Figure 1

Two-dimensional density plot of 'core genes' for the E. coli pan-genome. The plot illustrates the number of E. coli core genes for n = 2,...,32 genomes based on a maximum of 3,200 random combinations of genomes for each n. The density colors reflect the count of combinations giving rise to a certain number of core genes; that is, for n = 3, genome number 3 is compared to genomes 1 and 2, and the number of core genes is the number of genome 3 genes conserved in genomes 1 and 2. The green line is the fit to the exponential decay function by [3], and the red line is our proposed fit to a slightly modified decay function as explained in the Materials and methods.