The effect of host food quality on host and parasite fitness in an invertebrate-parasite system
Snell, Sara Jeanne
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Parasites achieve their fitness by reducing the fitness of the hosts they infect. The relationship between host and parasite fitness is often mediated by environmental conditions, such as the quality of food consumed by the host. We used the crustacean Daphnia dentifera and its virulent yeast parasite Metschnikowia bicuspidata to examine how the quality of the food consumed by the Daphnia affected the probability of the host being infected by the parasite and the fitness consequences of infection for the host (measured as offspring production and survival) and for the parasite (measured as the production of transmission spores within the host following infection). We fed Daphnia either high quality food (Ankistrodesmus falcatus) or low quality food (Oocystis sp.) before exposure to Metschnikowia spores, and then either high quality or low quality food after parasite exposure (according to a fully factorial design). We found that when hosts were fed high quality food as juveniles and adults, they were able to invest more energy in preventing loss of fitness due to parasites without limiting parasite growth; high quality food therefore benefits both parasite and host. High food quality benefited both host and parasite when hosts are infected. We saw that infected hosts that produced more offspring also contained more parasite spores. One possible explanation for this is the individuals that produced more offspring and spores were larger, allowing them to take in more resources.