Evolutionary dynamics of giant viruses and their virophages

Abstract

Giant viruses contain large genomes, encode many proteins atypical for viruses, replicate in large viral factories, and tend to infect protists. The giant virus replication factories can in turn be infected by so called virophages, which are smaller viruses that negatively impact giant virus replication. An example is Mimiviruses that infect the protist Acanthamoeba and that are themselves infected by the virophage Sputnik. This study examines the evolutionary dynamics of this system, using mathematical models. While the models suggest that the virophage population will evolve to increasing degrees of giant virus inhibition, it further suggests that this renders the virophage population prone to extinction due to dynamic instabilities over wide parameter ranges. Implications and conditions required to avoid extinction are discussed. Another interesting result is that virophage presence can fundamentally alter the evolutionary course of the giant virus. While the giant virus is predicted to evolve toward increasing its basic reproductive ratio in the absence of the virophage, the opposite is true in its presence. Therefore, virophages can not only benefit the host population directly by inhibiting the giant viruses but also indirectly by causing giant viruses to evolve toward weaker phenotypes. Experimental tests for this model are suggested.

Polintons, virophages and transpovirons: a tangled web linking viruses, transposons and immunity

“…A similar protective effect towards the host cells has been previously demonstrated in co-infection experiments with satellite and helper viruses of plants and animals [36–38], illuminating the generality of a phenomenon whereby inter-viral competition benefits the host. Theoretical models of the dynamic of tripartite host-parasite systems motivated by the discovery of virophages also suggest a mitigating effect of virophages on the giant virus growth and stabilization of the tripartite host-parasite system compared to the simple giant virus-host case [39–41]. More recently, this hypothesis has received direct, resounding support in experiments of Fischer and Hackl showing that, when marine flagellate C. roenbergensis is coinfected with CroV and Mavirus, in about 30% of the infected cells, multiple Mavirus genome copies integrate into the host genome [42].…”

Provirophages in the Bigelowiella genome bear testimony to past encounters with giant viruses

“…For instance, in the marine environment, the co-occurrence of giant viruses and virophages in the context of algal blooms may influence the overall carbon flux as proposed for Antarctic lakes (7). Nevertheless, such tripartite community networks remain poorly explored except on theoretical grounds (17,18).…”

Characteristics of virophages and giant viruses

“…It was recorded in Antarctica lakes that they affect the growth of blooming algae (Santini et al, 2013;Yau et al, 2011). Another model of the giant viruses and virophages spread, presented by Wodarz (Wodarz, 2013) is in opposition to that presented by Taylor and coworkers (Taylor et al, 2014) and describes this phenomenon (Taylor et al, 2014).…”