Genomic evolution of Francisella : metabolic innovation, endosymbiotic transitions to ticks, and biogeographic history

Ticks (Ixodida) are the second most important vectors of infectious diseases in vertebrates, after mosquitoes. Beyond vector roles, they maintain mutualistic associations with bacteria, including endosymbionts that provide essential B vitamins lacking in their blood-based diet. The most extensively studied endosymbionts belong to the genera Coxiella, Midichloria, and Francisella. The genus Francisella encompasses endosymbionts (FE), pathogens (FP), opportunistic pathogens (FO) and free-living environmental strains (FL), making it a powerful system for evolutionary and comparative genomic analyses. In this study, total DNA from six adult female ticks of the genera Hyalomma and Amblyomma was sequenced to generate new FE genomes. Seven deeply sequenced public metagenomes were also assembled, yielding 71 Francisella and three Allofrancisella strains. This dataset supported phylogenomic reconstruction and comparison of genomic features, including vitamin biosynthesis and virulence pathways, with a focus on transitions to tick endosymbiosis. A densely sampled MLST phylogeny was constructed to explore biogeographic patterns. Our results show that, except for FE, no ecological trait is monophyletic, supporting an origin of Francisella diversity from free-living ancestors. Biogeography suggests Palearctic and Afrotropical FE strains are derived and may involve horizontal transfers. Francisella comparative genomics reveals two contrasting profiles: environmental generalists and host-restricted specialists. These findings reinforce the role of tick FEs as nutritional mutualists, retaining key pathways such as riboflavin, shikimate, and biotin biosynthesis. In contrast, virulence is not ancestrally conserved but an innovation in pathogenic lineages, largely degraded in tick FEs. These results advance understanding of endosymbiont evolution and provide genomic insights with potential for disease control.