that the genetic instability resulting in somaclonal variation is caused by activation of transposons is currently very compelling [3]. In addition, the inserted transgenes are frequently silenced [4]. Even the most widely distributed commercial GM crops such as Roundup Ready soya were found to contain unexplained DNA sequences in the gene for herbicide resistance after ten years of cultivation [5] (see "Scrambled genome of Roundup Ready soya", this issue). Similar problems with GM corn, cotton or canola have not yet been studied. In fact, far too few studies have been carried out.

GM barley was found to be inferior to conventional barley in a number of genetic backgrounds and environmental conditions [6], reflecting the impacts of somaclonal variation, exacerbated by genetic modification.

Another related problem identified is 'error catastrophe' or 'extinction mutagenesis'. This refers to the accumulation of deleterious mutations that make populations go extinct. For example, the foot and mouth disease virus, treated with mutagens (base analogues fluoro-uracil and azacytidine), eventually became extinct [7]. Polio virus treated with the mutagenic drug ribavirin similarly went extinct [8]. Error catastrophe theory has led to a strategy of mutagenesis to control disease viruses.

The genetic changes activated in GM may be numerous and subtle, and may produce gradual loss in productivity of GM varieties or unexpected toxic plant products. Transposons have been shown to have powerful impacts on genetic stability. For example, the P transposon of the fruit fly, Drosophila, when activated under appropriate conditions, causes 'hybrid dysgenesis', a slow destruction of the genome receiving the transposon due to chromosome and gene mutation [9]. Gressel [10] has suggested that hyperactive transposons could be introduced into weed populations in order to eradicate them.

There has not been adequate study of ongoing transposition in GM crops. The threat of extinction mutagenesis has never been discussed in governmental reviews that led to the deregulation of experimental GM crops, nor has there been effort to examine the factors leading to subtle yield-depression in GM crops. It may only be a question of time until GM crops dramatically decrease yield and become extinct. Finally, little or no thought seems to have been given to the havoc that could be wreaked upon the human genome by GM crop retrotransposon running amok within humans and farm animals

References

  1. Labra M, Savini C, Bracale M, Pelucchi N, Columbo L, Bardini M and Sala F. Genomic changes in transgenic rice plants produced by infecting calli with Agrobacterium tumefacians. 2001 Plant Cell Reports 2001, On line reports DOI 10.1007/ s002990100329
  2. Agrawal G, Yamazaki M, Kobayashi M, Hirochika R, Miyao A and Hirochika H. Screening of rice viviparous mutants generated by endogenous retrotransposon Tos17 insertion tagging of a zeanthin epoxidase4 gene and a novel OsTATC gene. Plant Physiology, 2001, 125, 1248-57.
  3. Courtail B, Fenebach F, Ebehard S, Rhomer L, Chiapello H, Carilleri C and Lucas H. Tnt 1 transposition events are induced by in vitro transformation of Arabidopsis thaliana, and transposed copies integrated into genes" Mol Gen Genomics 2001, 265,32-42.
  4. Demeke T, Hucl P, Baga M, Caswell K, Leung N and Chibar R. Transgene inheritance and silencing in hexaploid spring wheat" Theor Appl Genet, 1999, 99, 947-53.
  5. Palevitz,B "DNA surprise: Monsanto discovers extra sequence in its Roundup Ready soybeans" The scientist 2000, 14, 20.
  6. Horvath H, Jensen L,Wong O, Kohl E, Ullrich S, Cochran J, Kannangara C, and von Wettstein D. Stability of transgene expression, field performance and recombination breeding of transformed barley lines, Theor Appl Genet. 2001,1-11
  7. Sierra S, Davila M, Lowenstein P and Domingo E. Response of foot and mouth disease virus to increased mutagenesis: Influence of viral load and fitness in loss of infectivity. J Virology, 2000, 74,8316-23
  8. Crotty S, Cameron C and Andino R. RNA virus error catastrophe: Direct molecular test by using ribavirin. Proc. Natnl.Acad.Sci USA, 2001, 98,6895-6900.
  9. Kidwell MG, Kidwell JF, and Sved JA. Hybrid dysgenesis in Drosophila melanogaster: a syndrome of aberrant traits including mutation, sterility, and male recombination. Genetics, 1977, 86:813-33.
  10. Gressel ,J "Molecular biology of weed control" 2000 Transgenic Res 9,355-82

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