Biology:Giant virus

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Short description: Very large DNA virus
"Girus" redirects here. It is not to be confused with Gyrus.

File:Comparison of the size of giant viruses to a common virus (HIV) and bacteria (E. coli).tif A giant virus, sometimes referred to as a girus, is a very large virus, some of which are larger than typical bacteria.[1][2] All known giant viruses belong to the phylum Nucleocytoviricota.[3]

Description

While the exact criteria used in the scientific literature varies, giant viruses are usually defined as those with virions larger than 200-300 nanometers in their smallest dimension (making them visible by light microscopy[4]) and a genome size greater than 200-300 kb pairs.[5][6][7][8] All known giant viruses belong to the Nucleocytoviricota, the nucleocytoplasmic large DNA viruses, meaning they are known to replicate in both the host cell nucleus and cytoplasm, and have large, double-stranded DNA genomes.[3][9] Although all giant viruses belong to this phylum, they are not a monophyletic group,[10] as some members of the Nucleocytoviricota, such as the genus Prasinovirus, are not giant viruses.

The large genomes of many giant viruses encode unusual genes that are not found in other viruses, including genes involved in glycolysis and the TCA cycle,[11] fermentation,[12] and the cytoskeleton.[13][14][15] Giant viruses from the deep ocean, terrestrial sources, and human patients contain genes encoding cytochrome P450 (CYP; P450) enzymes. The origin of these P450 genes in giant viruses remains unknown but may have been acquired from an ancient host.[16]

Giant viruses infect a large variety of eukaryotic hosts, such as amoebae, algae, lepidopterans, and humans, and are widely distributed in the natural environment.[17] Giant viruses may also be further infected by virophages, the first of which was discovered in the co-infecting Acanthamoeba castellanii mamavirus (ACMV).

Cryo-EM images of the giant viruses CroV and APMV. (A) Cryo-electron micrograph of four CroV particles. (B) Single CroV particle with concave core depression (white arrow). (C) Single APMV particle. Scale bars in (A–C) represent 2,000 Å.
Phylogeny of Nucleocytoviricota [18]

History

The term 'giant virus' was coined in the late 1990s to describe chloroviruses of the family Phycodnaviridae, which were found to have unusually large genomes.[17] The first of these, Paramecium bursaria chlorella virus 1, was discovered in 1981 by Russel H. Meints, James L. Van Etten, Daniel Kuczmarski, Kit Lee, and Barbara Ang, and was initially called HVCV (Hydra viridis Chlorella virus) since it was first found to infect Chlorella-like algae living inside Hydra viridissima.[19][20]

The term was further cemented in scientific literature after the characterization of Acanthamoeba polyphaga Mimivirus (APMV) in 2003 and its genetic sequencing in 2004 that revealed its genome was 1.2 million base pairs long, larger than the sequenced genomes of many bacteria.[17][21][22] It was initially discovered in 1992 infecting Acanthamoeba polyphaga and mistakenly identified as a gram-positive bacteria.[23] The emerging concept of the 'giant' virus led to the term 'girus' being coined to refer to the group in 2006.[24]

In 2008, the first virophage, Sputnik, was discovered parasitizing the Acanthamoeba polyphaga mimivirus (APMV), beginning a series of discoveries on giant virus parasitism. In 2011, Mavirus, which parasitizes the Cafeteria roenbergensis virus (CroV), was the first virophage found that was shown to be related to the large Maverick transposons or politons that are found in some eukaryotic cells.[25] In 2013, mobile genetic elements with segments homologous to genes in virophages, called transpovirons, were found in the genomes of several Mimivirus strains.[26]

Metagenomic samples of ocean water in the 2010s revealed that giant viruses are ubiquitous in the marine environment,[27] where they have been found to infect marine microzooplankton. The first of these was described in 1995 as BV-PW1, found in seawater samples collected from 1989 to 1991,[28] although its host was misidentified as belonging to the genus Bodo. The host was later correctly identified as being in the species Cafeteria roenbergensis in 2010, and the virus was renamed Cafeteria roenbergensis virus (CroV).[29] Several years later, in 2018, the Bodo saltans virus with a genome size of ~1.4 Mb that infected an isolate of Bodo saltans was cultured from a freshwater pond, and related viruses were found to be widespread in marine waters.[27]

Further notable discoveries include the largest known viral genome (~2.5 Mbp) of Pandoravirus salinus in 2013, the recovery of Pithovirus sibericum in a 30,000-year-old ice core in 2014, and the identification of the virus with the longest known virion, Megaklothovirus horridgei, in 2018.

Genetics and evolution

The genomes of giant viruses are the largest known for viruses, and contain genes that encode for important elements of translation machinery, a characteristic that had previously been believed to be indicative of cellular organisms. These genes include multiple genes encoding a number of aminoacyl tRNA synthetases, enzymes that catalyze the esterification of specific amino acids or their precursors to their corresponding cognate tRNAs to form an aminoacyl tRNA that is then used during translation.[9] The presence of four aminoacyl tRNA synthetase encoding genes in mimivirus and mamavirus genomes, both species within the Mimiviridae family, as well as the discovery of seven aminoacyl tRNA synthetase genes in the megavirus genome (including those in Mimiviridae) provide evidence that these large DNA viruses may have evolved from a shared cellular genome ancestor by means of genome reduction.[9]

The discovery and subsequent characterization of giant viruses has triggered debate on their evolutionary origins. The two main hypotheses are that they evolved from small viruses by picking up DNA from host organisms; or that they evolved from very complicated organisms via genome reduction, losing various functions including self-reproduction.[30] The possible complicated ancestral organism is also a topic of debate: by one proposal, it might represent a fourth domain of life,[9] but this has been largely discounted.[31][32][33] A molecular dating study shows that the divergence time of the last common ancestor of the giant viruses was within the last billion years, which is much younger than the origins of the known domains of life.[34]

Comparison of largest known giant viruses

Largest giant viruses with complete sequenced genomes as of June 2018
Giant virus name Genome Length Genes Capsid diameter (nm) Hair cover Genbank #
Pandoravirus salinus[35] 2,473,870 2500 proteins (predicted) ~500 KC977571
Tupanvirus[36] 1,500,000 1276–1425 proteins ≥450+550[37] KY523104
MF405918[38]
Bodo saltans virus[39] 1,385,869 1227 proteins (predicted) ~300 yes (~40 nm) MF782455
Megavirus chilense[40] 1,259,197 1120 proteins (predicted) 440 yes (75 nm) JN258408
Mamavirus[41] 1,191,693 1023 proteins (predicted) 500 yes (120 nm) JF801956
Mimivirus[22][42] 1,181,549 979 proteins 39 non-coding 500 yes (120 nm) NC_014649
M4[43] (Mimivirus "bald" variant) 981,813 756 proteins (predicted) 390 No JN036606
Cafeteria roenbergensis virus[44] 617,453 (730 kb) 544 proteins (predicted) 300 No NC_014637

The whole list is in the Giant Virus Toplist created by the Giant Virus Finder software.[45] As of June 11, 2018, there were 183 listed.[46]

Specific common features among giant viruses
Giant virus name Aminoacyl-tRNA synthetase Octocoral-like 1MutS 2Stargate[47] Known virophage[25] Cytoplasmic virion factory Host
Megavirus chilensis 7 (Tyr, Arg, Met, Cys, Trp, Asn, Ile) yes yes no yes Acanthamoeba (Unikonta, Amoebozoa)
Mamavirus 4 (Tyr, Arg, Met, Cys) yes yes yes yes Acanthamoeba (Unikonta, Amoebozoa)
Mimivirus 4 (Tyr, Arg, Met, Cys) yes yes yes yes Acanthamoeba (Unikonta, Amoebozoa)
M4 (Mimivirus "bald" variant) 3 (Met, Cys, Arg) yes yes Resistant yes Acanthamoeba (Unikonta, Amoebozoa)
Cafeteria roenbergensis virus 1 (Ile) yes no yes yes Phagotrophic protozoan (Heterokonta, Stramenopiles)

1Mutator S (MutS) and its homologs are a family of DNA mismatch repair proteins involved in the mismatch repair system that acts to correct point mutations or small insertion/deletion loops produced during DNA replication, increasing the fidelity of replication. 2A stargate is a five-pronged star structure present on the viral capsid forming the portal through which the internal core of the particle is delivered to the host's cytoplasm.

See also

References

  1. ↑ Reynolds, Kelly A. (2010). "Mysterious Microbe in Water Challenges the Very Definition of a Virus". Water Conditioning & Purification. http://www.wcponline.com/pdf/June_OnTap.pdf. 
  2. ↑ "Remarkable sequence similarity between the dinoflagellate-infecting marine girus and the terrestrial pathogen African swine fever virus". Virology Journal 6 (178). October 2009. doi:10.1186/1743-422X-6-178. PMID 19860921. 
  3. ↑ 3.0 3.1 Van Etten, James L. (July–August 2011). "Giant Viruses". American Scientist 99 (4): 304–311. doi:10.1511/2011.91.304. http://www.americanscientist.org/issues/feature/2011/4/giant-viruses. 
  4. ↑ Ian M. Watt (1997). The Principles and Practice of Electron Microscopy. Cambridge University Press. p. 6. ISBN 978-0-521-43591-8. https://books.google.com/books?id=Y6-sE4gUX-QC&pg=PA6. 
  5. ↑ Van Etten, James L. (July 2011). "Giant Viruses". American Scientist 99 (4): 304. doi:10.1511/2011.91.304. https://www.americanscientist.org/article/giant-viruses. Retrieved 19 February 2026. 
  6. ↑ Abergel, Chantal; Legendre, Matthieu; Claverie, Jean-Michel (November 2015). "The rapidly expanding universe of giant viruses: Mimivirus, Pandoravirus, Pithovirus and Mollivirus". FEMS Microbiology Reviews 39 (6): 779–796. doi:10.1093/femsre/fuv037. https://academic.oup.com/femsre/article/39/6/779/550971. Retrieved 19 February 2026. 
  7. ↑ "Cryo-EM reconstruction of the Cafeteria roenbergensis virus capsid suggests novel assembly pathway for giant viruses". Scientific Reports 7 (5484). 14 July 2017. doi:10.1038/s41598-017-05824-w. PMID 28710447. Bibcode: 2017NatSR...7.5484X. 
  8. ↑ Wilhelm, SW; Bird, JT; Bonifer, KS; Calfee, BC; Chen, T; Coy, SR; Gainer, PJ; Gann, ER et al. (17 March 2017). "A Student's Guide to Giant Viruses Infecting Small Eukaryotes: From Acanthamoeba to Zooxanthellae.". Viruses 9 (3): 46. doi:10.3390/v9030046. PMID 28304329. 
  9. ↑ 9.0 9.1 9.2 9.3 "Genomics of Megavirus and the elusive fourth domain of Life". Communicative & Integrative Biology 5 (1): 102–6. January 2012. doi:10.4161/cib.18624. PMID 22482024. 
  10. ↑ Shinn, GL; Bullard, BL (19 September 2018). "Ultrastructure of Meelsvirus: A nuclear virus of arrow worms (phylum Chaetognatha) producing giant "tailed" virions". PLOS ONE 13 (9). doi:10.1371/journal.pone.0203282. PMID 30231047. Bibcode: 2018PLoSO..1303282S. 
  11. ↑ "Dynamic genome evolution and complex virocell metabolism of globally-distributed giant viruses". Nature Communications 11 (1710). 2020. doi:10.1038/s41467-020-15507-2. PMID 32249765. Bibcode: 2020NatCo..11.1710M. 
  12. ↑ "A giant virus infecting green algae encodes key fermentation genes". Virology 518: 423–433. 2018. doi:10.1016/j.virol.2018.03.010. PMID 29649682. 
  13. ↑ Da Cunha V, Gaia M, Ogata H, Jaillon O, Delmont TO, Patrick Forterre P (2020). "Giant viruses encode novel types of actins possibly related to the origin of eukaryotic actin: the viractins". bioRxiv 10.1101/2020.06.16.150565.
  14. ↑ "High Transcriptional Activity and Diverse Functional Repertoires of Hundreds of Giant Viruses in a Coastal Marine System". mSystems 6 (4). 2021. doi:10.1128/mSystems.00293-21. PMID 34254826. 
  15. ↑ "Discovery of Viral Myosin Genes With Complex Evolutionary History Within Plankton". Frontiers in Microbiology 12. 7 June 2021. doi:10.3389/fmicb.2021.683294. PMID 34163457. 
  16. ↑ "On the occurrence of cytochrome P450 in viruses". Proceedings of the National Academy of Sciences of the United States of America 116 (25): 12343–12352. June 2019. doi:10.1073/pnas.1901080116. PMID 31167942. Bibcode: 2019PNAS..11612343L. 
  17. ↑ 17.0 17.1 17.2 Schulz, F; Abergel, C; Woyke, T (28 July 2022). "Giant virus biology and diversity in the era of genome-resolved metagenomics". Nat Rev Microbiol 20 (12): 721–736. doi:10.1038/s41579-022-00754-5. PMID 35902763. https://www.nature.com/articles/s41579-022-00754-5. Retrieved 13 February 2026. 
  18. ↑ "A phylogenomic framework for charting the diversity and evolution of giant viruses". PLOS Biology 19 (10). 2021. doi:10.1371/journal.pbio.3001430. PMID 34705818. 
  19. ↑ Meints, Russel H.; Van Etten, James L.; Kuczmarski, Daniel; Lee, Kit; Ang, Barbara (September 1981). "Viral infection of the symbiotic chlorella-like alga present in Hydra viridis". Virology 113 (2): 698–703. doi:10.1016/0042-6822(81)90198-7. PMID 18635088. 
  20. ↑ Hoshina, Ryo; Shimizu, Mayumi; Makino, Yoichi; Haruyama, Yoshihiro; Ueda, Shin-ichiro; Kato, Yutaka; Kasahara, Masahiro; Ono, Bun-ichiro et al. (13 September 2010). "Isolation and characterization of a virus (CvV-BW1) that infects symbiotic algae of Paramecium bursaria in Lake Biwa, Japan" (in en). Virology Journal 7. doi:10.1186/1743-422X-7-222. ISSN 1743-422X. PMID 20831832. 
  21. ↑ "A giant virus in amoebae". Science 299 (5615): 2033. 2003. doi:10.1126/science.1081867. PMID 12663918. 
  22. ↑ 22.0 22.1 "The 1.2-megabase genome sequence of Mimivirus". Science 306 (5700): 1344–50. November 2004. doi:10.1126/science.1101485. PMID 15486256. Bibcode: 2004Sci...306.1344R. 
  23. ↑ "Giant Viruses" (in en). 2017-02-06. https://www.americanscientist.org/article/giant-viruses. 
  24. ↑ Claverie, Jean-Michel; Ogata, Hiroyuki; Audic, Stéphane; Abergel, Chantal; Suhre, Karsten; Fournier, Pierre-Edouard (April 2006). "Mimivirus and the emerging concept of "giant" virus". Virus Research 117 (1): 133–144. doi:10.1016/j.virusres.2006.01.008. PMID 16469402. https://arxiv.org/ftp/q-bio/papers/0506/0506007.pdf. 
  25. ↑ 25.0 25.1 "A virophage at the origin of large DNA transposons". Science 332 (6026): 231–4. April 2011. doi:10.1126/science.1199412. PMID 21385722. Bibcode: 2011Sci...332..231F. 
  26. ↑ Desnues, C; La Scola, B; Yutin, N; Fournous, G; Robert, C; Azza, S; Jardot, P; Monteil, S et al. (2012). "Provirophages and transpovirons as the diverse mobilome of giant viruses". Proc Natl Acad Sci USA 109 (44): 18078–83. doi:10.1073/pnas.1208835109. PMID 23071316. Bibcode: 2012PNAS..10918078D. 
  27. ↑ 27.0 27.1 Deeg, C.M.; Chow, E.C.T.; Suttle, C.A. (2018). "The kinetoplastid-infecting Bodo saltans virus (BsV), a window into the most abundant giant viruses in the sea". eLife 7. doi:10.7554/eLife.33014. PMID 29582753. 
  28. ↑ D.R. Garza; C.A. Suttle (1995). "Large double-stranded DNA viruses which cause the lysis of a marine heterotrophic nanoflagellate (Bodo sp.) occur in natural marine viral communities". Aquatic Microbial Ecology 9 (3): 203–210. doi:10.3354/ame009203. 
  29. ↑ Fischer, M. G.; Allen, M. J.; Wilson, W. H.; Suttle, C. A. (2010). "Giant virus with a remarkable complement of genes infects marine zooplankton". Proceedings of the National Academy of Sciences 107 (45): 19508–13. doi:10.1073/pnas.1007615107. PMID 20974979. PMC 2984142. Bibcode: 2010PNAS..10719508F. http://pubman.mpdl.mpg.de/pubman/item/escidoc:2272841/component/escidoc:2272838/PNAS_107_2010_19508.pdf. 
  30. ↑ Bichell, Rae Ellen. "In Giant Virus Genes, Hints About Their Mysterious Origin". All Things Considered. https://www.npr.org/sections/health-shots/2017/04/06/522478901/in-giant-virus-genes-hints-about-their-mysterious-origin. 
  31. ↑ "Giant viruses with an expanded complement of translation system components". Science 356 (6333): 82–85. April 2017. doi:10.1126/science.aal4657. PMID 28386012. Bibcode: 2017Sci...356...82S. https://escholarship.org/content/qt0kf9t6gn/qt0kf9t6gn.pdf?t=oruwia. 
  32. ↑ "Virus Genomes from Deep Sea Sediments Expand the Ocean Megavirome and Support Independent Origins of Viral Gigantism". mBio 10 (2): e02497-02418. March 2019. doi:10.1128/mBio.02497-18. PMID 30837339. 
  33. ↑ "Origin of giant viruses from smaller DNA viruses not from a fourth domain of cellular life". Virology 466–467 (2014): 38–52. 2014. doi:10.1016/j.virol.2014.06.032. PMID 25042053. 
  34. ↑ "Host-calibrated time tree caps the age of giant viruses". Molecular Biology and Evolution 42 (2). 2025. doi:10.1093/molbev/msaf033. PMID 39976376. PMC 11840718. https://academic.oup.com/mbe/article/42/2/msaf033/8026316. 
  35. ↑ Pereira Andrade, Ana Cláudia dos Santos; Victor de Miranda Boratto, Paulo; Rodrigues, Rodrigo Araújo Lima; Bastos, Talita Machado; Azevedo, Bruna Luiza; Dornas, Fábio Pio; Oliveira, Danilo Bretas; Drumond, Betânia Paiva et al. (2019-02-19). "New Isolates of Pandoraviruses: Contribution to the Study of Replication Cycle Steps". Journal of Virology 93 (5). doi:10.1128/jvi.01942-18. PMID 30541841. 
  36. ↑ "Tailed giant Tupanvirus possesses the most complete translational apparatus of the known virosphere". Nature Communications 9 (1). February 2018. doi:10.1038/s41467-018-03168-1. PMID 29487281. Bibcode: 2018NatCo...9..749A. 
  37. ↑ head and tail, respectively
  38. ↑ soda lake and deep ocean species of Tupanvirues, respectively
  39. ↑ "The kinetoplastid-infecting Bodo saltans virus (BsV), a window into the most abundant giant viruses in the sea". eLife 7. March 2018. doi:10.7554/eLife.33014. PMID 29582753. 
  40. ↑ "Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae". Proceedings of the National Academy of Sciences of the United States of America 108 (42): 17486–91. October 2011. doi:10.1073/pnas.1110889108. PMID 21987820. Bibcode: 2011PNAS..10817486A. 
  41. ↑ "Viruses with more than 1,000 genes: Mamavirus, a new Acanthamoeba polyphaga mimivirus strain, and reannotation of Mimivirus genes". Genome Biology and Evolution 3: 737–42. 2011. doi:10.1093/gbe/evr048. PMID 21705471. 
  42. ↑ "Breaking the 1000-gene barrier for Mimivirus using ultra-deep genome and transcriptome sequencing". Virology Journal 8 (1). March 2011. doi:10.1186/1743-422X-8-99. PMID 21375749. 
  43. ↑ Boyer, M.; Azza, S.; Barrassi, L.; Klose, T.; Campocasso, A.; Pagnier, I.; Fournous, G.; Borg, A. et al. (2011). "Mimivirus shows dramatic genome reduction after intraamoebal culture". Proceedings of the National Academy of Sciences 108 (25): 10296–301. doi:10.1073/pnas.1101118108. PMID 21646533. Bibcode: 2011PNAS..10810296B. 
  44. ↑ Fischer, M. G.; Allen, M. J.; Wilson, W. H.; Suttle, C. A. (2010). "Giant virus with a remarkable complement of genes infects marine zooplankton". Proceedings of the National Academy of Sciences 107 (45): 19508–13. doi:10.1073/pnas.1007615107. PMID 20974979. PMC 2984142. Bibcode: 2010PNAS..10719508F. http://pubman.mpdl.mpg.de/pubman/item/escidoc:2272841/component/escidoc:2272838/PNAS_107_2010_19508.pdf. 
  45. ↑ "Giant Virus Toplist". PIT Bioinformatics Group, Department of Computer Science. Eötvös University. 2015-03-26. http://pitgroup.org/giant-virus-toplist/. 
  46. ↑ "Giant Virus Toplist". PIT Bioinformatics Group. 26 March 2015. https://pitgroup.org/giant-virus-toplist/. 
  47. ↑ Sugden, Bill, ed (May 2008). "Distinct DNA exit and packaging portals in the virus Acanthamoeba polyphaga mimivirus". PLOS Biology 6 (5): e114. doi:10.1371/journal.pbio.0060114. PMID 18479185. 



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