Antimicrobial peptide genes are over-expressed in uninfected nub1flies. (A-F) Reporter β-galactosidase staining (CecA1-lacZ) in fat body and intestine of uninfected flies. Strong β-galactosidase expression was observed in fat body and posterior midgut (arrows) in nub1 mutant background (B,D) but not in wild type background (A,C). Elevated reporter gene expression was confirmed using RNA interference against nub in the fat body (CecA1-lacZ/c564-Gal4;UAS-dsnub/+) (F), in comparison with control flies (CecA1-lacZ/c564-Gal4; +/+) (E). (G,H) Quantification of CecA1, Dipt and Drs mRNA levels in extracts of whole flies by RT-qPCR after pre-treatment with a cocktail of antibiotics (A+) or untreated control (A–) (G), and in RelE20 mutant background (H). (I) Quantification of CecA1 and Dipt mRNA levels by RT-qPCR in extracts of dissected intestines. The data are mean values; n = 3 (G), n = 4 (H,I). Statistical significance was calculated using paired t-test, *P <0.05, **P <0.01, ***P <0.001. fb, fat body; hg, hind gut; mg, midgut; pv, proventriculus; ns, not significant.