Evolutionary scenario between Rickettsia felis and one of its ancestors. (A) Phylogenetic tree of six Rickettsia (extracted from ). The numbers on the edges give the reversal distance between the genomes on the vertices, which could be either a current species or an ancestor (R 1, R 2, R 3, R 4 and R 5). (B) The optimal sequence of reversals to transform the ancestor R 2 into Rickettsia felis (proposed by Blanc et al.  with the help of the software GRIMM). The two common interval breaks are indicated by the "comma" signs.