Phylogenetic tree colored according to operon copy number. Each organism name on the tree is followed by the approximate size of its genome in megabases, (MB), and the number of rRNA operons found in the genome. The color of the lettering is decided by the number of operons. Fourteen distinct colors were used with each assigned to a specific number of operons. As the operon number increases the color used generally becomes darker. The darkish shade of green is used for 13 or more copies. This figure shows the upper quartile, for the full image please see Additional file 1.