Despite the modest number of isolated species, the morphological
diversity of dsDNA archeoviruses strongly surpasses
that of the numerous known species of
bacterioviruses. From the 452 species of dsDNA bacterioviruses,
97% are head-tail phages assigned to the order
Caudovirales, 2% have tail-less icosahedral virions and
o1% are pleomorphic viruses.
Archeoviruses encompass all these morphotypes and have
many more morphotypes that are not observed among
viruses of Bacteria or Eukarya. The caudoviruses may have
appeared in the archaeal virosphere as a result of a relatively
recent cross-domain spreading event from the bacterial
domain, mainly into the phylum Euryarchaeaota. However,
the recent finding of the provirus Nvie-Pro1 in the genome
of a member of the phylum Thaumarchaeaota, deeply
branching in the phylogenetic tree of the Archaea, suggests
the existence of archaeal head-tail viruses before the speciation
of Euryarchaeota and Crenarchaeota.