20.3B: Horizontal Gene Transfer

Learning Objectives

• Explain how horizontal gene transfer can make resolution of phylogenies difficult

Horizontal gene transfer (HGT) is the introduction of genetic material from one species to another species by mechanisms other than the vertical transmission from parent(s) to offspring. These transfers allow even distantly-related species (using standard phylogeny) to share genes, influencing their phenotypes. It is thought that HGT is more prevalent in prokaryotes, but that only about 2% of the prokaryotic genome may be transferred by this process. Some researchers believe these estimates are premature; the actual importance of HGT to evolutionary processes must be viewed as a work in progress. As the phenomenon is investigated more thoroughly, it may be revealed to be more common. Many evolutionists postulate a major role for this process in evolution, thus complicating the simple tree model. A number of scientists believe that HGT and mutation appear to be (especially in prokaryotes) a significant source of genetic variation, which is the raw material for the process of natural selection. These transfers may occur between any two species that share an intimate relationship, thus adding a layer of complexity to the understanding or resolution of phylogenetic relationships.

Figure \(\PageIndex{1}\): Mechanisms of prokaryotic and eukaryotic horizontal gene transfer: Horizontal gene transfer is the introduction of genetic material from one species to another species by mechanisms other than the vertical transmission from parent(s) to offspring. These transfers allow even distantly-related species (using standard phylogeny) to share genes, influencing their phenotypes. Examples of mechanisms of horizontal gene transfer are listed for both prokaryotic and eukaryotic organisms.

HGT in Prokaryotes

The mechanism of HGT has been shown to be quite common in the prokaryotic domains of Bacteria and Archaea, significantly changing the way their evolution is viewed. These gene transfers between species are the major mechanism whereby bacteria acquire resistance to antibiotics. Classically, this type of transfer was thought to occur by three different mechanisms:

• Transformation: naked DNA is taken up by a bacteria.
• Transduction: genes are transferred using a virus.
• Conjugation: the use a hollow tube called a pilus to transfer genes between organisms.

More recently, a fourth mechanism of gene transfer between prokaryotes has been discovered. Small, virus-like particles called gene transfer agents (GTAs) transfer random genomic segments from one species of prokaryote to another. GTAs have been shown to be responsible for genetic changes, sometimes at a very high frequency compared to other evolutionary processes. The first GTA was characterized in 1974 using purple, non-sulfur bacteria. These GTAs, which are thought to be bacteriophages that lost the ability to reproduce on their own, carry random pieces of DNA from one organism to another. The ability of GTAs to act with high frequency has been demonstrated in controlled studies using marine bacteria. Gene transfer events in marine prokaryotes, either by GTAs or by viruses, have been estimated to be as high as 10¹³ per year in the Mediterranean Sea alone. GTAs and viruses are thought to be efficient HGT vehicles with a major impact on prokaryotic evolution.

HGT in Eukaryotes

Although it is easy to see how prokaryotes exchange genetic material by HGT, it was initially thought that this process was absent in eukaryotes. After all, prokaryotes are only single cells exposed directly to their environment, whereas the sex cells of multicellular organisms are usually sequestered in protected parts of the body. It follows from this idea that the gene transfers between multicellular eukaryotes should be more difficult. Indeed, it is thought that this process is rarer in eukaryotes and has a much smaller evolutionary impact than in prokaryotes. In spite of this fact, HGT between distantly-related organisms has been demonstrated in several eukaryotic species. It is possible that more examples will be discovered in the future.

In plants, gene transfer has been observed in species that cannot cross-pollinate by normal means. Transposons or “jumping genes” have been shown to transfer between rice and millet plant species. Furthermore, fungal species feeding on yew trees, from which the anti-cancer drug TAXOL® is derived from the bark, have acquired the ability to make taxol themselves; a clear example of gene transfer.

In animals, a particularly interesting example of HGT occurs within the aphid species. Aphids are insects that vary in color based on carotenoid content. Carotenoids are pigments made by a variety of plants, fungi, and microbes, which serve a variety of functions in animals who obtain these chemicals from their food. Humans require carotenoids to synthesize vitamin A and we obtain them by eating orange fruits and vegetables: carrots, apricots, mangoes, and sweet potatoes. On the other hand, aphids have acquired the ability to make the carotenoids on their own. According to DNA analysis, this ability is due to the transfer of fungal genes into the insect by HGT, presumably as the insect consumed fungi for food. A carotenoid enzyme called a desaturase is responsible for the red coloration seen in certain aphids. Furthermore, it has been shown that when this gene is inactivated by mutation, the aphids revert back to their more common green color.

Figure \(\PageIndex{1}\): HGT within the aphid species: (a) Red aphids get their color from red carotenoid pigment. Genes necessary to make this pigment are present in certain fungi. Scientists speculate that aphids acquired these genes through HGT after consuming fungi for food. If genes for making carotenoids are inactivated by mutation, the aphids revert back to (b) their green color. Red coloration makes the aphids much more conspicuous to predators, but evidence suggests that red aphids are more resistant to insecticides than green ones. Thus, red aphids may be more fit to survive in some environments than green ones.

Key Points

• It is thought that HGT is more prevalent in prokaryotes than eukaryotes, but that only about 2% of the prokaryotic genome may be transferred by this process.
• Many scientists believe that HGT and mutation appear to be (especially in prokaryotes) a significant source of genetic variation, which is the raw material for the process of natural selection.
• HGT in prokaryotes occurs by four different mechanisms: transformation, transduction, conjugation, and via gene transfer agents.
• HGT occurs in plants through transposons (jumping genes), which transfer between different species of plants.
• An example of HGT in animals is the transfer (through consumption) of fungal genes into insects called aphids, which allows the aphids the ability to make carotenoids on their own.

Key Terms

• transformation: the alteration of a bacterial cell caused by the transfer of DNA from another, especially if pathogenic
• transduction: horizontal gene transfer mechanism in prokaryotes where genes are transferred using a virus
• conjugation: the temporary fusion of organisms, especially as part of sexual reproduction