Drivers of host specificity in an aquatic crustacean host

Background/Question/Methods

Transmission is determined by more than the relationship between a single parasite and single host. Often, multiple species in a community can be impacted by a parasite and in turn, impact parasite transmission. Less susceptible host species can still become infected and in some cryptic interactions, not become infected but still suffer fitness consequences from parasite challenges. Parasite abundance is influenced by both direct and indirect interactions with its primary host and other community members. The variation in these hosts’ immune systems can influence parasite transmission. We focus on two coexisting freshwater zooplankton species (Daphnia pulicaria and Daphnia dentifera) and other species in the genus (Daphnia magna and Daphnia pulex) that vary in their susceptibility to a fungal parasite, Metschnikowia bicuspidata, to better understand drivers of immune differences in these parasite-host community interactions and their fitness consequences. An experiment was done to observe Metschnikowia spores in Daphnia guts to test if gut volume is correlated with decreased spore penetration and if this relationship is affected by host species, genotype, and age. In a second experiment, we tested the fitness consequences of parasite exposure on two co-existing species, one highly susceptible (D. dentifera) and one thought to be unsusceptible (D. pulicaria).

Results/Conclusions

Intra- and interspecific variation in spore penetration was observed within and between both host species. We observed mechanisms that influenced differences in gut penetrability between species. Surprisingly, the results of my experiments also showed that D. pulicaria can become infected with Metschnikowia in rare cases. However, there was no difference in fitness between D. pulicaria individuals exposed to Metschnikowia spores and those that were unexposed, while D. dentifera exposed to the parasite showed a decline in fitness. This research furthers our knowledge in host specificity and the factors that influence it in four invertebrate host species. These interactions between Daphnia hosts and Metschnikowia give us a clearer understanding of what it means to be a suitable/unsuitable host, which is key to our understanding of disease patterns and their effect on community ecology.