sperm and egg Interactions
When sperm from multiple males compete to fertilize eggs, natural selection is expected to favor sperm traits that enhance a male’s competitive fertilization success (Jamieson 2009). Thus, the aim of our work is to test hypotheses, which govern ‘optimal’ conditions for sperm swimming and fertility using fishes with different reproductive patterns, such as monogamous vs. promiscuous mating (illustrated Fig. 1). Within this context, we will explore the different ways in which ejaculate traits (i.e. sperm form and function), pre-zygotic selection mechanisms, and competitive and non-competitive environments influence fertilization dynamics and shape the evolution of sexual characteristics.
From Myers, Senior, Zadmajid, Sørensen, Butts: 2020
Fig 1: Realistic representation of spawning dynamics for various fish species (e.g. Atlantic cod Gadus morhua, Pacific salmon Oncorhynchus spp., and Red snapper Lutjanus campechanus). In each scenario, females create unique fertilization micro-environments by expelling their distinct ovarian fluid (OF) along with an egg batch. In turn, these maternally-derived substances can potentially impact the outcome of a fertilization event by modifying sperm trajectories.
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Now previous models of gamete biology have focused mostly on aqueous environments that do not simulate natural spawning conditions; i.e. sperm activation media lack ovarian fluid and other egg-derived compounds. However, when these maternally-derived compounds are incorporated during spawning they can change the outcome of a fertilization event; i.e. we demonstrated that ovarian fluid influenced sperm velocity based on genetic relatedness between mates (Butts et al. 2012a). Thus, when testing hypotheses, we will shift from the classic sperm activation medium paradigm (water only) to more natural spawning media that encompass interactions between gametes and their associated fluids (illustrated in Fig. 2). Results obtained will not merely be of theoretical interest, i.e. for sexual selection and sperm competition theory, but of practical importance for hatchery production.
Fig. 2: A broad schematic that illustrates gamete development and fertility for externally fertilizing fish. Maternal development is in the upper section, while paternal development is in lower section, each represented with a histological image and cross-section of the mature gonad. Each female creates a unique fertilization micro-environment by expelling their distinct ovarian fluid along with an egg batch (top right). In turn, ovarian fluid can directly impact the outcome of a fertilization event by modifying sperm trajectories (bottom right). Abbreviations: GV: germinal vesicle; MO: mature oocyte. From Zadmajid, Myers, Sørensen, Butts, 2019. Theriogenology, 132: 144-152