Analysis of the immune-inducible transcriptome from microbial stress resistant, rat-tailed maggots of the drone fly Eristalis tenax

Background: The saprophagous and coprophagous maggots of the drone fly Eristalis tenax (Insecta, Diptera) have evolved the unique ability to survive in aquatic habitats with extreme microbial stress such as drains, sewage pools, and farmyard liquid manure storage pits. Therefore, they represent suitable models for the investigation of trade-offs between the benefits resulting from colonization of habitats lacking predators, parasitoids, or competitors and the investment in immunity against microbial stress. In this study, we screened for genes in E. tenax that are induced upon septic injury. Suppression subtractive hybridization was performed to selectively amplify and identify cDNAs that are differentially expressed in response to injected crude bacterial endotoxin (LPS). Results: Untreated E. tenax maggots exhibit significant antibacterial activity in the hemolymph which strongly increases upon challenge with LPS. In order to identify effector molecules contributing to this microbial defense we constructed a subtractive cDNA library using RNA samples from untreated and LPS injected maggots. Analysis of 288 cDNAs revealed induced expression of 117 cDNAs corresponding to 30 novel gene clusters in E. tenax. Among these immune-inducible transcripts we found homologues of known genes from other Diptera such as Drosophila and Anopheles that mediate pathogen recognition (e. g. peptidoglycan recognition protein) or immune-related signaling (e. g. relish). As predicted, we determined a high diversity of novel putative antimicrobial peptides including one E. tenax defensin. Conclusion: We identified 30 novel genes of E. tenax that were induced in response to septic injury including novel putative antimicrobial peptides. Further analysis of these immune- related effector molecules from Eristalis may help to elucidate the interdependency of ecological adaptation and molecular evolution of the innate immunity in Diptera.