Giant viruses blur clear line between life-forms

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Thanks:SHOJI KODAMA, Nikkei senior staff writer

April 17, 2014 12:00 am JST

Pithovirus is over 10 times larger than the influenza virus and rivals the size of some bacteria. (Courtesy of Chantal Abergel, IGS, CNRS-AMU)

TOKYO — Giant viruses discovered over the past decade are redefining the notion of life.

     The latest giant virus, which had been trapped in the Siberian permafrost for 30,000 years, is so big that it can be seen with a regular light microscope and is larger than many types of bacteria.

     Scientists may need to rethink evolution and life-form classification because of the giants. They differ from common viruses in numerous ways.

Frozen surprise

In 2003, scientists identified the Mimivirus, which is 0.75 micron in size, three times bigger than the largest viruses known at the time. It was so large that it was first mistaken for a bacterium. Investigation under an electron microscope revealed its true identity.

     Last year, another type of large virus was discovered in ocean sediments off the coast of Chile. Called Pandoravirus, these viruses are around 1 micron in length.

     And then came the report of the Pithovirus this March.

     Finding viable viruses in a 30,000-year-old sample of frozen soil from Siberia is news in itself. But the virus that the French scientists discovered is 1.5 microns long and 0.5 micron wide.

     Pithovirus is over 10 times the size of the influenza virus. Also, its oblong shape is unusual, as viruses tend to take on more geometric forms.

      Viruses are not defined as life forms because they cannot synthesize proteins on their own and must parasitize living cells to replicate. But the newly discovered giant viruses are very different.

     Mimivirus has both RNA and DNA, which breaks the presumed rule that viruses only have one or the other. It remains unclear whether this RNA is active when the virus invades cells to replicate, but its mere existence goes against conventional wisdom.

     “The discovery of Mimivirus shattered the definition of viruses,” said Hiroyuki Ogata of the Tokyo Institute of Technology.

     In addition, although Mimiviruses do not have ribosomes, which are the molecular machines used by cells to synthesize proteins, the giant viruses do have a number of genes that code for protein synthesis. “These viruses have over 100 kinds of proteins, so they must have some complex means of making copies,” Ogata said.  

     It is not just size that separates these giant viruses. The Pandoravirus genome, for example, is roughly 2.5 million base pairs long, which is more than that of some small bacteria. Also, parts of the virus are surrounded by lipid molecules, as found with bacteria.

     Science now classifies life forms into three domains: bacteria, archaea (which look like bacteria, but have some fundamental differences) and eukaryotes.

     The giant viruses may require the addition of a fourth domain of life, argue French microbiologist Jean-Michel Claverie and his colleagues who identified Mimivirus and helped find the other giant viruses.

Which came first?

The information gleaned from genome analysis has given support to the theory that viruses may have played a major role in the evolutionary development of eukaryotes, the domain of life that includes fungi, plants and humans.

     Eukaryotic cells have a nucleus, unlike bacteria and archaea. The new line of thinking is that this nucleus evolved from a distant, symbiotic relationship between viruses and archaea. In fact, poxviruses and eukaryotes share a close ancestry of genes that code for enzymes involved in DNA replication.

     “The spate of discoveries of giant viruses further supports the hypothesis,” said theory proponent Masaharu Takemura of the Tokyo University of Science.

     Analysis of the Mimivirus genome, for example, places the viruses on the phylogenetic tree somewhere before the branching of eukaryotes and archaea from their common ancestor.

     The existence of giant viruses blurs the line that until now clearly separated viruses from bacteria and other life forms. At the same time, genetic analysis makes clear that even among the giant viruses, there are major differences in evolutionary ancestry.

     “The giants have triggered a sea change in thinking about the relationship between viruses and living cells,” Takemura said.

     It may even upend the notion that viruses evolved after the origin of cells.

     Whatever the answer, studies of these giant viruses should provide important clues to help solve the riddles of the evolution of life.

 

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