Viruses, as we all know, are microscopic creatures that require the cellular machinery of their host organisms to replicate. Often considered nature’s ultimate parasites, the discovery in 2008 of organisms that can infest and exploit viruses, acting essentially as parasites, was striking. Named virophages, these tiny viruses are found within giant viruses, such as those belonging to the Mimiviridae family, which in turn are known to infect unicellular eukaryotes. Now a comprehensive phylogenomic analysis of known virophages, published in Virology Journal, provides insight into not only the evolution of virophages, but also of viruses as a whole.
Sequences from virophages such as Sputnik, the first such organism to be isolated, Mavirus, a parasite of the Cafeteria roenbergensis virus, and the Organic Lake Virophage, isolated from an Antarctic organic lake, were compared with those from Polintons and Mavericks, eukaryotic self-replicating transposable elements associated with giant viruses.
Eugene Koonin who led this study explained, “Between the known virophages there are six conserved genes, arranged in a similar way. Five of these have counterparts in the Polintons and Mavericks, but their sequence and arrangement is sufficiently different to discount suggestions that Polintons evolved from a Mavirus-like ancestor. Rather our data suggests that Maviruses have evolved from a fusion between a Politon/Maverick like transposable element and an unknown virus.”
Additionally, the analysis also included sequence information from other viruses, including adenoviruses (which infect animals), bacteriophages (which infect bacteria), transpovirons (which infect viruses and swap information between the giant virus and its amoeba host) and a Tetrahymena transposable element (Tlr1). The picture that emerged was a complex network of swapped genes.
Didier Raoult, one of the first people to identify transpovirons, commented, “It appears that viruses have evolved from non-viral genetic elements and vice versa on more than one occasion. Viral evolution is more complex than we thought.”