Exploring the role of microbiota in mediating sexually dimorphic infection outcomes in mealworm beetles

Sexually dimorphic responses to pathogenic infections in animals may stem from sex-specific differences in their life history and immune investment. Recent evidence highlights that such sex-specific variations in immune responses can also be critically regulated by the microbiota. However, direct experiments to test how the microbiota jointly impacts sex-specific immunity and vulnerability to pathogens are still limited. To this end, we used Tenebrio molitor beetles to first establish that the sexes appear to differ in their microbiota composition and infection responses. Females were more vulnerable to bacterial infections and carried a higher bacterial load than males. When we depleted the microbiota, only females improved their post-infection survival, leading to a decrease in the level of sex-specific divergence in infection outcomes. Males, on the other hand, remained unaffected. Microbiota recolonization (via feeding on faecal matter) of microbiota-depleted females increased their susceptibility to infection again, restoring the strong sexual dimorphism. We thus found a potential association between microbiota and infection responses. We also found reduced expression of an antimicrobial peptide, tenecin 1, in females, which could be associated with their higher infection susceptibility, but such immune gene versus phenotypic associations were not consistent across microbiota manipulations. Immune strategies that are required to mediate the plausible links between microbiota and infection response might thus vary with microbiota manipulations, warranting future investigations.