The distribution and impact of viral lineages in domains of life

Living organisms can be conveniently classified into three domains, Archaea, Bacteria, and Eukarya (Woese et al., 1990). The three domains are united by several features that support the common origin of life including the presence of ribosomes, double-stranded DNA genomes, a nearly universal genetic code, physical compartments (i.e., membranes), and the ability to carry out metabolism and oxidation-reduction reactions. In comparison, other types of genetic material and particles (e.g., viruses, plasmids, and other selfish genetic elements) are often excluded from the definition of “life” (for opposing views see Raoult and Forterre, 2008; Forterre, 2011, 2012a). However, they can still influence the evolution of cellular organisms, and in conjunction, establish complex life cycles.