Many of the viruses listed in probably have a common evolutionary ancestor, perhaps arising before the divergence of the major eukaryotic kingdoms (26
). These viruses, which include the phycodnaviruses, poxviruses, asfarviruses, iridoviruses, ascoviruses, and the Mimiviruses, are referred to as nucleocytoplasmic large dsDNA viruses (NCLDVs) (25
). Recently, another large NCLDV, named Marseillevirus, that is distantly related to the iridoviruses and ascoviruses was isolated from an amoeba (6
). NCLDVs contain 9 common genes, and 177 additional genes are present in at least two of the virus families (71
Although the hypothesis of a common ancestor for the NCLDVs is generally accepted, there is disagreement on the size and morphology of its ancestral virus and how it diverged into the different virus families. A recent maximum-likelihood reconstruction of NCLDV evolution produced a set of 47 conserved genes, which are considered the minimum genome for the common ancestor; NCLDVs then evolved by losing some of these common genes and acquiring new genes from their hosts and bacterial endosymbionts as well as by gene duplications (71
). Another scenario suggests the ancestral NCLDV was a huge virus or even a cellular organism that evolved primarily via genome contraction (51
). Finally, Filee et al. (15
) proposed that NCLDVs evolved from a small DNA virus by gene acquisition from cells.
The origin of NCLDVs is controversial. For example, some researchers have suggested that NCLDVs should be considered the fourth kingdom of life (12
), others have suggested that NCLDV genes arose from the original gene pool that led to prokaryotes and eukaryotes (26
), and still others have suggested that horizontal gene transfer has driven the evolution of their genomes (39
). These suggestions have stimulated controversy over whether the tree of life should include these viruses.1
Another interesting hypothesis is that primitive NCLDVs gave rise to the eukaryotic nucleus or vice versa (5
). Regardless, some viruses, including the NCLDVs, have a long evolutionary history, and viruses probably contributed to the emergence and subsequent structure of modern cellular life forms (29
It is becoming difficult to classify some of these large viruses into distinct families. Recent phylogenetic analysis of the DNA polymerase protein from four putative phycodnaviruses illustrates this problem. (a
) The DNA polymerase from three phycodnaviruses, CeV01, PpV01, and PoV01 (), is more similar to Mimivirus than to the other phycodnaviruses (41
) The HcDNAV polymerase indicates its closest relative is African swine fever virus (50
). Therefore, it is clear that giruses, like the DNA phages (23
), have exchanged genes for eons.
Contributing to the uncertainty about NCLDV evolution is the discovery that the structure of the PBCV-1 major capsid protein (MCP) resembles MCPs from other smaller dsDNA viruses with hosts in all three domains of life, including human adenoviruses, bacteriophage PRD1, and a virus infecting an archaeon, Sulfolobus solfataricus
. This similarity suggests that these three viruses might have a common evolutionary ancestor with the NCLDVs, despite the lack of amino acid sequence similarity among their MCPs (32
All NCLDVs are assembled in virus factories located in the cytoplasm. The role of the nucleus in the replication of NCLDVs varies. For example, poxviruses (43
) and Mimivirus (46
) carry out their entire life cycle in the cytoplasm. In contrast, the nucleus is probably essential for replication of the phycodnaviruses and other NCLDVs. However, the nuclear role in virus replication probably differs, even among the phycodnaviruses.
The four viruses in that are not NCLDVs are a polydnavirus, WSSV, and two bacteriophages, PhageG and 201
2-1. WSSV, which causes huge economic losses to the shrimp industry, is an enigma because it is not obviously related to known viruses. Large bacteriophages, referred to as jumbo phages, resemble smaller phages that may have acquired increased genome functions over evolutionary time.