It has been realized that viral particles are by far the most abundant biological entities on our planet.
Times are changing and viruses, once only considered as side-products of cellular evolution, are now at the center of many debates on the early evolution of life on our planet
The recent studies have confirmed that:
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| A typical virus replication cycle picture thankfully shared from wikipedia.org |
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| Some bacteriophages inject their genomes into bacterial cells (not to scale) *picture thankfully shared from wikipedia.org |
If you get sick with the flu,
for example, every infected cell in your airway produces about 10,000
new viruses. The total number of flu viruses in your body can rise to
100 trillion within a few days. That’s over 10,000 times more viruses
than people on Earth.
If there can be so many viruses in a single person, how many viruses
are there in total on our planet? I’ve hunted around for a number, and
the one I’ve seen most often is 1031. As in, 10000000000000000000000000000000. As in over 10 million times more viruses than there are stars in the universe.
As in, if you were to stack one virus on top of another, you’d create a
tower that would stretch beyond the moon, beyond the sun, beyond Alpha
Centauri, out past the edge of the Milky Way, past neighboring galaxies,
to reach a height of 200 million light years.
*by Carl Zimmer
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Noroviruses. Photo by Charles D. Humphrey,
Centers for Disease Control and Prevention |
Times are changing and viruses, once only considered as side-products of cellular evolution, are now at the center of many debates on the early evolution of life on our planet
The recent studies have confirmed that:
- Viruses have played and still playing a major innovative role in cellular organisms.
- Their position in the universal tree of life.
- Viruses are no more confused with their virions, but can be viewed as a complex living entities that transform the infected cell into a novel organism - the virus - producing virions.
Viruses and their relation to cellular organism:
Now living organisms are divided into three primary lineage (Woese and Fox 1977);
1. Bacteria (Eubacteria) (Bacteriophages)
(Archaeoviruses)
3. Eukarya (Eukaryotes)
(Eukaryoviruses)
Three cellular domain concept Old dichotomy of virus
from the virus perspective 1. Prokaryotes 1. Bacteria (Eubacteria) (Bacteriophages)
(Bacterioviruses) 2. Eukaryotes
2. Archaea (Archaebacteria) (Viruses)(Archaeoviruses)
3. Eukarya (Eukaryotes)
(Eukaryoviruses)
This trinity concept has now been corroborated by comparative biochemistry and comparative genomics.
Some archaeoviruses, bacterioviruses and eukaryoviruses share homologous capsid proteins and/or ATPases for protein packaging, suggesting that they all evolved from a common virus that existed at the time of LUCA of even before.
If infected archaea and bacteria are indeed transformed into bona fide viruses, one can conclude that infected eukaryotic cells in which viral factories have taken control of the cellular machinery became viruses themselves, the viral factory being in that case the equivalent of the nucleus. By adopting this viewpoint, one should finally consider viruses as cellular organisms. They are of course a particular form of cellular organism, since they do not encode their own ribosomes and cell membranes, but borrow those from the cells in which they live.
To take into account the idea that viruses represent a bona fide form of life, Didier Raoult and myself have recently proposed to divide the living world into two major groups of organisms, ribosome encoding-organisms (the descendants of LUCA, archaea, bacteria and eukarya) and capsid-encoding organisms (the viruses) (Raoult and Forterre 2008).
Organism: An ensemble of integrated organs (molecular and cellular) producing individuals evolving through natural selection.
Viruses encode two different "organs":
1. replicon: allowing genome replication,
2. capsid
Virus structure:
*pictures thankfully shared from; http://www.twiv.tv/virus-structure/
Viruses are ancient:
The ubiquitous existence of viruses infecting
members of the three cellular domains strongly suggests that viruses originated before LUCA, when cells still had genomes made of RNA
and not DNA (DNA, which is a chemically modified form of RNA, could
have appeared only after the emergence of complex proteins capable of
modifying RNA,
Some archaeoviruses, bacterioviruses and eukaryoviruses share homologous capsid proteins and/or ATPases for protein packaging, suggesting that they all evolved from a common virus that existed at the time of LUCA of even before.
Viruses are therefore very ancient.
Major role in biological evolution:
The
idea that viruses are very ancient and have co-evolved with the three
cellular lineages from the time of LUCA and even before has recently led
to several hypotheses posing that viruses have played a major role in
several critical evolutionary transitions. For instance:
it has been
suggested that DNA and DNA replication machineries first originated in
the viral world (Forterre 1999; Villarreal and DeFilippis 2000; Forterre 2002),
that virus-induced transition of cells with RNA genomes into cells with
DNA genomes triggered the emergence of the three cellular domains
(Forterre 2006), that the nucleus of eukaryotic cells originated from a
large DNA virus (Takemura 2001; Bell 2001),
or even that the selection pressure to prevent the entry of virions
promoted the evolution of cell walls (Jalasvuori and Bamford 2008).
All these hypotheses are not easily testable, but recent findings make
them reasonable. Indeed, it has been shown that cellular proteins
playing very important roles in modern organisms may have a viral
origin. For instance, phylogenetic analyses have revealed that the RNA
polymerase, DNA polymerase and DNA helicase that transcribe and
replicate DNA in modern mitochondria were recruited from a virus that
was originally integrated into the genome of the bacterium at the origin
of the mitochondria (Filée and Forterre 2005).
More recently, it has been shown that placentation in mammals is
initiated by a protein, syncitin, encoded by a retrovirus integrated in
mammalian chromosomes (De Parseval and Heidmann 2005; Prudhomme et al. 2005). There are many other examples of the role that viruses have played in recent cellular evolution (for reviews, see Ryan 2007; Brosius 2003; Villarreal 2005). Brosius wrote, for instance, that “the
interaction of hosts with retroviruses, retrotransposons and
retroelements is one of the eternal conflicts that drive the evolution
of life” (Brosius 2003).
Prangishvili and myself have recently extended his argument, concluding
that the conflict between cells and viruses has been (and still is) the
major engine of life evolution (Forterre and Prangishvili 2009).
*thankfully shared from: Defining life: The virus viewpoint by Patrick Forterre in NCBI article: Published online 2010 March 3
Nature of virus:
the intracellular phase of the virus
life cycle is the ontogenetically mature phase of viruses, where it shows the major physiological properties of oher organisms: metabolism, growth and reproduction.
Eukaryotic viruses that replicate in the cytoplasm form complex localized viral factories to replicate their genome and produce virion. The viral factory corresponds to the real viral organism, whereas the virion corresponds to the mechanism used by the virus to spread from one cell to others.
Eukaryotic viruses that replicate in the cytoplasm form complex localized viral factories to replicate their genome and produce virion. The viral factory corresponds to the real viral organism, whereas the virion corresponds to the mechanism used by the virus to spread from one cell to others.
If infected archaea and bacteria are indeed transformed into bona fide viruses, one can conclude that infected eukaryotic cells in which viral factories have taken control of the cellular machinery became viruses themselves, the viral factory being in that case the equivalent of the nucleus. By adopting this viewpoint, one should finally consider viruses as cellular organisms. They are of course a particular form of cellular organism, since they do not encode their own ribosomes and cell membranes, but borrow those from the cells in which they live.
To take into account the idea that viruses represent a bona fide form of life, Didier Raoult and myself have recently proposed to divide the living world into two major groups of organisms, ribosome encoding-organisms (the descendants of LUCA, archaea, bacteria and eukarya) and capsid-encoding organisms (the viruses) (Raoult and Forterre 2008).
Now newer perspective in defining:
Life: Mode of existence of ribosomes encoding organisms (cells) and capsid encoding organisms (viruses) and their ancestors.
Organism: An ensemble of integrated organs (molecular and cellular) producing individuals evolving through natural selection.
Viruses encode two different "organs":
1. replicon: allowing genome replication,
2. capsid
Virus structure:
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Helical Symmetry |
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Icosahedral Symmetry |
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Enveloped with helical nucleocapsid (influenza virus) |
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Enveloped with icosahedral nucleocapsid (herpesvirus) |
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Complex symmetry (poxvirus) |
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