Genes are usually passed from parents to offspring. This familiar process, called vertical gene transfer, explains how genetic information moves through generations. But genes can sometimes take a different route: they can move between unrelated organisms, or between organisms that are not in a direct parent-offspring relationship.
This process is known as horizontal gene transfer (HGT). It is especially important in bacteria and other microorganisms, where a newly acquired gene can sometimes change an organism’s abilities almost immediately. Horizontal gene transfer helps explain the rapid spread of traits such as antibiotic resistance and has played a major role in the evolution of microbial life.
HGT is not the same as ordinary reproduction, and it does not mean that entire genomes routinely move from one organism to another. Usually, what moves is a piece of DNA carrying one or a small number of genes.
What is horizontal gene transfer?
Horizontal gene transfer is the movement of genetic material between organisms by means other than reproduction.
In vertical inheritance, an organism receives DNA from its parent or parents. A child inherits genetic material from its biological parents, for example, and a bacterial cell typically passes its DNA to daughter cells when it divides.
Horizontal transfer moves DNA across that usual lineage boundary. A bacterium can acquire a gene from another bacterium, for instance, even when the donor and recipient are not parent and offspring.
The transferred DNA may become part of the recipient’s chromosome, remain as a separate DNA molecule, or exist temporarily before being lost. If the acquired gene is functional and gives the recipient an advantage, natural selection can favor cells that retain it and pass it on to their descendants.
Horizontal transfer is therefore both a mechanism for gene exchange and a source of evolutionary change.
Why does horizontal gene transfer matter?
The importance of HGT is most apparent in microorganisms. Bacteria can acquire genes that allow them to use new nutrients, tolerate particular environmental conditions, evade harmful substances, or resist antibiotics.
Antibiotic resistance provides a particularly important example. A resistance gene that arises in one bacterial lineage does not necessarily have to wait for that lineage to reproduce and spread. Under the right circumstances, the gene can move into other bacteria through horizontal transfer. Those recipients may then be better able to survive exposure to the antibiotic.
HGT can also complicate the reconstruction of evolutionary history. Scientists often use genes to infer relationships among organisms, but a gene may have a different history from the organism carrying it. A gene obtained through horizontal transfer can therefore make one part of a genome trace back to a different lineage than another part.
This is one reason the evolutionary history of microorganisms can look more like a network of exchanges than a simple branching family tree.
The three major mechanisms of horizontal gene transfer
In bacteria, horizontal gene transfer is commonly divided into three major mechanisms: transformation, transduction, and conjugation. They differ in how DNA gets from one cell to another.
Transformation: taking up DNA from the environment
Transformation occurs when a cell takes up DNA that is present in its surroundings.
Cells can release DNA into the environment when they die or otherwise break apart. Some bacteria are capable of taking up this free DNA. Once inside the cell, the DNA may be degraded, maintained, or incorporated into the cell’s genetic material through processes such as recombination.
Transformation does not require a living donor cell to make direct contact with the recipient. The environment itself becomes the source of genetic material.
Whether transformation occurs depends on the biology of the organism and the conditions around it. Not every bacterium can readily take up environmental DNA.
Transduction: viruses carry DNA between cells
Transduction involves viruses that infect bacteria, known as bacteriophages or simply phages.
During a phage infection, viral particles reproduce inside a bacterial cell. In some cases, a phage particle can accidentally package bacterial DNA instead of, or in addition to, the DNA it normally carries. When that particle infects another bacterium, it can deliver the bacterial DNA to the new cell.
The transferred DNA can sometimes become incorporated into the recipient’s genome.
Transduction therefore uses a virus as an intermediary between bacterial cells. The virus does not necessarily “intend” to transfer bacterial genes; the transfer can arise from errors or particular features of the viral life cycle.
Conjugation: DNA passes between cells
Conjugation involves direct cell-to-cell contact and is often associated with plasmids.
A plasmid is a relatively small, usually circular DNA molecule that can replicate separately from the bacterial chromosome. Some plasmids carry genes that benefit their hosts, including genes associated with antibiotic resistance.
In a typical conjugation system, a donor cell containing a transferable plasmid establishes contact with a recipient cell and transfers a copy, or part of a copy, of the plasmid. The recipient can then possess genetic information that it did not previously have.
Conjugation is particularly significant because transferable plasmids can move among bacterial cells and, in some circumstances, across different bacterial species.
What are plasmids, and why are they important?
Plasmids are central to many discussions of bacterial gene transfer because they provide a convenient vehicle for carrying genes between cells.
Unlike the main bacterial chromosome, a plasmid is an independent DNA molecule. A plasmid may contain genes that are not essential for basic survival but can provide an advantage under particular conditions.
A plasmid can carry several genes together. If those genes can be transferred as a unit, a recipient cell may acquire multiple new capabilities in a single transfer event.
Not every plasmid is transferable between cells. Some lack the machinery needed for horizontal movement, while others can be mobilized by cellular systems that provide the necessary functions.
Plasmids also illustrate an important point about HGT: the movement of a gene does not necessarily require the movement of an entire chromosome. Small, mobile pieces of DNA can be powerful agents of evolutionary change.
How does transferred DNA become part of an organism?
Acquiring DNA is only the first step. For horizontal gene transfer to have a lasting effect, the transferred genetic material must generally persist and, when relevant, be expressed.
One possibility is integration into the chromosome. DNA from another source can sometimes be incorporated into the recipient’s genome through recombination. If the sequence is compatible with the recipient’s cellular machinery, its gene may then be transcribed and translated into a functional product.
Another possibility is that the DNA remains separate, as with a plasmid. In that case, the acquired genes can still be expressed without becoming part of the chromosome.
The outcome depends on the transferred DNA, the recipient organism, and the mechanism of transfer. Many pieces of DNA that enter a cell do not establish themselves permanently.
Horizontal gene transfer and antibiotic resistance
HGT is one of the reasons antibiotic resistance can spread through bacterial populations in ways that cannot be explained by mutation and reproduction alone.
A bacterial population can acquire resistance through genetic changes that arise within its own lineage. But an existing resistance gene can also arrive from another bacterial cell through horizontal transfer.
Once a resistance gene is present, exposure to an antibiotic can create strong selection for bacteria that carry it. Susceptible cells may be inhibited or killed while resistant cells survive and reproduce. Horizontal transfer and natural selection can therefore work together: HGT supplies genetic variation, while selection influences which variants persist.
This does not mean that every instance of antibiotic resistance is acquired horizontally. Resistance can arise through mutation, changes in gene regulation, and other mechanisms as well. HGT is one important route among several.
Is horizontal gene transfer limited to bacteria?
No. Horizontal gene transfer is best known and particularly influential in the microbial world, but genetic material can move horizontally in other organisms as well.
It occurs among archaea and has been documented in eukaryotes, including some single-celled organisms, plants, fungi, and animals. The frequency and biological importance vary greatly among groups.
In complex multicellular organisms, HGT is generally less central to everyday inheritance than it is in bacteria. For a horizontally acquired gene to become a stable part of a multicellular organism’s hereditary material, it must reach cells that contribute to future generations or otherwise enter a lineage capable of being inherited.
Some genes found in eukaryotic genomes have evidence of having originated through horizontal transfer, sometimes from microorganisms or other distantly related organisms. Such events can be difficult to identify with certainty because organisms also inherit genes vertically and because genes can be duplicated, lost, or extensively modified over evolutionary time.
Horizontal gene transfer versus mutation
Mutation and HGT both introduce genetic variation, but they do so in fundamentally different ways.
A mutation is a change in an organism’s existing DNA sequence. It can alter a gene, affect gene regulation, or change other parts of the genome.
Horizontal gene transfer, by contrast, can introduce genetic material that already exists in another organism. The recipient may therefore gain a complete functional gene or a collection of genes without having to evolve those sequences through a series of new mutations.
These processes can interact. A transferred gene may subsequently mutate and evolve within its new host. Conversely, mutations can affect whether transferred DNA works effectively in the recipient.
Does horizontal gene transfer make organisms the same?
No. Gene transfer does not erase the genetic differences between organisms.
A transferred gene represents only a small portion of an organism’s genetic information in most cases. The recipient retains its existing genome, cellular machinery, and evolutionary history.
A gene can also behave differently in a new genetic background. Regulatory sequences may not function in the same way, the recipient may lack proteins needed to use the gene, or the gene’s product may have different effects in its new host.
For HGT to produce a meaningful biological change, the transferred DNA must fit into the recipient’s cellular and ecological context. A gene that is useful in one organism or environment may provide little or no benefit in another.
The evolutionary significance of horizontal gene transfer
For much of biology, inheritance is represented as a tree: ancestors give rise to descendants, and lineages diverge over time. That model remains fundamental, but HGT adds connections between branches.
If a gene moves between distantly related organisms, the history of that gene can differ sharply from the history of the organisms themselves. A bacterial species might contain genes whose evolutionary origins lie in several different lineages.
This means microbial evolution is often better understood as involving both vertical inheritance and horizontal exchange.
HGT can contribute to adaptation by allowing useful genetic functions to spread through populations. It can also produce genetic mosaics in which different portions of a genome have different histories.
At the same time, HGT is not an unlimited shortcut to adaptation. Transfer events are constrained by ecological contact, compatibility between donor and recipient, the mechanisms available for DNA transfer, and whether the acquired genes are retained and expressed. Natural selection then determines whether those changes persist in a population.
How scientists recognize horizontal gene transfer
Scientists look for patterns in DNA and protein sequences that are difficult to explain by ordinary vertical inheritance alone.
One clue is a gene whose sequence is much more similar to genes from a distantly related organism than to genes from closely related organisms. Another is an unusual distribution of a gene across species, particularly when closely related organisms lack it but more distant organisms possess closely related versions.
Researchers also examine the organization of genomes, the characteristics of surrounding DNA, and evolutionary trees built from individual genes and from larger sets of genes. No single clue is always decisive. Gene duplication, gene loss, incomplete sampling, and other evolutionary processes can produce patterns that resemble HGT.
Strong evidence generally comes from combining multiple lines of genetic and evolutionary analysis.
A different way to think about heredity
Horizontal gene transfer changes the simple picture of genes moving only from parents to offspring. In microorganisms especially, genetic information can also move sideways across populations and sometimes across species boundaries.
Transformation can bring DNA in from the environment. Transduction can move bacterial DNA through viruses. Conjugation can transfer DNA directly between cells, often involving plasmids. Once acquired, transferred genes may persist, integrate into a chromosome, or disappear.
The broader lesson is that evolution is shaped not only by changes that arise within a lineage, but also by the movement of existing genetic information between lineages. For many microorganisms, understanding how life changes over time requires seeing heredity as both vertical inheritance through generations and horizontal exchange across them.
