Comparing the impacts of macronutrients on life-history traits in larval and adult Drosophila melanogaster: the use of nutritional geometry and chemically defined diets.

Protein and carbohydrate are the two major macronutrients that exert profound influences over fitness in many organisms, including Drosophila melanogaster. Our understanding of how these macronutrients shape the components of fitness in D. melanogaster has been greatly enhanced by the use of nutritional geometry, but most nutritional geometric analyses on this species have been conducted using semi-synthetic diets that are not chemically well defined. Here, we combined the use of nutritional geometry and chemically defined diets to compare the patterns of larval and adult life-history traits expressed across 34 diets systematically varying in protein:carbohydrate (P:C) ratio and in protein plus carbohydrate (P+C) concentration. The shape of the response surfaces constructed for all larval and adult traits differed significantly from one another, with the nutritional optima being identified at P:C 1:4 for lifespan (P+C 120 g l-1), 1:2 for egg-to-adult viability (120 g l-1), 1:1 for female body mass at adult eclosion (240 g l-1) and lifetime fecundity (360 g l-1), 2:1 for larval developmental rate (60 g l-1) and 8:1 for egg production rate (120 g l-1). Such divergence in nutritional optima among life-history traits indicates that D. melanogaster confined to a single diet cannot maximize the expression of these traits simultaneously and thus may face a life-history trade-off. Our data provide the most comprehensive and nutritionally explicit analysis of the impacts of macronutrients on life-history traits in D. melanogaster and support the emerging notion that the fundamental trade-offs among life-history traits are mediated by macronutrients.