Natural and sexual selection linkages across the biphasic life cycle: experimental insight from fish

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Animals have biphasic life cycles that alternate between diploid (adult=2 chromosome copies) and haploid (gametes=1 copy) phases. Across this cycle, selection can be conceptualized at four stages: natural and sexual selection in the diploid phase, natural and sexual selection in the haploid phase. These stages are the main drivers of evolution - eg, a male bird that does not survive to adulthood (1), or survives but does not acquire a mate (2), contributes no gametes to selection in the haploid phase, and no direct descendants in the next generation. He also leaves no descendants if he does mate but his sperm are incapable of fertilizing an egg (3) or winning paternity under sperm competition (4). The stages are usually studied independently, but selection stages on traits expressed during the diploid and haploid phases are interconnected. Improved knowledge of the connected opportunities for selection within biphasic life cycles enables clearer insights into ecological and evolutionary processes, with benefits to applied science. In that context, my research focuses on events leading up to and following the haploid phase. Using external fertilizing fish, we examine the interactions between male and female gametes experiencing both sexual and natural selection in the fertilization micro-environment. The long-term goal is to determine how selection connections associated with the haploid phase, influence fertilization and development in two study systems. Progress will be achieved from 3 short-term objectives: (1) How do diploid (adult) experiences influence sexual selection in the haploid phase (among gametes)? This objective fills key knowledge voids in my study systems with capelin and hybridizing salmon/trout. It will determine whether pre- and post-mating sexual selection complement each other, and if poor diploid diet reduces the ability of sexual selection in the haploid phase to reduce hybridization. (2) Can constraints imposed by natural selection impact the outcome of sexual selection in the haploid phase? Within the diploid phase, the trade-off between natural and sexual selection is well known, such as the `inconvenience' of elaborate male bird tail feathers to survival. However, within the haploid phase it is not. We have virtually no understanding of when and how natural selection modifies the outcomes of sexual selection among gametes. We will determine how the scope for female biasing of sperm competition (cryptic female choice) is impacted by natural selection constraints. (3) How do haploid (gamete) experiences influence natural selection in the diploid phase (development of offspring)? This can occur if for example there are epigenetic changes to individual gametes that affect embryos. While keeping the same father, targeted work will show how sperm exposure to either stressful salinity (capelin) or river acidification (salmon), or rival male seminal fluid under sperm competition, impact subsequent offspring development.