Archaeal viruses and bacteriophages: comparisons and contrasts

• 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.

Isolated archaeal viruses comprise only a few percent of all known prokaryotic viruses. Thus, the study of viruses infecting archaea is still in its early stages. Here we summarize the most recent discoveries of archaeal viruses utilizing a virion-centered view. We describe the known archaeal virion morphotypes and compare them to the bacterial counterparts, if such exist. Viruses infecting archaea are morphologically diverse and present some unique morphotypes. Although limited in isolate number, archaeal viruses reveal new insights into the viral world, such as deep evolutionary relationships between viruses that infect hosts from all three domains of life.