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Cellular Origins of Giant Viruses?

The largest virus to be sequenced prompts researchers to consider whether giant viruses were once full-fledged living organisms.

By | October 16, 2011

Megavirus particleWIKIMEDIA, CHANTAL ABERGEL

Floating in the coastal waters of Chile, a team of French researchers has discovered and isolated yet another giant virus: Megavirus chilensis. Ringing in at a whopping 1.3 million base pairs, the Megavirus genome is the largest viral genome to be sequenced to date. Encoding around 1,120 putative proteins—the viral genome lies well in the range of many bacterial genomes and has researchers once again scratching their heads over the evolutionary origins of such mega-scale viruses.

A comparison of the genomes of Megavirus and Mimivirus—the first giant virus discovered—led researchers to conclude that not only are they distant relatives that share a wide range of genes including those for DNA repair, transcription, and viral factory genes, but that they may have both descended from an ancestor with a much larger genome. In fact, the authors suggest that the common ancestor may have been a free-living eukaryotic cell—one of the very first eukaryotic organisms—evidenced by the similarity of some giant virus genes with their living eukaryotic counterparts, Ars Technica reports. “Giant viruses might thus be relicts of the first chapters of the history of life,” Carl Zimmer wrote in a Discover blog post.

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Avatar of: Ed Rybicki

Ed Rybicki

Posts: 82

October 18, 2011

The discovery of the Megavirus - and Mimivirus too - shows that the biology of our planet is still so vast, and so undiscovered, that the tyranny of the hypothesis is still not absolute; there is still room for "discovery science".

So unless "We think there are more big viruses out there" can be taken seriously as a hypothesis, discovery rules, OK?!!  At least, in the marine environment.

But seriously now: the important facts here are that Megavirus shares most - but definitely not all - its genome with Mimivirus, BUT at such a genetic remove (<50% identity for most proteins) that it is almost certainly a very distant relative.  It has also NOT sourced most of its genes by horizontal gene transfer from cellular organisms, meaning that what it has is what it started with, by and large.

Which complicates our picture of where big viruses and cells came from - because these viruses are right at the base of the eukaryote family tree, and it is not at all clear which came first.

I do love discovery virology.  Only they won't give me enough money to go out and play too...B-(

Avatar of:

Posts: 0

October 18, 2011

The discovery of the Megavirus - and Mimivirus too - shows that the biology of our planet is still so vast, and so undiscovered, that the tyranny of the hypothesis is still not absolute; there is still room for "discovery science".

So unless "We think there are more big viruses out there" can be taken seriously as a hypothesis, discovery rules, OK?!!  At least, in the marine environment.

But seriously now: the important facts here are that Megavirus shares most - but definitely not all - its genome with Mimivirus, BUT at such a genetic remove (<50% identity for most proteins) that it is almost certainly a very distant relative.  It has also NOT sourced most of its genes by horizontal gene transfer from cellular organisms, meaning that what it has is what it started with, by and large.

Which complicates our picture of where big viruses and cells came from - because these viruses are right at the base of the eukaryote family tree, and it is not at all clear which came first.

I do love discovery virology.  Only they won't give me enough money to go out and play too...B-(

Avatar of:

Posts: 0

October 18, 2011

The discovery of the Megavirus - and Mimivirus too - shows that the biology of our planet is still so vast, and so undiscovered, that the tyranny of the hypothesis is still not absolute; there is still room for "discovery science".

So unless "We think there are more big viruses out there" can be taken seriously as a hypothesis, discovery rules, OK?!!  At least, in the marine environment.

But seriously now: the important facts here are that Megavirus shares most - but definitely not all - its genome with Mimivirus, BUT at such a genetic remove (<50% identity for most proteins) that it is almost certainly a very distant relative.  It has also NOT sourced most of its genes by horizontal gene transfer from cellular organisms, meaning that what it has is what it started with, by and large.

Which complicates our picture of where big viruses and cells came from - because these viruses are right at the base of the eukaryote family tree, and it is not at all clear which came first.

I do love discovery virology.  Only they won't give me enough money to go out and play too...B-(

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