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Background  Host sexual dimorphism is being increasingly recognized to generate strong differences in the outcome of infectious disease, but the mechanisms underlying immunological differences between males and females remain poorly characterized. Here, we used  Drosophila melanogaster  to assess and dissect sexual dimorphism in the innate response to systemic bacterial infection.    Results  We demonstrated sexual dimorphism in susceptibility to infection by a broad spectrum of Gram-positive and Gram-negative bacteria. We found that both virgin and mated females are more susceptible than mated males to most, but not all, infections. We investigated in more detail the lower resistance of females to infection with  Providencia rettgeri , a Gram-negative bacterium that naturally infects  D. melanogaster . We found that females have a higher number of phagocytes than males and that ablation of hemocytes does not eliminate the dimorphism in resistance to  P. rettgeri , so the observed dimorphism does not stem from differences in the cellular response. The Imd pathway is critical for the production of antimicrobial peptides in response to Gram-negative bacteria, but mutants for Imd signaling continued to exhibit dimorphism even though both sexes showed strongly reduced resistance. Instead, we found that the Toll pathway is responsible for the dimorphism in resistance. The Toll pathway is dimorphic in genome-wide constitutive gene expression and in induced response to infection. Toll signaling is dimorphic in both constitutive signaling and in induced activation in response to  P. rettgeri  infection. The dimorphism in pathway activation can be specifically attributed to Persephone-mediated immune stimulation, by which the Toll pathway is triggered in response to pathogen-derived virulence factors. We additionally found that, in absence of Toll signaling, males become more susceptible than females to the Gram-positive  Enterococcus faecalis . This reversal in susceptibility between male and female Toll pathway mutants compared to wildtype hosts highlights the key role of the Toll pathway in  D. melanogaster  sexual dimorphism in resistance to infection.    Conclusion  Altogether, our data demonstrate that Toll pathway activity differs between male and female  D. melanogaster  in response to bacterial infection, thus identifying innate immune signaling as a determinant of sexual immune dimorphism.    Electronic supplementary material  The online version of this article (doi:10.1186/s12915-017-0466-3) contains supplementary material, which is available to authorized users.

The Toll pathway underlies host sexual dimorphism in resistance to both Gram-negative and Gram-positive bacteria in mated Drosophila