Feed a fever, starve a haemolymph fungus: Host resource quantity affects virulence in a Daphnia-Metschnikowia interaction

Background/Question/Methods

In order to survive, grow, and reproduce, many parasites must steal resources (energy and nutrients) from their hosts. The parasite’s virulence emerges from this theft, as infected hosts are left with less resources for their own survival, growth, and reproduction. However, if the parasite obtains its resources solely from the host, its virulence is inherently dependent upon the quality and quantity of food available to the host. We have explored the linkages among resource, host, and parasite in theoretical, laboratory, and field studies. Here we present results from a laboratory experiment testing the effects of resource quantity for Daphnia dentifera (host) on the virulence of Metschnikowia bicuspidata (parasite). After infecting hosts, we subjected them to one of four consistent food quantity treatments. We tracked host survival, reproduction, and total growth as well as production of parasites for all individual hosts. Uninfected hosts were used as controls for each food treatment.  
Results/Conclusions

Time to death for infected hosts decreased significantly with increasing food. Despite living for fewer days, hosts in higher food treatments were larger and carried more parasites at time of death. Infected hosts still reproduced, but at a reduced rate compared to uninfected hosts. This decrease in reproductive rate increased in magnitude with increasing food treatment. Infection and resource level interacted to affect both clutch size and frequency of reproductive efforts of hosts. In total, parasite virulence increased with food supply to the host. These results demonstrate that host resource ecology plays an important role in mediating the virulent effects of Metschnikowia on Daphnia.