Female reproductive fluid attracts more and better sperm: implications for within-ejaculate cryptic female choice

Mounting evidence shows that the female reproductive fluid (FRF) can differently affect sperm performance of different males by biasing paternity share among competing males. Here, we tested for the first time the potential of 'within-ejaculate cryptic female choice' mediated by the FRF in the zebrafish (Danio rerio). Using a recently developed sperm selection chamber, we separated and collected FRF-selected from non-selected sperm to compare the two subpopulations of sperm in terms of sperm number, viability, DNA integrity and fertilizing ability. We showed that the sperm attracted by FRF are more numerous, more viable and with higher DNA integrity. In addition, FRF-selected sperm fertilized more eggs, but if this is due to fertilization ability per se or numerical advantage remains to be tested. Our results suggest that FRF can select sperm with a better phenotype, highlighting the crucial and impactful role that FRF might play in the process of fertilization and post-mating sexual selection dynamics, along with the potential implications for sperm selection in assisted reproductive techniques.