Genetic variation in the cellular response of Daphnia magna (Crustacea: Cladocera) to its bacterial parasite

Abstract

Linking measures of immune function with infection, and ultimately, host and parasite fitness is a major goal in the field of ecological immunology. In this study, we tested for the presence and timing of a cellular immune response in the crustacean Daphnia magna following exposure to its sterilizing endoparasite Pasteuria ramosa. We found that D. magna possesses two cell types circulating in the haemolymph: a spherical one, which we call a granulocyte and an irregular-shaped amoeboid cell first described by Metchnikoff over 125 years ago. Daphnia magna mounts a strong cellular response (of the amoeboid cells) just a few hours after parasite exposure. We further tested for, and found, considerable genetic variation for the magnitude of this cellular response. These data fostered a heuristic model of resistance in this naturally coevolving host–parasite interaction. Specifically, the strongest cellular responses were found in the most susceptible hosts, indicating resistance is not always borne from a response that destroys invading parasites, but rather stems from mechanisms that prevent their initial entry. Thus, D. magna may have a two-stage defence—a genetically determined barrier to parasite establishment and a cellular response once establishment has begun.

Evolutionary ecotoxicology of pesticide resistance: a case study in Daphnia

“…A possible pathway would be through reduced activity of phenoloxidase, a key immune component in Daphnia (Pauwels et al 2010) shown to be less present under pesticide exposure (Campero et al 2008). Further, carbaryl may weaken the carapax, the physical barrier of the host, and a major defence against parasite spores (Auld et al 2010). The idea of a decrease in carapax strength is supported by our microarray analysis, revealing a downregulation of gene fragments related to cuticle proteins under direct carbaryl exposure (Jansen et al unpublished observations).…”

Identifying the Molecular Basis of Host-Parasite Coevolution: Merging Models and Mechanisms

“…[1a], [1b]), our synthesis of molecular mechanisms suggests that two-step processes are likely common and perhaps ubiquitous (see also Auld et al 2010). Recent models have begun to explore such two-step processes when both steps are mediated by traditional qualitative infection matrices.…”

Genetic architecture of resistance in Daphnia hosts against two species of host-specific parasites

“…This is not the case for microsporidian infections, which infect the host by shooting-in a harpoon-like manner-a tube into the host tissue and forcefully injecting their cytoplasm directly into the host cell. Thus, no barrier defence seems to exist against microsporidians (Wittner and Weiss, 1999) and the host's degree of resistance seems determined by genetic polymorphisms at the level of its within-host cell immune system, which is likely much more complex than the apparently simple genetics of the Pasteuria attachment step (Auld et al 2010). Although Pasteuria resistance is also influenced by the host's immune system (Hall and Ebert, 2012), the immune system's overall role in explaining variation in resistance is much less because it encounters only those parasites that were able to pass the barrier defence, that is, that attached to and penetrated into the host.…”