asiaticus encodes different proteins click here exhibiting eukaryotic domains, suggesting that amoebae-resisting bacteria widely use such eukaryotic motifs to manipulate the host cell (Schmitz-Esser et al., 2010). These eukaryotic domains include U-box and F-box, leucine-rich repeats (LRRs) and ankyrin repeats, among others. U-box and F-box motifs are likely interfering with the ubiquitin system involved in the degradation of proteins by the proteasome, whereas ankyrin proteins are likely controlling the interactions of the intracellular bacteria in its host cell. Finally, the LRRs domain, also largely

present in the genome of Protochlamydia amoebophila (Eugster et al., 2007), may be involved in decreasing recognition of the bacteria by the innate immune system. We hope that this review on symbionts of nematodes, ticks and amoebae will help the reader to understand the importance of the symbiont in

determining the virulence of its host, as exemplified with Wolbachia in nematodes; similarly, an amoebal endosymbiont may also be implicated in the pathogenesis of Acanthamoeba keratitis, by potentially exacerbating local inflammation. This review Akt inhibitor also recaps the importance of the host in the ecology of its endosymbiont, by directly impacting its survival in the environment, its dissemination and its mode of transmission to humans and animals. This is of paramount importance, because ecology strongly controls the gene content of the symbionts. Sympatric amoebal symbionts exhibit much larger genomes and much more frequent genes exchange events than those living in an allopatric environment in nematodes and ticks. Symbionts have also clearly played an important role by ‘feeding’ eukaryotes with significant amounts of

genetic information during evolution, (1) as previously exemplified by the identification of the role of an ancestral member of the Rickettsiales in the biogenesis of current mitochondria (Andersson et al., 1998) and (2) as recently exemplified by the acquisition by a fruit fly of a nearly complete wolbachial genome second content (Dunning Hotopp et al., 2007). The fact that at least one member of the Order Rickettsiales has been identified in all three eukaryote lineages discussed in this review further supports the hypothesis that an ancestral rickettsia was already intracellular more than one billion years ago, when it exchanged genes encoding an ADP/ATP transporter with an ancestral Chlamydiales (Greub & Raoult, 2003). Moreover, this explains why rickettsiologists are in the forefront of research on endosymbiont–host interactions. Other important lessons provided by studying symbionts are that (1) their diverse nature (large biodiversity encompassing several clades) as well as (2) their intimate relationship with their specific host provides no guaranty of their innocuousness towards other eukaryotes encountered by chance, for instance, in a modified ecosystem such as man-made water networks. M.T. and O.M. contributed equally to this work.