Nutritional symbionts enhance structural defence against predation and fungal infection in a grain pest beetle

Abstract

Many insects benefit from bacterial symbionts that provide essential nutrients and thereby extend the hosts' adaptive potential and their ability to cope with challenging environments. However, the implications of nutritional symbioses for the hosts' defence against natural enemies remain largely unstudied. Here, we investigated whether the cuticle-enhancing nutritional symbiosis of the saw-toothed grain beetle Oryzaephilus surinamensis confers protection against predation and fungal infection. We exposed age-defined symbiotic and symbiont-depleted (aposymbiotic) beetles to two antagonists that must actively penetrate the cuticle for a successful attack: wolf spiders (Lycosidae) and the fungal entomopathogen Beauveria bassiana. While young beetles suffered from high predation and fungal infection rates regardless of symbiont presence, symbiotic beetles were able to escape this period of vulnerability and reach high survival probabilities significantly faster than aposymbiotic beetles. To understand the mechanistic basis of these differences, we conducted a time-series analysis of cuticle development in symbiotic and aposymbiotic beetles by measuring cuticular melanisation and thickness. The results reveal that the symbionts accelerate their host's cuticle formation and thereby enable it to quickly reach a cuticle quality threshold that confers structural protection against predation and fungal infection. Considering the widespread occurrence of cuticle enhancement via symbiont-mediated tyrosine supplementation in beetles and other insects, our findings demonstrate how nutritional symbioses can have important ecological implications reaching beyond the immediate nutrient-provisioning benefits.

Keywords: Candidatus Shikimatogenerans silvanidophilus; Oryzaephilus surinamensis; Bacteroidetes; Cuticle; Mutualism; Sawtoothed grain beetle; Structural defence; Symbiosis.

Fig. 1.

Melanisation progression in symbiotic and aposymbiotic Oryzaephilus surinamensis beetles from day 1 to day 7 post-eclosion. Inverse red values of symbiotic (grey contours, n=108) and aposymbiotic (white contours, n=104) beetles in different age groups. Higher inverse red values reflect darker cuticular coloration. The horizontal line inside each contour represents the median. Significant differences (P<0.05) were observed between treatments in every age group, following Wilcoxon pairwise comparisons with the Benjamini–Hochberg P-adjustment method.

Fig. 2.

Cuticle thickness progression in symbiotic and aposymbiotic beetles from day 1 to day 7 post-eclosion. Mean cuticle thickness of symbiotic (grey contours, n=56) and aposymbiotic (white contours, n=46) beetles in different age groups. The horizontal line inside each contour represents the median. Significant differences (P<0.05) between treatments were observed in every age group, following Wilcoxon pairwise comparisons with the Benjamini–Hochberg P-adjustment method.

Fig. 3.

Impact of symbiont status and age on adult beetle defence against predatory wolf spiders. Survival probability (mean and 95% confidence interval) of symbiotic (green line and shaded area; green dots show single data points) and aposymbiotic (black line and shaded area; black dots show single data points) adult beetles of different ages as predicted by the generalised linear mixed effects model (GLMER). Both symbiont status (***P<0.001) and age (P<0.001) had a significant influence on survival probability (GLMER, spiders n=39, symbiotic n=62, aposymbiotic n=64).

Fig. 4.

Impact of symbiont status on handling time of larvae by predatory wolf spiders, and on larval mass. (A) Spiders (n=28) took significantly longer to handle symbiotic 5th instar larvae (grey contours, n=18) than they did with aposymbiotic larvae (white contours, n=17) (GLM, ***P<0.001). (B) Differences in mass of symbiotic (grey contours, n=19) and aposymbiotic (white contours, n=17) larvae were not significant (ANOVA, ns, P>0.05). The horizontal line inside contours indicates the median.

Fig. 5.

Survival probability of young (<24 h post-eclosion) and old (14 days post-eclosion) symbiotic and aposymbiotic beetles exposed to Beauveria bassiana spores. Mortality was significantly influenced by symbiont status and age (Cox mixed-effects model, P=0.01437 and P<0.001, respectively). Lines depict the mean and the shaded area the 90% confidence interval. Young beetles (dashed lines) of both treatment groups suffered significantly higher mortality than old beetles (solid lines; old aposymbiotic n=45, old symbiotic n=45; ***P<0.001), and young aposymbiotic beetles (dashed black line; n=45) suffered from an earlier onset of mortality as well as a higher mortality rate than young symbiotic beetles (dashed green line; n=45, ***P<0.001).