The nutritional geometry of parental effects: maternal and paternal macronutrient consumption and offspring phenotype in a neriid fly

1. Although the ecological and evolutionary importance of environmentally induced parental effects is now widely recognized, such effects are still typically studied by contrasting just two environments in a single parental sex. Yet, parental effects should generally be viewed as reaction norms, and a more complete understanding of their ecological role therefore requires examining continuously varying and interacting environmental variables in both parental sexes. 2. We used nutritional geometry to investigate linear, nonlinear and interactive effects of protein and carbohydrate in maternal and paternal larval diets on offspring juvenile development and viability and adult body size and shape in the fly Telostylinus angusticollis (Diptera: Neriidae). 3. We found that egg hatching success was enhanced by protein in the maternal larval diet but reduced by protein in the paternal larval diet, while other juvenile traits were unaffected by parental diets. 4. Maternal effects on offspring adult body size and head elongation (a secondary sexual trait in males) were mediated by linear and quadratic effects of protein, and were consistent in sons and daughters. In contrast, paternal effects on offspring body size and head elongation were mediated by carbohydrate effects or carbohydrate─protein interactions, and varied by offspring sex. 5. Our findings show that macronutrients in the parental larval diet can have complex, nonlinear and interactive effects on offspring traits, and that the effects of maternal and paternal diets can be strikingly different. Effects of parental diet on offspring represent important fitness consequences of variation in nutrient intake, with potential implications for the evolution of foraging and reproductive strategies.