The role of female reproductive fluid in intra-ejaculate sperm selection and its effects on early offspring development in zebrafish

In recent years, the role of female reproductive fluid (FRF) as a mediator of cryptic female choice has been proposed and investigated. Evidence is growing, in external and internal fertilizing species, that FRF can differentially affect sperm performance based on female and male identity and thus bias the outcome of sperm competition between different partners. Sperm selection mediated by FRF operates at different levels, from avoiding hybridization to minimize inbreeding or favoring fertilization by more compatible males. An intriguing but still unexplored possibility is that FRF may also select among sperm from the same ejaculate, thus extending the mechanisms and consequences of cryptic female choice mediated by the FRF to an intra-individual level. Studies on within-ejaculate sperm variation have revealed genetic and phenotypic differences among sperm from the same ejaculate, that can impact their ability to reach and fertilize the eggs, and, ultimately, have repercussions on offspring quality. If FRF can select the best or more compatible sperm within a single ejaculate (i.e., from the same male), this could have important consequences for female fitness through enhanced offspring quality. Here, the zebrafish (Danio rerio) was used to test whether within-ejaculate sperm selection mediated by the FRF can affect embryonic development and early larval traits. As a starting point, a literature research was done to recollect the state of the art on FRF and its known effects on gamete quality and sperm selection in fishes. Subsequently, two in vitro fertilization (IVF) experiments were conducted. A first experiment investigated whether FRF had a direct effect on early embryonic development when added after fertilization. Then, the subsequent experiment focused on FRF-mediated sperm selection and its effects on offspring. Using a split-clutch design and a recently developed sperm selection chamber, eggs from each female were split in two aliquots and fertilized with either sperm selected or not selected by FRF. Fertilization success, embryo survival, hatching rate, and larval morphology and locomotor activity were then analyzed and compared between treatments. This study contributes to a growing understanding of FRF as a potential medium for sperm selection and its potential evolutionary and applied implications.