Virtually all organisms can disseminate genetic material via vertical gene transfer in which genetic information is transmitted from parents to offspring. In contrast, the process of horizontal gene transfer — or lateral gene transfer — is not common to all species. In this type of gene transfer, genetic material is transmitted between organisms that are not parent and offspring. Instead, the two organisms are usually unrelated, and are often of different species.
Most eukaryotic organisms acquire genetic material mainly via vertical gene transfer, during which DNA recombination occurs. In contrast, it is thought that horizontal gene transfer is the most common way in which single-celled organisms, bacteria in particular, acquire new genetic material. This is important because bacteria reproduce without sexual recombination; therefore horizontal transfer allows bacterial species to maintain diversity.
Horizontal gene transfer in bacteria is a common phenomenon. There are three ways in which bacteria can exchange genetic material horizontally. All three of these occur naturally and can also be carried out in the laboratory to genetically engineer bacteria with new properties, such as the ability to synthesize non-native proteins.
Transformation is a process in which bacteria ingest naked DNA or RNA molecules and express them within the cell. In transduction, genetic material is transferred by bacteriophages, which are viruses that infect bacteria. Finally there is bacterial conjugation, in which genetic material, often plasmids, is transferred via intercellular contact. Conjugation is mediated by transposons and plasmids, two types of genetic material which are distinct from and independent of the bacterial genome.
There is some evidence that viruses can also transmit genetic information via horizontal gene transfer. One of the strongest indicators involves viruses called Mimivirus, Mamavirus, and Sputnik. Both Mimivirus and Mamavirus are known hosts of Sputnik, and a small number of genes are common to all three species. It is thought that Sputnik transferred genes between its two host viruses at some point during the evolutionary history of these three organisms.
Certain eukaryotes, including species of fungi, insects, and plants, also engage in horizontal gene transfer. One of the most well-known examples is of horizontal genetic transfer between bacteria and Saccharomyces cerevisiae, a species of yeast. Another example of genetic transfer between eukaryotes involves the azuki bean beetle and a bacteria called Wolbachia. In the plant world, parasites of a plant family called Rafflesiaceae have transferred genetic information to their hosts, and a species of algae has transferred genetic material to a species of sea slug that preys on the algae.