By the time of discovery of the Archaea there were known two viruses from the euryarchaeal genus Halobacterium. Both had double-stranded (ds) DNA genomes and resembled morphologically head-tail bacteriophages. Finding of other euryarchaeal viruses of the same type (for review, see [1]) gave the impression that dsDNA viruses of the Archaea are just a variety of ubiquitous head-tailed bacteriophages. This notion was supported by studies on molecular organisation of the halovirus øH [1]. Thus, isolation by Wolfram Zillig and colleagues of several dsDNA viruses of the Crenarchaea with unusual morphology was unexpected [2]. Even more unexpected were subsequent results from different laboratories leading to the perception that dsDNA viruses with diverse extraordinary morphotypes are not just an oddity but reflect a general picture of diversity of archaeal viruses (for review, see [3]). Not only the morphology, but also the genomes of these viruses are exceptional, and the vast majority of the genes are without detectable homologs in other viruses or cellular life forms (for review, see [4]).
Despite a modest number of isolated species, morphological diversity of known dsDNA viruses of the Archaea notably surpasses that of numerous known species of dsDNA viruses of Bacteria or Eukarya. Recent observations raise the possibility that occurrence among archaeal viruses of typical bacterial head-tailed phages resulted from cross-domain spreading and lead to a hypothesis that each of the three domains of life could originally be characterized by a unique set of associated dsDNA viruses [3].
References
1. Zillig, W., W.-D. Reiter, P. Palm, F. Gropp, H. Neumann, and M. Rattenberger 1988. in The Bacteriophages (R. Calender, ed.) pp.517-558, Plenum Press, New York.
2. Zillig, W., D. Prangishvili, C. Schleper, M. Elferink, I. Holz, S. Albers, D. Janekovic, and D. Goetz.1996. FEMS Microbiol. Rev. 18, 225-236.
3. Prangishvili, D., P. Forterre, and R. A. Garrett. 2006. Nat. Rev. Microbiol. 4, 837-848.
4. Prangishvili, D., R. A. Garrett, and E. V. Koonin. 2006. Virus Res., 117, 52-67.
