TY - JOUR T1 - Evolution and maintenance of microbe-mediated protection under occasional pathogen attack JF - bioRxiv DO - 10.1101/2020.01.24.917138 SP - 2020.01.24.917138 AU - A. Kloock AU - M.B. Bonsall AU - K.C. King Y1 - 2020/01/01 UR - http://biorxiv.org/content/early/2020/01/25/2020.01.24.917138.abstract N2 - Every host is colonized by a variety of microbes, some of which can protect their hosts from pathogen infection. As microbe-mediated protection can be costly, it is hypothesized, that these costs only pay off in the presence of a pathogen. However, pathogen presence naturally varies over time and can expand once pathogen infection spreads to a new host, as during occasional spill-over from reservoir hosts. We experimentally coevolved populations of Caenorhabditis elegans worm hosts with bacteria possessing protective traits (Enterococcus faecalis), in treatments varying the infection frequency with pathogenic Staphylococcus aureus every host generation, alternating host generations, every fifth host generation or never. We additionally investigated the effect of initial pathogen presence at the formation of the defensive symbiosis. Our results show that enhanced microbe-mediated protection evolved during host-protective microbe coevolution when faced with spill-over infections by a human pathogen occurring rarely over evolutionary history. Initial pathogen presence had no effect on the evolutionary outcome of microbe-mediated protection. We also found that protection was only effective at preventing mortality during the time of pathogen infection. Overall, our results suggest that resident microbes can be a form of transgenerational immunity against occasional pathogen attack or spill-over events. ER -