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Post-copulatory sexual selection in birds: sperm production, sperm selection and early development in birds

Final Report Summary - PCSSIB (Post-copulatory sexual selection in birds: sperm production, sperm selection and early development in birds)

The aims of PCSSIB were to establish, using birds as a model system (i) how sperm reach the ovum and whether those that fertilise ova are a random subset of those inseminated into a female, and (ii) to estimate the energetic costs of making sperm. We discovered that following insemination, the storage and utilization of sperm by the female is under female endocrine control, with sperm stored only in a specific period before ovulation, followed by a period of release and subsequent utilization. We found that sperm from different makes are stored in different sperm storage tubules, and that some selection of sperm occurs between release from the tubules and reaching the ovum since the sperm reaching the ovum differed in several ways from those inseminated. We found that, in contrast to the situation in humans and other mammals, one sperm is insufficient to create a developing embryo in birds. Instead, while a single sperm can achieve fertilization, additional sperm are essential for embryo development. This phenomenon, referred to as physiological polyspermy, is an integral art of avian reproduction. Despite being socially monogamous, the females f many bird species routinely copulate with more than one male, creating a situation known as ‘sperm competition’. Sperm competition results in the evolution of a wide range of traits, physiological, anatomical and behavioural, associated with reproduction. One of those traits is sperm morphology and across species those with higher levels of female promiscuity have longer sperm. We showed through our research that in a competitive scenario longer sperm are more likely to achieve fertilization and this is because longer sperm swim faster, and probably therefore occupy the sperm storage tubules before other sperm and hence have a competitive advantage when it comes to fertilization. We also identified the genetic basses for the sperm design in zebra finches and show that this is due to a ‘supergene’ or a genetic inversion on the Z chromsome. Ouyr grant has substantially advanced our understanding of the reproductive biology of both male and female birds and has implications for our understanding of human fertility.