Viruses that are highly similar in virion morphology to
bacterial head-tail viruses have been seldom observed in
archaea-rich habitats. They were never reported in extreme
geothermal environments (with temperatures above 80 1C)
and are rare in hypersaline waters. However, due to the biased approaches applied for
virus isolation, head-tail viruses constitute a significant
proportion of described species of haloarchaeal viruses.
Moreover, in silico analysis of the genomes of members of
the phyla Euryarchaeaota and Thaumarchaeota revealed
several examples of integrated proviruses, apparently of a
head-tail type.