Horizontal Gene Transmission: What It Is, Types And How It Works
Vertical gene transmission is the key to evolution, although not many know what this term exactly means. The “vertical” concept determines a top-down mechanism, or in other words, from parents to children. Thus, from a genetic point of view, children inherit half of the genome from their father and the other half from their mother “vertically”, from the parental generation to the successor, through clear statistical proportions.
In any case, this term is not only used in genetics: a disease is said to have a vertical transmission mechanism when the mother is capable of transmitting the pathogen to her offspring before, during or after childbirth. Infant ocular gonorrhea is a clear example of this, since the baby is infected by passing 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 due to the appearance of new adaptive characters. Something very different (and more complex to understand) is the horizontal transmission of genes, which presents its dominance at the microscopic level. Here we tell you everything about her.
Before delving fully into horizontal gene transmission, we must make some terms clear. In genetics, An allele is each of the ways in which a gene can manifest itself, which differs from the others in its sequence (genotype) and, therefore, it can 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 has to be equal to it for the character it encodes to appear (aa).
Let’s take an example: the allele (A) encodes green eyes, and the (a) encodes blue eyes. If a specimen has a gene with the 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 appear 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, since many traits are polygenic (they are encoded by more than one gene) and many others are linked to sex chromosomes, which reports variations between males and females. In any case, this express class is necessary to understand the basal mechanisms of genetic transmission.
What is horizontal gene transmission?
What we have shown you in the previous paragraphs corresponds to the typical inheritance mechanisms, that is, from parents to children. Genetic recombination and the transmission of different alleles over time causes variability in living beings and, therefore, a greater probability of adaptation in the face of the emergence of new environmental challenges.
Horizontal gene transmission (HGT) is something completely different. In her, an organism transfers genes or genomes to isolated cells or eukaryotic organisms, regardless of sexual reproduction, that is, to an organism or cell that is not its descendant. Although HGT has historically been associated with the world of bacterial microorganisms, today we know that it also occurs in animals, plants and, as surprising as it may seem, 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 efficient mechanism of evolution over the centuries. Thanks to this series of mechanisms, some of the primitive living beings could have “advanced” on the evolutionary scale, creating new genetic manifestations and “borrowing” those genes from other organisms with high biological efficiency.
Below, we show you the most common types of horizontal gene transmission in nature. Don’t miss them.
A typical process in bacteria, which can “harvest” DNA from other organisms, in this case from genome strands that are suspended in the environment Although it is one of the typical mechanisms of horizontal gene transmission, it is a rather inefficient method of genetic diversification, since only 1 in every 10,000 cells successfully integrate extrachromosomal DNA.
In any case, this method can also give rise to true production mechanisms that seem straight out of a novel. For example, if the integrated plasmid DNA contains the information necessary to synthesize a protein, the bacteria will begin to form it with its internal ribosomes, even if it was not in its initial “nature.” Thus, the microorganisms can be lysed and the proteins formed inside them 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 particularities, 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 alone cannot reproduce. In this process, a section of the bacteria’s genome can be integrated into future viruses.
Thus, the viruses descended from the infection infect a new bacteria and can inoculate the genetic segment of the previously affected one to the new host. One bacteria “donates” genetic information to another, with the virus being the transmitting vehicle.
3. Conjugation
In this case, the donation of genetic information from a donor bacteria to another recipient occurs, but through direct contact Here, plasmids come into play, extrachromosomal genetic sequences of the bacteria that replicate on their own and are presented in its cytoplasm in a circular shape.
Again, we will not get lost in specific language: the pilus of bacteria (small hairs formed by the pilin protein) interact with each other and allow the two involved to form a union 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 direct fusion, a bacterium is allowed to present new genetic information on a newly incorporated plasmid. Fascinating, right?
All of 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 a medium as complex as the human body itself?
As surprising as it may seem, Our DNA contains no more and no less than 165 genes of microbial origin This may not seem like much if we take into account that our genome is made up of about 30,000 genes, but it is really a non-negligible number and its discovery represented a scientific revolution.
Furthermore, many of these genes inherited by bacteria that live in symbiosis with us are not anecdotal, mind you. Without going any further, the ABO gene, which codes for the blood group of human beings, seems to have been inherited from the close relationship with microorganisms throughout our evolutionary structure.
Summary
Incredible true? The world of genetics seems to have been established on completely inconceivable foundations, since it is very difficult for us, as human beings, to understand how some living beings are capable of directly transmitting 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, since it is clear that these living beings have donated genes to us that remain with us today.
Although vertical gene transfer is the basis of inheritance and evolution, horizontal gene transfer does not fall short, especially in viruses and bacteria. Thanks to it, multiple adaptive mechanisms can be justified, such as resistance to antibiotics by many pathogenic bacterial strains.
