During feeding behavior, many animals are exposed to infection of various microorganisms from their digestive organs. Therefore, the immune system is important for survival against the infection. To elucidate the relationship between the immune system and feeding habitat, we conducted comparative RNA-seq analysis using Drosophila species.
Drosophila flies feed and breed on fermented fruits and sap, where a variety of bacteria and fungi propagate. Drosophila flies defend themselves from invading microorganisms with innate immune system. In our previous studies, we found that Drosophila virilis, which feeds on slime fluxes and decaying parts of trees, is more resistant to Penicillium fungus infection than D. melanogaster, which feeds on fermented fruits. To investigate the immune mechanism responsible for the difference in the antifungal resistance, we compared the expression patterns of immune-related genes in gut, in salivary gland and in fat body between the two species in response to the infection of Penicillium fungus. We found that D. virilis used mainly antibacterial peptide genes, Diptericin and Defensin, whereas D. melanogaster used mainly antifungal peptide genes, Drosomycin. Additionally, expressions of lysozyme genes were also up-regulated in the infected D. virilis. These results indicate that the immune system has been substantially differentiated during the evolution of D. melanogaster and D. virilis. The differences in the immune system may have been evolved as an adaptive response to microbial environments.