"Critics of agricultural biotechnology have often been dismissed as modern-day Luddites," proclaims Michael Syvanen in his commentary published in Nature Biotechnology(1). Syvanen, a professor of medical microbiology and immunology at the University of California, Davis, defends the concerns of biotechnology opponents in his article titled, "In search of horizontal gene transfer." His commentary is offered in response to assertions made by several researchers and the US Food and Drug Administration (FDA) that horizontal gene transfer from plants to bacteria is not feasible. The FDA comment was referring to plant cells carrying antibiotic-resistance genes added to assist with the identification of transformed cells. These cells are eventually cultured into adult plants and released into the field.

The impact of horizontal gene transfer concerns agricultural biotechnology critics who claim that novel genes inserted into domesticated organisms might be transferred to wild organisms_an example would be the transfer of antibiotic resistance marker genes to soil bacteria. Studies supporting this argument are based on in vitro experiments, comparative gene frequency studies, and investigations of reproductive strategies(2).

Syvanen's studies of bacterial and plant phylogeny indicate horizontal gene transfer is involved in evolutionary change. His work consists of numerous genetic studies dating back to the early 1980's, including recent genomic investigations on Archaea. Other researchers have reported evidence that functional chunks of DNA persist in the environment, and that significant quantities of plasmid DNA survived passage through a mouse digestive system and even stayed intact after macrophage ingestion. Although Syvanen concedes there are no confirmed studies of this type of gene transmission occurring in nature, he cautions that laboratory studies are evidence enough to warrant concern.

A disturbing consequence of horizontal gene transfer in bacteria is the potential for antibiotic resistance and other traits carried on plasmids to migrate from one type of bacterium to another. Evidence suggests genomic DNA is transmitted in nature. In bacteria, conjugation is the most likely mechanism responsible for gene transfer. Also, evidence of transfer of genetic material between unrelated prokaryotes and between Archaea and prokaryotes early in the evolution of prokaryotes may be explained by conjugation and the uptake of environmental DNA. In the introduction to Horizontal Gene Transfer, a book co-authored by Syvanen and Clarence Kado, reference is made to experimental evidence for plasmid transfer between E. coli and actinomycetes, Agrobacterium, and cyanobacteria(2). The rapid diversification of eucaryotic evolution may have been accompanied by extensive gene transfer.

The possibility that gene transfer occurs between distantly related organisms such as bacteria and plants, or animals and plants, is the subject of speculation and debate. The most extensively studied mechanism of gene transfer from bacteria to plants is found in Agrobacterium, which readily transmits a plasmid that induces tumor development when incorporated into plants. Laboratory studies show that the bacteria are capable of transcribing and translating plant genes as well(2).

Transposon-like elements are likely vehicles for cutting and pasting genomic DNA from one organism to another. Jeffrey Palmer of Indiana University suggests that a genetic parasite resembling a transposon can carry or "jump" a segment of yeast DNA into higher plants. Clarence Kado, a plant pathologist who has collaborated with Syvanen, believes viruses may be responsible for transmitting genes between eucaryotes. Also, a few scientists suspect that viral transmission of regulatory genes was involved in the macroevolution of metazoa. It is also believed that viruses may be responsible for the origin of introns in eucaryotic DNA. In fact, the theory that viruses are responsible for transmitting traits between unrelated organisms suggests that horizontal gene transfer could be a common event.

Molecular evolution estimates indicate that horizontal gene transmission is more likely to occur in microorganisms and plants. However, animal studies also provide evidence of the horizontal transmission of Drosophila P-elements. Syvanen likewise believes that gene transfer is probable, though not common, in humans and other animals.

Syvanen argues that some of the gene frequency studies, such as those examining neomycin phosphotransferase, provide weak evidence of the dangers of gene transfer because, "This gene is already ubiquitous, so it is far from clear that its presence in genetically engineered plants will add substantially to the existing danger." Though biotechnology opponents may use the implications from Syvanen's work to justify limiting the employment of GMOs, Syvanen himself does not condemn using GMOs in the field. He states, "If horizontal transfer is found out to occur naturally, perhaps the most significant outcome would be the reshaping of our evolutionary paradigms." However, Syvanen does argue, "If and when experiments prove that horizontal transfer occurs, the implications will be debatable. In such a scenario, some vectors might indeed create an unacceptable risk."

Sources:

1. Syvanen M. 1999. In search of horizontal gene transfer. Nature Biotechnology, 17(9): 833.

2. Syvanen M and Kado CI. 1998. Horizontal gene transfer. London: Chapman & Hall.

3. Syvanen M. 1994. Horizontal gene transfer: Evidence and possible consequences. Annual Review of Genetics 28:237-261.