Covid-19, how the new variants are evolving

Covid-19, how the new variants are evolving


The first signs of "convergent evolution" of Sars-CoV-2 variants have been observed. That is, it seems that at the moment there is no variant of Covid-19 that is taking over another, as in the past, but that there are rather several sub-variants that accumulate very similar mutations, all associated with a greater immunoresistance of the virus. .

This is a mechanism that scientists have defined evolutionary convergence, borrowing the name given to the phenomenon whereby different species living in the same type of environment, under the pressure of the same environmental pressures, evolve and develop structures or adaptations that lead them to closely resemble each other.

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What has happened in the last two years

It must be said that the oddities have not started now. The evolution of Covid-19 was bizarre right from the start. In the second half of 2020, after an initial period in which the virus has mutated very little, the three variants Alpha, Beta and Gamma emerged: three different strains in three different parts of the world, all with relatively similar mutations. In 2021 the scenario has changed: in the world several waves have begun to be registered, each dominated by a variant, with the scepter passed from Alpha to Delta and then from Delta to Omicron; in this case, however, each variant was significantly different from the one that preceded it. In short, we were no longer in a "continuous" evolution regime, in which a single strain progressively accumulates mutations, but rather in an almost discrete regime, made up of great evolutionary leaps, each of which related to a strain that almost seems to come out of nowhere. Finally, between the end of 2021 and the beginning of 2022 it seems that the situation has returned to the previous regime, with the diffusion of the sub-variants BA.1, BA.2, BA.3, BA.4 and BA.5 and their second generations, which all seem to have very similar forms in terms of mutations.

The present and the future: is Centaurus coming?

Right now, after the increase in infections of a few months ago due to the Omicron 5 variant (currently the most widespread and responsible, in the United States, of 80% of cases), and after a relatively "quiet" summer, the attention of experts is focused on a possible autumn wave: according to the authors of an Italian study published in the European Journal of Internal Medicine, the autumn wave could be dominated by an Indian variant, the so-called Centaurus, a second generation evolution of the Omicron BA.2 variant, codenamed BA2.75. It is apparently a very efficient variant in binding to human Ace2 receptors, which would potentially make it capable of spreading very quickly.

Homoplasia and evolutionary convergence: birds and bats

Thus we come to the issue of evolutionary convergence, recently addressed by New Atlas in a piece on the topic of the future of Sars-CoV-2 that put together the latest considerations of several virologists on the issue. This is not a new topic: already in June 2020, for example, a study published in the journal Infection, Genetics and Evolution by a group of researchers led by Francois Belloux, of the Institute of Genetics at University College London, had suggested that the evolution of Sars-CoV-2 in different parts of the world was characterized by high levels of homoplasia - that is, it was adapting to the human being in a similar way. Analyzing nearly 8 thousand Sars-CoV-2 sequences from countries of different geographic areas, the authors of the work had observed at least 198 mutations that emerged independently. "The phenomenon referred to in this work - commented to Agi Giuseppe Novelli, geneticist at the Tor Vergata University in Rome - is well known, it is 'evolutionary convergence' and is a common behavior among viruses. Homoplasia essentially concerns the fact that in evolution some biologically different structures, such as the wings of mammals, such as bats, and the wings of birds, achieve the same purpose even if they come genetically from different conditions. Homoplasia is therefore a sort of evolutionary convergence of a different biological origin ". In evolutionary biology, in short, the theory of evolutionary convergence explains how and why completely different organisms can evolve, separately but under the pressure of the same environmental pressures, similar traits.

To each his own spike

In the case of Sars-CoV-2, the same concept applies to the structure of the now famous spike protein: the Rbd, or receptor binding domain, which sits on top of the spike and helps the virus attach to certain receptors of human cells, thus playing a fundamental role in the infection and replication of the pathogen. A study (still in pre-print on BiorXive) recently showed that all of Omicron's new sub-variants have mutations on Rbd, which, according to experts, indicates that the virus is looking for (and finding) new strategies to evade immunity. Human. "The convergent evolution of Rbd - explained Yunlong Cao, one of the authors of the work - means that the Rbd mutations that have evolved from the most recent Omicron strains all converge towards the same sites. Observing such a pattern means that Sars-CoV-2 will evolve immuno-evasive mutations much more frequently than in the past, and that therefore the new mutations will be much more immuno-evasive ”.

Several other “variant hunters” arrived at the same results independently, so much so that the phenomenon was nicknamed The Great Convergence. What is still not clear, however, is how much there is to worry: "It is possible - says Cao - that the cells of the immune system continue to recognize the virus despite these mutations in the Rbd, since they are also based on other parts of the virus, still not subject to mutations. But further evolutions could also take place that completely neutralize the effectiveness of antibodies, cause high rates of infection and give rise to serious disease in some individuals ".

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