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Molecular insights into post‐mating immune response in a fish with internal fertilization

The tight connection between immunity and reproduction has been studied for decades. However, basic knowledge at the molecular level of the effect of mating on immune function is still lacking in many taxa. Determining whether and how the immune system is engaged after mating is a crucial step in understanding post‐mating mechanisms of reproduction and sexual selection. Here, we study the transcriptional changes in immunity‐related genes caused by the ejaculate in the female reproductive tract using a model species for sexual selection studies, the guppy Poecilia reticulata. To study changes triggered by the ejaculate only, rather than caused by mating, we used artificial inseminations to transfer ejaculate into females. We then compared gene expression in the reproductive tract (gonoduct and ovary) of females artificially inseminated either with ejaculate or with a control solution, after 1 hr and after 6 hr. Overall, contact with ejaculate caused short‐term changes in the expression of immune‐related genes in the female reproductive tract, with a complex pattern of up‐ and down‐regulation of immune‐related pathways, but with clear indication of a marked down‐regulation of the immune system shortly after ejaculate contact. This suggests a link between immune function and processes occurring between mating and fertilization in this species.