Host Diet Quality Alters Priority Effects of Coinfecting Parasites

Separate bodies of literature suggest that the ecology and evolution of host-parasite systems are influenced by the quality of a host's diet and priority effects (i.e., when the order of exposure to multiple parasites influences the outcome of the infection). Although resource competition is invoked as a mechanism responsible for priority effects, the effect of host diet quality on priority effects is unexplored. We tested the hypothesis that low-quality diets reduce priority effects between the bacterial parasite Pasteuria ramosa and the microsporidian Hamiltosporidium tvaerminnensis in their daphnid host. We found that P. ramosa benefits from prior residency when hosts are fed high phosphorus (i.e., high-quality) but not low phosphorus diets. We were unable to detect priority effects for H. tvaerminnensis. These results are consistent with evidence from single-infection experiments on the same parasites, suggesting that low phosphorus diets could reduce resource competition between these parasites. Our study shows how the host's environment can influence infection outcomes by moderating parasite interactions.