Fig. 4.—

Reconstruction of gains and losses of variable sRNAs in the evolution of the Escherichia coli–Shigella lineage. The cladogram shows the phylogenetic relationships among the 27 E. coli and Shigella genomes, as in figure 2. Pie charts on internal nodes of the tree show the proportion of the 23 variable sRNAs that were inferred to be present in the ancestral strain corresponding to the node. Ancestral sRNA content as represented in the pie charts is based on the inferences of sRNA gains and losses reported in table 1. Dark blue indicates the presence and red indicates the absence. Similarly, the bars on the right show the proportion of the 23 variable sRNAs that were inferred to be present and the absence in the corresponding extant E. coli and Shigella genomes.