Stoichiometric acclimation of a bacterial parasite to shifts in the diet quality of a single host.

A growing body of literature suggests that the effects of host diet on populations of parasites within a host (i.e., infrapopulations) are dictated by the elemental stoichiometry of the parasite, and new data suggest that the stoichiometric composition of parasites can shift in response to a host's diet. The consequences of this shift in parasite stoichiometry for host-parasite interactions are underexplored. Here, we present evidence that the diet quality of initial "rearing" hosts influences how infrapopulations in subsequent hosts respond to host diet. We reared infrapopulations of the microparasite Pasteuria ramosa in Daphnia magna fed high or low phosphorus (P) diets. Then, because this parasite is transmitted from a spore bank to individual hosts, we pooled spores from hosts within each diet treatment and used them to infect subsequent hosts. We found that P. ramosa had larger spores and achieved higher spore loads when the diet of their new host matched that of their rearing host. We explored the mechanism for this apparent acclimation to host diet by analyzing the stoichiometry of infrapopulations as stoichiometric trait distributions characterized by the mean, functional richness, functional evenness, and functional divergence of spores for each stoichiometric trait carbon (C), nitrogen (N), P, C:N, C:P, and N:P. Both rearing and subsequent host diets altered the stoichiometry of individual spores in subsequent hosts. Furthermore, functional richness in infrapopulation N content explained more variation in the spore load of subsequent hosts (80%) than either of our diet treatments or any other metric of infrapopulation stoichiometry. Our treatments also influenced the parasite's niche. The C:P and N:P trait distributions of infrapopulations consisting of spores exposed to low P diets for two within-host lifecycles exhibited lower functional evenness and greater functional divergence than those consisting of spores unexposed to low P diets or exposed for one life cycle. Our results demonstrate that parasite rapid acclimation to host diet quality is possible, and quantifying infrapopulation stoichiometry can inform our understanding of host diet effects on host-parasite interactions. Replicating this work across additional host-parasite systems will help resolve the role of such effects in natural systems.