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New Virophage Points to Evolutionary Relationship between Transposons and DNA Viruses

By a GenomeWeb staff reporter

NEW YORK (GenomeWeb News) – A pair of researchers from the University of British Columbia reported in the early, online version of Science yesterday that they have discovered a tiny virus that seems to prey on a much larger virus — all within a single-celled zooplankton.

Moreover, the team's genetic analyses hint that this new virophage, which they dubbed Mavirus, likely shares a common ancestor with a group of eukaryotic transposable elements known as Maverick/Polinton transposons. Those involved in the study say the findings are consistent with the notion that some DNA transposon sequences found in eukaryotes today represent vestiges of double-stranded DNA viruses that once lived within these cells, possibly offering protective benefits.

"On the basis of our data we conclude that Maverick/Polinton transposons may have originated from ancient relatives of Mavirus and thereby influenced the evolution of eukaryotic genomes," senior author Curtis Suttle, an oceanography, microbiology, and virology researcher at UBC, and co-author Matthias Fischer wrote, "although we cannot rule out alternative evolutionary scenarios."

Suttle and Fischer found the new double-stranded DNA virus in the free-living, single-celled flagellate Cafeteria roenbergensis. The zooplankton is known to be susceptible to a massive marine virus known as "C. roenbergensis virus," or CroV, they explained. But their new evidence indicates that CroV is itself vulnerable to assault by another virus — the miniscule Mavirus.

The pair found that C. roenbergensis is less likely to be killed by CroV if Mavirus is also kicking around in the cell, for example. And Mavirus doesn't seem to replicate independently of the much larger virus that it parasitizes, the researchers reported.

When the team analyzed the new virophage's 19,063 base genome, meanwhile, they found that it was more similar to Maverick/Polinton transposons that hop around eukaryote genomes than to another virophage, the Mimivirus-parasitizing virus Sputnik.

Of the 20 predicted protein-coding sequences in the Mavirus genome, seven had homology to Maverick/Polinton transposons. Among them is a gene with strong conservation to the Maverick/Polinton transposon rve-superfamily retroviral integrase coding gene.

From these and other findings, the researchers speculated that Maverick/Polinton transposons may have originated as ancient double-stranded viruses related to Mavirus that were snapped up by eukaryotic genomes at some point in evolutionary history — perhaps as a way of protecting these eukaryotic cells from viral pathogens.

"[T]he closest genetic relatives to Mavirus are mobile genetic elements found in single-celled and higher organisms," Suttle said in a statement. "This implies that over evolutionary time, organisms have co-opted the DNA from ancient relatives of Mavirus into their own genomes, presumably so that they could acquire immunity against giant viruses like CroV."

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