Drosophila melanogaster seminal fluid can protect the sperm of other males

Many internally-fertilizing animals produce seminal fluid which is transferred along with sperm during mating. Seminal fluid typically contains a diverse range of chemicals that coordinate sperm storage, moderate sperm motility, provide advantages in sexual selection and influence female physiology. Seminal fluid is well-studied in Drosophila melanogaster, a species in which it has been suggested to 'incapacitate' the sperm of rival males (e.g. by killing them) and thereby provide an advantage in sperm competition. This hypothesis has been tested several times over many years, but different studies have yielded conflicting conclusions. Here, I use fluorescent staining to directly measure the effects of D. melanogaster seminal fluid on the survival of sperm from the same male or from a rival. The results suggest that seminal fluid improves sperm survival, even if the sperm are from a different male. This study therefore provides strong evidence that seminal fluid does not kill rival sperm, and instead can actually protect them. This study also tested whether chemicals in the female reproductive tract harm sperm as in another Drosophila species, but found no evidence of this. These findings suggest that residual seminal fluid inside females could benefit the sperm of subsequent mates, affecting the outcome of sperm competition and influencing the evolution of ejaculates and mating systems.