Rapid advances of genomics and metagenomics lead not only to the rapid growth of sequence databases but to discovery of fundamentally novel types of genetic elements. The discovery and characterization of giant viruses that infect unicellular eukaryotes, in particular members of the family Mimiviridae infecting amoeba, over the last decade revealed a remarkable new class of agents that are typical viruses by structure and reproduction strategy but exceed many parasitic bacteria in size and genomic complexity. Like bacteria, the giant viruses (sometimes called giruses) possess their own parasites and their own mobilomes, i.e. communities of associated mobile genetic elements.
The first virus infecting a giant virus, the Sputnik virophage, was isolated from a mimivirus-infected acanthamoeba and shown to replicate within the mimivirus factories and partially inhibit the reproduction of the host mimivirus. The second virophage, named Mavirus, is a parasite of the Cafeteria roenbergensis virus (CroV), a distant relative of the mimiviruses. The third virophage genome was isolated from the Antarctic Organic Lake (hence OLV, Organic Lake Virophage) where it apparently controls the reproduction of its virus host classified as a phycodnavirus. The three well-characterized virophages possess small isocahedral virions and genomes of 20 to 25 kilobase encoding 21 to 26 proteins each. Although the virophages are similar in genome size and structure and are generally construed as related, only a minority of the virophage genes are homologous.
Analysis of the Mavirus genome resulted in the discovery that this virophage shared 5 homologous genes with the large, self-replicating eukaryotic transposable elements of the Maverick/Polinton class (hereinafter Polintons). The Polintons that are scattered among genome of diverse eukaryotes and reach high abundance in some protists, such as Trichomonas vaginalis, have long been considered ‘virus-like’ transposons because of their large size (20 kb and larger) and the presence of several genes that are common in viruses but not in other transposable elements. The Mavirus shows by far the closest affinity with the Polintons among the currently known viruses, and accordingly, it has been proposed that the Polintons evolved from the virophages.
In addition to the virophages, the giant viruses host several other groups of mobile elements. These include self-splicing introns, inteins, putative bacterial-type transposons and the most recently discovered novel linear plasmids named transpovirons. The transpovirons are highly abundant genetic elements associated with several giant viruses of the Mimiviridae family that contain only 6 to 8 genes two of which are homologous to genes of the Sputnik virophage, indicating multiple gene exchanges within the giant virus mobilome.
Virophages, polintons, and transpovirons: a complex evolutionary network of diverse selfish genetic elements with different reproduction strategies. (2013) Virology Journal, 10:158 doi: 10.1186/1743-422X-10-158
Recent advances of genomics and metagenomics reveal remarkable diversity of viruses and other selfish genetic elements. In particular, giant viruses have been shown to possess their own mobilomes that include virophages, small viruses that parasitize on giant viruses of the Mimiviridae family, and transpovirons, distinct linear plasmids. One of the virophages known as the Mavirus, a parasite of the giant Cafeteria roenbergensis virus, shares several genes with large eukaryotic self-replicating transposon of the Polinton (Maverick) family, and it has been proposed that the polintons evolved from a Mavirus-like ancestor. We performed a comprehensive phylogenomic analysis of the available genomes of virophages and traced the evolutionary connections between the virophages and other selfish genetic elements. The comparison of the gene composition and genome organization of the virophages reveals 6 conserved, core genes that are organized in partially conserved arrays. Phylogenetic analysis of those core virophage genes, for which a sufficient diversity of homologs outside the virophages was detected, including the maturation protease and the packaging ATPase, supports the monophyly of the virophages. The results of this analysis appear incompatible with the origin of polintons from a Mavirus-like agent but rather suggest that Mavirus evolved through recombination between a polinton and an unknownvirus. Altogether, virophages, polintons, a distinct Tetrahymena transposable element Tlr1, transpovirons, adenoviruses, and some bacteriophages form a network of evolutionary relationships that is held together by overlapping sets of shared genes and appears to represent a distinct module in the vast total network of viruses and mobile elements. The results of the phylogenomic analysis of the virophages and related genetic elements are compatible with the concept of network-like evolution of the virus world and emphasize multiple evolutionary connections between bona fide viruses and other classes of capsid-less mobile elements.