No support for cryptic choice by ovarian fluid in an external fertilizer

Does the Rainbow Trout Ovarian Fluid Promote the Spermatozoon on Its Way to the Egg?

The fertilization of freshwater fish occurs in an environment that may negatively affect the gametes; therefore, the specific mechanisms triggering the encounters of gametes would be highly expedient. The egg and ovarian fluid are likely the major sources of these triggers, which we confirmed here for rainbow trout (Oncorhynchus mykiss). The ovarian fluid affected significantly spermatozoa performance: it supported high velocity for a longer period and changed the motility pattern from tumbling in water to straightforward moving in the ovarian fluid. Rainbow trout ovarian fluid induced a trapping chemotaxis-like effect on activated male gametes, and this effect depended on the properties of the activating medium. The interaction of the spermatozoa with the attracting agents was accompanied by the "turn-and-run" behavior involving asymmetric flagellar beating and Ca²⁺ concentration bursts in the bent flagellum segment, which are characteristic of the chemotactic response. Ovarian fluid created the optimal environment for rainbow trout spermatozoa performance, and the individual peculiarities of the egg (ovarian fluid)-sperm interaction reflect the specific features of the spawning process in this species.

Ovarian fluid proteome variation associates with sperm swimming speed in an externally fertilizing fish

Sperm velocity is a key trait that predicts the outcome of sperm competition. By promoting or impeding sperm velocity, females can control fertilization via postcopulatory cryptic female choice. In Chinook salmon, ovarian fluid (OF), which surrounds the ova, mediates sperm velocity according to male and female identity, biasing the outcome of sperm competition towards males with faster sperm. Past investigations have revealed proteome variation in OF, but the specific components of OF that differentially mediate sperm velocity have yet to be characterized. Here we use quantitative proteomics to investigate whether OF protein composition explains variation in sperm velocity and fertilization success. We found that OF proteomes from six females robustly clustered into two groups and that these groups are distinguished by the abundance of a restricted set of proteins significantly associated with sperm velocity. Exposure of sperm to OF from females in group I had faster sperm compared to sperm exposed to the OF of group II females. Overall, OF proteins that distinguished between these groups were enriched for vitellogenin and calcium ion interactions. Our findings suggest that these proteins may form the functional basis for cryptic female choice via the biochemical and physiological mediation of sperm velocity.

The role of female reproductive fluid in sperm competition

The role of non-gametic components of the ejaculate (seminal fluid) in fertility and sperm competitiveness is now well established. Surprisingly, however, we know far less about female reproductive fluid (FRF) in the context of sexual selection, and insights into male-FRF interactions in the context of sperm competition have only recently emerged. Despite this limited knowledge, evidence from taxonomically diverse species has revealed insights into the effects of FRF on sperm traits that have previously been implicated in studies of sperm competition. Specifically, through the differential effects of FRF on a range of sperm traits, including chemoattraction and alterations in sperm velocity, FRF has been shown to exert positive phenotypic effects on the sperm of males that are preferred as mating partners, or those from the most compatible or genetically diverse males. Despite these tantalizing insights into the putative sexually selected functions of FRF, we largely lack a mechanistic understanding of these processes. Taken together, the evidence presented here highlights the likely ubiquity of FRF-regulated biases in fertilization success across a diverse range of taxa, thus potentially elevating the importance of FRF to other non-gametic components that have so far been studied largely in males. This article is part of the theme issue 'Fifty years of sperm competition'.