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Context-dependent selection at the gamete level in a broadcast spawning invertebrate
The aim of this thesis is to explore patterns of sexual selection acting on gametes in a marine broadcast spawning invertebrate, Mytilus galloprovincialis. In organisms with mating systems such as these (i.e., externally fertilising species) sexual selection is constrained to occur after gamete release, providing exciting opportunities to investigate process of post-ejaculatory sexual selection. Externally spawned gametes often experience highly dynamic environments (e.g., intertidal zones); changing conditions across reproductive episodes can result in very different gamete densities in the water column, which will almost certainly affect gamete interactions (e.g., sperm attraction by eggs), fertilisation success, and patterns of selection on sperm. Further, changes in sperm density could generate sexual selection on egg traits. My work will explore how changes in gamete density affect sperm-egg interactions, and selection on both sperm and egg traits. Lastly, in addition to natural variation in the biotic spawning environment, broadcast spawners also face rapidly changing abiotic conditions through climate change. While gametes and fertilisation appear be robust to these stressors, there are growing reasons to believe that these abiotic pressures may cause molecular changes in gametes that are transmitted to offspring. My work will also briefly touch on non-genetic parental effects on offspring fitness due to climate change.
The gametes of broadcast spawning species face variable and often turbulent external environments that can strongly influence the outcomes of fertilisation. Despite this, much of the research exploring sexual selection on sperm has typically ignored that externally spawned gametes experience highly dynamic environments. Furthermore, broadcast spawning invertebrates are a study system that provide a great opportunity to explicitly investigate patterns of post-ejaculatory sexual selection on egg traits, an area of study that has been largely unexplored. Finally, the fundamental mechanisms underlying gamete interactions and fertilisation are thought to be (almost) universally shared across animal taxa. Broadcast spawning is an ancestral reproductive strategy, preceding the evolution of internal fertilisation, and studying gamete interactions in broadcast spawners can provide important insights into patterns of sexual selection more generally across taxa.