AbstractSymbiotic interactions may change depending on the abundance of third parties like predators, prey, or pathogens. Third-party interactions with prey bacteria are central to the symbiosis between Dictyostelium discoideum social amoeba hosts and Paraburkholderia facultative bacterial symbionts. Symbiosis with inedible Paraburkholderia allows host D. discoideum to carry prey bacteria through the dispersal stage where host amoebae aggregate and develop into fruiting bodies that disperse spores. Carrying prey bacteria benefits hosts when prey bacteria are scarce but harms hosts when prey bacteria are plentiful. Symbiont-carrying hosts leave some bacteria behind; this could explain the harm to hosts if left-behind bacteria include uneaten prey bacteria. Thus, understanding both benefits and costs in this symbiosis requires measuring how many prey bacteria are eaten, carried, and left behind by infected hosts. We found that Paraburkholderia infection makes hosts leave behind both symbionts and prey bacteria. However, the number of prey bacteria left uneaten was small and did not explain why infected hosts produced fewer spores than uninfected hosts. Turning to the bacteria that are carried, we found that hosts carry more prey bacteria after developing in prey- poor environments than in prey-rich ones. This suggests that carriage is actively modified to ensure hosts have prey in the harshest conditions. Our results show that multifaceted interactions with third parties shape the evolution of symbioses in complex ways.
