The role of varying resources on Daphnia dentifera immune responses

Despite all that is known about Daphnia and their interactions with algal resources, questions remain as to how a changing resource environment influences a host’s susceptibility to parasites. Theory and empiricism have demonstrated that increasing resource quantity can positively, negatively, and even non-linearly correlate with susceptibility. The nature of this correlation depends on the complex dynamics between the host’s immune traits (which are assumed to be costly) and a parasite’s ability to evade that immune system and “steal” resources from the host. We used three separate assays to examine how resources influence host immune responses and infection outcomes in eight genotypes of Daphnia dentifera. We challenged Daphnia with the fungal parasite Metschnikowia bicuspidata at three concentrations of the green algae Ankistrodesmus falcatus. In the first assay, we investigated how this resource gradient influences the number of fungal spores consumed (a measure of encounter with the parasite), host gut penetrability (a measure of resistance to the parasite), and the haemocyte response (a measure of clearance of the parasite). In the second assay, we explored how these traits combined to determine overall susceptibility to infection. Finally, our third assay investigated the potential for tolerance in this system by comparing reproduction among hosts that managed to avoid, resist, or clear infection to those that developed late-stage infections. We found that host immune responses changed non-uniformly with resources: the number of fungal spores consumed decreased with increasing resources, gut penetrability showed no relationship with resources (but was strongly driven by host genotype), and haemocyte counts peaked at intermediate resource levels. Ultimately, overall susceptibility demonstrated a strong genotype by environment interaction, with some genotypes showing the highest proportion infected in high resource environments, others in low resource environments, and one genotype had the highest proportion infected at the intermediate resource level. In all resource environments, individuals that avoided, resisted, or cleared infection had higher reproduction than those that developed late-stage infections, suggesting that Daphnia hosts use resistance rather than tolerance with this parasite. Our results demonstrate the importance of integrating resource supply with immunological mechanisms and examining those effects across a range of genotypes that differ in their responses to the environment.