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.

Ergonomic growth phases of annual social insect societies strongly influence horizontally transmitted parasites. Herein, we focused on the impact of temporal changes in host demography on the population structure of a horizontally transmitted parasite. Seasonal fluctuations in prevalence and the occurrence of multiple infections of the gut parasite Crithidia bombi were analyzed in repeatedly sampled populations of two common bumblebee (Bombus spp.) species. Prevalence of C. bombi was greatest in the middle of the foraging season and coincided with the maximal occurrence of multiple infections. Both decline later in the season. The genetic structure of the parasite population also showed strong seasonal fluctuations with a drastic decline in effective population size and an increase in linkage disequilibrium when infection rates were highest. These effects are mainly attributable to significant changes in parasite allele frequencies leading to selection of specific alleles and increasing the frequency of homozygote genotypes in the middle of the season. Within host, competition between parasite genotypes might explain the observed pattern leading to selection of these alleles, and thus a boost of homozygote genotypes in the middle of the season. Toward the end of the season, selection appears to relax and we observed a recovery in linkage equilibrium, as well as an increase in effective population size. This might be explained by genetic exchange in these trypanosomes in natural populations.

The protozoan parasite Crithidia bombi and its host, the bumblebee Bombus terrestris, are used as a model system for the study of the evolutionary ecology of host-parasite interactions. In order to study these interactions we established a method for in vitro cultivation of single parasite strains. Additionally, a high-throughput method is developed for the determination of cell numbers in cultures by means of optical density (OD) measurements. The protocol for in vitro cultivation allowed for growing different strains on agar plates as well as in culture medium. A calibration curve for the relationship between cell number and OD has been developed. Subsequently, growth rates for different genotypes of C. bombi have been recorded. Significant differences in the growth rates and generation times between these genotypes were demonstrated. As this might be related to the virulence of the parasite, this relationship may be confirmed by in vivo growth rate determination. In comparison with conventional cell counting, the application of OD measurements allows for high-throughput experiments as the time taken to record each sample is reduced by a factor of 30. The in vitro cultivation method allows for controlled infection experiments in order to study host-parasite interactions.

Local adaptation within host‐parasite systems can evolve by several non‐exclusive drivers (e.g., host species‐genetic adaptation; ecological conditions‐ecological adaptation, and time‐temporal adaptation). Social insects, especially bumblebees, with an annual colony life history not only provide an ideal system to test parasite transmission within and between different host colonies, but also parasite adaptation to specific host species and environments. Here, we study local adaptation in a multiple‐host parasite characterized by high levels of horizontal transmission. Crithidia bombi occurs as a gut parasite in several bumblebee species. Parasites were sampled from five different host species in two subsequent years. Population genetic tools were used to test for the several types of adaptation. Although we found no evidence for local adaptation of the parasite toward host species, there was a slight temporal differentiation of the parasite populations, which might have resulted from severe bottlenecks during queen hibernation. Parasite populations were in Hardy‐Weinberg equilibrium and showed no signs of linkage disequilibrium suggesting that sexual reproduction is an alternative strategy in this otherwise clonal parasite. Moreover, high levels of multiple infections were found, which might facilitate sexual genetic exchange. The detection of identical clones in different host species suggested that horizontal transmission occurs between host species and underpins the lack of host‐specific adaptation.

Social insects are the target of numerous pathogens. This is because the high density of closely‐related individuals frequently interacting with each other enhances the transmission and establishment of pathogens. This high selective pressure results in the rapid evolution of immune genes, which might be counteracted by a reduced effective population size (Ne) lowering the effectiveness of selection. We tested the effect of Ne on the evolutionary rate of an important immune gene for the antimicrobial peptide Hymenoptaecin in two common central European bumblebee species: Bombus terrestris and Bombus lapidarius. Both species are similar in their biology and are expected to be under similar selective pressures because pathogen prevalence does not differ between species. However, previous studies indicated a higher Ne in B. terrestris compared to B. lapidarius. We found high intraspecific variability in the coding sequence but low variability for silent polymorphisms in B. lapidarius. Estimates of long‐ and short‐term Ne were three‐ to four‐fold higher Ne in B. terrestris, although the species did not differ in census population sizes. The difference in Ne might result in less efficient selection and suboptimal adaptation of immune genes (e.g. hymenoptaecin) in B. lapidarius, and thus this species might become less resistant and more tolerant, turning into a superspreader of diseases.