A proteomics approach to quantify adaptive variation in mammalian ejaculates

This project addresses the role of seminal fluid proteins in mammalian sperm competition. Sperm competition occurs when the ejaculates of different males compete to fertilize a given set of ova. This is a pervasive selection pressure in the evolution of ejaculate characteristics and other reproductive traits for diverse animal taxa. In particular, it is now well established that male investment in the sperm component of ejaculates is strongly influenced by sperm competition. However, sperm usually make up only a tiny fraction of total ejaculate volume (1-5% for humans) compared to seminal fluid. Since proteins in the seminal fluid are essential to male fertilization success, it is likely that male investment in non-sperm components of the ejaculate will also be critical to determining sperm competition outcomes. Hence, to maximize fertilization success, males should tailor both the sperm and seminal fluid components of their ejaculates according to competitive conditions. To test this, we will combine advanced quantitative proteomics techniques with carefully controlled experiments using rodent models. Recently, we have demonstrated remarkable diversity in the seminal fluid proteins of muroid rodents, consistent with rapid evolution under sperm competition. To test for evidence of adaptive plasticity in ejaculate components, as predicted by sperm competition theory, we propose a systematic exploration of how investment in these proteins varies within species according to competitive risk. For the first time in any animal group, the application of advanced quantitative proteomics methodologies will allow us to precisely quantify variation in both seminal fluid protein and sperm content of the ejaculate when sperm competition is experimentally manipulated under controlled conditions. We will test for adaptive variation in the relative and total amount of seminal fluid proteins in rodent ejaculates, and quantify their relationships with sperm numbers. Specifically, we will determine ejaculate investment strategies according to (1) cues of sperm competition risk at the time of mating, (2) population-average sperm competition risk or intensity and (3) male dominance status. As well as testing for predicted changes in the ejaculate content of proteins with known functions, our research programme will reveal if other seminal fluid proteins vary consistently in relation to competitive conditions, and thus identify further candidate proteins with likely functional roles in sperm competition. These findings will establish the role of seminal fluid proteins in adaptive variation of mammalian ejaculate composition. To assess the fitness consequences of different ejaculate investment strategies, we will determine reproductive success for males mating under competitive conditions. This will provide insight into the functional significance of adaptive variation in mammalian ejaculate composition. These findings will have general relevance to understanding the functional and evolutionary consequences of adaptive variation in mammalian ejaculate composition.