Resources Modulate Developmental Shifts but Not Infection Tolerance Upon Co-Infection in an Insect System

Energetic resources within organisms fuel both parasite growth and immune responses against them, but it is unclear whether energy allocation is sufficient to explain changes in infection outcomes under the threat of multiple parasites. We manipulated diet in flour beetles (Tribolium confusum) infected with two natural parasites and used a combination of transcriptomic and phenotypic assays to investigate the role of resources in shifting metabolic and immune responses after single and co-infection. Our results suggest that relatively benign, single-celled, eukaryotic gregarine parasites alter the within-host energetic environment and, by extension, juvenile development time, in a diet-dependent manner. While they do not affect host resistance to acute bacterial infection, the mRNA-seq results reveal that they stimulate the expression of an alternative set of immune genes and promote damage to the gut, ultimately contributing to reduced survival regardless of diet. Thus, energy allocation is not sufficient to explain the immunological contribution to co-infection outcomes, emphasising the importance of mechanistic insight for predicting the impact of co-infection across levels of biological organisation.