Horizontal gene transferDNA) 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
- Conjugation in which a living bacterial cell donates genetic material to another.
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.