Horizontal Gene Transfer and Symbiogenesis

How Lateral Gene Transfer Drives Genome Evolution

Nov 3, 2009

Horizontal gene transfer, also called lateral gene transfer, is a mechanism for evolution that goes beyond typical Darwinian theory.

In traditional evolutionary theory, genes mutate over time and a species slowly changes as these mutations accumulate in the population. Lateral gene transfer is a different proposed mechanism for evolution.

Transferred Genes Affect Cell Evolution

Transferred genes are genes that move from one species to another or from one individual to another. There are a variety of ways this can happen. In some cases, the gene moves through a virus, becoming part of the virus and then dropping into the genome of whatever organism the virus infects. In other cases, an organism can completely engulf another organism and incorporate that genome into their own.

Horizontal Gene Transfer in Prokaryotes Causes Antibiotic Resistance

In bacteria and archaebacteria, the transfer of genes is common. Since bacteria do not sexually reproduce, transferring genes between individuals is the only way to share genetic information. This process is called horizontal gene transfer or lateral gene transfer.

Even bacteria that are not closely related can share genes, which is how different bacterial species acquire immunity to medicines, such as antibiotics. When one bacteria develops immunity, it can share those protective genes with other bacteria and spread the antibiotic resistance.

Horizontal gene transfer in bacteria is accomplished through different mechanisms such as transformation (when only a small piece of naked DNA is transferred) and conjugation (the use of small circular DNA that move from one bacteria to another.)

Lateral Gene Transfer in Eukaryotes, Multicellular Organisms, Via Movable Elements

In multicellular organisms, the eukaryotes, horizontal gene transfer is a little more complex.

One form of horizontal gene transfer is the movement of genes via viruses or "jumping genes," movable elements that shift from one chromosome to another, sometimes between species. These movements of jumping genes are a concern with regard to genetically engineered crops, since some people worry that they will cause a modified gene to jump into other species.

Another method is the transfer of genes from bacteria to multicellular organisms. This has been seen with fungi, especially Saccharomyces cerevisiae, a yeast, which has picked up a variety of genes from bacterial species.

When two completely different organisms merge to form something new, this is called symbiogenesis. For many years, this was considered a heretical idea in biology, until Lynn Margulis discovered that mitochondria in animals and chloroplasts in plants were both originally symbiotic bacteria that had been incorporated into the larger organisms as organelles.

Now, some scientists, Lynn Margulis among them, believe that many – perhaps most – new species have developed in this way, via the acquisition of entire genomes instead of by a gradual series of mutations.

Whether horizontal gene transfer and symbiogenesis are driving forces behind all examples of speciation or not, these processes are undoubtably active in genome evolution. Continued study of lateral gene transfer will likely find more examples of the process at work in biology.

Keeling PJ, Palmer JD. Horizontal gene transfer in eukaryotic evolution. Nat Rev Genet. 2008 Aug;9(8):605-18.

Uprooting the Tree of Life by W. Ford Doolittle. Scientific American. February 2000, pp 90–95

Dr. Lynn Margulis and Dorion Sagan. Acquiring Genomes: A Theory of the Origins of Species. Basic Books. 2002.

The copyright of the article Horizontal Gene Transfer and Symbiogenesis in Genetics & Evolution is owned by Bridget Coila. Permission to republish Horizontal Gene Transfer and Symbiogenesis in print or online must be granted by the author in writing.