Horizontal gene transfer is any process in which an organism transfers genetic material (i.e. DNA) to another cell that is not its offspring. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g. its parent or a species from which it evolved. Most thinking in genetics has focussed on the more prevalent vertical transfer, but there is a recent awareness that horizontal gene transfer is a significant phenomenon.

Horizontal gene transfer is common among bacteria, even very distantly-related ones. For example, this process is thought to be a significant cause of increased drug resistance; when one bacterial cell acquires resistance, it can quickly transfer the resistance genes to many species. Also enteric bacteria appear to exchange genetic material with each other within the gut in which they live. Horizontal gene transfer can occur through the following three of the most common mechanisms:

• Transformation, in which cells take up naked DNA, i.e. DNA that is not contained in a cell. This process is relatively common in bacteria, but less common in higher eukaryotes. Transformation is often used to insert novel genes into bacteria for experiments, or for industrial or medical applications. See also molecular biology and biotechnology.
• Transduction in which a virus or bacteriophage inserts its genetic material into a cell. This process is used to insert novel genes into higher eukaryotes, where transformation is generally not practical
• Bacterial conjugation in which a living bacterial cell donates genetic material to another.

Analysis of DNA sequences suggests that horizontal gene transfer has also occurred within eukaryotes, from their chloroplast and mitochondrial genome to their nuclear genome. As stated in the endosymbiotic theory, chloroplasts and mitochondria probably originated as bacterial endosymbionts of a progenitor to the eukaryotic cell. There is also recent evidence that the adzuki bean beetle has somehow acquired genetic material from its (non-beneficial) endosymbiont Wolbachia.

Horizontal gene transfer is a potentially confounding factor in inferring phylogenetic trees based on the sequence of one gene. For example, given two distantly related bacteria that have exchanged a gene, a phylogenetic tree including those species will show them to be closely related because that gene is the same, even though most other genes have substantially diverged. For this reason, it is often ideal to use other information to infer robust phylogenies, such as the presence or absence of genes, or, more commonly, to include as wide a range of genes for phylogenetic analysis as possible.

For example, the most common gene to be used for constructing phylogenetic relationships in prokaryotes is the 16s rRNA gene, since its sequences tend to be conserved among members with close phylogenetic distances, but variable enough that differences can be measured. However, in recent years it has also been argued that 16s rRNA genes can also be horizontally transferred. Although this may be infrequent, validity of 16s rRNA-constructed phylogenetic trees must be reevaluated.

References

• This article gives the evidence that Wolbachia DNA is in the adzuki bean beetle genome. Natsuko Kondo, Naruo Nikoh, Nobuyuki Ijichi, Masakazu Shimada and Takema Fukatsu (2002) "Genome fragment of Wolbachia endosymbiont transferred to X chromosome of host insect", Proceedings of the National Academy of Sciences of the USA , 99 (22): 14280-14285". http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12386340&dopt=Abstract (this article is in a free journal)
• This article proposes using the presence or absence of a set of genes to infer phylogenies, in order to avoid confounding factors such as horizontal gene transfer. Snel B, Bork P, Huynen MA (1999) "Genome phylogeny based on gene content", Nature Genetics, 21(1) 66-67. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9916801&dopt=Abstract