FIG. 2.

Relationship between mutation rate and genome size, with major virus groups indicated. Values for viroids and bacteria, the two adjacent levels of biological complexity, are also plotted. The mutation rate is expressed as the number of substitutions per nucleotide per generation, defined as a cell infection in viruses (μ_s/n/c). We obtained bacterial mutation rates from a previous study (57) and divided them by 1.46 to convert the total rate into a substitution rate, as previously suggested (27). 1, Bacillus; 2, Deinococcus; 3, enterobacteria; 4, Helicobacter; 5, Mycobacterium; 6, Sulfolobus. The star indicates the solitary rate for a viroid (37), a subviral infectious agent constituted by small noncoding RNA. This rate is in substitutions per strand copying, since viroids do not have the equivalent of a generation.