Figure 1
From: On the need for widespread horizontal gene transfers under genome size constraint
Genome size restriction of free-living prokaryotes. The genomes of free-living prokaryotes (archaea and eubacteria, in red) appear to be restricted to a mere 10-fold range in size from 1.3 Mbp to 13 Mbp, while the sizes of free-living eukaryote genomes (in black) span almost 105 folds from 10 Mbp to near 106 Mbp. The lower ranges of obligate parasite or symbiont genomes, shown as light pink and grey bars, can be much smaller than free-living prokaryote and eukaryote genomes, respectively, due to the progressive loss of dispensable genes. Viruses and gene-free viroids (in light blue) further reduce the size of parasitic "genomes" down to a few hundred nucleotides only.