Intralocus sexual conflict over optimal nutrient intake and the evolution of sex differences in lifespan and reproduction

Despite widespread variation in lifespan across species, three clear patterns exist: sex differences in lifespan are ubiquitous, lifespan is commonly traded against reproduction, and nutrition has a major influence on these traits and how they trade-off. One process that potentially unites these patterns is Intralocus Sexual Conflict (IASC) over the optimal intake of nutrients for lifespan and reproduction. If nutrient intake has sex-specific effects on lifespan and reproduction and nutrient choice is genetically linked across the sexes, IASC will occur and may prevent one or both sexes from feeding to their nutritional optima. Here we determine whether this process is operating in the cricket Gryllodes sigillatus. We show that protein and carbohydrate intake have contrasting effects on lifespan and reproduction in the sexes and that there are strong positive intersexual genetic correlations for the intake of these nutrients under dietary choice. This divergence in nutrient effects, combined with the genetic architecture for nutrient choice is predicted to accelerate the evolutionary response of nutrient intake in males but constrain it in females, suggesting they are losing the conflict. Supporting this view, males and females were shown to regulate nutrient intake to a common ratio that was not perfectly optimal for lifespan or reproduction in either sex, especially in females. Our findings show that IASC over the optimal intake of nutrients is likely to be an important process generating sex differences in lifespan and reproduction and may help explain why females age faster and live shorter than males in G. sigillatus.The data from the 3 experiments are presented as separate worksheet in the Hunt et al_data file. In each worksheet, we used the following abbreviations for the variables measured: P = protein, C = carbohydrate, LS= lifespan, DRE = daily reproductive effort, LRE = lifetime reproductive effort. For each variable, if a "Z" is presented in front of the abbreviation it signifies that the variable underwent a Z – transformation prior to analysis. Funding provided by: Royal SocietyCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000288Award Number: UF0762844Funding provided by: Natural Environment Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000270Award Number: NE/G00949X/1Funding provided by: Australian Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000923Award Number: DP180101708Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IOS-1118160Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IOS-1654028This work consists of 3 main experiments. In Experiment 1, we examine how the intake of protein and carbohydrates influences lifespan and reproduction in male and female crickets. The data is analysed using response surface analysis and comparisons between the sexes (and across traits within the sexes) are characterised using sequential models and the calculation of angles between vectors and distances between nutritional optima. In Experiment 2, we examine the quantitative genetic basis of protein and carbohydrate intake in males and females using 9 inbred lines of crickets. These crickets were given the choice between two diets differing in the ratio of protein:carbohydrate but with the same caloric content. Genetic estimates were obtained using a Bayesian analysis. In Experiment 3, we examined the regulated intake point of males and females when given the choice between a broader range of diets differing the ratio of protein:carbohydrates and total caloric content. This data was analysed using multivariate analysis of variance (MANOVA) and analysis of covariance (ANCOVA). Data from Experiments 1 and 2 were used to estimate the strength of intralocus sexual conflict over the optimal intake of nutrients for lifespan. Data from Experiments 1 and 3 were used to examine how well male and female crickets regulated their intake of nutrients relative to the optima for lifespan and reproduction