Parasite resistance and parasite tolerance: insights into transgenerational immune priming in an invertebrate host.

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.