Fig. 3. Major events in the evolutionary history of the F7 system in γ-Proteobacteria.

Each major branch is identified by the type of its F7 system, and shading indicates the function of the system. Numbers in parentheses indicate the number of genomes from the designated class compared to the number of genomes present in the branch. Differences in these numbers indicate lateral gene transfers and are highlighted in Supplementary Fig. 1. The double division symbol is used to show that the protein clusters on each side of the symbol are located in different parts of the genome, the symbols plus and crosses indicate gene additions and losses, respectively. The presence and absence of other relevant systems in the genomes containing each stage are marked as complete (full circle), partial (half circle), or absent (empty circle).