Individual and Genotypic Variation in Diel Rhythms of Infection Risk in the Keystone Zooplankton Daphnia dentifera

Infection risk is determined by both parasite exposure and host susceptibility. In many host-parasite systems the within-host processes that mediate infection risk—behavior, which mediates exposure, and immune function, which mediates susceptibility—have a daily rhythm. However, how these daily rhythms interact to shape infection outcomes is unknown. My research takes advantage of a well-studied host-pathogen system, Daphnia dentifera-Metschnikowia bicuspidata, to test the core hypothesis: variation in the daily rhythms of host behavior and immune function mediates infection outcomes at the individual and population level. I first tested if D. dentifera has a circadian rhythm in feeding rate, as pathogen exposure happens during feeding in this system. Daphnia dentifera have a nocturnal circadian rhythm in feeding. I used this feeding rhythm to predict how infection risk would vary over a daily cycle if immune function was constant, had a rhythm in phase with feeding, or a rhythm out of phase with feeding. I tested these predictions by quantifying one arm of invertebrate immune expression (phenoloxidase, a precursor in the melanization pathway) over a daily cycle and found that phenoloxidase is relatively constant. Second, I used experimental exposures during the active and resting phase and found that infection risk was higher during the active phase. Finally, I tested how infection risk varies at the population level by measuring feeding rhythms in three genotypes. Genotypes had similar feeding rhythms in the absence of M. bicuspidata and all genotypes exhibit decreased feeding in the presence of M. bicuspidata (parasite mediated anorexia). The genotypes studied varied in overall infection probability, but they were all more likely to get infected at night. Together, these results show that there is a diel rhythm in infection risk, with a higher probability of infection at night, that varies among genotypes. Understanding how genotypic variation in diel rhythms shape infection outcomes is a necessary first step for connecting variation among individuals and genotypes to population level disease emergence