Fat body transcriptomic responses to dietary macronutrient balance and Metarhizium infection in the desert locust, Schistocerca gregaria.

Dietary macronutrient balance influences immunity and disease outcomes in most organisms, yet the molecular mechanisms underlying these effects remain poorly understood. We investigated how dietary protein-to-carbohydrate ratio shapes the fat body transcriptomic response of an insect, the desert locust (Schistocerca gregaria), following infection with the entomopathogenic fungus Metarhizium robertsii. Our earlier research showed that a carbohydrate-biased diet extends better survival from infection. We hypothesized that this was due to enriched immune functions, potentially due to more readily accessible energy to support immune responses, and/or because the fungal growth was slowed due to reduced availability of protein or other nutrients. RNA sequencing of fat body tissue collected 96 h post-infection followed by differential expression analysis showed that infection elicited a core transcriptional response, whereas direct dietary effects remained limited. Infection substantially enriched all immune pathways (humoral, cellular, and recognition and signaling) regardless of diet. These results provide support against the hypothesis that differences in survival among dietary treatments are due to differences in host immune pathway activation.