![]() As more Phycodnaviridae genomes are sequenced, there is likely to be an explosion of exciting gene discoveries with novel functions. 2006c) supports the idea that the Chlorovirus, Coccolithovirus and Phaeovirus genera diverged a long time ago. The finding that there are only 14 genes in common, from a pool of approximately 1000 genes, between three genomes from different genera of the Phycodnaviridae ( Allen et al. Phylogeny of the NCLDVs constructed by cladistic analysis indicates that the major families may have diverged prior to the divergence of the major eukaryotic lineages 2-3 billion years ago ( Iyer et al. The grouping of the NCLDVs is significant for two reasons: (i) as the name suggests, it implies a likely propagation mechanism for members of the Phycodnaviridae where replication is probably initiated in the nucleus and is completed in the cytoplasm (ii) the NCLDVs are proposed to have an ancient evolutionary lineage (e.g., Villarreal and DeFilippis 2000 lyer et al. Probably, the virus family Ascoviridae should also be included in this grouping because these viruses have clearly evolved from the Iridoviruses (see the chapter by V.G. ![]() There are five families in the NCLDV clade that include Poxviridae, Iridoviridae, Asfarviridae, Phycodnaviridae and Mimiviridae. 2006) and evolutionary analysis of their genomes places them within a major, monophyletic assemblage of large eukaryotic dsDNA viruses termed the Nucleo-Cytoplasmic Large DNA Viruses (NCLDVs) ( Iyer et al. Complete genome sequences have been obtained from representatives of the Chlorovirus, Coccolithovirus and Phaeovirus genera ( Dunigan et al. It is likely the Phycodnaviridae will eventually be split into several subfamilies and numerous genera as more isolates are characterized and the current nomenclature will probably become obsolete. However, phylogenetic analysis of the DNA polymerase gene from these viruses indicated they belong to a new genus ( Schroeder et al. Nomenclature anomalies have already crept into the system with the formation of the genus Coccolithovirus, a group of viruses that infect Emiliania huxleyi (an alga species in the class Prymnesiophyceae), which logically should have been classified within the genus Prymnesiovirus. Members of the Phycodnaviridae are currently grouped into six genera (named after the hosts they infect): Chlorovirus, Coccolithovirus, Prasinovirus, Prymnesiovirus, Phaeovirus and Raphidovirus ( Wilson et al. 2006), yet morphologically similar, family of large icosahedral viruses that infect marine or freshwater eukaryotic algae with dsDNA genomes ranging from 160 to 560 kb ( Van Etten et al. The Phycodnaviridae comprise a genetically diverse ( Dunigan et al. 2005b) and at least 100 others mentioned in the literature, it is certain that most phycodnaviruses, containing an almost infinite reservoir of genetic diversity, remain to be discovered. With only approximately 150 formal identifications ( Wilson et al. Given the number of potential hosts, it is incredible so few phycodnaviruses have been isolated to date. A literal translation of Phycodnaviridae is, DNA viruses that infect algae. The Phycodnaviridae consists of a family of viruses that infect these globally important players. Conservative estimates suggest there is somewhere between 100,000 to several million species of algae and that only approximately 40,000 have been identified. Phytoplankton (microalgae) form the base of the marine food web and their photosynthetic activities provide an important carbon sink that influences the global carbon cycle and even climate ( Charlson et al. ![]() Absorption of heat energy by the ocean and light energy by tiny floating marine plants, known as phytoplankton, are the engines that help regulate many aspects of the global environment. The illuminated region is only a small part of the 3.7-km mean depth of the ocean, yet it houses several of the great engines of planetary control ( Tett 1990).
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