Male ageing and the ejaculate: composition, competition and conflict

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Parents are becoming older. In the UK, the US and other developed countries, there is a trend for people to have children later, and with this trend comes a suite of potential difficulties. For women, many of these difficulties are relatively well known: there is a time limit imposed by the menopause, fertility declines with age and there are increased chances of genetic disorders in the children of older mothers. But less well known is that males also have a "biological clock"; sperm quality and fertility declines with age and the probability of certain genetic disorders in children of older men is also increased. These impacts of ageing don't just apply to humans but are likely ubiquitous across the animal kingdom. There is currently lots of research, in model organisms such as flies and mice, to help us understand the basic biology and evolution of ageing, the genes and molecules that cause it, and how we might develop interventions to slow ageing and promote health in people. Along with this type of research often comes insights into female reproductive ageing. This is because reproductive ageing in females is obvious: females often produce fewer offspring as they age and this is easy to quantify. Male reproductive ageing is, however, more cryptic. Male offspring-production occurs via females, so it is experimentally more challenging to measure. Moreover, there are several reasons why males might fail reproductively. For example, it could be because they don't mate, because they don't make effective sperm or seminal fluids, or because they fail to transfer ejaculates to females if they do mate. However, using the fruit fly, Drosophila melanogaster, it is now possible to measure many aspects of the semen, and new "proteomic" techniques mean that we can quantify the overall composition of the male ejaculate. Now is the ideal time to utilise these techniques, and our extensive and increasing knowledge about the biology of ageing in this species, to test how ageing impacts the male ejaculate and to explore the consequences of ejaculate ageing. This is what my project aims to do. In the first part, I will characterise how the ejaculate changes as males age. Do males produce fewer sperm or less seminal fluid, as they get older? Does the composition of the semen change? How much do they transfer to females? I will investigate how various important factors influence ejaculates in ageing males, such as nutrition, health and mating activity. I will also test whether interventions that increase lifespan, such as dietary restriction (a reduction in food intake without starvation), or the manipulation of genes involved in nutrient-sensing pathways, can lead to healthier ejaculates in ageing males. Combining the information gleaned from these experiments with evolutionary theory I will make predictions about how male ejaculate ageing should affect female fertility and reproductive success, and the outcome of sperm competition between males. In the second part of the project I will test evolutionary theories, to tell us how male ageing affects the adaptive reproductive decisions of males and females. For example, are females better off mating with young or old males? Does the sperm-competitive ability of males change as they age? By genetically manipulating male reproductive proteins I will be able to uncover the genes that causes such age-related changes. The goal of the project is to reveal the importance of age-related changes in the ejaculate, whether anything can be done to ameliorate negative effects of ejaculate ageing, and to help explain the age-mediated reproductive decisions that males and female make.