Horizontal gene transmission: what is it, types and how does it work?

Horizontal gene transmission is one of the most curious mechanisms of genetic inheritance.

Vertical gene transmission is the key to evolution, although not many people know exactly what this term means.although not many people know exactly what this term means. The concept "vertical" refers to a mechanism from top to bottom, or in other words, from parents to offspring. Thus, from a genetic point of view, children inherit half of their father's genome and half of their mother's genome in a "vertical" way, from the parental generation to the successor generation, by means of clear statistical proportions.

However, this term is not only used in genetics: a disease is said to have a vertical transmission mechanism when the mother is able to transmit the pathogen to her offspring before, during or after birth. Infantile ocular gonorrhea is a clear example of this, as the baby is infected when it passes through the mother's infected vaginal tract.

Vertical transmission is constant in living beings, both at the genetic and pathogenic levels. Thanks to the inheritance of genes and recombination, genetic variability arises in species, which in turn allows them to remain in ecosystems over time through the appearance of new adaptive traits. Something very different (and more complex to understand) is the horizontal transmission of genes, which is dominant at the microscopic level. Here we tell you all about it.

What are the bases of gene transmission?

Before we dive into horizontal gene transmission, we should clarify a few terms. In genetics, an allele is each of the forms in which a gene can manifest itself, which differs from the others in its sequence (genotype) and can therefore encode specific external modifications (phenotype).

In general, we understand a gene as the union of 2 alleles, one from the mother and one from the father. If an allele is dominant (A), it will manifest itself independently of the form of its partner, but if it is recessive (a), the copy must be equal to it for the character it encodes (aa) to appear.

Let us take an example: allele (A) encodes green eyes, and (a), blue eyes. If a specimen has a gene with alleles (AA), it is said to be homozygous dominant for the character and its eyes are green. If its genome is (aa), it will be homozygous recessive and its eyes will be blue. Finally, if the individual is (Aa) for the given gene, he is classified as heterozygous and his eyes will be green, since (A) prevails over (a).

Clearly this is not so simple in the real world, as many traits are polygenic (encoded by more than one gene) and many others are linked to sex chromosomes. many traits are polygenic (encoded by more than one gene) and many others are linked to the sex chromosomes, resulting in variations between males and females.which reports variations between males and females. In any case, this express class is necessary to understand the basic mechanisms of genetic transmission.

What is horizontal gene transmission?

What we have shown you in the previous paragraphs corresponds to typical inheritance mechanisms, i.e. from parents to offspring. Genetic recombination and the transmission of different alleles over time leads to variability in living organisms and, therefore, a greater probability of adaptation to new environmental challenges.

Horizontal gene transmission (HGT) is something completely different. In it, an organism transfers genes or genomes to isolated cells or eukaryotic organisms, outside of sexual reproduction, i.e. to an organism or cell that is not its offspring.that is, to an organism or cell that is not its offspring. Although HGT has historically been associated with the world of bacterial microorganisms, we now know that it also occurs in animals, plants and, surprisingly enough, even in humans.

Types of horizontal gene transfer

Horizontal gene transmission dominates the world of bacteria and viruses, and it is postulated that it may have been a very effective mechanism of evolution over the centuries. Thanks to this series of mechanisms, some of the primordial living beings could have "advanced" in the evolutionary scale, creating new genetic manifestations and "borrowing" those genes from other organisms with a high biological efficiency.

Here are the most common types of horizontal gene transmission in nature. Don't miss them.

1. Transformation

A typical process in bacteria, which can "harvest" DNA from other organisms, in this case from genome strands suspended in the environment.. Although this is one of the typical mechanisms of horizontal gene transmission, it is a rather inefficient method of genetic diversification, as only 1 in 10,000 cells successfully integrate extrachromosomal DNA.

However, this method can also give rise to real production mechanisms that seem to be straight out of a novel. For example, if the integrated plasmid DNA contains the information necessary to synthesize a protein, the bacterium will start to form it with its internal ribosomes, even if it was not in its initial "nature". Thus, microorganisms can be lysed and the proteins formed inside them can be extracted.

2. Transduction

A similar premise to the previous one, but in this case, it is mediated by viruses. We don't want to get lost in specifics, so we will summarize this intricate process in a couple of concepts: viruses enter host cells and "hijack" their replication mechanism to multiply their genome and form their protective capsids.since they cannot reproduce on their own. In this process, a section of the bacterial genome can be integrated into future viruses.

Thus, viruses descending from the infection infect a new bacterium and may inoculate the genetic segment of the previously affected bacterium into the new host. One bacterium "donates" genetic information to another, the virus being the transmitting vehicle.

3. Conjugation

In this case, the donation of genetic information from a donor bacterium to a recipient bacterium occurs, but through direct contact with the recipient bacterium.. This is where plasmids come into play, extrachromosomal genetic sequences of the bacterium that replicate on their own and present themselves in their cytoplasm in a circular form.

Again, we will not get lost in specific language: the bacterial pilus (small hairs formed by the pilin protein) interact with each other and allow the 2 involved to form a linkage bridge. Here, the plasmid self-replicates, allowing the copy of the genetic information to go to the recipient and the original plasmid to remain in the donor. Through a direct fusion, a bacterium is allowed to present new genetic information on a newly incorporated plasmid. Fascinating, isn't it?

Does it happen in humans?

All these mechanisms are more or less easy to imagine in primitive living beings, since, after all, many bacteria are malleable and resistant to exogenous and endogenous changes. But how is it possible for horizontal transmission to occur in an environment as complex as the human body itself?

Surprising as it may seem, our DNA contains no more and no less than 165 genes of microbial origin.. This may not seem much if we take into account that our genome is made up of some 30,000 genes, but it is actually a not inconsiderable number and its discovery was a scientific revolution.

Moreover, many of these genes inherited by bacteria that live in symbiosis with us are not anecdotal, to say the least. Without going any further, the ABO gene, which encodes the blood group of humans, seems to have been inherited from the close relationship with microorganisms throughout our evolutionary structure.

Summary

Incredible, isn't it? The world of genetics seems to have settled on a completely inconceivable basis, as it is very difficult for us, as human beings, to understand how some living beings are able to directly transmit genetic information to each other in the short and long term. Even more difficult is trying to understand how our interaction with microorganisms has affected us throughout evolution, as it is clear that these living things have donated genes to us that remain with us today.

While vertical gene transfer is the basis of inheritance and evolution, horizontal gene transfer is not far behind, especially in viruses and bacteria.especially in viruses and bacteria. Thanks to it, multiple adaptive mechanisms, such as Antibiotic resistance by many pathogenic bacterial strains, can be justified.

Bibliographic references:

• Abrahamovich, E. (2018). Studies on horizontal transfer of tetracycline resistance in sporulating bacteria isolated from honey bee hives.
• Types of genetic inheritance, IMEGEN. Retrieved February 20 from https://imegen.es/informacion-al-paciente/informacion-genetica-enfermedades-hereditarias/conceptos-genetica/tipos-herencia-genetica/.
• Horizontal gene transfer mediated by mobile elements, CSIC (CIB). Retrieved February 20 from https://www.cib.csic.es/es/project/transferencia-genetica-horizontal-mediada-por-elementos-moviles-alicia-bravo-y-manuel
• Bacterial transformation and selection, Khan Academy. Retrieved February 20 from https://es.khanacademy.org/science/biology/biotech-dna-technology/dna-cloning-tutorial/a/bacterial-transformation-selection#:~:text=The%20bacteria%20can%20collect%20DNA,reci%C3%A9n%20made%20to%20the%20bacteria.
• Valencia Cano, P. A. (2009). Evidence of horizontal transfer of antibiotic resistance genes from environmental bacteria (Bachelor's thesis, Quito: USFQ).