Differences in Cellular Immune Competence Explain Parasitoid Resistance for Two Coleopteran Species

Abstract

The immune defence of an organism is evolving continuously, causing counteradaptations in interacting species, which in turn affect other ecological and evolutionary processes. Until recently comparative studies of species interactions and immunity, combining information from both ecological and immunological fields, have been rare. The cellular immune defense in insects, mainly mediated by circulating hemocytes, has been studied primarily in Lepidoptera and Diptera, whereas corresponding information about coleopteran species is still scarce. In the study presented here, we used two closely related chrysomelids, Galerucella pusilla and G. calmariensis (Coleoptera), both attacked by the same parasitoid, Asecodes parviclava (Hymenoptera). In order to investigate the structure of the immune system in Galerucella and to detect possible differences between the two species, we combined ecological studies with controlled parasitism experiments, followed by an investigation of the cell composition in the larval hemolymph. We found a striking difference in parasitism rate between the species, as well as in the level of successful immune response (i.e. encapsulation and melanisation of parasitoid eggs), with G. pusilla showing a much more potent immune defense than G. calmariensis. These differences were linked to differences in the larval cell composition, where hemocyte subsets in both naïve and parasitised individuals differed significantly between the species. In particular, the hemocytes shown to be active in the encapsulation process; phagocytes, lamellocytes and granulocytes, differ between the species, indicating that the cell composition reflects the ability to defend against the parasitoid.

Geographic variation and trade‐offs in parasitoid virulence

“…Galerucella calmariensis mounted an overall poor immune defence, which confirms our previous results (Fors et al . ). In addition, the study shows that the weak defence was independent of the former host species of the parasitoid (Table ).…”

“…In all three Galerucella species, the general cell composition was in accordance with our previous finding (Fors et al . ), consisting of six hemocyte types: granulocytes, phagocytes, prohemocytes, oenocytoids, lamellocytes and lamellocyte precursors, with granulocytes being the most common. Notable was, however, that oenocytoids, which are in general very rare, were more common in G. tenella than in the other two species.…”

“…As previously shown, lamellocytes, phagocytes and granulocytes all participate in the encapsulation process in Galerucella , with lamellocytes being crucial for the capsule formation to be completed (Fors et al . ). In this study, we find the same pattern in a third species ( G. tenella ).…”

“…Differences in immune response in G. pusilla and G. calmariensis have previously been observed (Fors et al . ), as well as among population differences in parasitism rate (i.e. the proportion of Galerucella larvae killed by parasitism).…”

Selection by parasitoid females among closely related hosts based on volatiles: Identifying relevant chemical cues

“…In this system, the host species vary in resistance to parasitoid attack, and it is notable that G. calmariensis (that was selected by parasitoids originating from G. calmariensis in the trials) is the species showing the lowest resistance to parasitoid attack (Fors et al., , ). In fact, Fors et al.…”