What is lateral gene transfer?

Gene Transfer in Prokaryotes

The fact that genes may move between bacteria has been known since the experiments of Frederick Griffith with pneumonia-causing bacteria in the 1920’s. Griffith discovered the process of bacterial transformation, by which the organism acquires genetic material from its environment and expresses the traits contained on the DNA in its own cells. Bacteria may also acquire foreign genetic material by the process of transduction. In transduction a bacteriophage picks up a piece of host DNA from one cell and delivers it to another cell, where it integrates into the genome. This material may then be expressed in the same manner as any of the other of the host’s genes. A third mechanism,
conjugation, allows two bacteria that are connected by means of a cytoplasmic bridge to exchange genetic information.

With the development of molecular biology, evidence has accumulated that supports the lateral movement of genes between prokaryotic species. In the case of Escherichia coli, one of the most heavily researched bacteria on the planet, there is evidence that as much as 20 percent of the organism’s approximately 4,403 genes may have been transferred laterally into the species from other bacteria. This may explain the ability of E. coli, and indeed many other prokaryotic species, to adapt to new environments. It may also explain why, in a given bacterial genus, some members are pathogenic while others are not. Rather than evolving pathogenic traits, bacteria may have acquired genetic sequences from other organisms and then exploited their new abilities.

It is also now possible to screen the genomes of bacteria for similarities in genetic sequences and use this information to reassess previously established phylogenetic relationships. Once again, the majority of this work has been done in prokaryotic organisms, with the primary focus being on the relationship between the domains Archaea
and Bacteria. Several researchers have detected evidence of lateral gene transfer between thermophilic bacteria
and Archaea prokaryotes. Although the degree of gene transfer between these domains is under contention, there is widespread agreement that the transfer of genes occurred early in their evolutionary history. The fact that there
was lateral gene transfer has complicated accurate determinations of divergence time and order.

Gene Transfer in Eukaryotes

Although not as common as in prokaryotes, there is evidence of gene transfer in eukaryotic organisms as well. A mechanism by which gene transfer may be possible is the transposon. Barbara McClintock first proposed the existence of transposons, or mobile genetic elements, in 1948. One of the first examples of a transposon moving laterally between species was discovered in Drosophila in the 1950’s. A form of transposon called a P element was found to have moved from D. willistoni to D. melanogaster. What is interesting about these studies is that the movement of the P element was enabled by a parasitic mite common to the two species. This suggests that
parasites may play an important role in lateral gene transfer, especially in higher organisms. Furthermore, since the transposon may move parts of the host genome during transition, it may play a crucial role in gene transfer.

The completion of the Human Genome Project, and the technological advances in genomic processing that it developed, have allowed researchers to compare the human genome with the genomes of other organisms to look for evidence of lateral transfer. It is estimated that between 113 and 223 human genes may not be the result of vertical gene transfer but instead might have been introduced laterally from bacteria.

Implications

While the concept of lateral gene transfer may initially seem to be a concern only for evolutionary geneticists in their construction of phylogenetic trees, in reality the effects of lateral gene transfer pose concerns with regard to both medicine and agriculture, specifically in the case of transgenic plants.

Currently the biggest concern regarding lateral gene transfer is the unintentional movement of genes from genetically modified organisms (GMOs) into other plant species. Such transfer may occur by parasites, as appears to have occurred with Drosophila
in animals, or by dispersal of pollen grains out of the treated field. This second possibility holds particular significance for corn growers, whose crop is wind-pollinated. Genetically modified corn, containing the microbial insecticide Bt, may cross-pollinate with unintentional species, reducing the effectiveness of pest management strategies. In another case, the movement of herbicide-resistant genes
from a GMO to a weed species may result in the formation of a superweed.

On the beneficial side, lateral gene transfer may also play a part in medicine as part of gene therapy. A number of researchers are examining the possibility of using viruses, transposons, and other systems to move genes, or parts of genes, into target cells in the human body, where they may be therapeutic in treating diseases and disorders.

Key terms



gene transfer

:

the movement of fragments of genetic information, whole genes, or groups of genes between organisms




genetically modified organism (GMO)

:

an organism produced by using biotechnology to introduce a new gene or genes, or new regulatory sequences for genes, into it for the purpose of giving the organism a new trait, usually to adapt the organism to a new environment, provide resistance to pest species, or enable the production of new products from the organism




transposons

:

mobile genetic elements that may be responsible for the movement of genetic material between unrelated organisms





Bibliography

Bushman, Frederick. Lateral Gene Transfer: Mechanisms and Consequences. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 2001. Examines the ability of genes to move between organisms and its implications for the development of antibiotic resistance, cancer, and evolutionary pathways, including those of humans.

Gogarten, Maria B., Johann Peter Gogarten, and Lorraine C. Olendzenski, eds. Horizontal Gene Transfer: Genomes in Flux. New York: Springer, 2009. Collection of articles by researchers who provide an overview of horizontal gene transfer (HGT) concepts and specific case histories. Begins with an overview of terminology, concepts, and the implications of HGT on evolutionary thought and philosophy, followed by a discussion of molecular biology techniques for identifying, quantifying, and differentiating instances, and concluding with a section of case studies.

Hensel, Michael, and Herbert Schmidt, eds. Horizontal Gene Transfer in the Evolution of Pathogenesis. New York: Cambridge University Press, 2008. An overview of current knowledge relating to the evolution of microbial pathogenicity that focuses on the rearrangements of the genome resulting from horizontal gene transfer. Aimed at graduate students and researchers.

Rissler, Jane, and Margaret Mellon. The Ecological Risks of Engineered Crops. Cambridge, Mass.: MIT Press, 1996. Introduces the reader to the concept of transgenic crops and then discusses the potential environmental risks of gene flow between genetically modified organisms and nontarget species of plants. Suggests mechanisms of regulation to inhibit environmental risk.

Syvanen, Michael, and Clarence Kado. Horizontal Gene Transfer. 2d ed. Burlington, Mass.: Academic Press, 2002. Examines the process of gene transfer from an advanced perspective. Discusses the relationship between gene transfer and phylogenetic analysis, evolutionary theory, and taxonomy.