Wikipedia

Virosphere

Virosphere(virusdiversity, virus world, global virosphere) was coined to refer to all those places in which viruses are found or which are affected by viruses.[1][2]However, more recently virosphere has also been used to refer to the pool of viruses that occurs in all hosts and all environments,[3]as well as viruses associated with specific types of hosts (prokaryoticvirosphere,[4]archaealvirosphere,[5]Invertebratevirosphere),[6]type ofgenome(RNAvirosphere,[7]dsDNAvirosphere)[8]or ecological niche (marine virosphere).[9]

Viral genome diversity

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The scope of viral genome diversity is enormous compared to cellular life. Cellular life including all known organisms have double stranded DNA genome. Whereas viruses have one of at least 7 different types ofgenetic information,namely dsDNA, ssDNA, dsRNA, ssRNA+, ssRNA-, ssRNA-RT, dsDNA-RT. Each type of genetic information has its specific manner ofmRNAsynthesis.Baltimore classificationis a system providing overview on these mechanisms for each type of genome. Moreover, in contrast to cellular organisms, viruses don't have universallyconserved sequencesin their genomes to be compared by.[citation needed]

Viral genome size varies approximately 1000 fold. Smallest viruses may consist of only from 1–2 kb genome coding for 1 or 2 genes and it is enough for them to successfully evolve and travel through space and time by infecting and replicating (make copies of their own) in its host. Two most basic viral genes arereplicasegene andcapsidproteingene, as soon as virus has them it represents a biological entity able to evolve and reproduce in cellular life forms. Some viruses may have only replicase gene and use capsid gene of other e.g.endogenous virus.Most viral genomes are 10-100kb, whereasbacteriophagestend to have larger genomes carrying parts of genome translation machinery genes from their host. In contrast, RNA viruses have smaller genomes, with maximum 35kb bycoronavirus.RNA genomes have higher mutation rate, that is why their genome has to be small enough in order not to harbour to many mutations, which would disrupt the essential genes or their parts.[10]The function of the vast majority of viral genes remain unknown und the approaches to study have to be developed.[11]The total number of viral genes is much higher, than the total number of genes of threedomainsof life all together, which practically means viruses encode most of the genetic diversity on the planet.[12]

Viral host diversity

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Viruses arecosmopolites,they are able to infect every cell and every organism on planet earth. However different viruses infect different hosts. Viruses are host specific as they need to replicate (reproduce) within a host cell. In order to enter the cell viral particle needs to interact with a receptor on the surface of its host cell. For the process of replication many viruses use their own replicases, but for protein synthesis they are dependent on their host cellprotein synthesismachinery. Thus, host specificity is a limiting factor for viral reproduction.[citation needed]

Some viruses have extremely narrow host range and are able to infect only 1 certain strain of 1 bacterial species, whereas others are able to infect hundreds or even thousands of different hosts. For examplecucumber mosaic virus(CMV) can use more than 1000 different plant species as a host.[13]Members of viral families likeRhabdoviridaeinfect hosts from different kingdoms e.g. plants and vertebrates.[14]And members of generaPsimunavirusandMyohalovirusinfect hosts from different domains of life e.g. bacteria and archaea.[15]

Viral capsid diversity

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Capsidis the outer protecting shell or scaffold of a viral genome. Capsid enclosing viral nucleic acid make up viral particle or a virion. Capsid is made of protein and sometimes has lipid layer harboured from the host cell while exiting it. Capsid proteins are highly symmetrical and assemble within a host cell by their own due to the fact, that assembled capsid is more thermodynamically favourable state, than separate randomly floating proteins. The most viral capsids haveicosahedralor helical symmetry, whereas bacteriophages have complex structure consisting of icosahedral head and helical tail including baseplate and fibers important for host cell recognition and penetration.[16]Viruses of archaea infecting hosts living in extreme environments like boiling water, highly saline or acidic environments have totally different capsid shapes and structures. The variety of capsid structures of Archaeal viruses includes lemon shaped virusesBicaudaviridaeof family andSalterprovirusgenus, spindle formFuselloviridae,bottle shapedAmpullaviridae,egg shapedGuttaviridae.[5]

Capsid size of a virus differs dramatically depending on its genome size and capsid type.Icosahedral capsids are measured by diameter, whereas helical and complex are measured by length and diameter. Viruses differ in capsid size in a spectrum from 10 to more than 1000 nm. The smallest viruses are ssRNA viruses likeParvovirus.They haveicosahedralcapsid approximately 14 nm in diameter. Whereas the biggest currently known viruses arePithovirus,MamavirusandPandoravirus.Pithovirus is a flask-shaped virus 1500 nm long and 500 nm in diameter, Pandoravirus is an oval-shaped virus1000nm (1 micron) long and Mamavirus is an icosahedral virus reaching approximately 500 nm in diameter.[17]Example of how capsid size depends on the size of viral genome can be shown by comparing icosahedral viruses - the smallest viruses are 15-30 nm in diameter have genomes in range of 5 to 15 kb (kilo bases or kilo base pairs depending on type of genome), and the biggest are near 500 nm in diameter and their genomes are also the largest, they exceed1Mb (million base pairs).[citation needed]

Viral evolution

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Viral evolutionor evolution of viruses presumably started from the beginning of the second age of RNA world, when different types of viral genomes arose through the transition from RNA- RT –DNA, which also emphasises that viruses played a critical role in the emergence of DNA and predate LUCA[18][19]The abundance and variety of viral genes also imply that their origin predatesLUCA.[20]As viruses do not share unifying common genes they are considered to bepolyphyleticor having multiple origins as opposed to one common origin as all cellular life forms have.[21][22]Virus evolution is more complex as it is highly prone tohorizontal gene transfer,genetic recombinationandreassortment.Moreover viral evolution should always be considered as a process ofco-evolutionwith its host, as a host cell is inevitable for virus reproduction and hence,evolution.[citation needed]

Viral abundance

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Viruses are the most abundant biological entities, there are 10^31 viruses on our planet.[23][24]Viruses are capable of infecting all organisms on earth and they are able to survive in much harsher environments, than any cellular life form. As viruses can not be included in the tree of life there is no separate structure illustrating viral diversity and evolutionary relationships.[25]However, viral ubiquity can be imagined as a virosphere covering the whole tree of life.[citation needed]

Nowadays we are entering the phase of exponential viral discovery. Thegenome sequencingtechnologies includinghigh-throughputmethods allow fast and cheap sequencing of environmental samples. The vast majority of the sequences from any environment, both from wild nature and human made, reservoirs are new.[26][27]It practically means that during over 100 years of virus research from the discovery of bacteriophages - viruses of bacteria in 1917 until current time we only scratched on a surface of a great viral diversity. The classic methods likeviral cultureused previously allowed to observe physical virions or viral particles usingelectron microscope,they also allowed to gathering information about their physical and molecular properties. New methods deal only with the genetic information of viruses.[citation needed]

See also

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References

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  1. ^"World Wide Words: Virosphere".World Wide Words.Retrieved2023-04-13.
  2. ^Suttle, Curtis (2005)."The viriosphere: the greatest biological diversity on Earth and driver of global processes".Environmental Microbiology.7(4): 481–482.Bibcode:2005EnvMi...7..481S.doi:10.1111/j.1462-2920.2005.803_11.x.ISSN1462-2912.PMID15816923.S2CID40555592.
  3. ^Abroi, Aare; Gough, Julian (2011)."Are viruses a source of new protein folds for organisms? – Virosphere structure space and evolution".BioEssays.33(8): 626–635.doi:10.1002/bies.201000126.ISSN1521-1878.PMID21633962.S2CID6680980.
  4. ^Krupovic, Mart; Prangishvili, David; Hendrix, Roger W.; Bamford, Dennis H. (2011)."Genomics of Bacterial and Archaeal Viruses: Dynamics within the Prokaryotic Virosphere".Microbiology and Molecular Biology Reviews.75(4): 610–635.doi:10.1128/mmbr.00011-11.PMC3232739.PMID22126996.
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  12. ^Rohwer, Forest; Barott, Katie (2013-03-01)."Viral information".Biology & Philosophy.28(2): 283–297.doi:10.1007/s10539-012-9344-0.ISSN1572-8404.PMC3585991.PMID23482918.
  13. ^Palukaitis, Peter; Roossinck, Marilyn J.; Dietzgen, Ralf G.; Francki, Richard I.B. (1992-01-01)."Cucumber MOSAIC Virus".Advances in Virus Research.41:281–348.doi:10.1016/S0065-3527(08)60039-1.ISBN9780120398416.ISSN0065-3527.PMID1575085.
  14. ^Hogenhout, Saskia A.; Redinbaugh, Margaret G.; Ammar, El-Desouky (June 2003)."Plant and animal rhabdovirus host range: a bug's view".Trends in Microbiology.11(6): 264–271.doi:10.1016/s0966-842x(03)00120-3.ISSN0966-842X.PMID12823943.
  15. ^Dyall-Smith, Mike; Palm, Peter; Wanner, Gerhard; Witte, Angela; Oesterhelt, Dieter; Pfeiffer, Friedhelm (March 2019)."Halobacterium salinarum virus ChaoS9, a Novel Halovirus Related to PhiH1 and PhiCh1".Genes.10(3): 194.doi:10.3390/genes10030194.PMC6471424.PMID30832293.
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  25. ^"V-table – the interactive structured virosphere"(PDF).dpublication.com.6 December 2019.Retrieved18 September2021.
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