Viruses have always been a unique topic in microbiology. They are found in almost all environments on the Earth, even in extreme conditions [1]. Viruses are super small, not seen with a naked eye, notorious for being responsible for countless diseases in human and other organisms. Also, due to fact that many people hear the viruses are not a form of life, there is something soulless in their nature, which makes them more sinister for us.Indeed, there are still discussions about classification of viruses and their origins. The most popular theories are to be told in this article.
Virus-first theory claims that viruses originated from RNP world. Firstly, peptides and RNAs together formed coacervatesand gave rise First Universal Common Ancestor (FUCA) which had the earliest encoding system. Viruses used capside proteins as their very first membrane. These proteins enabled viruses’ motility, protection, and stabilization of their genetic material. It is supposed that this pattern was taken by first cells to form own membranes by additional binding to lipid domains. Moreover, “after cell” theories cannot explain how would cells develop membranes without such a help from viruses [1,2,4]. Another evidence lies in retroviruses’ existence that penetrated into our genome and retained there which means the viruses could integrate to cellular genomes (8% of human genome consists of retroviruses) [1,2,3,4]. On the other hand, some assume that capsids of viruses were much more helpful in parasitizing cells due to allowance of transfer own genetic material – this view supports “viruses after cells” theory [1,2,4].
According to this theory, viruses evolved from ancient cells(RNA cells), that existed before Latest Universal Common Ancestor (LUCA) evolved into modern cells. The viruses’genomes of Archaea, Bacteria, and Eukarya do not containhomologs, meaning viruses did not rise directly from the LUCA. Yet, some viruses that infect even distantly related hosts from all three domains of life share some conserved proteins, indicating these viruses existed probably before the LUCA modification.Also, the RNA cells were the first to parasitize and propagate viaparticles or vesicles. This pattern could be taken by the first viruses, aligning to their today’s parasitic life [1,2]. Reduction theory has own downside as it does not explain the first appearance of capsidic membranes and does not discuss the first appearance of cellular membranes [4].
General explanation of this theory is following – the viruses originated from genetic material that was able to move between cells and acquire genes via horizontal gene transfer. However,escape theory has two versions. First, the viruses arose from modern cells that originated from the LUCA, in other words, they arose from post-LUCA cells. Second version is similar to developed reduction theory, specifically when talking about origin of viruses from the RNA cells. But this version uses virus strategy as the main evidence as the infection rate of viruses was the most important factor for their successful proliferation and persistence [1,3].
In this version an ancient ribocell (RNA cell) could produce virus only if it divided unequally. As a result, two new cells were produced, but one of them did not have protein coat, while the other one lacked translation machinery. The coatless cell was destined to die, whereas other one could invade another cell, together becoming a viroribocell (ribocell harboring virion). The viroribocell replicated and became much more like a virus. And, if lysed, it could release virions that infected other cells. So, the first viruses had two phases in their life cycle – intracellular (multiplication) and extracellular (transmissible virions). Then, the virions continued cycle of replication by lysing or budding. And what is interesting, the viruses prefer the budding over lysing today. The viruses go out of host cells without immediate lysing it, pinching off some of its membrane to make one own. Also, research showed that short budding delays lead to more rapid virus emergence in the ribocells and if budding delay is shorter than lysing delay, the virus emerges more rapidly. Thisevidence is an important point for the escape hypothesis being one of the most probable theories [3,4].
Coevolution theory suggests the viruses being the early genetic replicators needed to meet entities with primitive translational machinery (e.g. FUCA-like) to enable their propagation [1]. The exchange of various modules was possible between themthrough lateral gene transfers and it could contribute tocoadaptation of the cells and the first viruses through evolution.Besides, looking at viruses’ nature today, this symbiosis could be a crucial factor for such coevolutionary interactions as it is also notable in host-parasite interaction today [4]. Another evidence derived from presence of endogenous retroviruses in human organism. They can protect human organism, including developing fetus, against infections induced by some exogenous retroviruses. For example, some plant RNA viruses can delay symptoms of abiotic stress produced by environment. Thus,evolutionary origin of defence mechanisms against viruses can be thought as a response to such excessive number of virus-cell interactions that are already present in retrovirus-cell interactions [1,2,3,4]. Additionally, the coevolution theory is supported by essential role of viruses in biosphere. That, viruses take part in evolution by introducing themselves into biosystems: cells acquire resistance by enhancing or reducing molecular work. So, when bigger epidemy comes, only virus-resistant, more adapted organisms survive. Thus, cells, being necessary for viruses, can use them as essential tools for own evolution [4].
Limitations that blur our way to the truth
As evolution of viruses started long before us and technology era, we have scarce molecular data and limited understanding of ancient Earth conditions that allowed viruses to arise. Also, theirgenomes could have serious mutations as they can evolve rapidly in response to environmental changes. Besides, we cannot exclude inner genomic mutations that are caused by spontaneous recombination, mutation, or gene transfers. Some theories are supported by conclusions of viruses pathogenic or non-pathogenic nature. So, it is believed that by calculating the ratio of these two broad groups of viruses we can understand whether viruses originated before or after cells. However, it is not impossible due to the significant number of viruses lineagesand those viruses that have already extinct in evolution history[2,4].
It is truly understandable why we should know how the viruses originated and evolved. Having this knowledge, people could propose which viruses are especially dangerous to humans and start inventing specific medicines as a prevention means.
REFERENCES
https://doi.org/10.1016/b978-0-12-816331-3.00001-5
