The effect of plant breeding at the DNA level� how different is it from genetic engineering?

dna20.gif - 5863 Bytes

In 2003, as controversy was surrounding the use of transgenic maize as food aid for Africa, Roland Lesseps, S.J., of Kasisi Catholic Church, Lusaka, Zambia, asserted that, �Insertion of DNA can cause deletions and rearrangements of the original DNA at the insertion site. This information helps us understand that GE is significantly different from conventional breeding techniques.� (Promotio Iustitiae, Social Justice Secretariat, Society of Jesus)

The goal of this page is to evaluate the statement made by Fthr. Lesseps and others opposed to genetic engineering. First, some background information:

Charles Darwin wrote in his Origin of Species (1859) that our current plants and animals were the result of "a form of Selection, which may be called Unconscious, and which results from every one trying to possess and breed from the best individuals..." and that the changes have been so great that �... in a vast number of cases, we cannot recognize ... the wild parent-stocks of the plants which have been longest cultivated in our flower and kitchen-gardens." Image used with permission of The Natural History Museum, London.

Take Darwin's test
These are "wild parent-stocks" of some of our crops. Can you identify the modern crop derived from each of these? Afterwards, click on each photo to determine if you were correct.


Photo credits: Left, Peggy Lemaux, UC-Berkeley; Center, Raul Coronado, M�xico; Right, Dr. John Meade, Weed Scientist Emeritus, and is used with permission of the Rutgers Cooperative Extension Service.

Did you know?
Some of our crops never existed in Nature! Click on each photo to learn more about it.

So, what about the DNA?
Human selection over the years has resulted in drastic changes to the way plants look, grow, and taste. What did this do to their DNA?

  • Crop plants have "jumping genes" that mimic the effect of transgene insertion
    Not unexpectedly, such changes have also altered the DNA of crop plants. In first place, plants are continuously altered by the effects of transposable elements jumping in and out of genes, where they �can alter gene expression or serve as sites of chromosome breakage or rearrangement� (Wessler, 2001) just like transgenes, and without ill effects to the plants or those of us who consume them. In addition, retrotransposons continuously insert themselves between genes (San Miguel et al., 1996). Because retrotranspon sequences are found in current EST databases, we know that their movement was not just a thing of the past, but something that continues to the present. Other parts of the genome are also subject to change. Repetitive DNA sequences (microsatellites) have a tendency to change in copy number (e.g., Lian et al., 2004), thus making them highly variable in plant populations and explaining their popularity as a molecular marker in plant breeding programs.

  • Different varieties of the same crop can have drastic differences in DNA amount
    All this means that different varieties of the same crop differ greatly in the amounts of DNA they have. Such changes is likely to have resulted in evolutionary improvements in plant adaptation (e.g., Ceccarelli et al., 1997; Shirasu et al., 2000) and in breeding progress, as evidenced by the large differences in genome size within varieties of the same crop. For example, different varieties of maize can differ by as much as 42% in their DNA content; different varieties of chili pepper differ by 25%, and different soybean varities differ by 12% (Graham et al., 1994; Mukherjee & Sharma, 1990; Rayburn et al., 1989). This means that, for soybean, different varieties vary by over 100 million base pairs of DNA, dwarfing the few thousand base pairs that transgenes add to genomes. The take home message is that crops safely tolerate large changes to their DNA.

  • Genes themselves can move from one location in the genome to another
    Comparisons between closely related grass species reveals not all genes are in their original location, indicating they have moved about (Lai et al., 2004). Genes also get transferred between organellar genomes, and from organellar to nuclear genomes or the reverse (Adams et al. 2002; Cummings et al. 2003), even between mitochondria of unrelated, sexually incompatible plant species (Bergthorsson et al. 2003)!

  • Different varieties of the same crop have do not have the same genes
    Different individuals of the same species differ by the number of transposon and retrotransposons they contain, a phenomenon vividly illustrated by Fu & Dooner (2002). This finding has since been extended to another part of the maize genome (Song and Messing, 2003). What this means is that different individuals within the same species do not even have to have the same number of genes.

    Again, this result is not altogether surprising, cytoplasmic male sterility in crop plants has long known to result from the creation of new genes in the mitochondria, along with novel fertility restorer genes in the nucleus (Schnable and Wise, 1998). We now know that at least some plants can create totally new genes with new functions by mixing and matching bits and pieces of other genes (Jiang et al., 2004).

    The bottom line is that individuals within a species can tolerate different gene numbers without endangering the animals that consume them.

  • Nature's own transgenics: Gene transfer between sexually incompatible organisms takes place often enough to say it is not unnatural
    A final argument made is that conventional plant breeding does not involve transfer of DNA between completely unrelated organisms. While such a statement is true, it must also be acknowledged that DNA of unrelated species does get transferred and incorporated into plant genomes anyway. For example, plantain bananas contain the entire DNA content of the banana streak virus, rice contains DNA from the rice tungro bacilliform virus, and tomato has DNA from the tobacco vein clearing virus. Some tobaccos even have genes in them from Agrobacterium rhizogenes (reviewed in Harper et al., 2002). The true extent of such natural interspecific gene transfer will become more apparent as additional genomes are sequenced.

  • Conventional breeding is not precise at all
    Large amounts of DNA from other species can be brought in during the breeding process. For example, Young and Tanksley (1989) looked at tomato cultivars into which the Tm-2 gene had been introgressed from wild species, and found wild species DNA left in modern tomato cultivars. The amount of DNA from the wild species ranges from 4 to 51 cM long� enough for several genes to have come along from the wild species.

Conclusions
Plant genomes are incredibly variable and fluid. We now know that extensive variation in DNA content is normal within a species, that DNA movement and rearrangements are common and natural phenomena, and that individuals within a species can naturally have different numbers of genes in them. We also know that genes get transferred between unrelated, sexually incompatible species-- not very often; but often to say the phenomenon is natural. The nature of these changes is similar to what happens during genetic engineering, except that the changes brought about by domestication and breeding are much larger in scale than the changes from engineering.

Most of the changes in the DNA of crop plants are without ill effects to the plant, to the environment, or to herbivores. Given the similarities in the nature of the DNA changes and the differences in magnitude of these changes, genetic engineering should not pose hazards different from those posed by those changes brought about from breeding and domestication, which history has shown are usually too small to be concerned about.

Additional Reading

Literature Cited

  • Adams, K.L., Y.L. Qiu, M. Stoutemyer, and J.D. Palmer. 2002. Punctuated evolution of mitochondrial gene content: High and variable rates of mitochondrial gene loss and transfer to the nucleus during angiosperm evolution. Proc. Natl. Acad. Sci. USA 99:9905-9912.
  • Bergthorsson U., K.L. Adams, B. Thomason, and J.D. Palmer. 2003. Widespread horizontal transfer of mitochondrial genes in flowering plants. Nature 424:197-201.
  • Ceccarelli, M., T. Giordani, L. Natali, A. Cavallini, and P.G. Cionini. 1997. Genome plasticity during seed germination in Festuca arundinacea. Theor.Appl.Genet. 94:309-315.
  • Cummings, M.P., J.M. Nugent, R.G. Olmstead, and J.D. Palmer. 2003. Phylogenetic analysis reveals five independent transfers of the chloroplast gene rbcL to the mitochondrial genome in angiosperms. Current Genetics 43:131-138.
  • Fu, H.H., and H.K. Dooner. 2002. Intraspecific violation of genetic colinearity and its implications in maize. Proc. Natl. Acad. Sci. USA 99:9573-9578.
  • Graham, M.J., C.D. Nickell, and A.L. Rayburn. 1994. Relationship between genome size and maturity group in soybean. Theor. Appl. Genet. 88:429-432.
  • Harper, G., R. Hull, B. Lockhart, and N. Olszewski. 2002. Viral sequences integrated into plant genomes. Ann. Rev. Phytopathol. 40:119-136.
  • Jiang, N., Z. Bao, X. Zhang, S.R. Eddy, and S.R. Wessler. 2004. Pack-MULE transposable elements mediate gene evolution in plants. Nature 431:569-573.
  • Lai, J., J. Ma, Z. Swignonov�, W. Ramakrishna, E. Linton, V. Llaca, B. Tanyolac, Y.-J. Park, O.-Y. Jeong, J.L. Bennetzen, and J. Messing. 2004. Gene loss and movement in the maize genome. Genome Res. 14:1924-1931.
  • Lian, C., R. Oishi, N. Miyashita, and T. Hogetsu. 2004. High somatic instability of a microsatellite locus in a clonal tree, Robinia pseudoacacia. Theor. Appl. Genet. 108:836-841.
  • Mukherjee, S., and A.K. Sharma. 1990. Intraspecific variation of nuclear DNA in Capsicum annuum L. Proc. Indian Acad. Sci. 100:1-6.
  • Rayburn, A.L., J.A. Auger, E.A. Benzinger, and A.G. Hepburn. 1989. Detection of intraspecific DNA content variation in Zea mays L. by flow cytometry. J. Exp. Bot. 40:1179-1183.
  • San Miguel, P. et al. 1996. Nested retrotransposons in the intergenic regions of the maize genome. Science 274:765-768.
  • Schnable, P.S., and R.P. Wise. 1998. The molecular basis of cytoplasmic male sterility and fertility restoration. Trends Plant Sci. 3:175-180.
  • Shirasu, K., A.H. Schulman, T. Lahaye, and P. Shulze-Lefert. 2000. A contiguous 66-kb barley DNA sequence provides evidence for reversible genome expansion. Genome Res. 10:908-915.
  • Song, R., and J. Messing. 2003. Gene expression of a gene family in maize based on noncollinear haplotypes. Proc. Natl. Acad. Sci. USA. 100:9055-9060.
  • Wessler, S.R. 2001. Plant transposable elements. A hard act to follow. Plant Physiol. 125:149-151.
  • Young, N.D., and S.D.Tanksley. 1989. RFLP analysis of the size of chromosomal segments retained around the Tm-2 locus of tomato during backcross breeding. Theor. Appl. Genet. 77:353-359.