Cross-fertilization is predicted to facilitate the short-term response and the long-term persistence of host populations engaged in antagonistic coevolutionary interactions. Consistent with this idea, our previous work has shown that coevolving bacterial pathogens (Serratia marcescens) can drive obligately selfing hosts (Caenorhabditis elegans) to extinction, while the obligately outcrossing and partially outcrossing populations persisted. We focused the present study on the partially outcrossing (mixed mating) and obligately outcrossing hosts, and analyzed the changes in the host resistance/avoidance (and pathogen infectivity) over time. We found that host mortality rates increased in the mixed mating populations over the first ten generations of coevolution when outcrossing rates were initially low. However, mortality rates decreased after elevated outcrossing rates evolved during the experiment. In contrast, host mortality rates decreased in the obligately outcrossing populations during the first ten generations of coevolution, and remained low throughout the experiment. Therefore, predominant selfing reduced the ability of the hosts to respond to coevolving pathogens compared to outcrossing hosts. Thus, we found that host-pathogen coevolution can generate rapid evolutionary change, and that host mating system can influence the outcome of coevolution at a fine temporal scale.

Coevolved Bacteria Exposed to Ancestral Hosts Over TimeFigure 1 DataDryad Coevolved Bacteria Against Ancestral Hosts Over Time.xlsMortality Rate DataFigure 2 dataDryad Mortality Rate Data.xlsMean Outcrossing RatesFigure 3 dataDryad Harmonic Mean Outcrossing Rates Evolution.xls