but to date only about 50 viruses have been reported that
infect this large and diverse group of organisms (Prangishvili
et al. 2006). Most archaeal viruses have been isolated from
either extreme thermoacidophiles or extreme halophiles
(Prangishvili et al. 2006; Porter et al. 2007). This work was
pioneered by the late Wolfram Zillig, but was not systematically
addressed until the work of Prangishvili and Dyall-
Smith, respectively. Early virus isolates of extreme halophiles
(haloviruses) were of the head-and-tail type, the same morphology
observed in more than 90% of described bacteriophages;
more recent isolates have included representatives of
spindle-shaped and spherical morphotypes (Porter et al.
2007; Pietila et al. 2009). In contrast, none of the viruses of
extremely thermophilic crenarchaea are of the head-and-tail
type, but show a fascinating variety of unique morphologies
and genomes, indicating that we have only just begun to
appreciate the diversity of archaeal viruses (Prangishvili et al.
2006).