Evolutionary Convergence of Nutritional Symbionts in Ticks.

Symbiosis with bacteria is essential for the survival of animals with an obligate blood-feeding lifestyle. In ticks, two distinct bacterial lineages, Coxiella-like and Francisella-like endosymbionts, have independently evolved into nutritional symbionts, converging on a key biochemical function for the tick's survival and growth: the production of three B vitamins. In this study, we carried out comparative analyses across multiple tick species and characterised remarkable similarities in their tissue localisation, particularly in organs important for nutrient metabolism and maternal transmission to progeny. In these organs, both symbionts colonise similar intracellular niches, residing within membrane-bound, replicative vacuoles that occupy a substantial part of the cytoplasm of tick cells. Despite extensive genomic reduction, both symbionts have retained pathways for the biosynthesis of B vitamins and, in some cases, chorismate, a precursor used for the production of serotonin by ticks. However, differences exist: while Coxiella-like endosymbionts lack the ability to synthesise heme, Francisella-like endosymbionts possess a complete heme biosynthesis pathway and may potentially provide ticks with this essential cofactor. Overall, these phenotypic and genomic characteristics reveal a broad convergence among symbiotic interactions across major tick families, highlighting the essential role of symbiosis in tick nutrition, feeding behaviour, blood intake and subsequently in pathogen transmission.