Influence of host nutritional condition on post-infection traits in the association between the manipulative acanthocephalan Pomphorhynchus laevis and the amphipod Gammarus pulex

Sophie Labaude, Frank Cézilly, Xavier Tercier, Thierry Rigaud

Several parasites with complex life-cycles induce phenotypic alterations in their intermediate hosts. According to the host manipulation hypothesis, such phenotypic alterations are supposed to increase the fitness of the parasite at the expense of that of its intermediate hosts through increasing the probability of transmission to next hosts. Although the phenomenon has received a large attention, the proximate factors modulating the occurrence and intensity of host manipulation remain poorly known. It has however, been suggested that the amount of energy reserves in the intermediate host might be a key parameter, although its precise influence on the intensity of manipulation remains unclear. Dietary depletion in the host may also lead to compromise with other parasite traits, such as probability of establishing or growth or virulence. Here, we address the question through performing experimental infections of the freshwater amphipod Gammarus pulex with two different populations of the acanthocephalan fish parasite Pomphorhynchus laevis, and manipulation of host nutritional condition. Following exposure, gammarids were given either a "standard" diet (consisting of elm leaves and chironomid larvae) or a "deprived" food treatment (deprived in proteins), and infection parameters were recorded. Once parasites reached the stage at which they become infective to their definitive host, refuge use (a behavioural trait presumably implied in trophic transmission) was assessed, and metabolic rate was measured. Infected gammarids exposed to the deprived food treatment showed a lower metabolic rate, indicative of a lower body condition, compared to those exposed to the standard food treatment. Parasite size was smaller, and, depending on the population of origin of the parasites, intensity of infection was lower or mortality was higher in deprived hosts. However, food treatment had no effect on either the timing or intensity of behavioural modifications. Overall, while our results suggest that acanthocephalan parasites develop better in hosts in good condition, no evidence was found for an influence of host nutritional condition on host manipulation by parasites.