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Introduction: Species of the ananassae subgroup of Drosophilidae are highly resistant to parasitoid wasp infections. We have previously shown that the genes encoding cytolethal distending toxin B (CdtB) and the apoptosis inducing protein of 56 kDa (AIP56) were horizontally transferred to these fly species from prokaryotes and are now instrumental in the anti-parasitoid immune defense of Drosophila ananassae. Herewe describe a new family of genes, which encode proteins with hemolysin E domains, heretofore only identified in prokaryotes. Hemolysin E proteins are pore-forming toxins, important virulence factors of bacteria.

Methods: Bioinformatical, transcriptional, and protein expressional studies were used.

Results: The hemolysin E-like genes have a scattered distribution among the genomes of species belonging to several different monophyletic lineages in the family Drosophilidae. We detected structural homology with the bacterial Hemolysin E toxins and showed that the origin of the D. ananassae hemolysin E-like genes (hl1-38) is consistent with prokaryotic horizontal gene transfer. These genes encode humoral factors, secreted into the hemolymph by the fat body and hemocytes. Their expression is induced solely by parasitoid infection and the proteins bind to the developing parasitoids.

Conclusions: Hemolysin E-like proteins acquired by horizontal gene transfer and expressed by the primary immune organs may contribute to the elimination of parasitoids, as novel humoral factors in Drosophila innate immunity.