Male gut microbiome mediates post-mating sexual selection in Drosophila

When females mate with multiple males, sexual selection continues after mating and favors males with more competitive ejaculates. While these traits are physiologically costly and sensitive to internal state, the effects of the gut microbiome on male reproductive performance remains underexplored. Here, we tested how gut microbial communities influence sperm competition outcomes and the traits underpinning them by inoculating male Drosophila melanogaster with fully factorial synthetic consortia of five common commensal bacteria. We found highly context-dependent and non-additive microbial effects, along with two general patterns: Acetobacter-dominated microbiomes enhanced sperm transfer and increased paternity shares by 20-27% relative to germ-free males, whereas species richness strongly predicted sperm viability and female sperm storage. These results indicate that gut microbes contribute to multiple components of ejaculate performance through distinct pathways, potentially including nutrient provisioning for spermatogenesis, acetate-mediated energy metabolism, and microbe-dependent modulation of female sperm use. By integrating microbial ecology with reproductive biology, our findings establish the gut microbiome as an overlooked but fundamental modulator of sexually selected traits. This perspective helps clarify how non-genetic and community-based factors generate phenotypic variation under strong selection and provides a framework for future work on the mechanisms, transmission routes, and evolutionary consequences of host-microbe interactions in reproduction.