Exploring the effects of genetic variation in host algal resource acquisition traits on disease prevalence in a competitive Daphnia disease system

Background/Question/Methods

Disease dynamics can be significantly altered as a result of interactions between the focal host and its competitors and resources. In addition, intraspecific variation in feeding traits can influence competitive ability and alter the outcome of competitive interactions. We used the experimentally tractable disease system of the fungus Metschnikowia bicuspidata and its host Daphnia dentifera to ask how intraspecific variation in resource acquisition traits influences disease prevalence, both in the presence and absence of interspecific competition.  Daphnia dentifera commonly co-occurs and competes with Daphnia pulicaria, which consumes the parasite spores but does not become infected. As a result, D. pulicaria may reduce disease prevalence via a dilution effect. A 5-population mathematic model was combined with laboratory experiments in which clones of D. dentifera, which vary in their feeding rates and sensitivity to resources, were infected with Metschnikowia both alone and in the presence of D. pulicaria under two resource levels.  Disease prevalence in D. dentifera was measured in 8 treatments (high and low quality food x sensitive vs. insensitive host D. dentifera genotypes x presence/absence of D. pulicaria). 

Results/Conclusions

Our model demonstrates the existence of various biologically relevant equilibria, both disease-free and endemic. Theoretical analysis and simulations suggest that the feeding traits of the host and the available algal resources have a significant effect on disease prevalence. Laboratory assays in which multiple genotypes of both species of Daphnia were raised on high and low quality resources have demonstrated that some genotypes are “powerful”, meaning they grow quickly under high food conditions but suffer from reduced growth rates when resource levels decline, whereas others are “efficient” and have growth rates that are less sensitive to changes in resource conditions. These differences in resource sensitivity are predicted to influence the outcome of competition.  Our model indicates that the presence of the competitor D. pulicaria can reduce disease prevalence in D. dentifera via a dilution effect, but the strength of this dilution varies depending upon the feeding traits of both competitors. Laboratory experiments to test these model predictions are ongoing.