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Are Viruses Alive? The Replicator Paradigm Sheds Decisive Light on an Old but Misguided Question

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Are Viruses Alive? The Replicator Paradigm Sheds Decisive Light on an Old but Misguided Question

Eugene V Koonin et al. Stud Hist Philos Biol Biomed Sci.

Abstract

The question whether or not "viruses are alive" has caused considerable debate over many years. Yet, the question is effectively without substance because the answer depends entirely on the definition of life or the state of "being alive" that is bound to be arbitrary. In contrast, the status of viruses among biological entities is readily defined within the replicator paradigm. All biological replicators form a continuum along the selfishness-cooperativity axis, from the completely selfish to fully cooperative forms. Within this range, typical, lytic viruses represent the selfish extreme whereas temperate viruses and various mobile elements occupy positions closer to the middle of the range. Selfish replicators not only belong to the biological realm but are intrinsic to any evolving system of replicators. No such system can evolve without the emergence of parasites, and moreover, parasites drive the evolution of biological complexity at multiple levels. The history of life is a story of parasite-host coevolution that includes both the incessant arms race and various forms of cooperation. All organisms are communities of interacting, coevolving replicators of different classes. A complete theory of replicator coevolution remains to be developed, but it appears likely that not only the differentiation between selfish and cooperative replicators but the emergence of the entire range of replication strategies, from selfish to cooperative, is intrinsic to biological evolution.

Keywords: Evolvability; Host-parasite coevolution; Replicators; Selfish elements; Viruses.

Figures

Figure 1
Figure 1. The diversity of replicators: replicative autonomy vs selfishness-cooperativity
Although the distribution of different groups of replicators in this plane can be viewed as continuous, four classes are delineated by the criteria of presence or absence of signals for replication and/or transposition and the respective protein machinery. The specific positions of different replicators on the plane can be defined only qualitatively. The classes of replicators are denoted as follows: C, chromosomes (including organellar genomes); I, inteins; In, (self-splicing) introns; MI, mini-inteins; O, organellar genomes; P, plasmids; QR, quasi-replicators; RT, retrotransposons; T, (DNA) transposons; V, viruses; Vi, viroids. The three colors denote virion-less selfish elements, viruses and cellular life forms.
Figure 2
Figure 2. Replicators, their vehicles and resource production
As in Figure 1, the specific positions of different groups of replicators along the resource production axis is determined only qualitatively. However, on the vehicle axis, there are only three distinct positions for the two types of vehicles and the replicators without vehicles. The designations for the classes of replicators are as in Figure 1.
Figure 3
Figure 3. Communities of interacting replicators
The 5 classes of replicators along the mobility/selfishness axis are denoted according to (Jalasvuori and Koonin 2015). The arrows denote both physical fusion (integration) and parasitic, commensal or symbiotic relationships between different classes of replicators.

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