These are mainly vertically transmitted but according PF-02341066 concentration to the host-symbiont association, horizontal transfers may occur within and between species on different evolutionary time scales [6–9]. An extremely diverse group of bacterial taxa is involved in facultative symbiosis, with
a wide range of both hosts and phenotypes. Some facultative endosymbiotic bacteria confer direct fitness benefits such as protection against natural enemies [10, 11], host-plant specialization  or thermal tolerance . Others, like the alphaproteobacterium Wolbachia and the Bacteroidetes Cardinium, manipulate host reproduction to enable their spread and maintenance in host populations despite deleterious effects (for review see Stouthamer et al. ). Among the CX-4945 clinical trial symbiotic bacteria, the gammaproteobacterium genus Arsenophonus has
particular characteristic features with regard to lineage diversity, host spectrum and the symbiotic relationships established with its host. It thus constitutes a good model to study the evolutionary process shaping symbiotic associations. The diversity of Arsenophonus host species is particularly large, including insects, other arthropods (such as ticks) and plants . This can be explained by the symbiont’s transmission routes since this vertically transmitted bacterium can also be acquired by horizontal transfer within and among species [16, 17]. Moreover, some strains can be cultivated on cell-free cultures . Arsenophonus-host relationships range from parasitism to mutualism, with the induction of various phenotypes such as reproductive manipulation Progesterone (male-killing) , phytopathogenicity  or obligatory mutualism [21, 22]. However, in most reported symbiotic associations, the impact
of this symbiont on the host phenotype remains unknown. Based on rRNA gene analysis, phylogenetic studies have revealed an extremely high diversity of bacterial lineages forming a monophyletic group . In addition, the Arsenophonus phylogeny encompasses several other host-specific sub-clusters with lower divergence associated to ticks, plants, triatomine bugs, whiteflies, several genera of hippoboscids and ants, but no co-speciation pattern within clades. Beside these bacterial lineages that click here cluster according to host taxonomy, a number of closely related Arsenophonus strains infect unrelated host species. Moreover, the same host species sometimes harbors several Arsenophonus lineages, a pattern that is probably due to the Arsenophonus’s ability to be horizontally transferred, as recently demonstrated in the hymenopteran parasitoids of the family Pteromalidae . Previous studies have shown that whitefly species can host different strains of several bacteria [15, 23, 24] , and they thus appear to be particularly relevant to investigating Arsenophonus diversity and evolution.