Highlights

• Viruses infecting archaea are so far the least studied virus group.
• Archaeal viruses are morphologically more diverse than bacterial ones.
• Archaeal viruses display unique morphotypes such as virions resembling lemons.
• Virion structures reveal the relationships between archaeal and bacterial viruses

Most viruses infecting archaea have nothing in common with those infecting bacteria, although they are still considered as “bacteriophages” by many virologists, just because archaea and bacteria are both prokaryotes (without nucleus).For historical reasons, bacteriophage is widely used to refer to viruses of bacteria (and sometimes even archaea). The problem with such nomenclature is that it artificially divides the virosphere into two camps, with viruses of bacteria and archaea on one hand and viruses of eukaryotes on the other.Archaea, the third domain of life, same as the two other domains, Bacteria and Eukarya, is parasitised by viruses. Known species of archaeal viruses have been isolated from environments with extreme conditions – extremely high temperatures, exceeding 80 °C, or extremely high salinity. Known archaeal viruses have DNA genomes, most of which are double stranded. Morphological diversity of archaeal viruses exceeds that of bacterial viruses (bacteriophages).Archaeal viruses from extreme thermal environments are enveloped and have diverse unusual morphologies, indicative of multiple independent origins. Most known viruses from highly saline waters resemble bacteriophages and share an ancestry with them. Most of the genes of hyperthermophilic archaeal viruses have no homologues in other known viruses. The knowledge on viruses of Archaea contributes to the understanding of the origin and evolution of viruses and virus–host interactions