Do viruses have viruses?

The short answer to the question: -- YES.

Following on from the previous posting about Pandoraviruses, the virological world has become much more complex than we could have imagined -- a network of diverse genetic elements with different reproduction strategies and lifestyles -- the transpovirons, polintons, and virophages.

Comparison of the genome architectures of the virophages, polintons, some viruses, and transpovirons. Homologous genes are marked by the same colors. (CLICK TO ENLARGE)

The rapid advances of genomics and the growth of sequence databases has led to the discovery of fundamentally novel types of genetic elements. The giant viruses, Mimiviruses that infect amoeba, possess their own parasites and communities of associated mobile genetic elements. The first virus infecting a giant virus, the Sputnik virophage, was shown to replicate within the mimivirus factories and partially inhibit the reproduction of the host Mimivirus. A second isolated virophage was named Mavirus and the third virophage genome was isolated from the Antarctic Organic Lake (hence OLV, Organic Lake Virophage). The virophages possess small isocahedral virions and genomes of 20 to 25 kilobase encoding 21 to 26 proteins.

Analysis of the Mavirus genome resulted in the unexpected discovery that this virophage shared 5 homologous genes with the large, self-replicating eukaryotic transposable elements called Maverick/Polintons. 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. In a recent paper published in the Virology Journal, the authors sought to decipher the evolutionary relationships between the three known virophages, the Maverick/Polintons, transpovirons and bona fide viruses.

Their conclusion was that the virophages and related genetic elements formed a vast network of evolutionary relationships with multiple connections between bona fide viruses and other classes of non-virion mobile genetic entities -- a genetic soup.

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