Immune response to sympatric and allopatric parasites in a snail-trematode interaction

Erik E Osnas,Curtis M Lively

doi:10.1186/1742-9994-2-8

Abstract

BackgroundThe outcome of parasite exposure depends on the (1) genetic specificity of the interaction, (2) induction of host defenses, and (3) parasite counter defenses. We studied both the genetic specificity for infection and the specificity for the host-defense response in a snail-trematode interaction (Potamopyrgus antipodarum-Microphallus sp.) by conducting a reciprocal cross-infection experiment between two populations of host and parasite.ResultsWe found that infection was greater in sympatric host-parasite combinations. We also found that the host-defense response (hemocyte concentration) was induced by parasite exposure, but the response did not increase with increased parasite dose nor did it depend on parasite source, host source, or host-parasite combination.ConclusionThe results are consistent with a genetically specific host-parasite interaction, but inconsistent with a general arms-race type interaction where allocation to defense is the main determinant of host resistance.

Highlights

The outcome of parasite exposure depends on the (1) genetic specificity of the interaction, (2) induction of host defenses, and (3) parasite counter defenses
The significant three-way interaction between dose, host, and parasite indicates that the strength of the two-way interaction increased with dose (Fig. 1a vs. b)
There was a significant effect of parasite exposure on hemocyte count (F = 3.71, d.f. = 2, 57, P = 0.03; Table 2; Fig. 2)

Summary

Introduction

The outcome of parasite exposure depends on the (1) genetic specificity of the interaction, (2) induction of host defenses, and (3) parasite counter defenses. One approach tends to emphasize the induction and cost of defense against parasites, while the other approach tends to emphasize the genetic basis and specificity required for successful infection. Both avenues have been productive, but they need not be seen as mutually exclusive [1,2,3]. Kurtz et al [5] showed that the immune response of grasshoppers was reduced in foreign environments, even though body mass, a measure of general condition, was not reduced They interpreted this to mean that the grasshoppers require less immune defense in the face of foreign, and presumably locally adapted, parasites.

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Discussion

Conclusion