HORIZONTAL GENE TRANSFER: MECHANISMS AND EXAMPLES - BIOLOGY - 2024

The horizontal gene transfer or lateral gene transfer is the exchange of genetic material between organisms, which does not occur in families. This event takes place between individuals of the same generation and can occur in unicellular or multicellular beings.

Horizontal transfer occurs through three main mechanisms: conjugation, transformation, and transduction. In the first type, the exchange of long DNA fragments is possible, while in the last two the transfer is restricted to small segments of the genetic material.

A bacterium. THG is common in these organisms. Source pixabay.com

The opposite concept is vertical gene transfer, where genetic information passes from an organism to its offspring. This process is widespread in eukaryotes, like plants and animals. In contrast, horizontal transfer is common in microorganisms.

In eukaryotes, horizontal transfer is not as common. However, there is evidence of exchange of this phenomenon, including the ancestor of humans, who obtained certain genes through viruses.

What is horizontal gene transfer?

During reproduction, eukaryotic organisms pass their genes from one generation to their offspring (children) in a process known as vertical gene transfer. Prokaryotes also perform this step, but through asexual reproduction through fission or other mechanisms.

However, in prokaryotes there is another way of exchanging genetic material called horizontal gene transfer. Here, the DNA fragments are exchanged between organisms of the same generation and can pass from one species to another.

Horizontal transfer is relatively common among bacteria. Take the example of genes that cause resistance to antibiotics. These important DNA fragments are transferred normally between bacteria of different species.

These mechanisms involve significant medical complications when treating infections.

Mechanisms

There are three fundamental mechanisms by which DNA can be exchanged by horizontal transfer. These are conjugation, transformation, and transduction.

Conjugation

Gene transfer by means of conjugation is the only type that involves direct contact between the two bacteria.

However, it should not be compared to gene exchange through sexual reproduction (where there is usually a contact between the organisms involved), since the process is very different. Among the main differences is the absence of meiosis.

During conjugation, the passage of genetic material from one bacterium to another takes place through physical contact established by a structure called pili. This works as a connecting bridge, where the exchange occurs.

Although bacteria do not differentiate into sexes, the organism that carries a small circular DNA known as factor F (fertility f) is known as "male". These cells are the donors during conjugation, passing the material to another cell that lacks the factor.

Factor F DNA consists of about 40 genes, which control the replication of the sexual factor and the synthesis of the sexual pili.

The first evidence of the conjugation process comes from the Lederberg and Tatum experiments, but it was Bernard Davis who finally demonstrated that contact was necessary for transfer.

Transformation

The transformation involves the taking of a naked DNA molecule found in the environment close to a host bacteria. This piece of DNA comes from another bacterium.

The process can be carried out naturally, since bacteria populations normally undergo transformation. Similarly, the transformation can be simulated in the laboratory to force bacteria to take up DNA of interest that is found outside.

Theoretically, any piece of DNA can be taken. However, the process has been observed to involve small molecules.

Transduction

Finally, the transduction mechanism occurs by means of a phage (virus) that carries DNA from a donor bacterium to a recipient. As in the previous case, the amount of transferred DNA is relatively small, since the ability of the virus to carry DNA is limited.

Usually, this mechanism is restricted to bacteria that are phylogenetically close, since the virus that carries the DNA must bind to specific receptors on the bacteria in order to inject the material.

Examples

Endonucleases are enzymes that have the ability to break phosphodiester bonds within a polynucleotide chain, from the inside - that's why they are known as "endo". These enzymes do not cut anywhere, they have specific sites to do so, called restriction sites.

The amino acid sequences for the enzymes EcoRI (in E. coli) and RSRI (in Rhodobacter sphaeroides) have a sequence of nearly 300 amino acid residues, which are 50% identical to each other, clearly indicating a close evolutionary kinship.

However, thanks to the study of other molecular and biochemical characteristics, these two bacteria are very different and are very little related from a phylogenetic point of view.

Furthermore, the gene that codes for the enzyme EcoRI uses very specific codons that are different from those normally used by E. coli, so it is suspected that the gene did not originate in this bacterium.

Horizontal gene transfer in evolution

In 1859, the British naturalist Charles Darwin revolutionized the biological sciences with his theory of evolution through natural selection. In his iconic book, The Origin of Species, Darwin proposes the metaphor of the tree of life to illustrate the genealogical relationships between species.

Today, phylogenies are a formal representation of this metaphor, where it is assumed that the transmission of genetic information occurs vertically - from parents to children.

We can apply this vision without major inconvenience to multicellular organisms and we will obtain a branched pattern, as Darwin proposes.

However, this representation of branches without fusions is difficult to apply to microorganisms. When comparing the genomes of different prokaryotes, it is clear that there is extensive gene transfer between lineages.

Thus, the pattern of relationships is more like a network, with branches connected and fused together, thanks to the prevalence of horizontal gene transfer.

References

1. Gogarten, JP, & Townsend, JP (2005). Horizontal gene transfer, genome innovation and evolution. Nature Reviews Microbiology, 3 (9), 679.
2. Keeling, PJ, & Palmer, JD (2008). Horizontal gene transfer in eukaryotic evolution. Nature Reviews Genetics, 9 (8), 605.
3. Pierce, BA (2009). Genetics: A conceptual approach. Panamerican Medical Ed.
4. Russell, P., Hertz, P., & McMillan, B. (2013). Biology: The Dynamic Science. Nelson Education.
5. Sumbali, G., & Mehrotra, RS (2009). Principles of microbiology. McGraw-Hill.
6. Syvanen, M., & Kado, CI (2001). Horizontal gene transfer. Academic Press.
7. Tortora, GJ, Funke, BR, & Case, CL (2007). Introduction to microbiology. Panamerican Medical Ed.