2.7—Mobile DNA drives evolution
Movement of mobile-DNA is a major driver of chromosome variability.
Analysis of Peer-Reviewed Research:
Much, if not most, changes that occur to plant chromosomes during evolution is generated by dynamic rearrangement of plant DNA orchestrated by mobile parasitic DNA. Typical plant genomes can contain about 90 percent of their DNA as the remnants of the past movement and multiplication of mobile DNA, reflecting the huge amount of mobile DNA multiplication that occurs during plant evolution. Often this movement is associated with rearrangement or scrambling of genes. Plant chromosomes are clearly very tolerant of structural variation during the course of their history. Although most copies of parasitic DNA are inactive, many food crops, for example soybean, maize, and rice, possess active copies of mobile DNA. All these evolutionary processes that dynamically change plant chromosomes generate much more genetic novelty than does the introduction of a single new trait by genetic engineering. Possible activation of plant mobile DNA by insertion of a transgene does not introduce a new risk to food when a plant is genetically engineered, because chromosomal change is happening all the time in the plant DNA of our food supply.
Mutations can also add to this shuffling of chromosomal DNA when plants grow in the field. Soybeans for instance, have been found growing in the field with an altered flower color caused by mobile DNA inserting into the genes involved in flower color. When new crop hybrids are made using conventional breeding methods, a large amount of genetic alteration is created in the new hybrid varieties when completely new sets of chromosomes are brought together by cross-pollination. All these many different processes that dynamically change chromosomes generate much more genetic novelty than does the introduction of a single new trait by genetic engineering.
For over 70 years or so, thousands and thousands of new food varieties have been introduced into the food supply and the varied and highly improbable theoretical risks from random gene movement and parasitic DNA activation that might be caused by techniques used in conventional breeding have been put to the test of practical experience. No harm to consumers of food has been seen over 70 years from a large amount of theoretically hazardous genetic variation.
Arguments put forward in Genetic Roulette about the possibility of adverse effects from transgenes influencing plant mobile DNA fail to bear in mind this long history of breeding of safe crops from plants which are subject to much DNA rearrangement which poses similar theoretical risks.
This section of Genetic Roulette also doesn’t consider the protection against unexpected effects provided by scrutiny of new varieties during the long process of new seed development and registration, typically taking 10 years for a new genetically engineered crop variety to reach customers.
Transgenic GM crops are carefully selected to be free of unintended genetic subjected to extensive safety analysis by regulatory agencies. Thus, whatever the chances are that a plant mobile DNA created some undesired change in a crop, they are greatly reduced by breeding, variety selection and regulatory scrutiny. There is no evidence that these hypothetical risks have ever caused any harm to humans, and the fact that Genetic Roulette can only speculate about what might occur is testimony to the overall safety of plant breeding. It is noteworthy that not only does Jeffrey Smith not have peer-reviewed scientific papers to support his views, all of the testimony in this section about risks comes from well known anti-GM activists who, like Smith, seem blindly committed to blocking the adoption of GM technology.
1. Plants contain numerous mobile genetic elements that move about their genomes and cause mutations. We are exposed to a wide variety of the random activities of parasitic DNAs such as transposons when we eat food (Adams and Wendel 2005, Jiang and others 2004, Leitch and Leitch 2008, Morgante M and others , 2005, Wendel and Wessler 2000, Yamashita and Tahara 2006). Peer-reviewed scientific studies have fully documented that commonly consumed crops contain numerous copies of many different mobile DNA sequences, and that active movement of these DNAs occurs in food crops (Jiang and others 2003, Kalendar and others 2000, Moon and others 2006, Tsugane and others 2006, Wendel and Wessler 2000; Zabala, Vodkin 2008).
2. DNA from viruses is frequently found in plant chromosomes. Non-GM rice chromosomes have several random fragments of DNA from a virus called Rice tungro bacilliform virus randomly inserted in them. This virus is related to the virus used to provide promoters for many GM crops. Virus DNA fragments are present in the genome of potato. Similarly many Banana streak virus DNA fragments are found in banana chromosomes. Other virus DNA inserts are widely distributed among the chromosomes of many other plants (Gayral and others 2008, Harper and others 2002, Hansen and others 2005, Staginnus Richert-Pöggeler 2006) (as was discussed in section 2.5). The promoters from these virus fragments pose the same kind of risks as those imagined by Smith for transgene DNA inserts, but no harmful consequences for human health have been linked to these virus DNA inserts in plant chromosomes.
3. No adverse health effect has ever been attributed to mobile gene movement or insertion of DNA in crop plants. The type of risk postulated in this section by Smith to be linked with transgene insertion is an extremely low probability event. In our food we are already exposed to similar extremely low probability genetic risks from numerous viruses and mobile DNA insertions occurring in non-GM food crops. This background of existing food risk is triggered by the continual movement of mobile DNA to new places in chromosomes, and vast numbers of viruses coming into contact with the chromosomes of plants growing in farmers fields. Exposure of plants to mutation causing radiations adds to this background risk. Compared to this background level of existing risk those promoted by Genetic Roulette are miniscule.
4. Genetic Roulette suggests that biological and non-biological stresses occurring during creation of genetically manipulated crops may trigger parasitic mobile DNA activity. Activation of mobile DNA triggered by the biological stress of cross-pollination during conventional breeding of wheat has been reported by scientists (Grandbastien 1998, Feldman M, Levy AA (2005),and cross-pollination between different species is recognized as a major “genome shock” that subjects plant genomes to radical rearrangements including mobile DNA activation (Chen and Ni 2006). It is also asserted by Smith that tissue culture used for genetic engineering induces movement of mobile DNA within plant DNA; however, as discussed in section 2.2, tissue culture is frequently used in conventional breeding, so people are already eating food with this risk even if they avoid eating GM produce. The generation of transgenic crops is unlikely to significantly add to this background exposure to activated parasitic mobile DNAs in existing food crops. There is no evidence these hypothetical risks cause harm to humans.
Adams K L and Wendel J F. (2005) Novel patterns of gene expression in polyploid plants. Trends in Genetics 21(10):539- 543. Genes are silenced in hybrid plants but there is not a common or predictable pattern.
Chen ZJ and Ni Z (2006). Mechanisms of genome rearrangements and gene expression changes in plant polyploids. Review article. BioEssays 28:240-252.Newly created crosses between different plant species undergo numerous genetic changes.
Feldman M, Levy AA (2005) Allopolyploidy-a shaping force in the evolution of wheat genomes. Cytogenet Genome Res 109(1–3): 250–258.Cereal hybridization is a stress that cause dramatic genome changes.
Gayral P, and other (2008) A single Banana streak virus integration event in the banana genome as the origin of infectious endogenous pararetrovirus. J Virol. 82(13):6697-710.
Grandbastien M-A (1998) Activation of plant retrotransposons under stress conditions. Trends Plant Sci 3:181–187
Harper G and others (2002). Review. Viral sequences integrated into plant genomes. Annual Review of Phytopathology 40:119–36. Numerous bits of viruses are found inside the chromosomes of plants that we eat.
Hansen CN, Harper G, Heslop-Harrison JS. (2005) Characterisation of pararetrovirus-like sequences in the genome of potato (Solanum tuberosum. Cytogenet Genome Res. 110(1-4):559-65.
Jiang N, and others (2003). An active DNA transposon family in rice. Nature 421:163–167. Mobile genes called mPing and Pong are active in rice.
Jiang N and others (2004) Pack-MULE transposable elements mediate gene evolution in plants Nature 431, 569-573. Mobile DNAs in rice carry fragments of more than 1000 cellular genes.
Kalendar R and others (2000). Genome evolution of wild barley (Hordeum spontaneum) by BARE-1 retrotransposon dynamics in response to sharpe microclimate divergence. Proceedings of the National Academy of Sciences of the USA 97:6603-6607
Leitch AR and Leitch IJ (2008. Genome plasticity and the diversity of polyploid plants.(2008).Science 320:481-483 The success of flowering plant is partly attributable to their highly plastic genomes which can withstand large scale changes in structure over just a few generations.
Morgante M and others (2005) Gene duplication and exon shuffling by helitron-like transposons generate intraspecies diversity in maize. Nat Genet. 37(9):997-1002.
Moon S and others .(2006) Identification of active transposon dTok, a member of the hAT family, in rice. Plant Cell Physiol. 47(11):1473-83.
Staginnus C, Richert-Pöggeler KR (2006) Endogenous pararetroviruses: two-faced travelers in the plant genome. Trends Plant Sci. 11(10):485-91.
Staginnus C, and other (2007) Endogenous pararetroviral sequences in tomato (Solanum lycopersicum) and related species. BMC Plant Biol. 7:24.
Tsugane K, and others (2006) Plant J. 45(1):46-57. An active DNA transposon nDart causing leaf variegation and mutable dwarfism and its related elements in rice.
Wendel JF and Wessler S (2000) Commentary.Retrotransposon-mediated genome evolution on a local ecological scale. Proceedings of the National Academy of Sciences of the USA 97:6250-6252
Yamashita H, Tahara M. (2006) Plant Mol Biol. 61(1-2):79-94. A LINE-type retrotransposon active in meristem stem cells causes heritable transpositions in the sweet potato genome.
Zabala G, Vodkin L (2008). A putative autonomous 20.5 kb-CACTA transposon insertion in an F3′H allele identifies a new CACTA transposon subfamily in Glycine max. BMC Plant Biology. Research article Open Access www.biomedcentral.com/1471-2229/8/124 Soybeans harbour active mobile genes that insert disruptively and cause mutations.
Genetic engineering activates mobile DNA, called transposons, which generate mutations
- In plant DNA, mobile elements called transposons move from place to place and can lead to mutations.
- The tissue culture process used in genetic engineering activates transposons, and is a major factor for the resulting genome-wide mutations.
- Transgenes in commercial GM crops tend to be inserted near transposons.
- This insertion might alter the transgene expression.
Plants contain many different types of parasitic DNA that often increase in numbers by inserting an extra copy of themselves in new places in chromosomes. Genetic Roulette claims that the interactions of newly introduced genetically engineered DNA with the numerous existing mobile DNA parasites might have unexpected hazardous effects. (Although the correct scientific term used for many of these mobile DNAs is transposon we will mostly refer to transposons using the more understandable descriptive terms mobile or parasitic DNA.)