Abstract
The innate immune system which helps individuals to combat pathogens comprises a set of genes representing four immune system pathways (Toll, Imd, JNK and JAK/STAT). There is a lack of immune genes in social insects (e.g. honeybees) when compared to Diptera. Potentially, this might be compensated by an advanced system of social immunity (synergistic action of several individuals). The bumble bee, Bombus terrestris, is a primitively eusocial species with an annual life cycle and colonies headed by a single queen. We used this key pollinator to study the temporal dynamics of immune system gene expression in response to wounding and bacterial challenge.Antimicrobial peptides (AMP) (abaecin, defensin 1, hymenoptaecin) were strongly up-regulated by wounding and bacterial challenge, the latter showing a higher impact on the gene expression level. Sterile wounding down-regulated TEP A, an effector gene of the JAK/STAT pathway, and bacterial infection influenced genes of the Imd (relish) and JNK pathway (basket). Relish was up-regulated within the first hour after bacterial challenge, but decreased strongly afterwards. AMP expression following wounding and bacterial challenge correlates with the expression pattern of relish whereas correlated expression with dorsal was absent. Although expression of AMPs was high, continuous bacterial growth was observed throughout the experiment.Here we demonstrate for the first time the temporal dynamics of immune system gene expression in a social insect. Wounding and bacterial challenge affected the innate immune system significantly. Induction of AMP expression due to wounding might comprise a pre-adaptation to accompanying bacterial infections. Compared with solitary species this social insect exhibits reduced immune system efficiency, as bacterial growth could not be inhibited. A negative feedback loop regulating the Imd-pathway is suggested. AMPs, the end product of the Imd-pathway, inhibited the up-regulation of the transcription factor relish, which is necessary for effector gene expression.
Highlights
Host-parasite interactions are characterized by a permanent challenge between pathogenicity factors of the parasite and resistance mechanisms of the host
B. terrestris Antimicrobial peptides (AMP) expression was regulated in different intensities pending on treatment (Table 1)
E. coli injection had a higher impact on AMP expression compared to sterile injection
Summary
Host-parasite interactions are characterized by a permanent challenge between pathogenicity factors of the parasite and resistance mechanisms of the host. Social insects seem to be especially prone to parasite attacks, as their colonies frequently contain a high density of individuals, usually showing highly related genotypes, facilitating high transmission rates. Social insects have an advanced system for the maintenance of nest homeostasis which may aid the transmission of certain parasitic species within the colony. Despite these characteristics enhancing parasite intrusion several mechanisms preventing parasites from ‘breaking into the fortress’ might exist. These range from behavioural adaptations, such as a highly advanced system of nest defence and nestmate recognition to hygienic behaviour like self- and allogrooming. Group effects might exist, known as social immunity [4]
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