Viruses: Evolution’s cheaters

Viruses can be considered genetic parasites that hijack the resources of their cellular hosts. They are evolution’s cheaters. And they are doing well with this strategy: viruses parasitize all forms of cellular life and are the most abundant and diverse entities on the planet.

So, given the ubiquity and abundance of viruses, the story of evolution is perhaps better framed as a story of virus-host coevolution. In fact, some mathematical models of replicator evolution suggest that parasitic genetic elements are a necessity of the system. Yet the origin of viruses remains somewhat mysterious.

Three scenarios for the evolution of viruses

All forms of cellular life have associated viruses. But from where and when did viruses arise? Multiple scenarios have been proposed for the origin of viruses, and the relative popularity of each of these scenarios has shifted over time. Let’s consider the three principal scenarios.

The virus early scenario – According to the virus early hypothesis, viruses originate from the precellular stage of evolution and are the direct descendants of the first replicons.

The reductive virus scenario – This scenario proposes that viruses result from the degeneration of an ancestral cell that lost its autonomy, becoming a host-dependent genetic parasite.

The escaped gene scenario – In this scenario, viruses have evolved on multiple, independent occasions from host genes that escape towards a selfish replication and infectivity strategy.

Hallmark viral replication genes

Krupovic et al. argue that any effort to determine the origin of viruses must focus on the core viral genes responsible for the key virus-specific functions: genome replication and virion formation. And by applying a laser-focus on what they refer to as the hallmark viral replication genes (RNA-dependent RNA polymerase, Reverse transcriptase, Protein-primed family B DNAP, Rolling-circle replication endonuclease, and Superfamily 3 helicase), the authors propose a chimeric scenario for viral replication.

A chimeric scenario for the origin of viruses

While the scenarios described above might seem mutually exclusive, Krupovic et al. propose a ‘chimeric’ scenario in which the virus replication machinery originated from the primordial pool of genetic elements, but the structural proteins were acquired from the virus hosts at various stages of evolution.

In support of this chimeric scenario, the authors highlight the absence of closely related homologs of viral replication proteins among extant cellular organisms and cite examples of apparent recruitment of ancestral cellular proteins in the evolution of viral capsids.

Encompassing a broad body of literature, the authors describe a model in which relatively cooperative genetic parasites recruited host structural protein to evolve into the diverse, efficient biological entities that we today know as viruses.