Sperm velocity in a promiscuous bird across experimental media of different viscosities.

Geir Rudolfsen,Holger Schielzeth,Tim Schmoll,Oddmund Kleven

doi:10.1098/rspb.2020.1031

Abstract

In species with internal fertilization, the female genital tract appears challenging to sperm, possibly resulting from selection on for example ovarian fluid to control sperm behaviour and, ultimately, fertilization. Few studies, however, have examined the effects of swimming media viscosities on sperm performance. We quantified effects of media viscosities on sperm velocity in promiscuous willow warblers Phylloscopus trochilus. We used both a reaction norm and a character-state approach to model phenotypic plasticity of sperm behaviour across three experimental media of different viscosities. Compared with a standard medium (Dulbecco's Modified Eagle Medium, DMEM), media enriched with 1% or 2% w/v methyl cellulose decreased sperm velocity by up to about 50%. Spermatozoa from experimental ejaculates of different males responded similarly to different viscosities, and a lack of covariance between elevations and slopes of individual velocity-by-viscosity reaction norms indicated that spermatozoa from high- and low-velocity ejaculates were slowed down by a similar degree when confronted with high-viscosity environments. Positive cross-environment (1% versus 2% cellulose) covariances of sperm velocity under the character-state approach suggested that sperm performance represents a transitive trait, with rank order of individual ejaculates maintained when expressed against different environmental backgrounds. Importantly, however, a lack of significant covariances in sperm velocity involving a cellulose concentration of 0% indicated that pure DMEM represented a qualitatively different environment, questioning the validity of this widely used standard medium for assaying sperm performance. Enriching sperm environments along ecologically relevant gradients prior to assessing sperm performance will strengthen explanatory power of in vitro studies of sperm behaviour.

Highlights

In species with internal fertilization, spermatozoa typically have to migrate through the female genital tract to reach and eventually fertilize eggs
Under the reaction norm (RN) approach, we tested for effects of experimental media viscosities on sperm velocity by means of linear mixed effects models and used random regression analyses to test for variation in phenotypic plasticity among ejaculates
To test for potential trade-offs between average sperm velocity and the response in sperm velocity to the cellulose gradient, we evaluated the covariance between the ejaculate random intercept term and the cellulose-by-ejaculate random slope term in our random intercept and slope model

Summary

Background

In species with internal fertilization, spermatozoa typically have to migrate through the female genital tract to reach and eventually fertilize eggs. Besides cellulose concentration as our fixed treatment effect, we were mostly interested in among-ejaculate random variation in sperm velocity in order to test the idea that spermatozoa from ejaculates of different males may specialize in their performance in different swimming environments and thereby trade-off high velocity in a low-viscosity environment with their ability to show high velocity in a high-viscosity environment, or vice versa. Note that we had sampled only a single experimental ejaculate per male, such that our ejaculate identity variance term includes both among-individual variation and amongejaculate variation caused by uncontrolled environmental effects (e.g. seasonal plasticity in sperm phenotype). Under the RN approach, we tested for effects of experimental media viscosities on sperm velocity by means of linear mixed effects models and used random regression analyses to test for variation in phenotypic plasticity among ejaculates. We note that the multi-response model estimates separate residual variances for each environment, unlike the random slope model of the RN approach that fits a single residual variance and assumes residuals to be identically distributed across all observations

Results

Findings

Discussion