A review of: Colson P, Gimenez G, Boyer M, Fournous G, Raoult D (2011) The giant Cafeteria roenbergensis virus that infects a widespread marine phagocytic protist is a new member of the fourth domain of life. PLoS ONE 6(4)
The genome of a new nucleo-cytoplasmic large DNA virus (NCLDV), Cafeteria roenbergensis (CroV) was recently released. A virus of the protist, Cafeteria roenbergensis, a marine flagellate. In this study, a phylogenetic tree of known NCLDVs by comparing four universal NCVOGS (orthologous genes) was constructed and a phyletic analysis of the gene content of CroV in comparison with other NCLDVs and common genes found in bacteria, archaea and eukarya was carried out. The aim; to determine whether CroV supports the ‘fourth domain of life’ hypothesis.
The tree indicated that CroV was most closely related to the Mimiviridae family but appears to branch off much earlier in the clade, effectively forming two sub-families within the family which suggests ancient separation between CroV and Mimivirus.
In the phyletic analysis, 24 CroV proteins were found to be significantly similar to an ORF from the genome of Acanthamoeba castellanii, one of the amoebal hosts of the Mimivirus and phylogenetically very different from Cafeteria roenbergensis. This surprise result implies some gene transfer between the NCLDVs and their hosts and strongly suggests common ancestry between CroV and the other sub-family of the Mimiviridae.
There were 47 NCLDV core genes identified from previous research, of which 5 were found to be common to all NCLDVs and were all found in CroV. There appears to be a core genome within the NCLDVs and this infers a monophyletic origin for this group of giant viruses. As well as this, a number of genes found in the NCLDVs which encode for a set of informational proteins were also found to be present in bacteria, archaea and eukarya indicating a common ancestry. These two pieces of evidence therefore justify the idea of a fourth domain of life consisting of these giant viruses.
The discovery of the Mimivirus in 2003 and other NCLVDs, including CroV, has increased understanding but also challenged the definitions of viruses, particularly in terms of the size of the genome and of the virus itself, fuelling the question over whether viruses are alive or not. This has lead to the debate over whether there is a fourth domain of life and, by extension, whether viruses have had a greater role in the evolution of life than previously thought. Although this study appears to focus heavily on placing CroV in the Mimiviridae family, it does provide some convincing evidence in support of the ‘fourth domain of life’ hypothesis.
3 comments:
Hi Rachelle
Well done on doing this one on your own the other day.
Its really interesting that some of the NCLVD genes are found in all the other domains. Inportant genes too.
I did have a copy of this paper but I have misplaced it somewhere. How big is the CroV virus genome again?
Rachelle - this is a great summary of this major paper discussed on Friday. As well as its general biological significance, it's important to remember that C. roenbergensis is one of th most important predatory protists - grazing on bacteria. Now we now there is a virus that will affect the protist population - further study will probably reveal genetic variation in both host and virus and KTW dynamics as discussed in earlier posts. And just to add a twist, earlier this year Suttle discovered the "virophage" Mavirus, which "infects" CroV. Here's a link http://www.microbemagazine.org/index.php/07-2011-current-topics/3544-second-virophage-found-in-large-marine-dna-virus
Thanks. The genome of the CroV virus is an incredible 730 kb of which there are 544 believed ORFs. According to the authors, this makes CroV the second largest of currently known viruses.
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