Multihost interactions in the plankton: Influences of Daphnia host diversity on the common bacterial parasite Pasteuria ramosa

Background/Question/Methods

Most parasites can infect multiple host species, yet studies still typically focus on pairings between a single host and single parasite species. This has limited our understanding of the factors driving host-parasite dynamics in nature. How readily can parasites move between host species? Does parasite fitness differ between hosts? Are there tradeoffs in ability to exploit different hosts? And how does host species community composition influence dynamics of parasitism in natural populations? This talk will focus on interactions between the virulent bacterial parasite Pasteuria ramosa and its Daphnia and Ceriodaphnia hosts. We carried out a lab experiment in which we quantified the ability of Pasteuria isolates to infect 10 genotypes of Daphnia dentifera and 8 genotypes of Ceriodaphnia. This study also measured how host species identity influenced host and parasite fitness. We also carried out a large field survey in which we quantified the densities of four common host species (Daphnia dentifera, D. pulicaria, D. retrocurva, and Ceriodaphnia) as well as the prevalence of Pasteuriainfections in each of those host species. We carried out this survey in 15 lake populations in southeast Michigan between July and November 2013.

Results/Conclusions

Pasteuria isolated from Daphnia dentifera can infect Ceriodaphnia, but at a much lower rate. Infection prevalence in the 10 D. dentifera genotypes ranged from 5-79%, whereas infection prevalence in Ceriodaphnia ranged from 0-12.5%, with only 3 of 8 genotypes showing any infections. Even in successful infections, Pasteuria isolated from Daphnia had lower fitness when infecting Ceriodaphnia, yielding only 1/3 as many spores per infection on average. Both host species suffered substantial reductions in fecundity as a result of Pasteuria infections, with infected hosts of both species showing similar fecundity (~5 offspring/female). However, since uninfected D. dentifera had twice the fecundity of uninfected Ceriodaphnia, D. dentifera fitness was more strongly impacted by infection. Analyses of patterns of infections in natural lake populations are ongoing, but there was substantial variation among the 15 lake populations in host species composition and levels of disease. Some populations showed no disease, some had epidemics in only a single host species, while others had epidemics in three host species. Future analyses will focus on synchrony in infection dynamics in multiple host species, and on relationships between the relative frequencies of the different host species and patterns of disease prevalence.