Tuesday, January 4, 2011
A 4th domain of life?
From a paper in PLoS ONE just before Christmas: Phylogenetic and Phyletic Studies of Informational Genes in Genomes Highlight Existence of a 4th Domain of Life Including Giant Viruses.
The discovery of Mimivirus, with its very large genome content, made it possible to identify genes common to the three domains of life (Eukarya, Bacteria and Archaea) and to generate controversial phylogenomic trees congruent with that of ribosomal genes, branching Mimivirus at its root.
One of the things I like to do with research papers is to see which news organizations pick up the story and how they present it. I was a little surprised to see this one cropping up on Fox News's 'The Science Breakthroughs that 2011 may bring'. It's actually a nice little report. Fair and balanced you might say.
In October, researchers announced the discovery of the world's second giant virus, dubbed CroV. This virus, which infects single-cell marine creatures, is considered enormous due to the size of its genome – approximately 730,000 base pairs, or genetic building blocks, more than double the size of the largest known "normal" virus. Mimivirus, the king of giant viruses so far, has 1.2 million base pairs.
More giant viruses are likely on their way to being found. Mimivirus and CroV are only distant relatives of each other, suggesting that others exist, according to Matthias Fischer who described CroV for his doctoral dissertation at the University of British Columbia. Gunnar Bratbak, of the University of Bergen in Norway, confirmed that his lab has isolated and is studying additional giant viruses.
Viruses, which rely on the machinery of infected cells to reproduce and so aren't considered "alive," are not included in the three domains of life: eukaryotes, prokaryotes and the most ancient, archaea. However, the surprising contents of these giant viruses' genomes endow them with features similar to cells and allow them to play a more active role in replicating themselves.
Research published Dec. 2 in the journal PLoS ONE reconstructs evolutionary relationships between the three domains and viruses, arguing that viruses are entitled to a domain for themselves. Viruses and all other organisms share a common set of genes involved in DNA processing, according to the researchers at Université de la Méditerranée in France.
Bratbak does not believe viruses are "alive" but that they are an important part of life and evolution, and excluding them from the domains is tricky, he told LiveScience in an e-mail.
"Viruses are not only a 'process' that aid evolution by shuffling genes around, but they are also evolving and obeying the same laws of evolution as the other domains," Bratbak wrote. "Thus the more we learn, the harder it gets to define 'domain' without including viruses among them. I am not sure we are there yet."
Previous mimivirus post that has a link to a very readable Discover magazine article on the topic.