Selection and Evaluation of Tissue Specific Reference Genes in Lucilia sericata during an Immune Challenge

The larvae of the common green bottle fly Lucilia sericata (Diptera: Calliphoridae) have been used for centuries to promote wound healing, but the molecular basis of their antimicrobial, debridement and healing functions remains largely unknown. The analysis of differential gene expression in specific larval tissues before and after immune challenge could be used to identify key molecular factors, but the most sensitive and reproducible method qRT-PCR requires validated reference genes. We therefore selected 10 candidate reference genes encoding products from different functional classes (18S rRNA, 28S rRNA, actin, β-tubulin, RPS3, RPLP0, EF1α, PKA, GAPDH and GST1). Two widely applied algorithms (GeNorm and Normfinder) were used to analyze reference gene candidates in different larval tissues associated with secretion, digestion, and antimicrobial activity (midgut, hindgut, salivary glands, crop and fat body). The Gram-negative bacterium Pseudomonas aeruginosa was then used to boost the larval immune system and the stability of reference gene expression was tested in comparison to three immune genes (lucimycin, defensin-1 and attacin-2), which target different pathogen classes. We observed no differential expression of the antifungal peptide lucimycin, whereas the representative targeting Gram-positive bacteria (defensin-1) was upregulated in salivary glands, crop, nerve ganglion and reached its maximum in fat body (up to 300-fold). The strongest upregulation in all immune challenged tissues (over 50,000-fold induction in the fat body) was monitored for attacin-2, the representative targeting Gram-negative bacteria. Here we identified and validated a set of reference genes that allows the accurate normalization of gene expression in specific tissues of L. sericata after immune challenge.