Immune stimulation alters chemical profiles of Tribolium castaneum

I.Rapid recognition of disease cues is essential for preventing pathogenic infections and for disease management in group-living animals. Healthy individuals across taxa can detect illness in other conspecifics and adjust their responses to limit further infections of themselves and the group. However, little is known about potential changes in chemical phenotypes due to disease, which may mediate these responses. We here asked whether individual immune experience resulting from wounding or the injection of heat-killed bacteria of Bacillus thuringiensis (i.e., immune priming) leads to changes in the chemical profiles of adult red flour beetles (Tribolium castaneum). This group-living insect species is a well-studied example for both immune priming as a form of innate immune memory and niche construction via external immunity, i.e., the secretion of quinone-containing antimicrobials into the flour. Upon interaction with wounded conspecifics, naive beetles were previously found to not only up-regulate immunity, but moreover reduce gene expression of the evolutionary capacitor HSP90, an effect that has the potential to enhance adaptability. We here used gas chromatography-flame-ionisation detection (GC-FID) to study the composition of stink gland secretions and cuticular hydrocarbons (CHCs) of immune-primed and wounded beetles compared to controls. The overall profiles as well as target compounds of the stink gland secretions showed transient, slight changes after these treatments, particularly in wounded females. Priming and wounding led to pronounced changes in CHC profiles with increases in the proportion of methyl-branched alkanes. Furthermore, we found sex-specific differences, that were particularly pronounced in the CHCs, although the changes due to immune stimulation were overall similar in both sexes. We suggest that CHCs are potential candidates for the transfer of information related to individual immunological experience into the group.