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 challenged eight genotypes of Daphnia dentifera with the fungal parasite Metschnikowia bicuspidata at three concentrations of the green algae Ankistrodesmus falcatus to determine 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 hemocyte response (a measure of clearance of the parasite). We then explored how these traits combined to determine overall susceptibility to infection. We also investigated the potential for tolerance in this system by comparing reproduction among hosts that managed to avoid, resist, or clear infection to those that eventually succumbed to infection.
Results/Conclusions
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 hemocyte counts peaked at intermediate resource levels. Ultimately, overall susceptibility demonstrated a strong genotype by environment interaction, with some genotypes showing the highest infection prevalence in high resource environments, others in low resource environments, and one genotype had the highest prevalence at the intermediate resource level. In all resource environments, individuals that avoided, resisted, or cleared infection had higher reproduction than those that succumbed to infection, 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.
