Sexual selection, one of Darwin’s seminal theories, has become a very familiar evolutionary force. Essentially, it occurs when some individuals are more successful in reproduction, either because they are better at competing for access to mates or are more attractive to potential mates. Perhaps less widely realised outside of evolutionary biology is that sexual selection continues after mating, with sperm from multiple males often competing for fertilisations, and females (or their eggs) exhibiting ‘cryptic’ choice for preferred sperm. Although these gamete interactions and their evolutionary outcomes are crucial for a wide range of taxa, it is often very difficult to observe them directly. I have developed a method to distinguish sperm from individual males when they compete with rival males, by dyeing them with fluorescent mitochondrial dye. This is an exciting development, as it provides the potential to explore the way rival sperm interact and affect each other in a wide range of competitive scenarios.

I am applying the mitochondrial dye method to the broadcast-spawning blue mussel, Mytilus galloprovincialis, as a putative model system. As an external fertiliser, M. galloprovincialis is very amenable to in vitro crosses of different combinations of gametes; moreover, it has a crucial advantage for our mitochondrial dye technique; paternal mitochondrial inheritance. Sperm mitochondria are transferred to the egg at fertilisation, which provides us with the unique opportunity to determine the competitive success of sperm from individual males at the point of fertilisation. Usually, competitive fertilisation success can only be inferred by genetically determining the paternity of resulting embryos, which is costly and risks confounding the competitive success of sperm with factors that can separately influence the survival of embryos after fertilisation.

The capacity to directly measure competitive fertilisation success provides enormous potential to address unresolved questions in gamete-level sexual selection. In my research, I have shown that the mitochondrial dye technique can reliably be used in M. galloprovincialis to determine fertilisation success in sperm competition scenarios. I am currently using the technique to explore the way females can use chemical attractants released by eggs to alter sperm swimming behaviour and bias fertilisations toward sperm from different preferred males. Furthermore, I aim to extend the technique beyond simple two-male competitive scenarios in M. galloprovincialis, and examine the outcomes and selective implications of biologically realistic competitions between sperm from many males for fertilisation of eggs from many females. The insights yielded by these methods could have important implications for our understanding of gamete interactions across many taxa, and of major evolutionary events in animal reproduction, such as the transition from ancestral broadcast spawning to internal fertilisation.

Edit (17/03/16): This study has now been published: Fluorescent sperm offer a method for tracking the real-time success of ejaculates when they compete to fertilise eggs

Contributed by Rowan Lymbery, PhD student at the University of Western Australia