The innate immune system of Drosophila is activated by ingestion of microorganisms. D. melanogaster breeds on fruits fermented by Saccharomyces cerevisiae, whereas D. virilis breeds on slime flux and decaying bark of tree housing a variety of bacteria, yeasts, and molds. In this study, it is shown that D. virilis has a higher resistance to oral infection of a species of filamentous fungi belonging to the genus Penicillium compared to D. melanogaster. In response to the fungal infection, a transcriptome profile of immune-related genes was considerably different between D. melanogaster and D. virilis: the genes encoding antifungal peptides, Drosomycin and Metchnikowin, were highly expressed in D. melanogaster whereas, the genes encoding Diptericin and Defensin were highly expressed in D. virilis. On the other hand, the immune-induced molecule (IM) genes showed contrary expression patterns between the two species: they were induced by the fungal infection in D. melanogaster but tended to be suppressed in D. virilis. Our transcriptome analysis also showed newly predicted immune-related genes in D. virilis. These results suggest that the innate immune system has been extensively differentiated during the evolution of these Drosophila species. 1. Introduction In natural environments, Drosophila species feed and breed on fermenting fruits, slime fluxes on decaying parts of tree, and so forth, where biochemical processes of bacteria and fungi are extremely active [1–3]. Therefore, Drosophila species are exposed to a huge number of microorganisms throughout their developmental stages. Feeding on decaying or fermented materials results in the ingestion of a wide variety of microorganisms in their digestive organs. Recent studies on larval immune response of D. melanogaster to oral infection of bacteria and fungi showed that the fat body mediated systemic immune response including antimicrobial peptide (AMP) production was triggered by infections of Gram-negative bacterial species such as Pseudomonas entomophila and Erwinia carotovora carotovora 15 (Ecc15) and of a dimorphic fungal species, Candida albicans [4–6]. AMPs are cationic small secretory peptides that exhibit a wide range of activities against bacteria, fungi, and/or viruses, playing an essential role in the innate immune system of Drosophila [7]. To date, seven AMP families, that is, Attacin, Cecropin, Defensin, Diptericin, Drosocin, Drosomycin, and Metchnikowin, have been identified in Drosophila melanogaster [7]. According to Sackton et al. (2007), it was indicated by their sequence analysis of the 12
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