In accordance with the traditional view viruses are not organisms as organisms are comprised of cells and unlike the cells, viruses bear only one type of nucleic acid, either DNA or RNA. Again, if we trace back to the discovery of viruses, we learn that viruses assimilate to their virions which are capsid encoding organisms. However, there are DNA viruses present that possess both RNA (messenger RNA) and DNA. This wrongful assimilation of viruses to their corresponding virions have led biologists and evolutionists underestimating the capability of viruses in producing new genes “de novo” (Forterre, 2017). This is so because virus’s assimilation to its virion make it an inert and passive object dependent on their cellular hosts.
In the opinion of Moreira and López-García (2009), viruses are results of cell evolutions based on the logic that although some viruses are capable of encoding their own distinct polymerases (mostly those are error-prone) while their functionalities and expressions require the cell machinery. This can be defined as “produced but not self-produced”. Alternatively, “not living, but lived entities”. Moreira and López-García (2009), further presented that in a sterile planet under all the essential physicochemical requirements and inseminations of populations with all viral lineages, there will be a progressive decay of the molecules breeding those viruses.
Keeping all other requirements intact except inseminations of populations with all bacterial species, however, is more likely to result in self-maintenance, reproduction, evolution and colonization amongst the bacteria in a stable manner. This research further states that viruses are polyphyletic as members of various viral families possess distinct and separate kinds of capsid constituents, gene contents and nucleic acids indicating various evolutionary origins. On contrary, an overwhelming number of empirical evidences also show that each and every cellular life share a single and common origin inferring a tree as the source of all cellular species. But this sensible scientific pervasiveness remains unattainable for viruses in the absence of any common traits or characteristics prevailing amongst virus families or between viruses and cells making a taxonomic scheme which considers all these entities as artificial and ineligible for taxonomic practices.
Now, we can look into the replicator paradigm as described by Koonin and Starokadomskyy (2016). The authors explained that replicators bear a strong linkage between the genome and the replicon. It is important to note that a replicator does not necessarily have to be a single uniform replicon and not all replicons possess similar levels of autonomy. This atrial replicative autonomy has been identified as an integral trait that is based on a particular evolutionary strategy evolving around a unique trajectory. The level of autonomy can be measured by the enzymes and various protein factors contained n the replication machinery. These factors bear encoded replicator genome and dedicated transposition or replication signals.
The researchers skilfully present through their experimental study that even quasi-replicators, for instance mini-inteins, Open Reading Frame (ORF)-less Group I self-splicing introns, restriction-modification modules and prokaryotic toxin-antitoxin (TA) do not possess any specific transposition or replication signals and yet they are comprised of properties that aids in promoting survival. They are, in some cases, capable of aiding in survival of the replicators on which they parasitize. The researchers further explained that the cellular replicators chromosomes bear accessory proteins as well as structural RNAs and encode all proteins involved in the replication which corresponds to the opposite end of spectrum resulting in maximum autonomy.
Also, there prevail all the other diverse replicators such as plasmids, transposable elements, viruses and organelle genomes originating from chloroplasts and mitochondria. It is to be noted that the span of these non-cellular replicators ranges from around one kilobase to over two mega bases. These spans are roughly similar with one kilobase of small transposons as well as satellite viruses and giant viruses of two mega base sized. Additionally, these non-cellular replicators differ widely when it come to protein complements that they encode.
From the above discussion it is evident that whether or not viruses are living can be both yes or no depending on the definition of life or the traits that make a being qualify to the state of being living and hence becomes arbitrary. In the realm of biology or biological world, no system of replication can evolve without the inevitable emergence of parasites. There exists a complex evolution of parasitic replicators and parasite-host coevolution does involve an inevitable arms race as well as various cooperation. As demonstrated by Moreira and López-García (2009), along the selfishness-cooperativity axis, there prevails a selfish extreme by lytic viruses whereas the remaining parasitic replicators spanned along a broader range.
The authors mentioned that these selfish replicators contribute to an intrinsic and central part of lifeforms. Hence, although we cannot conclude on the aliveness of viruses, considering the replicator paradigm, we can conduct experimental and theoretical study pertinent to the interactions occurring between the replicator community synchronizing the key drivers accounting for evaluation. However, this viewpoint shades light upon two critical aspects. Firstly, if replicators do not confine with the realm of biology, then life cannot be explained in terms of replicators.
Secondly, the replication of life is subject to energy production and resources acquisition as the key components of life formation. This is true as Schroedinger (2003) explained that the biological manifestation is associated with the complementarity present between replication and metabolism constituting the information dualism (entropy and energy). Needless to say, here the use of metabolism also refers to the broader spectrum of production of energy.
As inferred by Moreira and López-García (2009), precluding on multiple reasons viruses should be called living beings and so are the intracellular bacterial parasites. This seems plausible considering the case prokaryotic cells that contain at most one order of magnitude less viruses than the marine waters. Again, in the words of Koonin and Starokadomskyy (2016), besides the direct descendants of LUCA including archaea, eukarya and bacteria, viruses and related evolutionary mobile elements form an inevitable componential structure of biosphere. Even in the current century, the history of coevolution existing between a diverse and wide range of viruses, cells and other mobile elements continue to be indecipherable against a strong foundational block-chain.
Most empirical studies show that researchers have put inordinate amount of efforts in analysing the viromes originating in different environments. Those research works reflect that the sequential evidences deriving from analysis of such viromes correspond to completely unknown an undiscovered virus (Forterre, 2017). This diverts the data analysis procedure resulting on more focus on small sized sets of sequences retrieved from already recognised and known groups of viruses. Hence, it may be suggested that new and newer efforts should be directed towards the isolation of emerging and unresolved systems of virus and host.
It might be helpful considering the infections caused by viruses amongst the understudied organisms bears a potentially less explored area of concern and research trend. This is necessarily because of the chances that those understudied organisms constituting maximum percentage of biodiversity on our planet earth. A few examples of such organisms include recently discovered archaeal lineages or diverse kinds of phyla of protists that can be grouped under the eukaryotic domain. Hence, substantial emphasis should be attached on those organisms that are seemingly wide spreading within different environmental conditions.
Forterre, P. A. T. R. I. C. K. (2017). The Origin, Nature and Definittion Of Viruses (and Life): New Concepts and Controversies. Institute Pasteur, 1, 15-26.
Koonin, E. V., & Starokadomskyy, P. (2016). Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question. Studies in history and philosophy of science part C: Studies in history and philosophy of biological and biomedical sciences, 59, 125-134.
Moreira, D., & López-García, P. (2009). Ten reasons to exclude viruses from the tree of life. Nature Reviews Microbiology, 7(4), 306-311.
Schroedinger, E. (2003). What is life?: With “mind and matter” and “autobiographical sketches”. Cambridge: Cambridge University Press.
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