Fluorescent sperm offer a method for tracking the real-time success of ejaculates when they compete to fertilise eggs.

Rowan A Lymbery,Jonathan P Evans,W Jason Kennington

doi:10.1038/srep22689

Abstract

Despite intensive research effort, many uncertainties remain in the field of gamete-level sexual selection, particularly in understanding how sperm from different males interact when competing for fertilisations. Here, we demonstrate the utility of broadcast spawning marine invertebrates for unravelling these mysteries, highlighting their mode of reproduction and, in some species, unusual patterns of mitochondrial inheritance. We present a method utilising both properties in the blue mussel, Mytilus galloprovincialis. In mytilids and many other bivalves, both sperm and egg mitochondria are inherited. We exploit this, using the vital mitochondrial dye MitoTracker, to track the success of sperm from individual males when they compete with those from rivals to fertilise eggs. We confirm that dying mitochondria has no adverse effects on in vitro measures of sperm motility (reflecting mitochondrial energetics) or sperm competitive fertilisation success. Therefore, we propose the technique as a powerful and logistically tractable tool for sperm competition studies. Importantly, our method allows the competitive fertilisation success of sperm from any male to be measured directly and disentangled from confounding effects of post-fertilisation embryo survival. Moreover, the mitochondrial dye has broader applications in taxa without paternal mitochondrial inheritance, for example by tracking the dynamics of competing ejaculates prior to fertilisation.

Highlights

Despite intensive research effort, many uncertainties remain in the field of gamete-level sexual selection, in understanding how sperm from different males interact when competing for fertilisations
A critical turning point in the field of sexual selection was the recognition that females often mate with multiple males, or their eggs are exposed to sperm from multiple males, meaning that sexual selection can continue after gamete release[3]
This occurs as sperm competition, where ejaculates from rival males compete for fertilisations[4], and cryptic female choice, where females influence the outcome of such contests[5,6]

Summary

Introduction

Many uncertainties remain in the field of gamete-level sexual selection, in understanding how sperm from different males interact when competing for fertilisations. A critical turning point in the field of sexual selection was the recognition that females often mate with multiple males, or their eggs are exposed to sperm from multiple males, meaning that sexual selection can continue after gamete release[3] This occurs as sperm competition, where ejaculates from rival males compete for fertilisations[4], and cryptic female choice, where females influence the outcome of such contests[5,6]. Both of these mechanisms of sexual selection are widespread across most sexually reproducing taxa and constitute important evolutionary forces acting on both sexes[7,8,9]. Because broadcast spawning is likely the ancestral animal reproductive strategy[22,23], the selective forces shaping this form of reproduction may yield insights into early evolutionary transitions, such as anisogamy to isogamy and external to internal fertilisation (for a recent theoretical model of this “sexual cascade” of events, see ref. 24)

Methods

Results

Conclusion