Parasites impose different selection regimes on their hosts, which respond by increasing their resistance and/or tolerance. Parental challenge with parasites can enhance the immune response of their offspring, a phenomenon documented in invertebrates and termed transgenerational immune priming. We exposed two parental generations of the model organism Daphnia magna to the horizontally-transmitted parasitic yeast Metschnikowia bicuspidata, and recorded resistance- and tolerance-related traits in the offspring generation. We hypothesized that parentally-primed offspring will increase either their resistance or their tolerance to the parasite. Our susceptibility assays revealed no impact of parental exposure on offspring resistance. Nonetheless, different fitness-related traits, which are indicative of tolerance, were altered. Specifically, maternal priming increased offspring production and decreased survival. Grandmaternal priming positively affected age at first reproduction and negatively affected brood size at first reproduction. Interestingly, both maternal and grandmaternal priming significantly reduced within-host parasite proliferation. Nevertheless, Daphnia primed for two consecutive generations had no competitive advantage in comparison to unprimed ones, implying additive maternal and grandmaternal effects. Our findings do not support evidence of transgenerational immune priming from bacterial infections in the same host species, thus emphasizing that transgenerational immune responses may not be consistent even within the same host species.

We challenged D. magna hosts from two different clones with the exclusively horizontally-transmitted parasitic yeast Metschnikowia bicuspidata for two consecutive generations and assessed their offspring’s resistance and tolerance. We measured infection rate, spore production, and life-history traits such as age at first reproduction, brood size at first reproduction, offspring production, and survival. We selected the most parsimonious model using the "MuMIN" package in r. Data were visualized with the r package “ggplot2”.