Intra- and interspecific immune variation in Daphnia

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Patrick J. Wilson, Evolution, Ecology, and Behavior, University of Illinois Urbana-Champaign, Urbana-Champaign, IL, Tara Stewart Merrill, Ecology and Evolutionary Biology, University of Colorado at Boulder, Boulder, CO and Carla E. Cáceres, School of Integrative Biology, University of Illinois at Urbana-Champaign, Urbana, IL

Background/Question/Methods
Variation of immune traits in a host population can shape the outcome of a pathogen’s transmission. Natural populations of zooplankton host species show variation in their susceptibility to pathogens, but the innate immune mechanisms that drive these differences have not been well defined. We focus on two species, Daphnia dentifera and Daphnia pulicaria to ask how barriers can prevent a pathogen from entering a host (resistance traits) and how internal responses to pathogen invasion (clearance traits) differ within and between populations of D. dentifera and D. pulicaria. We observed individuals from natural populations in Indiana and conducted laboratory experiments on multiple clonal lines of both species to quantify their interactions with and defenses against the fungal pathogen Metschnikowia bicuspidata.
Results/Conclusions
Field data from Indiana showed high variability in the prevalence of Metschnikowia in D. dentifera, while D. pulicaria was not observed to be infected with the fungal pathogen. Intra- and interspecific variation in immunity was observed within and between both host species. We observed mechanisms that allow D. pulicaria to prevent infections through resistance traits that lessen gut penetration and large hemocyte responses that lead to high clearance, while D. dentifera showed varying degrees of susceptibility between clonal lines in lab caused by variation in both resistance and clearance traits. This research furthers our understanding of intra- and interspecific variation in innate immune function for two invertebrate species, and how that variation influences disease dynamics in aquatic communities. Pathogen transmission is reliant on a pathogen’s ability to overcome host defenses, forming a vital connection between variation in host immunity and disease dynamics. The interactions between these hosts and Metschnikowia help demonstrate why some hosts support pathogens while others do not, a key for better understanding disease patterns across species. This work is an important first step in quantifying the amount of intra- and interspecific variation that is needed to better predict sustained disease transmission in a natural population.