Quantity versus quality: Effects of diet protein-carbohydrate ratios and amounts on insect herbivore gene expression

High-fat and high-sugar diets induce rapid adaptations of fat storage in the house fly Musca domestica L

Dietary change can be a strong evolutionary force and lead to rapid adaptation in organisms. High-fat and high-sugar diets can challenge key metabolic pathways, negatively affecting other life history traits and inducing pathologies such as obesity and diabetes. In this study, we use experimental evolution to investigate the plastic and evolutionary responses to nutritionally unbalanced diets. We reared replicated lines of larvae of the housefly Musca domestica on a fat-enriched (FAT), a sugar-enriched (SUG), and a control (CTRL) diet for thirteen generations. We measured development time in each generation and larval growth and fat accumulation in generations 1, 7, and 13. Subsequently, all lines were reared for one generation on the control diet to detect any plastic and evolutionary changes. In the first generation, time to pupation decreased on a fat-rich diet and increased on a sugar-rich diet. The fat-rich diet increased fat accumulation and, to a lesser extent, the dry weight of the larvae. Multigenerational exposure to unbalanced diets caused compensatory changes in development time, dry weight, and absolute and relative fat content, although pattern and timing depended on diet and trait. When put back on a control diet, many of the changes induced by the unbalanced diets disappeared, indicating that the diet has large plastic effects. Nevertheless, fat-evolved lines still grew significantly larger than the sugar-evolved lines, and sugar-evolved lines had consistently lower fat content. This can be an effect of parental diet or an evolutionary change in nutrient metabolism as a consequence of multigenerational exposure to unbalanced diets.

Population divergence in nutrient-temperature interactions in Pieris rapae

The interaction between larval host plant quality and temperature can influence the short-term physiological rates and life-history traits of insect herbivores. These factors can vary locally, resulting in local adaptation in responses to diet and temperature, but the comparison of these interactions between populations is infrequently carried out. In this study, we examine how the macronutrient ratio of an artificial diet determines the larval growth, development, and survival of larval Pieris rapae (Lepidoptera: Pieridae) at different temperatures between two invasive North American populations from different climatic regions. We conducted a fully factorial experiment with three temperature treatments (18°C, 25°C, and 32°C) and three artificial diet treatments varying in terms of the ratio of protein to carbohydrate (low protein, balanced, and high protein). The effects of diet on life-history traits were greater at lower temperatures, but these differed between populations. Larvae from the subtropical population had reduced survival to pupation on the low-protein diet in the cold temperature treatment, whereas larval survival for the temperate population was equally high for all temperature and diet treatments. Overall, both populations performed more poorly (i.e., they showed slower rates of consumption, growth, and development, and had a smaller pupal mass) in the diet with the low protein ratio, but larvae from the temperate population were less sensitive to diet ratio changes at all temperatures. Our results confirm that the physiological and life-history consequences of imbalanced nutrition for insect herbivores may depend on developmental temperatures, and that different geographic populations of P. rapae within North America vary in their sensitivity to nutritional balance and temperature.

Temperature- and Diet-Induced Plasticity of Growth and Digestive Enzymes Activity in Spongy Moth Larvae

Temperature and food quality are the most important environmental factors determining the performance of herbivorous insects. The objective of our study was to evaluate the responses of the spongy moth (formerly known as the gypsy moth) [Lymantria dispar L. (Lepidoptera: Erebidae)] to simultaneous variation in these two factors. From hatching to the fourth instar, larvae were exposed to three temperatures (19 °C, 23 °C, and 28 °C) and fed four artificial diets that differed in protein (P) and carbohydrate (C) content. Within each temperature regime, the effects of the nutrient content (P+C) and ratio (P:C) on development duration, larval mass, growth rate, and activities of digestive proteases, carbohydrases, and lipase were examined. It was found that temperature and food quality had a significant effect on the fitness-related traits and digestive physiology of the larvae. The greatest mass and highest growth rate were obtained at 28 °C on a high-protein low-carbohydrate diet. A homeostatic increase in activity was observed for total protease, trypsin, and amylase in response to low substrate levels in the diet. A significant modulation of overall enzyme activities in response to 28 °C was detected only with a low diet quality. A decrease in the nutrient content and P:C ratio only affected the coordination of enzyme activities at 28 °C, as indicated by the significantly altered correlation matrices. Multiple linear regression analysis showed that variation in fitness traits in response to different rearing conditions could be explained by variation in digestion. Our results contribute to the understanding of the role of digestive enzymes in post-ingestive nutrient balancing.