Poor starts and silver spoons: how diet shapes sex-specific fitness from birth to death

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'..methinks I would not grow so fast, Because sweet flowers are slow ...' (Shakespeare, Richard III, Act II Scene 4)There is huge variation in the way that organisms live their lives. For example, mayflies famously live and reproduce as adults for only 1 day, while turtles live and reproduce for many decades. However, what is becoming increasingly clear is that males and females from the same species can also exhibit striking variation in these life histories, and may differ in lifespan, growth rate and duration of reproduction.It is thought that the expression of contrasting life histories by males and females allows each sex to increase their reproductive success. The ultimate reasons for this stem from fundamental differences between the sexes in reproductive investment, with females typically investing more in fewer, larger eggs and males less in their more numerous sperm. The consequences that flow from this basic difference are profound and dictate the contrasting ways in which each sex achieves reproductive success over their whole lifetimes.For example, males are predicted to compete strongly with each other for access to matings and fertilisations. This can lead to a high risk / high pay off 'live fast die young' strategy that is energetically demanding and hence requires a high carbohydrate (C) diet. On the other hand, females are expected to benefit from adopting a 'live slow die old' strategy contingent on the acquisition of high protein (P) to support high fecundity and promote survival. Experiments are consistent with this idea but we have surprisingly little insight into the effects on reproductive success of sex-specific responses to diets across the whole lifespan. Nor do we understand how each sex responds differently to diet. Our aim in this research programme is to address these major gaps.A major problem in achieving the next steps has been a lack of model systems and manipulations for the direct experimental tests that are urgently needed. Fortunately, the fruit fly model system, in combination with recent advances in 'Nutritional Geometry' (diets formulated with precise ratios of C:P) offer powerful tools to address this gap. For example, we can vary diets across the life course in a defined manner, generate large cohorts for high-resolution tests of survival and reproductive success and use genetic and molecular genetic reagents to test effects of manipulating nutrient sensing pathways. In addition, we have evolved populations of flies on different P and C ratios over many generations, which offers a unique opportunity to examine whether these diets favour the success of females versus males, respectively. Our overarching aim is to identify how and why sex differences in responses to diets occur, as follows:1. We will test the direct effect on the reproductive success of males and females of varying diets during development and adulthood. We will manipulate diets with varying C:P ratios and test the effect on lifespan and reproductive success for each sex separately.2. We will investigate the mechanisms associated with the sex-specific responses. In this we will use molecular and genetic tools to measure the impact of altered nutrient sensing pathways on male and female reproductive success.3. We will measure the reproductive success of males and females from lines with an evolutionary history of exposure to high and low C and P. This will test the idea that the balance of male to female success has evolved. Tests of the expression of nutrient sensing genes will also reveal evolved changes in these underlying pathways.The work will provide the first direct tests of the consequences and mechanisms underpinning sex differences in responses to diet across the whole life course. The work is expected to result in a major advance in our fundamental understanding of why and how males and females exhibit different life histories.