Transcriptomic response of Hermetia illucens L. (Diptera: Stratiomyidae) to wounding and Gram-negative bacterial infection

The larvae of the black soldier fly (BSFL), Hermetia illucens L. (Diptera: Stratiomyidae), are of economic interest due to their use as livestock feed component. Unraveling their response to a bacterial infection will allow us to gain a better insight into their biology. In the current study, we used RNA-Seq analysis to unravel the transcriptomic response of BSFL to wounding and infection by a Gram-negative bacterium, Pseudomonas protegens Pf-5. Five-day-old BSFL were subjected to three treatments, i.e. untreated, PBS-injection, and bacteria-injection (5000 CFU per larva) and samples were collected at three time points (2 h, 6 h, and 13 h) post- treatment. Wounding induced expression of genes encoding pathogen recognition molecules and signaling pathway genes such as PGRP-SA, and Relish, and antimicrobial peptides (AMPs) such cecropin, defensin, and attacin. At 2 h, wounding resulted in a significant upregulation of immunity-related genes whereas genes encoding for resilin and cuticle proteins were significantly downregulated. At 6 h, the expression of immunity-related genes reduced in response to wounding whereas their expression increased in infected larvae. At 13 h, the expression of immunity-related genes reduced drastically in response to wounding, while their expression increased significantly in infected larvae. Conversely, the expression of metabolism-related genes, such as trypsin and chymotrypsin, was significantly upregulated in wounded larvae at 13 h, while their expression was significantly reduced in infected larvae. Increased investments in immunity-related processes in infected BSF larvae correlated with the downregulation of genes associated with metabolic processes indicative of a trade-off. Various immunity-related genes, including those encoding cecropin, defensin, attacin, and PGRP-SA, were consistently induced only during pathogen infection, indicating their role in immunity against Gram-negative bacteria. In this study, we report multiple genes that are significantly upregulated post-bacterial infection in BSFL that may be utilized as biomarkers to monitor insect health in mass production facilities.