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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 ...
, realm is the highest taxonomic rank established for
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 by the
International Committee on Taxonomy of Viruses The International Committee on Taxonomy of Viruses (ICTV) authorizes and organizes the taxonomic classification of and the nomenclatures for viruses. The ICTV has developed a universal taxonomic scheme for viruses, and thus has the means to ap ...
(ICTV), which oversees virus taxonomy. Six virus realms are recognized and united by specific
highly conserved In evolutionary biology, conserved sequences are identical or similar sequences in nucleic acids ( DNA and RNA) or proteins across species ( orthologous sequences), or within a genome ( paralogous sequences), or between donor and receptor taxa ( ...
traits: * ''
Adnaviria ''Adnaviria'' is a realm of viruses that includes archaeal viruses that have a filamentous virion (i.e. body) and a linear, double-stranded DNA genome. The genome exists in A-form (A-DNA) and encodes a dimeric major capsid protein (MCP) that co ...
'', which contains archaeal filamentous viruses with A-form double-stranded (ds) DNA genomes encoding a unique alpha-helical major capsid protein; * ''
Duplodnaviria ''Duplodnaviria'' is a realm of viruses that includes all double-stranded DNA viruses that encode the HK97 fold major capsid protein. The HK97 fold major capsid protein (HK97 MCP) is the primary component of the viral capsid, which stores the v ...
'', which contains all dsDNA viruses that encode the HK97-fold major capsid protein; * ''
Monodnaviria ''Monodnaviria'' is a realm of viruses that includes all single-stranded DNA viruses that encode an endonuclease of the HUH superfamily that initiates rolling circle replication of the circular viral genome. Viruses descended from such viruses ...
'', which contains all single-stranded DNA (ssDNA) viruses that encode a HUH superfamily endonuclease and their descendants; * ''
Riboviria ''Riboviria'' is a realm of viruses that includes all viruses that use a homologous RNA-dependent polymerase for replication. It includes RNA viruses that encode an RNA-dependent RNA polymerase, as well as reverse-transcribing viruses (with e ...
'', which contains all RNA viruses that encode
RNA-dependent RNA polymerase RNA-dependent RNA polymerase (RdRp) or RNA replicase is an enzyme that catalyzes the replication of RNA from an RNA template. Specifically, it catalyzes synthesis of the RNA strand complementary to a given RNA template. This is in contrast to ...
and all viruses that encode
reverse transcriptase A reverse transcriptase (RT) is an enzyme used to generate complementary DNA (cDNA) from an RNA template, a process termed reverse transcription. Reverse transcriptases are used by viruses such as HIV and hepatitis B to replicate their genom ...
; * '' Ribozyviria'', which contains hepatitis delta-like viruses with circular, negative-sense ssRNA genomes; * and ''
Varidnaviria ''Varidnaviria'' is a realm of viruses that includes all DNA viruses that encode major capsid proteins that contain a vertical jelly roll fold. The major capsid proteins (MCP) form into pseudohexameric subunits of the viral capsid, which store ...
'', which contains all dsDNA viruses that encode a vertical jelly roll major capsid protein. The rank of realm corresponds to the rank of
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined ** Domain of definition of a partial function ** Natural domain of a partial function **Domain of holomorphy of a function * ...
used for cellular life, but differs in that viruses in a realm do not necessarily share a common ancestor based on
common descent Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal com ...
nor do the realms share a common ancestor. Instead, realms group viruses together based on specific traits that are highly conserved over time, which may have been obtained on a single occasion or multiple occasions. As such, each realm represents at least one instance of viruses coming into existence. While historically it was difficult to determine deep evolutionary relations between viruses, in the 21st century methods such as
metagenomics Metagenomics is the study of genetic material recovered directly from environmental or clinical samples by a method called sequencing. The broad field may also be referred to as environmental genomics, ecogenomics, community genomics or microb ...
and
cryogenic electron microscopy Cryogenic electron microscopy (cryo-EM) is a cryomicroscopy technique applied on samples cooled to cryogenic temperatures. For biological specimens, the structure is preserved by embedding in an environment of vitreous ice. An aqueous sample so ...
have enabled such research to occur, which led to the establishment of ''Riboviria'' in 2018, three realms in 2019, and two in 2020.


Naming

The names of realms consist of a descriptive first part and the suffix -''viria'', which is the suffix used for virus realms. The first part of ''Duplodnaviria'' means "double DNA", referring to dsDNA viruses, the first part of ''Monodnaviria'' means "single DNA", referring to ssDNA viruses, the first part of ''Riboviria'' is taken from ribonucleic acid (RNA), and the first part of ''Varidnaviria'' means "various DNA". For
viroid Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economi ...
s, the suffix is designated as -''viroidia'', and for
satellites A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotop ...
, the suffix is -''satellitia'', but as of 2019 neither viroid nor satellite realms have been designated.


Realms


''Duplodnaviria''

''
Duplodnaviria ''Duplodnaviria'' is a realm of viruses that includes all double-stranded DNA viruses that encode the HK97 fold major capsid protein. The HK97 fold major capsid protein (HK97 MCP) is the primary component of the viral capsid, which stores the v ...
'' contains double-stranded DNA (dsDNA) viruses that encode a major capsid protein (MCP) that has the HK97 fold. Viruses in the realm also share a number of other characteristics involving the capsid and capsid assembly, including an icosahedral capsid shape and a terminase enzyme that packages viral DNA into the capsid during assembly. Two groups of viruses are included in the realm: tailed bacteriophages, which infect prokaryotes and are assigned to the order ''
Caudovirales ''Caudovirales'' is an order of viruses known as the tailed bacteriophages (''cauda'' is Latin for "tail"). Under the Baltimore classification scheme, the ''Caudovirales'' are group I viruses as they have double stranded DNA (dsDNA) genomes ...
'', and herpesviruses, which infect animals and are assigned to the order ''
Herpesvirales The ''Herpesvirales'' is an order of dsDNA viruses (Baltimore group I) with animal hosts, characterised by a common morphology consisting of an icosahedral capsid enclosed in a glycoprotein-containing lipid envelope. Common infections in humans ...
''. The relation between caudoviruses and herpesviruses is not certain, as they may either share a common ancestor or herpesviruses may be a divergent clade from within ''Caudovirales''. A common trait among duplodnaviruses is that they cause latent infections without replication while still being able to replicate in the future. Tailed bacteriophages are ubiquitous worldwide, important in marine ecology, and the subject of much research. Herpesviruses are known to cause a variety of epithelial diseases, including
herpes simplex Herpes simplex is a viral infection caused by the herpes simplex virus. Infections are categorized based on the part of the body infected. Oral herpes involves the face or mouth. It may result in small blisters in groups often called col ...
,
chickenpox Chickenpox, also known as varicella, is a highly contagious disease caused by the initial infection with varicella zoster virus (VZV). The disease results in a characteristic skin rash that forms small, itchy blisters, which eventually scab ...
and
shingles Shingles, also known as zoster or herpes zoster, is a viral disease characterized by a painful skin rash with blisters in a localized area. Typically the rash occurs in a single, wide mark either on the left or right side of the body or fac ...
, and Kaposi's sarcoma.


''Monodnaviria''

''
Monodnaviria ''Monodnaviria'' is a realm of viruses that includes all single-stranded DNA viruses that encode an endonuclease of the HUH superfamily that initiates rolling circle replication of the circular viral genome. Viruses descended from such viruses ...
'' contains single-stranded DNA (ssDNA) viruses that encode an
endonuclease Endonucleases are enzymes that cleave the phosphodiester bond within a polynucleotide chain. Some, such as deoxyribonuclease I, cut DNA relatively nonspecifically (without regard to sequence), while many, typically called restriction endonuclease ...
of the HUH superfamily that initiates rolling circle replication and all other viruses descended from such viruses. The prototypical members of the realm are called CRESS-DNA viruses and have circular ssDNA genomes. ssDNA viruses with linear genomes are descended from them, and in turn some dsDNA viruses with circular genomes are descended from linear ssDNA viruses. CRESS-DNA viruses include three kingdoms that infect prokaryotes: '' Loebvirae'', '' Sangervirae'', and ''
Trapavirae ''Pleolipoviridae'' is a family of DNA viruses that infect archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially cl ...
''. The kingdom '' Shotokuvirae'' contains eukaryotic CRESS-DNA viruses and the atypical members of ''Monodnaviria''. Eukaryotic monodnaviruses are associated with many diseases, and they include
papillomavirus ''Papillomaviridae'' is a family of non- enveloped DNA viruses whose members are known as papillomaviruses. Several hundred species of papillomaviruses, traditionally referred to as "types", have been identified infecting all carefully inspected ...
es and
polyomavirus ''Polyomaviridae'' is a family of viruses whose natural hosts are primarily mammals and birds. As of 2020, there are six recognized genera and 117 species, five of which are unassigned to a genus. 14 species are known to infect humans, while othe ...
es, which cause many cancers, and
geminivirus ''Geminiviridae'' is a family of plant viruses that encode their genetic information on a circular genome of single-stranded (ss) DNA. There are 520 species in this family, assigned to 14 genera. Diseases associated with this family include: br ...
es, which infect many economically important crops.


''Riboviria''

''
Riboviria ''Riboviria'' is a realm of viruses that includes all viruses that use a homologous RNA-dependent polymerase for replication. It includes RNA viruses that encode an RNA-dependent RNA polymerase, as well as reverse-transcribing viruses (with e ...
'' contains all RNA viruses that encode an
RNA-dependent RNA polymerase RNA-dependent RNA polymerase (RdRp) or RNA replicase is an enzyme that catalyzes the replication of RNA from an RNA template. Specifically, it catalyzes synthesis of the RNA strand complementary to a given RNA template. This is in contrast to ...
(RdRp), assigned to the kingdom ''
Orthornavirae ''Orthornavirae'' is a kingdom of viruses that have genomes made of ribonucleic acid (RNA), those genomes encoding an RNA-dependent RNA polymerase (RdRp). The RdRp is used to transcribe the viral RNA genome into messenger RNA (mRNA) and to repli ...
'', and all reverse transcribing viruses, i.e. all viruses that encode a
reverse transcriptase A reverse transcriptase (RT) is an enzyme used to generate complementary DNA (cDNA) from an RNA template, a process termed reverse transcription. Reverse transcriptases are used by viruses such as HIV and hepatitis B to replicate their genom ...
(RT), assigned to the kingdom ''
Pararnavirae ''Revtraviricetes'' is a class of viruses that contains all viruses that encode a reverse transcriptase. The group includes all ssRNA-RT viruses (including the retroviruses) and dsDNA-RT viruses. It is the sole class in the phylum ''Artverviric ...
''. These enzymes are vital in the viral life cycle, as RdRp transcribes viral mRNA and replicates the genome, and RT likewise replicates the genome. ''Riboviria'' mostly contains eukaryotic viruses, and most eukaryotic viruses, including most human, animal, and plant viruses, belong to the realm. Most widely known viral diseases are caused by viruses in ''Riboviria'', which includes
influenza virus ''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ' ...
es, HIV,
coronavirus Coronaviruses are a group of related RNA viruses that cause diseases in mammals and birds. In humans and birds, they cause respiratory tract infections that can range from mild to lethal. Mild illnesses in humans include some cases of the co ...
es, ebolaviruses, and the
rabies virus Rabies virus, scientific name ''Rabies lyssavirus'', is a neurotropic virus that causes rabies in humans and animals. Rabies transmission can occur through the saliva of animals and less commonly through contact with human saliva. ''Rabies lys ...
, as well as the first virus to be discovered, ''
Tobacco mosaic virus ''Tobacco mosaic virus'' (TMV) is a positive-sense single-stranded RNA virus species in the genus ''Tobamovirus'' that infects a wide range of plants, especially tobacco and other members of the family Solanaceae. The infection causes characteri ...
''. Reverse transcribing viruses are a major source of horizontal gene transfer by means of becoming endogenized in their host's genome, and a significant portion of the human genome consists of this viral DNA.


''Varidnaviria''

''
Varidnaviria ''Varidnaviria'' is a realm of viruses that includes all DNA viruses that encode major capsid proteins that contain a vertical jelly roll fold. The major capsid proteins (MCP) form into pseudohexameric subunits of the viral capsid, which store ...
'' contains DNA viruses that encode MCPs that have a
jelly roll fold The jelly roll or Swiss roll fold is a protein fold or supersecondary structure composed of eight beta strands arranged in two four-stranded sheets. The name of the structure was introduced by Jane S. Richardson in 1981, reflecting its resemblance ...
folded structure in which the jelly roll (JR) fold is perpendicular to the surface of the viral capsid. Many members also share a variety of other characteristics, including a minor capsid protein that has a single JR fold, an ATPase that packages the genome during capsid assembly, and a common
DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to crea ...
. Two kingdoms are recognized: ''
Helvetiavirae ''Alphasphaerolipovirus'' is a genus of double stranded DNA viruses that infect haloarchaea. The genus contains four species. Taxonomy The genus contains the following species: * ''Haloarcula hispanica icosahedral virus 2'' * ''Haloarcula hispa ...
'', whose members have MCPs with a single vertical JR fold, and ''
Bamfordvirae ''Bamfordvirae'' is a kingdom of viruses. This kingdom is recognized for its use of double jelly roll major capsid proteins. It was formerly known as the PRD1-adenovirus lineage. The kingdom is named after Dennis H. Bamford who first promoted ...
'', whose members have MCPs with two vertical JR folds. Marine viruses in ''Varidnaviria'' are ubiquitous worldwide and, like tailed bacteriophages, play an important role in marine ecology. Most identified eukaryotic DNA viruses belong to the realm. Notable disease-causing viruses in ''Varidnaviria'' include
adenovirus Adenoviruses (members of the family ''Adenoviridae'') are medium-sized (90–100 nm), nonenveloped (without an outer lipid bilayer) viruses with an icosahedral nucleocapsid containing a double-stranded DNA genome. Their name derives from ...
es, poxviruses, and the
African swine fever virus ''African swine fever virus'' (ASFV) is a large, double-stranded DNA virus in the '' Asfarviridae'' family. It is the causative agent of African swine fever (ASF). The virus causes a hemorrhagic fever with high mortality rates in domestic pigs; ...
. Poxviruses have been highly prominent in the history of modern medicine, especially ''Variola virus'', which caused
smallpox Smallpox was an infectious disease caused by variola virus (often called smallpox virus) which belongs to the genus Orthopoxvirus. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (WHO) c ...
. Many varidnaviruses are able to become endogenized, and a peculiar example of this are
virophage Virophages are small, double-stranded DNA viral phages that require the co-infection of another virus. The co-infecting viruses are typically giant viruses. Virophages rely on the viral replication factory of the co-infecting giant virus for th ...
s, which confer protection for their hosts against
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 ...
es during infection.


''Adnaviria''

Realm ''Adnaviria'' unifies archaeal filamentous viruses with linear A-form double-stranded DNA genomes and characteristic major capsid proteins unrelated to those encoded by other known viruses. The realm currently includes viruses from three families, '' Lipothrixviridae'', '' Rudiviridae'', and '' Tristromaviridae'', all infecting hyperthermophilic archaea. The nucleoprotein helix of adnaviruses is composed of asymmetric units containing two MCP molecules, a homodimer in the case of rudivirids and a heterodimer of paralogous MCPs in the case of lipothrixvirids and tristromavirids. The MCPs of ligamenviral particles have a unique α-helical fold first found in the MCP of rudivirid Sulfolobus islandicus rod-shaped virus 2 (SIRV2). All members of the ''Adnaviria'' share a characteristic feature in that the interaction between the MCP dimer and the linear dsDNA genome maintains the DNA in the A form. Consequently, the entire genome adopts the A form in virions. Like many structurally related viruses in the two other realms of dsDNA viruses (''Duplodnaviria'' and ''Varidnaviria''), there is no detectable sequence similarity among the capsid proteins of viruses from different tokiviricete families, suggesting a vast undescribed diversity of viruses in this part of the virosphere.


''Ribozyviria''

''Ribozyviria'' is characterised by the presence of genomic and antigenomic ribozymes of the ''
Deltavirus Hepatitis D is a type of viral hepatitis caused by the hepatitis delta virus (HDV). HDV is one of five known hepatitis viruses: A, B, C, D, and E. HDV is considered to be a satellite (a type of subviral agent) because it can propagate only in ...
'' type. Additional common features include a rod-like structure and a RNA-binding "delta antigen" encoded in the genome.


Origins

In general, virus realms have no genetic relation to each other based on common descent, in contrast to the three domains of cellular life—
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 ...
,
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 ...
, and Eukarya—which share a common ancestor. Likewise, viruses within each realm are not necessarily descended from a common ancestor since realms group viruses together based on highly conserved traits, not common ancestry, which is used as the basis for the taxonomy of cellular life. As such, each virus realm is considered to represent at least one instance of viruses coming into existence. By realm: * ''Adnaviria'' its origin is unknown, but it has been suggested that viruses of ''Adnaviria'' have potentially existed for a long time, as it is thought that they may have infected the last archaeal common ancestor. * ''Duplodnaviria'' is either monophyletic or polyphyletic and may predate the
last universal common ancestor The last universal common ancestor (LUCA) is the most recent population from which all organisms now living on Earth share common descent—the most recent common ancestor of all current life on Earth. This includes all cellular organisms; th ...
(LUCA) of cellular life. The exact origin of the realm is not known, but the HK97-fold MCP encoded by all members is, outside the realm, only found in
encapsulin The encapsulins are a family of bacterial proteins that serve as the main structural components of encapsulin nanocompartments. There are several different encapsulin proteins, including EncA, which forms the shell, and EncB, EncC, and EncD, whic ...
s, a type of nanocompartment found in bacteria, although the relation between ''Duplodnaviria'' and encapsulins is not fully understood. * ''Monodnaviria'' is polyphyletic and appears to have emerged multiple times from bacterial and archaeal circular plasmids, which are extra-chromosomal DNA molecules that live inside of bacteria and archaea and which self-replicate. * ''Riboviria'' is monophyletic or polyphyletic. The reverse transcriptase of kingdom ''Pararnavirae'' likely evolved on a single occasion from a
retrotransposon Retrotransposons (also called Class I transposable elements or transposons via RNA intermediates) are a type of genetic component that copy and paste themselves into different genomic locations ( transposon) by converting RNA back into DNA throu ...
, a type of self-replicating DNA molecule that replicates via
reverse transcription A reverse transcriptase (RT) is an enzyme used to generate complementary DNA (cDNA) from an RNA template, a process termed reverse transcription. Reverse transcriptases are used by viruses such as HIV and hepatitis B to replicate their genom ...
. The origin of the RdRp of ''Orthornavirae'' is less certain, but they are believed to originate from a bacterial group II intron that encodes reverse transcriptase or to predate the LUCA being descendants of the ancient
RNA world The RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins. The term also refers to the hypothesis that posits the existen ...
and precede reverse transcriptases of cellular life. A larger study (2022) where new lieneages (phyla) were described, was in favor of the hypothesis that RNA viruses descend from the RNA world, suggesting that retroelements of cellular life originated from an ancestor related to the phylum ''
Lenarviricota ''Lenarviricota'' is a phylum of RNA viruses that includes all positive-strand RNA viruses that infect prokaryotes. Some members also infect eukaryotes. Most of these viruses do not have capsids, except for the genus '' Ourmiavirus''. The name of ...
'' and that members of a newly discovered '' Taraviricota'' lineage (phylum) would be the ancestors of all RNA viruses. * ''Ribozyviria'' it has been suggested this realm of viruses appears to have derived from retrozymes (a family of
retrotransposon Retrotransposons (also called Class I transposable elements or transposons via RNA intermediates) are a type of genetic component that copy and paste themselves into different genomic locations ( transposon) by converting RNA back into DNA throu ...
s) or one of the other groups of
mobile genetic elements Mobile genetic elements (MGEs) sometimes called selfish genetic elements are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In ...
that descended from retrozymes (i.e.
viroid Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economi ...
s and
satellites A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotop ...
). But it was also proposed that they may have originated from
viroids Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economi ...
with capsid protein capture. * ''Varidnaviria'' is either monophyletic or polyphyletic and may predate the LUCA. The kingdom ''Bamfordvirae'' is likely derived from the other kingdom ''Helvetiavirae'' via
fusion Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole. Fusion may also refer to: Science and technology Physics *Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...
of two MCPs to have an MCP with two jelly roll folds instead of one. The single jelly roll (SJR) fold MCPs of ''Helvetiavirae'' show a relation to a group of proteins that contain SJR folds, including the
Cupin superfamily The cupin superfamily is a diverse superfamily of proteins named after its conserved barrel domain (''cupa'' being the Latin term for a small barrel). The superfamily includes a wide variety of enzymes as well as non-enzymatic seed storage p ...
and
nucleoplasmin Nucleoplasmin, the first identified molecular chaperone is a thermostable acidic protein with a pentameric structure. The protein was first isolated from Xenopus species Functions The pentameric protein participates in various significant cellul ...
s. However, another scenario was later proposed in which the ''Bamfordvirae'' and ''Helvetiavirae'' kingdoms would originate independently from
mobile genetic elements Mobile genetic elements (MGEs) sometimes called selfish genetic elements are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In ...
such as
plasmids A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...
with the capture of two different cellular proteins when it was detected that the ''Bamfordvirae'' DJR-MCP protein is homologous to the bacterial DUF 2961 protein, leading to a revision of the realm ''Varidnaviria''. According to this scenario, the ''Bamfordvirae'' DJR-MCP fold evolved from the bacterial DUF 2961 protein, while the ''Helvetiavirae'' SJR-MCP evolved from the SJR-MCP related to the '' Portogloboviridae'' family, which in turn originated from the Cupin superfamily and the nucleoplasmins. While the realms generally have no genetic relation to each other, there are some exceptions: * Viruses in the family '' Podoviridae'' in ''Duplodnaviria'' encode a DNA polymerase that is related to the DNA polymerases encoded by many members of ''Varidnaviria''. * Eukaryotic viruses in the kingdom '' Shotokuvirae'' in ''Monodnaviria'' were created on multiple occasions by recombination events that combined the DNA of ancestral plasmids with complementary DNA (cDNA) of positive sense RNA viruses in ''Riboviria'', by which ssDNA viruses in ''Shotokuvirae'' obtained capsid proteins from RNA viruses. * The family '' Bidnaviridae'' in ''Monodnaviria'' was created via integration of a parvovirus (of ''Monodnaviria'') genome into a
polinton Polintons (also called Mavericks) are large DNA transposons which contain genes with homology to viral proteins and which are often found in eukaryotic genomes. They were first discovered in the mid-2000s and are the largest and most complex kno ...
, a virus-like self-replicating DNA molecule, which are related to viruses in ''Varidnaviria''. Furthermore, bidnaviruses encode a receptor-binding protein inherited from
reoviruses ''Reoviridae'' is a family of double-stranded RNA viruses. Member viruses have a wide host range, including vertebrates, invertebrates, plants, protists and fungi. They lack lipid envelopes and package their segmented genome within multi-layered ...
in the realm ''Riboviria''.


Subrealm

In virology, the second highest taxonomy rank established by the ICTV is subrealm, which is the rank below realm. Subrealms of viruses use the suffix -''vira'', viroid subrealms use the suffix -''viroida'', and satellites use the suffix -''satellitida''. The rank below subrealm is kingdom. As of 2019, no taxa are described at the rank of subrealm.


History

Prior to the 21st century, it was believed that deep evolutionary relations between viruses could not be discovered due to their high mutation rates and small number of genes making discovering these relations more difficult. Because of this, the highest taxonomic rank for viruses from 1991 to 2017 was order. In the 21st century, however, various methods have been developed that have enabled these deeper evolutionary relationships to be studied, including metagenomics, which has identified many previously unidentified viruses, and comparison of highly conserved traits, leading to the desire to establish higher-level taxonomy for viruses. In two votes in 2018 and 2019, the ICTV agreed to adopt a 15-rank classification system for viruses, ranging from realm to species. ''Riboviria'' was established in 2018 based on phylogenetic analysis of the RNA-dependent polymerases being monophyletic, ''Duplodnaviria'' was established in 2019 based on increasing evidence that tailed bacteriophages and herpesviruses shared many traits, ''Monodnaviria'' was established in 2019 after the relation and origin of CRESS-DNA viruses was resolved, and ''Varidnaviria'' was established 2019 based on the shared characteristics of member viruses.


See also

*
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


Further reading

* {{Taxonomic ranks Virus taxonomy