Host genetic variation in feeding rate mediates a fecundity cost of parasite resistance in a Daphnia-parasite system

Organisms face numerous challenges over their lifetimes, including from competitors and parasites, and experience selection to maximise their fitness in the face of these various pressures. However, selection can rarely maximise individual ability to cope with all challenges, and trade-offs therefore emerge. One such trade-off is the cost of resisting parasitic infection, whereby hosts that have a high intrinsic capacity to resist parasitic infection have comparatively low fitness in the absence of the parasite, and spatio-temporal variation in the relative strength of parasite- and non parasite-mediated selection is thought to maintain diversity in host resistance. Here, we test for, and find, a simple cost of resistance in the freshwater host Daphnia magna and its sterilising bacterial parasite, Pasteuria ramosa that is shaped by ecology as opposed to immunity. We uncovered significant genetic variation in Daphnia feeding rate, and show that rapid-feeding Daphnia genotypes have high fecundity in the absence of the parasite, but are more likely to go on to suffer sterilising infection when exposed to the parasite. This feeding rate-mediated cost of resistance can explain the persistence of parasite-susceptible genotypes. Further, we found evidence of infection induced anorexia in Pasteuria-infected hosts. It follows that reduced feeding in infected hosts means that high parasite prevalence could result in greater host food availability; this could reduce intra-specific competition and mask the cost of resistance in nature. Summary statement Costs of Daphnia immunity to a sterilising bacterial parasite are mediated by feeding ecology and not immunity, and infection-induced anorexia can further alter the relative strength of parasitism and host-host competition.