Relationship between the genome size and the number of protein-coding genes in 623 complete archaeal and bacterial genomes, based on data in IMG version 2.5 (March 2008). The line points to I. hospitalis having the smallest genome among independently replicating organisms. The genomes of obligate parasites/symbionts are represented by grey circles. The shaded region of genome sizes spans the transition between obligate symbionts/parasites and free-living organisms.
Podar et al. Genome Biology 2008 9:R158 doi:10.1186/gb-2008-9-11-r158