Comparative genomics of Rickettsiella bacteria reveal variable metabolic pathways potentially involved in symbiotic interactions with arthropods
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Abstract
Members of the Rickettsiella genus (order: Legionellales ) are emerging as widespread bacteria associated with insects, arachnids, and crustaceans. While some Rickettsiella strains are highly virulent pathogens, others are maternally inherited endosymbionts that manipulate arthropod phenotypes, including the induction of defensive symbiosis and cytoplasmic incompatibility. However, the genomic diversity of Rickettsiella remains largely unexplored, and their genetic potential to induce complex phenotypes in arthropods is only partially understood. In this study, we sequenced five new Rickettsiella genomes isolated from three tick species. Through comparative genomics, we observed that Rickettsiella members share similar metabolic capabilities, and collectively lack virulence genes from pathogenic Legionellales. Additional analysis of Rickettsiella genomes revealed significant variability in metabolic properties related to endosymbiosis. Specifically, their capacity to biosynthesize certain B vitamins and heme varies, suggesting a functional role of some Rickettsiella strains in the nutrition of their arthropod hosts. Some Rickettsiella genomes harbour homologs of Wolbachia cif genes, the cause of Wolbachia -induced cytoplasmic incompatibility, suggesting that Rickettsiella may use a similar molecular mechanism to manipulate the reproduction of their arthropod hosts. Phylogenomics further revealed that tick-borne Rickettsiella exhibit distinct evolutionary origins within the genus, indicating that Rickettsiella have undergone repeated horizontal transfers between ticks and other arthropods.
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This study significantly advances our understanding of Rickettsiella, a poorly characterized but widespread group of bacterial associates in arthropods, by providing five new metagenome-assembled genomes isolated from three tick species. The authors conduct comprehensive comparative and phylogenomic analyses that reveal the diversity of tick-associated Rickettsiella and highlight their potential roles.A central finding of the study is that tick-associated Rickettsiella are not monophyletic but are instead scattered across the Rickettsiella clade, consistent with repeated horizontal transfers among arthropods. The authors interpret genomic features such as moderate genome reduction, pseudo-genization, low mobile element content, and intermediate completeness scores as consistent with relatively recent symbiotic associations. …
This study significantly advances our understanding of Rickettsiella, a poorly characterized but widespread group of bacterial associates in arthropods, by providing five new metagenome-assembled genomes isolated from three tick species. The authors conduct comprehensive comparative and phylogenomic analyses that reveal the diversity of tick-associated Rickettsiella and highlight their potential roles.A central finding of the study is that tick-associated Rickettsiella are not monophyletic but are instead scattered across the Rickettsiella clade, consistent with repeated horizontal transfers among arthropods. The authors interpret genomic features such as moderate genome reduction, pseudo-genization, low mobile element content, and intermediate completeness scores as consistent with relatively recent symbiotic associations. Interestingly in one tick species lacking Coxiella -the typical nutritional symbiont of ticks- Ricketsiella is found as the sole bacterial symbiont that could fill nutritional functions. These results on rick-associated Rickettsiella evolutionary origins echoe patterns seen in other symbiont-arthropod systems where nutritional symbiont distribution and evolutionary history reflect frequent turnover and replacements (e.g. Mao & Bennett 2020; Martin Říhová et al. 2020). Another major finding is the variability in metabolic capacities across strains, particularly regarding B vitamin and heme biosynthesis pathways. This heterogeneity supports the hypothesis that different Rickettsiella strains may confer distinct functional benefits to their hosts, possibly contributing to nutritional supplementation - a function already attributed to other bacterial endosymbionts in blood-feeding arthropods. The detection of homologs to Wolbachia’s cif genes in some Rickettsiella genomes from two strains also suggests the potential for reproductive manipulation by this endosymbionts akin to cytoplasmic incompatibility (CI). Rather than over-interpreting these findings, the authors draw cautious parallels with other intracellular bacteria such as Coxiella, Legionella, Wolbachia, and Rickettsia. Similarly, the study does not attempt to definitively resolve the functional role of the metabolic genes in host interactions but instead lays out clear hypotheses and comparative patterns that can guide future research. It emphasizes the importance of integrating genomics with transcriptomics, metabolomics, and in vivo assays to directly test symbiont function.
Overall, this study brings substantial data to the field of arthropod microbiome research, particularly within the complex tick endosymbiont community. Ticks harbor a diverse array of bacterial associates—some pathogenic, others mutualistic—yet the ecological and evolutionary implications of many remain poorly understood. This work helps fill key knowledge gaps for Rickettsiella, a genus that includes both pathogenic and mutualistic representatives across arthropods.
In summary, the study offers novel insights into the diversity and evolutionary biology of Rickettsiella. While rich in descriptive and comparative content, it remains appropriately cautious when interpreting how genomic potential translates to host phenotypes. In doing so, it provides an essential genomic foundation for future research into the complex interplay between ticks and their microbial partners.
References
Anna M. Floriano, Adil El-Filali, Julien Amoros, Marie Buysse, Hélène Jourdan-Pineau, Hein Sprong, Robert Kohl, Ron P. Dirks, Peter Schaap, Jasper Koehorst, Bart Nijsse, Didier Bouchon, Vincent Daubin, Fabrice Vavre, Olivier Duron (2025) Comparative genomics of Rickettsiella bacteria reveal variable metabolic pathways potentially involved in symbiotic interactions with arthropods. bioRxiv, ver.3 peer-reviewed and recommended by PCI Evolutionary Biology https://doi.org/10.1101/2024.12.03.626579
Mao, M., & Bennett, G. M. (2020). Symbiont replacements reset the co-evolutionary relationship between insects and their heritable bacteria. The ISME journal, 14(6), 1384-1395. https://doi.org/10.1038/s41396-020-0616-4
Martin Říhová, J., Gupta, S., Darby, A. C., Nováková, E., & Hypša, V. (2023). Arsenophonus symbiosis with louse flies: multiple origins, coevolutionary dynamics, and metabolic significance. Msystems, 8(5), e00706-23. https://doi.org/10.1128/msystems.00706-23
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