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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., (© 2020 John Wiley & Sons Ltd.)

Filarial nematodes of the Dipetalonema lineage are widespread parasites and include some species that are transmitted by ticks. In this study, we conducted a large molecular survey of ticks in French Guiana, South America, to understand the overall diversity of tick-borne filarioids in this remote region largely covered by dense tropical forests. Out of 682 ticks belonging to 22 species and 6 genera, 21 ticks (3.1%) of the species Amblyomma cajennense, A. oblongoguttatum, A. romitii, Ixodes luciae and Rhipicephalus sanguineus sensu lato were positive for infection by filarioids. Molecular typing and phylogenetic analysis identified all these filarioids as members of the Dipetalonema lineage. While the filarioid of R. sanguineus sensu lato is a previously described species, the canine worm Cercopithifilaria bainae Almeida & Vicente, 1984, all other filarioids detected in this study are related but distinct to already known species in the genera Cercopithifilaria, Cruorifilaria and Dipetalonema. Their vertebrate host range may include a wide variety of mammals present in French Guiana, but dogs, capybaras, and opossums are the best candidate hosts for some of these filarioids. Although the detection of members of the Dipetalonema lineage in ticks of significant medical or veterinary interest is of concern, the risk of contracting a tick-borne filarial infection is still largely unknown. The pathogenicity of these filarioids, their epidemiology, developmental cycles, and mechanisms of transmission by South American tick species now require further study.