There is no doubt that, in recent months, everything related to viruses has taken on an unprecedented topicality. Terms such as IgG, IgM, PCR, antigens or cellular immunity, which were previously reserved almost exclusively for scientific forums, are now commonly used in our conversations. We believe that we are living a historic moment. But a brief look at the near past shows us that this situation is not extraordinary.
There are still 37 million people in the world who are infected with the AIDS virus; Ebola outbreaks continue to occur in Africa; and viruses that we previously believed to be typical of remote places, such as dengue, Zika or Chikungunya, can now spread near us.
If we look to the future, the situation is not reassuring either. Our hyper-connected and densely populated world is extremely conducive to the spread of viruses. In addition, we are leaving less and less free space for wildlife, with the consequences that this has on exposure to new pathogens.
Thus, it is not surprising that the most widespread view of viruses is that of mere disease-causing agents. But viruses are much more than that. They have accompanied life from its origins and have contributed to shaping it, to the point that we can affirm that the biosphere that currently exists would not be as it is if it were not for its interaction with viruses.
How did viruses arise?
There are several hypotheses to explain the origin of viruses. The most widely accepted defends that they are direct descendants of the set of primitive replicators. That is, of the molecules with genetic information transmissible through a copying process that populated the Earth before the appearance of cellular life.
There is broad consensus among scientists that these early replicators were RNA molecules capable of catalyzing some simple reactions. Their meeting in compartments, possibly of inorganic origin, facilitated cooperation between them, at the same time that genetic parasites emerged. This distinction between cooperating elements and parasites is what would later give rise to the appearance of the viral world and the cellular world, as we now know them.
The existence of viruses that store their genetic information in RNA genomes, as opposed to cells that always store it in DNA, appears to be proof of this ancestral hypothesis of viruses. The fact that some viruses possess an activity that is capable of transforming RNA into DNA is also considered proof of their age and their possible role in relevant evolutionary transitions.
Another alternative for the origin of viruses is the regressive hypothesis, which argues that they are products of the degeneration of ancestral cells that lost their autonomy and evolved into a parasitic form.
And we also have the escape hypothesis, which states that viruses come from cellular genes that have acquired the capacities for autonomous replication – outside the context of the rest of the genome – and infectivity.
Whatever their origin, the truth is that viruses have co-evolved with life and have contributed to the emergence of innovations, to the point of being considered the most important evolutionary engine that exists in nature.
Viruses as selective pressure
Selective pressure is nothing more than something that limits the survival of a population and, therefore, favors the individuals who best resist that specific circumstance.
In the case of bacteria, infection by their characteristic viruses, bacteriophages, gives an advantage to those that have some defense mechanism. For example, the existence of a receptor with which the virus cannot interact. Or the presence of restriction endonucleases and the CRISPR system that degrade viral genetic material preventing its multiplication.
In turn, the existence of these resistant bacteria implies a selective pressure on the viral population, which favors viruses that better resist bacterial defense mechanisms. When these cycles are repeated in time, there is a concerted evolution between the pathogen and the host, so that the evolution of both is accelerated.
In the case of viruses that infect animals, a similar pattern occurs that, among other things, has led to the emergence of the adaptive immune system of vertebrates. Viral populations, for their part, often reduce their virulence over time. That is, viruses become less aggressive in order to remain multiplying longer in their host. In these cases, a peaceful coexistence between the two can be achieved, generating the so-called viral reservoirs, animal species that carry viruses without presenting symptoms of disease.
When, by some circumstance, one of these viruses jumps into the human population, a new disease may arise for our species. This is what has happened with COVID-19, AIDS, and new strains of the flu virus. In the short term, the introduction of these new viruses is going to be very negative. But in the long run, it is these situations that are shaping us. And in the end, as the saying goes, what doesn’t kill us makes us stronger.
Viruses as generators of genetic diversity
Viruses are great inventors of genes. This is because when they copy their genomes, they produce many mutations that are passed on to their progeny. In addition, some viral genomes have a propensity to mix with each other, which can lead to changes much more drastic than point mutations.
Many viruses have the ability to integrate their genome into the host genome, multiplying at the same time as it and thus transmitting itself to offspring. This is how many viral genes can pass into the cellular world.
Without going any further, many bacteriophage genomes, integrated into bacterial genomes, provide bacteria with such interesting capacities as that of resisting the action of antibiotics or the production of toxins that give them an advantage over other competing bacteria.
Sometimes the bacteriophage genome is separated from the cellular genome. In this process, part of the host’s genetic material is taken away, which can now be transmitted to other bacteria. Viruses, therefore, constitute a powerful horizontal gene transfer mechanism that allows evolutionary innovations that arise in a species to be shared by the entire community.
Vertebrate genomes also have a good portion of DNA of viral origin. In the case of humans, this can represent up to 10% of the genome. And it’s not junk DNA, it performs important functions. Sometimes helping to regulate gene expression (the genes that are expressed in each cell type at all times) and other times causing new functions to emerge.
The list of cellular functions, activities and processes that have arisen or been modified thanks to the action of viruses is much longer than what can be described in this article. The message that derives from all this is that, to a large extent, we are the result of interaction with the viruses that have affected us throughout our history. Something that, in the short term, can cause us problems, but that, throughout evolution, has contributed to making us human.
Ester Lázaro Lázaro, Scientific Researcher of Public Research Bodies. Specialized in virus evolution, Astrobiology Center (INTA-CSIC)
This article was originally published on The Conversation. Read the original.
