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Towards an evolutionary ecology of spermatogenesis

Final Report Summary - SPERM EVOLUTION (Towards an evolutionary ecology of spermatogenesis)

The essentials of sperm function differ little from one species to another, and yet there is tremendous diversity among species in the type and number of sperm a male produces. My research programme aims to understand how these
traits, which are fundamental to male fertility, evolve. Specifically, I aim to identify selective forces acting on sperm, spermatogenesis and the testis, and adopt a multidisciplinary approach to test the working hypothesis that post-copulatory sexual selection has been a major force during sperm and testis evolution. Such a research programme is needed, because evolutionary biologists have traditionally treated the testis as a “black box” (ignoring the details of its function) whereas more clinically-orientated research has usually not considered how evolutionary history shapes the complex machinery of spermatogenesis. The time is ripe for a more integrative programme, and in this project I aimed to develop Macrostomum flatworms as a model system for studying the evolutionary biology of sperm, spermatogenesis and the testis. We have investigated these phenomena using a wide variety of techniques, with principal findings including (i) the discovery of an unusual hypodermic self-insemination behaviour exhibited by M. hystrix flatworms as a form of reproductive assurance under conditions of low mate availability; (ii) the discovery that the speed of spermatogenesis is plastically modulated in M. lignano in response to social environmental conditions, a novel finding in any taxon; and (iii) the documenting of large-scale genome-wide expression changes in response to differing sperm competition levels, including describing genomic reaction norms for gametogenesis. Ongoing genome and transcriptome studies begun during the project will enable us to study the molecular basis of how variation in spermatogenesis traits is achieved over both ecological and evolutionary timescales. The results of the project are therefore enabling us to build a more complete “evolutionary ecology of spermatogenesis” beyond a past focus on simple measures such as gross testis size, with wide implications for our understanding of the male gamete and its production by the testis. In the long-term, I therefore expect the results of the project to help bridge evolutionary and basic biomedical research perspectives on male fertility.