Context-dependence in the symbiosis between Dictyostelium discoideum and Paraburkholderia

Symbiotic interactions change with environmental context. Measuring these
context-dependent effects in hosts and symbionts is critical to
determining the nature of symbiotic interactions. We investigated
context-dependence in the symbiosis between social amoeba hosts and their
inedible Paraburkholderia bacterial symbionts, where the
context is the abundance of host food
bacteria. Paraburkholderia have been shown to harm hosts
dispersed to food-rich environments, but aid hosts dispersed to food-poor
environments by allowing hosts to carry food bacteria. Through measuring
symbiont density and host spore production, we show that this food context
matters in three other ways. First, it matters for symbionts, who suffer a
greater cost from competition with food bacteria in the food-rich context.
Second, it matters for host-symbiont conflict, changing how symbiont
density negatively impacts host spore production. Third, data-based
simulations show that symbiosis often provides a long-term fitness
advantage for hosts after rounds of growth and dispersal in variable
food-contexts, especially when conditions are harsh with little food.
These results show how food context can have many consequences for
the Dictyostelium-Paraburkholderia symbiosis and that
both sides can frequently benefit.