Microbial community structure reveals instability of nutritional symbiosis during the evolutionary radiation of Amblyomma ticks.

Mutualistic interactions with microbes have facilitated the adaptation of major eukaryotic lineages to restricted diet niches. Hence, ticks with their strictly blood-feeding lifestyle are associated with intracellular bacterial symbionts through an essential B vitamin supplementation. In this study, examination of bacterial diversity in 25 tick species of the genus Amblyomma showed that three intracellular bacteria, Coxiella-like endosymbionts (LE), Francisella-LE and Rickettsia, are remarkably common. No other bacterium is as uniformly present in Amblyomma ticks. Almost all Amblyomma species were found to harbour a nutritive obligate symbiont, Coxiella-LE or Francisella-LE, that is able to synthesize B vitamins. However, despite the co-evolved and obligate nature of these mutualistic interactions, the structure of microbiomes does not mirror the Amblyomma phylogeny, with a clear exclusion pattern between Coxiella-LE and Francisella-LE across tick species. Coxiella-LE, but not Francisella-LE, form evolutionarily stable associations with ticks, commonly leading to co-cladogenesis. We further found evidence for symbiont replacements during the radiation of Amblyomma, with recent, and probably ongoing, invasions by Francisella-LE and subsequent replacements of ancestral Coxiella-LE through transient co-infections. Nutritional symbiosis in Amblyomma ticks is thus not a stable evolutionary state, but instead arises from conflicting origins between unrelated but competing symbionts with similar metabolic capabilities.

Novel Rickettsia genotypes in ticks in French Guiana, South America.

Rickettsia are obligate intracellular bacteria often associated with ticks and best known for causing human diseases (rickettsiosis), including typhus fever and sporadic cases of serious infection. In this study, we conducted a large survey of ticks in French Guiana to understand the overall diversity of Rickettsia in this remote area largely covered by dense rainforests. Out of 819 individuals (22 tick species in six genera), 252 (30.8%) samples were positive for Rickettsia infection. Multilocus typing and phylogenetic analysis identified 19 Rickettsia genotypes, but none was 100% identical to already known Rickettsia species or strains. Among these 19 genotypes, we identified two validated Rickettsia species, Rickettsia amblyommatis (spotted fever group) and Rickettsia bellii (bellii group), and characterized a novel and divergent Rickettsia phylogenetic group, the guiana group. While some tick hosts of these Rickettsia genotypes are among the most common ticks to bite humans in French Guiana, their potential pathogenicity remains entirely unknown. However, we found a strong association between Rickettsia genotypes and their host tick species, suggesting that most of these Rickettsia genotypes may be nonpathogenic forms maintained through transovarial transmission.