Discussion

Hosts are typically faced with many parasites. Here, I discuss the model system of the bumblebee, Bombus terrestris, and its gut parasites, Crithidia, that illustrates a number of important problems. On one hand, this parasite is highly variable and genetic exchange allows for rapid strain formation. On the other, the host has a fixed genomic repertoire for its main effectors, the anti-microbial peptides. Seemingly, host defences track parasites by variable gene expression. Furthermore, these insects show immune priming and trans-generational immune protection of offspring.