Developmental responses to variable diet composition in a butterfly: the role of nitrogen, carbohydrates and genotype

Understanding the evolution of herbivore-plant interactions requires detailed information on proximate responses to relevant dietary variability and genetic variance, if any, associated with these responses. We measured the behavioral and developmental responses of Pieris rapae larvae to variation in the nitrogen (N) and carbohydrate (CARB) content of chemically-defined diets, using differences in the average responses of sibling groups to estimate underlying genetic variance. Larval P. rapae responded to dietary reductions in both N and CARB with increased feeding, but these responses were inadequate to compensate for dietary N deficiencies within the range of N found in host plants. As a result, larvae on reduced N diets exhibited lower relative growth rates and longer development times, whereas larvae on reduced CARB diets maintained normal developmental trajectories. We also report evidence for genetic variation underlying (a) compensatory feeding responses to CARB and N availability, (b) N-driven variation in growth rate and (c) CARB-driven variation in larval duration. Our results highlight N availability as a key factor in the growth and development of this herbivorous butterfly, with CARB availability being less constraining and sufficiently addressed by changes to larval consumption rate. Furthermore, our study reveals standing genetic variance associated with larval responses to macronutrient availability, suggesting continued potential for herbivore-plant co-evolution in this system despite a putative history of strong directional selection.