although global gene content and detailed sequence similarity comparisons with the previously described megaviridae members clearly classify pgv within the family ( including the partially sequenced organic lake phycodnaviruses ( olpv ) , chrysochromulina erecina virus , and phaoecystis pouchetti virus ) , the finding of a pgv - associated virophage came as a surprise . complete virophage genomes have now been described , but only 3 correspond to identified and isolated the pgv virophage ( pgvv ) is the third , and presumably not the last , of a rapidly growing series . except for their genome size in the 20 kb range , these new types of satellites viruses have little in common in terms of gene content , although they all code for a major capsid protein and one dna primase . consistent with the fact that it was never observed in pgv - infected cultures , pgvv is the first example of a virophage lacking the information to make a capsid protein , the essential building block of a virus particle . once multiplied in the pgv virus factory , the pgvv genome is thus packaged ( in multiple copies ) alongside the pgv genome , and propagated through the pgv virion , either as an integrated or free viral  plasmid .  the precise molecular structure(s ) the finding of a virophage associated to pgv is already teaching us the important lesson that virophages are not solely associated to dna viruses with micron - sized particles and 1 mb genome sizes , but can be found with large dna viruses of more reasonable ( poxvirus - like ? ) proportion . it is thus likely that many of such associations have been overlooked in the past , and that virophages might have played a fundamental role in the evolution of many more viruses than just the megaviridae . if this is true , they could be ( and have been ) the main vehicle of gene transfers between eukaryotic viruses , and indirectly between these viruses and their hosts . they may also be responsible for the sporadic occurrences of mobiles elements such as self - splicing introns , inteins , and transpovirons . another lesson is that , like all parasites , the virophages are submitted to the irreversible phenomenon of reductive evolution , condemning them to disappear as individual biological entities , eventually saving some of their genes by integrating them into the genome of their companion virus , themselves undergoing a similar process vis -  - vis their cellular host . the first published phylogenetic tree including mimivirus 2 and using a concatenation of 7 universally conserved protein sequences , already pointed out that :  we could now build a tentative tree of life , within which mimivirus appears to define a new branch distinct from the three other domains .  elsewhere in the same article , mimivirus was also shown to be part of the broad family of the nucleocytoplasmic large dna viruses ( ncldv ) , branching near the middle of the previously defined iridovirus , phycodnaviruses , poxviruses , and asfarviruses lineages . put together , these 2 results suggested in a subliminal way that all the large dna viruses were in fact defining a domain distinct from the 3 established cellular domains . as additional genomes of megaviridae became available , molecular phylogenies computed with an increasing number of universal proteins associated to basic functions ( dna clamp loaders , ribonucleotide reductases , aminoacyl - trna synthetases , dna polymerases ) kept clustering the megaviruses in their own clade , clearly separate from the cellular domains , thus contributing an additional domain rooted in between archaea and eukarya in the tree of life . despite receiving increasing support from various authors , others remain strongly opposed to this view . in agreement with the notion of a fourth domain of life anchored by the largest dna viruses , the dna polymerase of pgv nicely clusters with the other megaviridae homologs , exhibiting no affinity with any of the main cellular lineages ( fig .  1 ) . based on the presence of a vestigial protein translation system , i previously argued that the genome of today 's giant viruses derived from an ancestral cellular organism through the irreversible process of reductive evolution experienced by all parasites . dna viruses exhibiting a whole range of genomic complexity could have been generated through this continuous process : viruses would have lost translation first ( the ribosome ) , then transcription ( the rna polymerase ) , then dna replication ( the dna polymerase ) , following an evolutionary scenario whereby they become increasingly dependent from their host . the vast range of size and complexity among today 's dna viruses might thus be the results of differences in their evolution rates ( the largest ones experiencing the least evolutionary pressure ) . in this context , the lack of affinity of the giant viruses with any of today 's cellular domains , as well as the huge proportion of the viral genes without cellular homolog , would suggest that the lineage of the ancestral cellular organism that gave rise to giant viruses became extinct as a cellular life form , and only managed to survive as a parasitic  fourth domain . the recently described giant pandoravirus might represent an independent instance of the same scenario . the tree was produced using the default option on the mafft server ( url : mafft.cbrc.jp ) from the multiple alignment of 25 dna polymerase b sequences ( 510 ungapped positions , excluding the inteins ) . branches with bootstrap values < 80 are collapsed . despite infecting eukaryotic hosts from vastly divergent phyla , the viruses do not show any phylogenetic affinity with a specific eukaryotic group , and cluster ( in red ) separately from the 3 cellular domains : eukarya ( green ) , archaea ( purple ) , and eubacteria ( blue ) . this strongly supported topology suggests that the common ancestor of the megaviridaelargely predated the radiation of the eukaryotes .
the viral nature of the first  giant virus ,  mimivirus , was realized in 2003 , 10 y after its initial isolation from the water of a cooling tower in bradford , uk . 
 soon after its genome was sequenced , the mining of the global ocean sampling environmental sequence database revealed that the closest relatives of mimivirus , only known to infect acanthamoeba , were to be found in the sea . 
 these predicted marine mimivirus relatives remained elusive until 2010 , with the first genomic characterization of a virus infecting a heterotrophic unicellular eukaryote , the microflagellate grazer cafeteria roenbergensis . 
 the genome analysis of a virus ( pgv ) infecting the common unicellular algae phaeocystis globosa now shows that it is a bona fide member of the mimivirus family ( i.e. , the megaviridae ) , extending the realm of these giant viruses to abundant blooming phytoplankton species . despite its smaller genome size ( 460 kb encoding 434 proteins ) 
 , pgv exhibits the most intriguing feature of the previously characterized megaviridae : an associated virophage . 
 however , the 19-kb virophage genome , devoid of a capsid gene , is packaged in the pgv particle and propagated as a  viral plasmid ,  the first ever described . 
 the pgv genome also exhibits the duplication of  core genes ,  normally present as single copies and a putative new type of mobile element . 
 in a dna polymerase phylogeny including representatives of the three cellular domains , pgv and the other megaviridae cluster into their own clade deeply branching between domains archaea and eukarya domains , thus exhibiting the topology of a fourth domain in the tree of life .