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''Pandoravirus'' is a
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of
giant virus A giant virus, sometimes referred to as a girus, is a very large virus, some of which are larger than typical bacteria. All known giant viruses belong to the phylum '' Nucleocytoviricota''. Description While the exact criteria as defined in the ...
, first discovered in 2013. It is the second largest in physical size of any known viral genus. Pandoraviruses have double stranded DNA genomes, with the largest
genome size Genome size is the total amount of DNA contained within one copy of a single complete genome. It is typically measured in terms of mass in picograms (trillionths (10−12) of a gram, abbreviated pg) or less frequently in daltons, or as the total ...
(2.5 million base pairs) of any known viral genus.


Discovery

The discovery of Pandoraviruses by a team of French scientists, led by husband and wife Jean-Michel Claverie and Chantal Abergel, was announced in a report in the journal ''
Science Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earliest archeological evidence ...
'' in July 2013. Other scientists had previously observed the pandoravirus particles, but owing to their enormous size they were not expected to be viruses. Patrick Scheid, a parasitologist from the Central Institute of the Bundeswehr Medical Service in
Koblenz Koblenz (; Moselle Franconian: ''Kowelenz''), spelled Coblenz before 1926, is a German city on the banks of the Rhine and the Moselle, a multi-nation tributary. Koblenz was established as a Roman military post by Drusus around 8 B.C. Its nam ...
, Germany, found one in 2008, in an
amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...
living in the
contact lens Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmeti ...
of a woman with
keratitis Keratitis is a condition in which the eye's cornea, the clear dome on the front surface of the eye, becomes inflamed. The condition is often marked by moderate to intense pain and usually involves any of the following symptoms: pain, impaired e ...
. Its development within the amoebal host was documented extensively. Unlike in other cases with such giant viruses, the large particles within ''Acanthamoeba'' were not mistaken for bacteria. The authors initially termed them "endocytobionts". '' Mimivirus'', a nucleocytoplasmic large
DNA virus A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and ...
with a genome size of about 1.1 megabases, was described in 1992 but not recognized as a virus until 2003. ''
Megavirus Megavirus is a viral genus containing a single identified species named ''Megavirus chilensis'', phylogenetically related to Acanthamoeba polyphaga Mimivirus (APMV). In colloquial speech, Megavirus chilensis is more commonly referred to as jus ...
'', discovered in seawater off the coast of Chile in 2011, has a genome size of approximately 1.2 megabases. The prior discovery of these viruses prompted a search for other types of large amoeba-infecting viruses, which led to the finding of two species; '' Pandoravirus salinus'', found in
seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
taken from the coast of Chile, with a genome size of ~2.5 megabases, and '' Pandoravirus dulcis'', found in a shallow freshwater pond in
La Trobe University La Trobe University is a public research university based in Melbourne, Victoria, Australia. Its main campus is located in the suburb of Bundoora. The university was established in 1964, becoming the third university in the state of Victoria a ...
,
Melbourne, Australia Melbourne ( ; Boonwurrung/Woiwurrung: ''Narrm'' or ''Naarm'') is the capital and most populous city of the Australian state of Victoria, and the second-most populous city in both Australia and Oceania. Its name generally refers to a metr ...
, with a 1.9 megabase genome.


Description

Pandoraviruses are oval in shape and are about 1 
micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
 (1000 nanometers) in length. Other viruses range from 25 to 100 nanometers. In addition to being large physically, Pandoraviruses have a large
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
made up of 2,500 genes, compared to only 10 genes on average in other viruses. For example, the
Influenza A '' A virus'' (''IAV'') causes influenza in birds and some mammals, and is the only species of the genus ''Alphainfluenzavirus'' of the virus family ''Orthomyxoviridae''. Strains of all subtypes of influenza A virus have been isolated from wild ...
virus contains 7 genes and HIV contains only 9 genes. Gene content varies among species of Pandoravirus, with '' Pandoravirus salinus'' containing 2,500 genes and '' Pandoravirus dulcis'' containing about 1,500 genes. Pandoraviruses were originally mistaken for
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
; however, they lack some of the characteristics of bacteria, such as the ability to make their own proteins. The dissimilarity of the remaining genes to any cellular genes led researchers to speculate that this virus represents a previously unknown branch of the 
tree of life The tree of life is a fundamental archetype in many of the world's mythological, religious, and philosophical traditions. It is closely related to the concept of the sacred tree.Giovino, Mariana (2007). ''The Assyrian Sacred Tree: A Hist ...
. However, other experts have called this proposal premature because there is very little evidence supporting the idea. They are also seemingly capable of moving on their own, unlike other viruses.


Replication

Pandoraviruses have double stranded DNA. Like most giant viruses, Pandoraviruses have a viral life cycle. They lack the ability to make their own proteins, rely on the host cells for ATP (energy) and reproduction, and also do not contain
ribosome Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to fo ...
s or produce energy to divide. Under the microscope, scientists observed the virus enter the amoeba through fusion with membrane vacuoles, and integrate their DNA into the host cells. The host cell replicates the viral particles and eventually splits open, releasing the viral particles. The process of replication lasts 10–15 hours. Viral replication and assembly happens simultaneously. In other words, viral DNA is replicated within the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
of the host cell and assembled into new viral particles followed by lysis of the host cell.


Prevalence in the environment

Pandoraviruses do not seem to be harmful to humans. They are mostly found in marine environments, infecting amoebae. One reason for their only relatively recent discovery is because they exist in environments that are not well studied. Pandoraviruses, like other marine viruses, prey on
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
, which are organisms that live in the
water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
and form the basis of the food chain for other marine species. More study and research needs to be done in order to confirm the prevalence of Pandoraviruses in different environments. Currently, not much is known about their role in marine ecosystems.


Phylogenetic affinities

Approximately 93% of Pandoravirus genes are not known from any other microbes, suggesting that they belong to an as of yet undescribed "fourth domain" aside from
Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
,
Archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeba ...
, and
Eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
s.
Virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...
es are not widely considered to belong within these three domains, although they have been proposed as one in the past by some biologists.


Comparison with other giant viruses

Other giant viruses such as the ''Mimivirus'', ''
Pithovirus ''Pithovirus'', first described in a 2014 paper, is a genus of giant virus known from two species, ''Pithovirus sibericum'', which infects amoebas and ''Pithovirus massiliensis''. It is a double-stranded DNA virus, and is a member of the nucle ...
,'' and ''
Megavirus Megavirus is a viral genus containing a single identified species named ''Megavirus chilensis'', phylogenetically related to Acanthamoeba polyphaga Mimivirus (APMV). In colloquial speech, Megavirus chilensis is more commonly referred to as jus ...
'', have much smaller genomes. For example, ''Mimivirus'', considered one of the largest giant viruses, has a genome size of 1.1 million base pairs compared to 2.5 million base pairs for Pandoraviruses. Another feature that is different in Pandoraviruses compared to other giant viruses is the replication cycle. Pandoraviruses infect
amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...
s, which are single celled eukaryotes. Pandoravirus enters amoebas through phagocytic vacuoles, then fuses with the membrane vacuole of the amoeba. This leads to viral particles to be released into the cytoplasm of the amoeba.


See also

*
DNA virus A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and ...
*
Introduction to viruses A virus is a tiny infectious agent that reproduces inside the cells of living hosts. When infected, the host cell is forced to rapidly produce thousands of identical copies of the original virus. Unlike most living things, viruses do not ...
*
List of viruses This is an index of lists of virus taxa. By taxonomic rank * List of higher virus taxa, i.e. all taxa above the rank of family * List of virus families and subfamilies * List of virus genera :''This is a list of genera of biological viruses. See ...
*
Microbiology Microbiology () is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompasses numerous sub-disciplines including virology, bacteriology, ...
*
Virology Virology is the scientific study of biological viruses. It is a subfield of microbiology that focuses on their detection, structure, classification and evolution, their methods of infection and exploitation of host cells for reproduction, the ...
*
Virus classification Virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification systems used for cellular organisms. Viruses are classified by phenotypic characteristics, such as morphology, nucleic ...


References


External links


Viralzone: ''Pandoravirus''
{{Taxonbar, from=Q14084932 2013 in science Nucleocytoplasmic large DNA viruses Unaccepted virus taxa