Host developmental stage effects on parasite resistance and tolerance

Hosts can defend themselves against parasites by either preventing or limiting infections (resistance), or limiting parasite-induced damage (tolerance). However, it remains underexplored how these defense types vary over host development with shifting patterns of resource allocation priorities. Here, we studied the role of developmental stage on resistance and tolerance in Atlantic salmon (Salmo salar). This anadromous fish has distinct life stages related to living in fresh and sea water. We experimentally exposed one-year old salmon, either at the freshwater stage or at the stage transitioning to the marine phase, to the trematode Diplostomum pseudospathaceum. Using 56 pedigreed families and multivariate animal models, we show that developmental transition is associated with reduced resistance, but does not affect tolerance. Furthermore, by comparing tolerance slopes (host fitness against parasite load) based on additive genetic effects among infected and unexposed control relatives, ...