Immune stimulation alters chemical profiles of Tribolium castaneum

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.

Food quality influences density-dependent fitness, but not always density-dependent habitat selection, in red flour beetles (Coleoptera: Tenebrionidae)

Density-dependent habitat selection models inherently rely on the negative relationship between population density and mean fitness in different habitats. Habitats differing in quality, such as different food sources or habitat structure, can have different strengths of density-dependent relationships, which can then affect patterns of density dependence in habitat selection. We tested the hypothesis that density dependence in fitness dictates the patterns in density-dependent habitat selection: individuals should prefer higher-quality habitat over lower-quality habitat. We used controlled experiments with red flour beetles (Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)) to measure density dependence of fitness and to examine density-dependent habitat selection by beetles in wheat (Triticum Linnaeus (Poaceae)), corn (Zea mays Linnaeus (Poaceae)), and soy (Glycine max (Linnaeus) Merrill (Fabaceae)) flour habitats. Despite large differences in fitness between habitats (fitness was the highest in wheat flour, lower in corn flour, and zero in soy flour), beetles showed only weak preference for wheat over corn flour and for corn over soy flour, but showed strong preference for wheat over soy flour. These preferences were the strongest at low density. This study gives insight into the relationship between habitat quality and density-dependent habitat selection in flour beetles.

Artificial selection on walking distance suggests a mobility-sperm competitiveness trade-off

Securing matings is a key determinant of fitness, and in many species, males are the sex that engages in mate searching. Searching for mates is often associated with increased mobility. This elevated investment in movement is predicted to trade-off with sperm competitiveness, but few studies have directly tested whether this trade-off occurs. Here, we assessed whether artificial selection on mobility affected sperm competitiveness and mating behavior, and if increased mobility was due to increased leg length in red flour beetles (Tribolium castaneum). We found that, in general, males selected for decreased mobility copulated for longer, stimulated females more during mating, and tended to be better sperm competitors. Surprisingly, they also had longer legs. However, how well males performed in sperm competition depended on females. Males with reduced mobility always copulated for longer than males with high mobility, but this only translated into greater fertilization success in females from control populations and not the selection populations (i.e. treatment females). These results are consistent with a mate-searching/mating-duration trade-off and broadly support a trade-off between mobility and sperm competitiveness.