Abstract
The Rhabdochlamydiaceae family is one of the most widely distributed within the phylum Chlamydiae, but most of its members remain uncultivable. Rhabdochlamydia 16S rRNA was recently reported in more than 2% of 8,534 pools of ticks from Switzerland. Shotgun metagenomics was performed on a pool of five female Ixodes ricinus ticks presenting a high concentration of chlamydial DNA, allowing the assembly of a high-quality draft genome.About 60% of sequence reads originated from a single bacterial population that was named “Candidatus Rhabdochlamydia helvetica” whereas only few thousand reads mapped to the genome of “Candidatus Midichloria mitochondrii,” a symbiont normally observed in all I. ricinus females. The 1.8 Mbp genome of R. helvetica is smaller than other Chlamydia-related bacteria. Comparative analyses with other chlamydial genomes identified transposases of the PD-(D/E)XK nuclease family that are unique to this new genome. These transposases show evidence of interphylum horizontal gene transfers between multiple arthropod endosymbionts, including Cardinium spp. (Bacteroidetes) and diverse proteobacteria such as Wolbachia, Rickettsia spp. (Rickettsiales), and Caedimonas varicaedens (Holosporales). Bacterial symbionts were previously suggested to provide B-vitamins to hematophagous hosts. However, incomplete metabolic capacities including for B-vitamin biosynthesis, high bacterial density and limited prevalence suggest that R. helvetica is parasitic rather than symbiotic to its host.The identification of novel Rhabdochlamydia strains in different hosts and their sequencing will help understanding if members of this genus have become highly specialized parasites with reduced genomes, like the Chlamydiaceae, or if they could be pathogenic to humans using ticks as a transmission vector.
Highlights
Ticks are the most common arthropod vector of human and animal diseases (Moutailler et al 2016).They frequently carry mutualist symbionts; all females and nearly 50% of the males of the European tick Ixodes ricinus carry the Rickettsiales symbiont “Candidatus Midichloria mitochondrii.” The high prevalence of this symbiont suggests an obligate association between the two species, but the role of the symbiont in the biology of I. ricinus remains unknown (Ahantarig et al 2013; Moutailler et al 2016)
About 60% of sequence reads originated from a single bacterial population that was named “Candidatus Rhabdochlamydia helvetica” whereas only few thousand reads mapped to the genome of “Candidatus Midichloria mitochondrii,” a symbiont normally observed in all I. ricinus females
The single base of 18S rRNA sequence enabling to distinguish both species was identical to I. ricinus variant, confirming that all five ticks in the pool belonged to I. ricinus species
Summary
Ticks are the most common arthropod vector of human and animal diseases (Moutailler et al 2016).They frequently carry mutualist symbionts; all females and nearly 50% of the males of the European tick Ixodes ricinus carry the Rickettsiales symbiont “Candidatus Midichloria mitochondrii.” The high prevalence of this symbiont suggests an obligate association between the two species, but the role of the symbiont in the biology of I. ricinus remains unknown (Ahantarig et al 2013; Moutailler et al 2016). The high prevalence of this symbiont suggests an obligate association between the two species, but the role of the symbiont in the biology of I. ricinus remains unknown (Ahantarig et al 2013; Moutailler et al 2016). As confirmed by whole genome sequencing, ticks are incapable of de novo heme synthesis and acquire heme from exogenous source (Gulia-Nuss et al 2016; Perner et al 2016). Chlamydiae DNA was identified in approximately 0.89% of I. ricinus ticks in Switzerland based on a large-scale screen of 62,889 I. ricinus ticks sampled from 172 different sites (Croxatto et al 2014; Pilloux et al 2015). Based on the 16S rRNA gene sequence of 359 positive samples, over 29%
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