Phylogenetic tree colored according to genome size. Each organism name on the tree is followed by the approximate size of its genome in megabases and the number of rRNA operons found in the genome. The color of the lettering is decided by the size of the genome. Twelve distinct colors were used with each assigned to a genome size range. The lightest color was used for genomes up to 1 MB. Subsequently, colors were assigned to genome size ranges in increments of 0.5 MB. Genomes larger than 6 MB were all colored green. This figure shows the upper quartile, for the full image please see Additional file 2.