Abstract
We explored variation in infection outcomes and processes explaining that variation in multiple taxa of Daphnia exposed to the pathogen, Australozyma monospora (formerly Metschnikowia bicuspidata) . Three laboratory assays were conducted to explore (1) variation in likelihood of infection and pathogen load, (2) host encounter and defense traits, and the influence of host maturity on these traits, and (3) the fitness costs of preventing infection. We found considerable among-taxon variation in susceptibility as well as the pathogen’s ability to reproduce following successful infection. Notably, in both the most susceptible ( D. dentifera ) and least susceptible ( D. pulicaria ) host species, we discovered that host barriers and immune responses, but not encounter rates, play an important role in shaping infection outcomes. Although differences in body size could underlie variation in barriers and immune responses between the two taxa, they could not explain infection differences across the broader set of host taxa. Finally, we found that the least susceptible taxon did not exhibit a cost of resistance, whereas the most susceptible taxon-suffered fitness declines when preventing infection. These results underscore the complexity of taxon-level differences in host–pathogen interactions and provide a framework for understanding how variation among and within taxa can shape disease outbreaks.