Giant virus with a remarkable complement of genes infects marine zooplankton.
The predominant theory about the origin of giant viruses is that they have acquired most of their DNA from horizontal gene transfer with cellular organisms, however little research has actually been done into the genomes of giant viruses.
Fischer et al (2010) look at the DNA and function of CroV, a giant virus which infects the bacterivorous marine flagellate Cafeteria roenbergensis. Only one other giant virus had previously been studied at this detail, and this was a freshwater Mimivirus.
As well as mapping out the genome of CroV the researchers used fluorescent in situ hybridisation to see which genes were expressed during infection which would show which genes were functional.
Although they found that half of CroV’s genes were similar to those previously found in eukaryotes, bacteria, archaea and other giant viruses, this paper shows the importance of studying the genetics of viruses alongside their hosts, as they could not verify that eukaryotic genes found in the virus were from C. roenbergensis due to the lack of knowledge of the host genome.
Fischer et al did find that CroV is far less dependent on host cell components than smaller viruses, and has the ability to make its own tRNA and tRNA modifying enzymes. Most of the genes atypical to viruses were expressed during infection suggesting that these genes were not simply non- functional DNA from horizontal gene transfer with cellular organisms, and this virus undertakes its own processes rather than relying on the host more than other viruses.
CroV also has DNA repair genes thought to be an adaptation to the high radiation surface waters where the host lives. These included DNA repair genes primarily found in bacteria and euryarchaeotes, but thought to have been acquired from another giant virus as it is most similar to those found in the Mimivirus previously studied. Also found were regions of DNA relating to glycoprotein biosynthesis that may be needed to create a protective outer coating to the virus.
Although this paper does not answer the question of whether CroV acquired most of its huge genome from horizontal gene transfer, the impression is given that the author thinks it did not. As much of CroV's DNA is more similar to the Mimivirus previously studied than it's host, the author believes that much of CroV's DNA is viral in origin, existing in an ancestor before the line had contact with eukaryotes.
Even though it does not answer the question directly, this paper may have brought us closer to understanding the origin of viruses and transposons in eukaryotes, as more recent work done on this virus revealed the Mavirus virophage which can only survive in the presence of CroV and showed that the origin of certain types of transposons is most likely viral.
The gaps in this study such as comparisons between the virus and host genomes, the function of repetitive DNA and ubiquitin in viruses, and a more in-depth analysis of the processes occurring during infection, would make interesting further work into this and other giant viruses. Further research on the ecological relationships this giant virus may have with its host and its parasite, and how the population dynamics of one would effect the other two would also be facinating, as this is only the second known example of a virus parasitising another virus.
Fischer M.G., Allen M.J., Wilson W.H., Suttle C.A. (2010) Giant virus with a remarkable complement of genes infects marine zooplankton. Proc. Natl. Acad. Sci. U.S.A. 107(19508)
http://www.pnas.org/content/early/2010/10/15/1007615107
Fischer M.G., Suttle C.A. (2011) A virophage at the origin of large DNA transposons. Science 332(6026):231–234.
Hi Hannah,
ReplyDeleteThis paper got me thinking:
The CroV virus is less dependent on the host due to its genome coding for metabolic processes, which is a rare trait amongst viruses. I wonder if this shows promise for bridging a gap between viruses and other larger microbes, or free-living microbes.
Technically, a virus cannot reproduce and survive without a host, however this virus has already taken up genes that are vital for an independent organism to survive. Horizontal gene transfer will no doubt continue to occur for this linage of virus, which will give the opportunity for more genes that enable independent living to become established as part of the genome permanently. I understand the paper you reviewed suggested the CroV virus did not gain a lot of its DNA from other organisms such as bacteria, supporting the idea that viruses are evolving without the aid of their host.
A separate note on the gene transfer amongst viruses: I recently read that there is a new term emerging “virophage”, which works on the same principle as bacteriophage but the virus infects other viruses as opposed to bacteria. This was mentioned in another paper by Fischer, M. and Suttle, C (2011), who stated that these virophages cannot replicate without the virus host.
The point I am bringing up is more about evolution and speciation, with the idea being that organisms is constantly evolving and developing in fill new niches as they open up (Bilton, 2012). Indeed they could come a point when the specimens currently classified as ‘a virus’ becomes able to survive outside a host, thus becoming a free-living organism, at which point the virophage will replace the role of the virus.
It would be interesting to carry out further research into the possibility of a virus crossing over classification into an independent organism, however since evolution is an extremely slow process, it is unlikely that we will see a full change over occur in our lifetimes.
Bilton, D. (2012) Lecture 1 BIO3109 Speciation and Diversity, University of Plymouth Unpublished
Fischer M.G., Suttle C.A. (2011) A virophage at the origin of large DNA transposons. Science 332(6026):231–234.