Endosymbiotic bacteria likely provide ticks with nutrients not present in vertebrate blood. Two common putative nutritional symbionts in ticks are Coxiella-like and Francisella-like endosymbionts (CLEs and FLEs). Both symbionts evolved from pathogenic ancestors and have lost the genes that likely made their ancestors virulent. Independent evolution of nutrient-provisioning endosymbionts from pathogenic bacteria in multiple lineages of ticks indicate that this process occurs efficiently in ticks. Rapid attenuation of virulence, and the ability of bacteria to invade ovaries and salivary glands that facilitate tick-to-tick transfer could contribute to this speedy transformation. Interestingly, unlike insects, most tick lineages do not form long-term symbiotic partnerships with a specific bacterium. This apparent lack of selection pressure to maintain long-term symbionts suggest that ticks have ready access to a large pool of bacteria—probably pathogens encountered in vertebrate blood, that could easily be domesticated into nutrient-provisioning endosymbionts. Justification: We describe how pathogenic bacteria evolved into putative nutrient-provisioning endosymbionts in ticks. We also discuss how constant replacement of endosymbionts could be beneficial to ticks. In our opinion, these insights are appropriate for this forum.
