Horizontal gene transfer (HGT) is the movement of genetic material between organisms other than through ordinary parent-to-offspring inheritance. It can occur between closely related strains or across species boundaries, allowing recipients to acquire existing genes rather than generate new variants solely through mutation. HGT is especially important in bacteria and archaea, but also occurs in eukaryotes. Together with vertical inheritance, it produces genomes whose different regions may have different evolutionary histories. (pmc.ncbi.nlm.nih.gov)
Transfer and inheritance
“Horizontal” describes the relationship between donor and recipient, not a particular molecular mechanism. Transfer may introduce a new gene, replace an existing sequence, or deliver a larger segment containing several genes. Acquired material can subsequently pass vertically to the recipient’s descendants. Consequently, horizontal acquisition and vertical inheritance are complementary processes rather than mutually exclusive categories. (pmc.ncbi.nlm.nih.gov)
Entry into a recipient does not necessarily establish a lasting genetic change. Incoming DNA must persist through autonomous replication or integration into a chromosome. It may instead be degraded or lost. Experiments that recorded incoming DNA in bacterial cells demonstrated that transfer can be detected even when the introduced plasmid cannot replicate in the recipient, illustrating the distinction between DNA entry and stable inheritance. (nature.com)
Principal mechanisms
Three classical routes are recognized in bacteria:
- Natural transformation: a competent recipient takes up extracellular DNA. Imported sequences can become incorporated into the chromosome through homologous recombination. This route can exchange chromosomal variants and introduce additional genes when recombination occurs at suitable flanking sequences.
- Conjugation: DNA passes between cells through specialized transfer machinery requiring donor–recipient contact. Plasmids are important vehicles, although not every plasmid can transfer independently. Conjugative plasmids encode their own transfer functions; mobilizable plasmids rely on machinery supplied by another element.
- Transduction: a bacteriophage carries bacterial DNA to another cell. The transferred material can subsequently become established in the recipient.
These routes differ in their requirements for contact, extracellular DNA, and biological vectors, and therefore connect different sets of potential donors and recipients. (nature.com)
Mobile DNA also links movement within a genome to movement between organisms. A transposable element can relocate genes between a chromosome and a plasmid; the plasmid may then carry those genes into another species. Transposition alone is therefore not necessarily horizontal transfer. Experimental soil communities have demonstrated successive chromosome-to-plasmid, plasmid-to-chromosome, and interspecies movements of resistance-associated DNA. (nature.com)
Barriers and establishment
Transfer is constrained by both molecular compatibility and cellular defenses. Restriction–modification systems can attack foreign DNA, while CRISPR–Cas systems can recognize incoming genetic elements through sequence-specific targeting. Some plasmids encode anti-defense functions that help them overcome these barriers; experimental work has identified such functions concentrated near the region transferred first during conjugation. (nature.com)
Persistence also depends on interactions between acquired DNA and its host. A plasmid may impose a reproductive cost, fail to replicate reliably, or be lost during cell division. Natural selection can favor improved host–plasmid compatibility. In experimental bacterial populations, compensatory chromosomal mutations increased the persistence of an antibiotic-resistance plasmid and also improved retention of other plasmids. Thus, successful acquisition depends not only on transfer frequency but on subsequent evolution. (nature.com)
Evolutionary and ecological effects
HGT provides a route to adaptation by redistributing genetic functions already present elsewhere. Transfer can alter metabolism, environmental tolerance, or interactions with other organisms. In experimental evolution involving two divergent Bacillus subtilis lineages, repeated DNA transfer replaced substantial portions of the recipient’s core genome and produced measurable changes in physiology and reproductive performance. (arxiv.org)
The spread of antibiotic resistance is a particularly important consequence. Resistance genes can travel on plasmids and other mobile elements, allowing their dissemination independently of the spread of a single bacterial clone. Nevertheless, transfer rates, selective advantages, and long-term persistence are distinct quantities: environmental selection does not invariably increase gene exchange. Experiments with mercury-resistance plasmids found that positive selection could reduce conjugation and inhibit interspecies transfer. (nature.com)
Gene acquisition is also spatially structured within chromosomes. A comparative study of 80 bacterial species found transferred genes concentrated in genomic hotspots, many associated with mobile elements or recombination-prone flanking regions. HGT therefore changes both gene content and genome organization. (nature.com)
Transfer involving eukaryotes
HGT is not restricted to microorganisms exchanging genes with other microorganisms. Agrobacterium transfers DNA into plant cells, and bacterial-derived DNA containing expressed genes has been identified in cultivated sweet potato. Comparative and experimental investigations have also documented acquisitions from bacteria and other eukaryotes in ochrophytes, a lineage containing diverse algae. These examples establish eukaryotic HGT without implying that its frequency is uniform across eukaryotic groups. (pubmed.ncbi.nlm.nih.gov)
Detection and interpretation
Researchers infer historical transfers using phylogenetics, gene distributions, sequence composition, and genomic context. A gene tree that conflicts with an organismal tree can indicate transfer, but alternative explanations and reconstruction errors must be assessed. DNA sequencing can provide evidence that a candidate foreign sequence is physically integrated into the recipient genome. (pmc.ncbi.nlm.nih.gov)
Contamination is a major concern, particularly when sequencing organisms alongside associated microbes. Reanalysis of a tardigrade genome showed that bacterial contamination had inflated an earlier estimate of foreign genes. Evidence of integration, appropriate sequencing coverage, and independent validation is therefore important. At broader evolutionary scales, phylogenetic networks can represent horizontal connections alongside vertical descent without discarding the tree-like component of organismal history. (pure.ed.ac.uk)