Quasispecies & crab mentality
In the last 15 months, not even a single SARS-CoV-2 variant has remained dominant for long. Why? I have tried to answer this question using the quasispecies concept and crab mentality.
Put a single crab in a bucket, it will easily climb out of it, but none will escape if you place a few more crabs in the same bucket. Simply because, as one crab tries to escape, other crabs will pull it back inside. So the fight is about “if I can’t have it, neither can you.”
Let’s apply the same thing to SARS-CoV-2 variants. Many variants have come and disappeared, and none has remained dominant for very long. That’s because the viral population evolution is shaped by natural selection. Only those that are beneficial to the virus are positively selected over time, and those that are lethal or deleterious are removed. [1]
I came across the “quasispecies concept” from Dr. Arinjay Banerjee’s tweets. Very well explained. Follow him @sci_questions on Twitter.
Let’s say a coronavirus exists as different variants — A+B+C. The variants exist in different quantities — 4A + 2B + 1C. When you sequence a patient sample, your sequencing is likely to pick up the most represented variant — in this case: 4A.
As the virus circulates, due to multiple factors, this ‘represented’ population may evolve and change to become 2A + 4B +2C. Thus, if you sequence this sample from a patient — your sequence will likely identify 4B as the circulation strain.
What is Quasispecies? (Journal of Virology, 2002)
Quasispecies refers to an equilibrium process of mutation and natural selection that generates a population of variable genomes. These genetic variants are organized around one or a set of genotypes of the highest fitness known as master sequences. [2]
Apart from viral fitness, SARS-CoV-2 mutations also improve their susceptibility to antibody neutralization since many therapies rely on antibody neutralization (e.g., vaccination, monoclonal antibodies, and plasma).[1]
Viruses are not independent entities in the quasispecies. Mutational couplings link them. The idea is to make the entire population a cooperative structure to evolve collectively as a single unit. Therefore, natural selection is no longer directed toward the single fittest variant but the quasispecies in their entirety — to help it evolve and maximize its average replication rate. [2]
So, suppose a variant becomes more virulent or develops a high fatality rate. In that case, it is not in the quasispecies population interest because high fatality will reduce the population’s replication rate. So others will pull it down. This cycle will continue with the “crab mentality.”
References:
[1] Badley AD. The Yin and Yang of SARS-CoV-2 Mutation and Evolution. Mayo Clin Proc. 2021 Feb 3:S0025–6196(21)00089–6. doi: 10.1016/j.mayocp.2021.01.023. Epub ahead of print. PMID: 33715846; PMCID: PMC7857040.
[2] Holmes EC, Moya A. Is the quasispecies concept relevant to RNA viruses? J Virol. 2002 Jan;76(1):460–5. doi: 10.1128/jvi.76.1.460–462.2002. PMID: 11739715; PMCID: PMC135735.
