Note: DuPont scientists conducted a review of current scientific literature related to horizontal gene transfer and biotech crops. This information will be updated from time-to-time. We welcome new scientific information and, of course, your perspective.

Table of Contents

1. Introduction - Horizontal Gene Transfer
2. Background - Evidence of HGT
3. Horizontal Gene Transfer and Biotech Crops
4. The Potential Impacts of Horizontal Gene Transfer
5. Impacts of HGT from Biotech Crops
6. Future Steps

Genes move among different species in nature. Sometimes gene transfer occurs between two closely related species via typical routes of reproduction, such as cross pollination of plants and interbreeding of animals. Other times genes move between different species, such as bacteria and plants, through a process unrelated to reproduction that is known as horizontal gene transfer (HGT). HGT also can occur between two closely related species. Therefore, the factor that distinguishes HGT from reproductive gene transfer is the mechanism by which is occurs, not the degree of similarity between the two species.

There are three distinct mechanisms of HGT:

• Conjugation - direct exchange of DNA between cells in physical contact with each other.
• Transduction - transfer of DNA through a virus. Some infecting viruses are able to move DNA between the organisms they infect.
• Transformation - direct uptake of DNA from the environment. DNA is found in soil, water, and;digestive tracts.

Considering the millions of species that co-exist in close physical contact and the universal presence of DNA in environments, the trivial amount of HGT that occurs is actually remarkable. Organisms are much more likely to reject foreign DNA than accept it, and evolution has provided them with a number of barriers to prevent the uptake and incorporation of foreign DNA.

The simple transfer of DNA into a new cell does not constitute HGT. First, for HGT to occur, the DNA must contain an entire gene. Secondly, HGT depends on the movement of the gene into a cell and on its ability to survive there. Cells will often degrade the DNA as soon as it enters, preventing complete HGT.

Regarding genes incorporated into plants through biotechnology, the important question is not whether these genes can move to other organisms, but whether they provide sufficient advantage to make maintaining them worthwhile. Concerns about environmental impacts of HGT involving these genes are germane only in those rare instances where a gene would spread through a population.

The World Health Organization (1993) and the U.S. Food and Drug Administration (1992) concluded there is no risk in consuming the DNA of biotech crops. The basis of this decision was that mammals have always ingested DNA from plants, animals and other sources, and the proportion of transgenic DNA within native DNA of a biotech crop, for example, is barely detectable.

HGT is most common among bacteria, especially those that are closely related to each other, occurring by all three mechanisms described above. For example, two well-known and closely related bacteria, E. coli and Salmonella, have transferred hundreds of genes back-and-forth over time.  » More

HGT from plants to bacteria, or vice versa, is less common but there are indications that it does occur. Scientists have not been able to demonstrate HGT from higher organisms to bacteria under natural conditions, but they have other indications that such HGT has occurred over the millennia bacteria and higher organisms have coexisted. For example, based on DNA similarities, a number of bacterial genes appear to be have originated in higher organisms. Other studies of genetic similarities indicate some plant genes may have bacterial origins.

Finally, while we are certain that small segments of plant DNA from ingested food are taken up routinely by intestinal cells in animals, there is no evidence entire plant genes have become incorporated into the genetic material of animals.  » More

Innumerable opportunities exist for HGT to occur because:

• Organisms are continually exposed to large amounts of DNA from ingested food and organisms decaying in the environment,
• Millions of species co-exist in close physical contact with each other, and
• Many microorganisms cause infections by inserting their DNA into host cells.

The universality of DNA as the genetic material provides the basis for crop biotechnology. Virtually all cells of all organisms contain DNA that is made from the same chemical building blocks.  » More

All organisms that are consumed or that decay in the environment release DNA that is available for uptake by other cells. DNA from a biotech crop is just as likely to be taken up as one from any other organism is to be taken up by cells. The most important variable determining which bits of DNA will be taken up is their relative frequency. The number of genes from biotech-derived plants that are available for uptake is insignificant compared to the number of genes generally available in the environment.  » More

The possibility that HGT might have an impact depends on which cells are involved and what happens after successful HGT.

First, the potential for negative impacts depends on the gene's ability to function in the recipient cell. For example, most genes incorporated into plants through biotechnology would not be functional in bacterial cells, because bacteria do not respond appropriately to the activating portion of the gene.

In addition, to have an impact, a significant number of cells must acquire the new gene. This depends on the rate of HGT, the nature of the gene and, in most cases, environmental influences. If maintaining the new gene is difficult for a cell, the gene will not persist. On the other hand, if the new gene provides the cells that have acquired it a competitive advantage, the gene will spread through the population. The inherent advantage, disadvantage or neutrality of possessing a certain gene depends on environmental factors and, therefore, can vary. A gene that provides a competitive advantage in one environment may be difficult to maintain in another environment.  » More

There are two related factors that indicate HGT involving genes from biotech-derived plants is virtually non-existent: The barely perceptible rate of plant gene transfer to soil bacteria, intestinal bacteria or intestinal cells; and the comparatively trivial number of genes from biotech-derived plants in decaying or ingested plant material. This is confirmed by extensive field studies that have revealed no HGT from biotech crops to soil microorganisms, and laboratory studies on HGT of genes from biotech-derived plants to intestinal bacteria and intestinal cells. The most likely, but difficult, mode of HGT from biotech crops to other organisms is through the mechanism called transformation. Even under optimal laboratory conditions, the rate of successful transformation of bacterial cells and intestinal cells with genes from plants approaches zero.

For more information on HGT from biotech crops to:

• Soil microbes
• Intestinal bacteria
• Intestinal cells

However, HGT can have an impact if the gene provides a competitive advantage to microorganisms living in soil or digestive tracts. Genes for crop traits, such as insect resistance or herbicide tolerance, would provide no competitive advantage to either soil or intestinal microorganisms. Some biotech crops contained an antibiotic resistance (AR) gene, linked to the crop trait gene that enabled crop developers to identify plant cells that had taken up the crop trait gene. In theory, if soil or intestinal microorganisms acquired the AR gene via HGT from the biotech crop, they would have a competitive advantage if exposed to that antibiotic.  » More

A number of factors, in addition to miniscule levels of HGT from plants to microbes, decrease the potential for the AR gene in biotech crops to spread through microbial population, such as

• In some biotech crops, the AR gene can only be activated in plant cells and, therefore, is not functional in microbes.
• The AR gene often does not include the signal that allows microbes to replicate it.

More importantly, the number of AR microbes created through HGT of AR genes from biotech plants is negligible when compared to the millions of microbes in soils or intestines that naturally contain the same AR gene.  » More

Companies are increasingly using the methods of biotechnology to construct genes with traits that further minimize the already small chance of gene transfer from biotech crops to other organisms. For example, they are deleting the genetic material from genes that allows bacteria to make copies so that any instance of HGT from plants to bacteria will not spread through the microbial population. In addition, even though a number of authoritative scientific bodies, such as the World Health Organization and American Medical Association, have concluded AR marker genes present no risk to human and animal health or the environment, biotech crop developers are phasing out the use of AR markers and developing new marker genes for identifying the cells containing genes from biotech-derived plants.