Extending the Nutritional Geometric Framework to Three Dimensions

Abstract

Nutrition has pervasive effects on an organism’s fitness, influencing traits across life history stages from early development and reproduction to ageing and lifespan. Consequently an understanding of these effects, and the mechanisms underpinning them, is of high importance in understanding variation in fitness. The current framework for studying nutrition (Nutritional Geometric Framework NGF) is limited in its usefulness in emerging questions in nutrition, and expanding the taxonomic reach of studies, because nutrients can only be studied in two dimensions (i.e. two nutrients can be studied simultaneously). In this thesis, I present an extension to the NGF with increased dimensionality, allowing more macronutrients to be manipulated simultaneously. To achieve this I apply principles of multivariate selection analysis to macronutrient consumption, allowing accurate analysis of the linear and non-linear effects of individual macronutrients, alongside correlational effects caused by interactions between macronutrients. I then test this framework in a study of reproduction (Chapter 1) and lipid deposition and obesity (Chapter 2) in the cockroach Nauphoeta cinera, a species where nutritional effects have been examined previously, to test the practicality of the new framework, and assess new insight gained by this extended framework. In Chapter 1 I found all three macronutrients (protein (P), carbohydrates (C) and lipids (L)) to have substantial effects on fitness-related traits (pheromone expression and attractiveness in males and offspring number and gestation time in females), with sex-specific effects in trait maximisation, and new insight into traits compared to previous work using only two macronutrients (P and C). I also showed that males and females regulate their balanced macronutrient intake to almost exactly the same point when given a choice between diets. In Chapter 2 I examined the effect of these macronutrients on lipid deposition in male and female cockroaches. Again I found large effects of all three macronutrients on lipid deposition, with sex-specific effects of different macronutrients maximising lipid deposition in males and females, despite the fact that both sexes regulate their intake to almost the same point. Across both experiments the shared regulated intake point was a closer match to female traits than male traits, suggesting the possibility for intralocus sexual conflict over the optimal intake of macronutrients. 3 Collectively, my thesis illustrates the effectiveness of the new multidimensional framework, and demonstrates that extra insight can be gained in previously studied areas that only examine the effects of two nutrients. Furthermore, it opens the door for wider application of the nutritional geometric methodology to longstanding questions in ageing and obesity, providing a more relevant framework for nutritionally complex species.