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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 are Mimiviruses that infect
the protist Acanthamoeba and that are themselves infected by the virophage Sputnik.
This paper 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 towards increasing its basic
reproductive ratio in the absence of the virophage, the opposite is true 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 towards
weaker phenotypes. Experimental tests for this model are suggested.