Title: Ticks Facilitate Pathogen-Symbiont Transitions
Abtract: Ticks transmit pathogenic bacteria and also contain beneficial endosymbionts that are closely related to pathogens. For example, the human Q-fever pathogen Coxiella burnetii and the closely related Coxiella-like endosymbionts (CLEs) are found in ticks worldwide. Similarly, Francisella tularensis, which causes Rabbit fever in humans, as well as Francisella-like endosymbionts (FLEs) occur in several ticks. However, it is not clear whether the pathogens evolved from symbionts or vice versa. By analyzing the genomes of tick-borne pathogens and endosymbionts, we found that the human pathogen C. burnetiilikely evolved from a tick-associated non-pathogenic ancestor, and that the host shift was facilitated through horizontal acquisition of metabolic and virulence genes (Smith et al. 2015; Moses et al. 2017). In contrast to CLEs, we discovered that FLEs evolved from a mammalian pathogen and have lost most virulence genes in the process (Gerhart et al. 2016; Gerhart et al. 2018). These results reveal that ticks could facilitate pathogen-symbiont transitions in either direction. In addition to advancing basic knowledge about how bacteria adapt to their environments, our approach could be used to develop new therapeutic strategies against difficult-to-treat pathogens such asC. burnetii. For instance, using our comparative genomics approach we recently identified heme and biotin biosynthesis pathways as promising targets for developing new therapeutic agents to treat C. burnetii infections (Moses et al. 2017).