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Nutrition is vital to health, but while the link between diet and body nutritional composition is well explored in humans and other vertebrates, this information is not well understood in insects, despite the vital roles they play in ecosystems, and their increasing use as experimental models. Here we used Nutritional Geometry to explore the rapid physiological response to ingested nutrients in the haemolymph nutritional profile of Spodoptera littoralis caterpillars. We ask whether blood nutrients are maintained homeostatically in the face of variable nutritional intake, or if regulation is more flexible for some nutrients than others (allostasis), which allows animals to adapt to stress by responding in a way that prioritises efficiency of responses in the face of trade-offs. Caterpillars were placed on 1 of 20 diets, systematically varying in their nutrient ratios (protein: carbohydrate) and density (calorie content), and their consumption was measured. After 48 h, caterpillars were bled, and the macronutrient (protein, carbohydrates and lipids) and nutrient metabolite (amino acids and simple sugars) content of the haemolymph was measured. Proteins comprised 93% of the haemolymph macronutrient pool on average and their concentration increased with protein eaten. The amino acid (AA) pool was dominated by five AAs, and the total pool increased with total nutrient intake. However, the ratio of essential to non-essential AAs increased as the proportion of protein consumed increased. Carbohydrates were tightly controlled, increasing only on the most extreme carbohydrate intakes. Simple sugars were dominated by glucose and trehalose, and overall, the simple sugar pool showed high levels of homeostasis. Rather than strict homeostasis of blood nutritional properties, an allostatic model seemed to be a better fit for blood nutrient regulation in this generalist herbivore. This flexibility in response to the nutritional composition of the diet may, in part, explain how this

Recent research has suggested that the outcome of host-parasite interactions is dependent on the diet of the host, but most previous studies have focussed on ‘top down’ mechanisms i.e. how the host’s diet improves the host immune response to drive down the parasite population and improve host fitness. In contrast, the direct impacts of host nutrition on parasite fitness or the mechanisms underpinning these effects are relatively unexplored. Here, using a model host-pathogen system (Spodoptera littoralis caterpillars and Xenorhabdus nematophila, an extracellular bacterial blood parasite), we explore the effects of host dietary macronutrient balance on pathogen growth rates both in vivo and in vitro, allowing us to compare pathogen growth rates both in the presence and absence of the host immune response. In vivo, high dietary protein resulted in lower rates of bacterial establishment, slower bacterial growth, higher host survival and slower speed of host death; in contrast, the energy content and amount of carbohydrate in the diet explained little variation in any measure of pathogen or host fitness. In vitro, we show that these effects are largely driven by the impact of host dietary protein on host hemolymph (blood) osmolality (i.e. its concentration of solutes), with bacterial growth being slower in protein-rich, high osmolality, hemolymphs, highlighting a novel ‘bottom up’ mechanism by which host diet can impact both pathogen and host fitness.

Nutrition is vital to health and the availability of resources has long been acknowledged as a key factor in the ability to fight off parasites, as investing in the immune system is costly. Resources have typically been considered as something of a "black box", with the quantity of available food being used as a proxy for resource limitation. However, food is a complex mixture of macro- and micronutrients, the precise balance of which determines an animal's fitness. Here we use a state-space modelling approach, the Geometric Framework for Nutrition (GFN), to assess for the first time, how the balance and amount of nutrients affects an animal's ability to mount an immune response to a pathogenic infection. Spodoptera littoralis caterpillars were assigned to one of 20 diets that varied in the ratio of macronutrients (protein and carbohydrate) and their calorie content to cover a large region of nutrient space. Caterpillars were then handled or injected with either live or dead Xenorhabdus nematophila bacterial cells. The expression of nine genes (5 immune, 4 non-immune) was measured 20 h post immune challenge. For two of the immune genes (PPO and Lysozyme) we also measured the relevant functional immune response in the haemolymph. Gene expression and functional immune responses were then mapped against nutritional intake. The expression of all immune genes was up-regulated by injection with dead bacteria, but only those in the IMD pathway (Moricin and Relish) were substantially up-regulated by both dead and live bacterial challenge. Functional immune responses increased with the protein content of the diet but the expression of immune genes was much less predictable. Our results indicate that diet does play an important role in the ability of an animal to mount an adequate immune response, with the availability of protein being the most important predictor of the functional (physiological) immune response. Importantly, however, immune gene expression responds quite di

Nutrition is vital to health and the availability of resources has long been acknowledged as a key factor in the ability to fight off parasites, as investing in the immune system is costly. Resources have typically been considered as something of a “black box”, with the quantity of available food being used as a proxy for resource limitation. However, food is a complex mixture of macro- and micronutrients, the precise balance of which determines an animal's fitness. Here we use a state-space modelling approach, the Geometric Framework for Nutrition (GFN), to assess for the first time, how the balance and amount of nutrients affects an animal's ability to mount an immune response to a pathogenic infection. Spodoptera littoralis caterpillars were assigned to one of 20 diets that varied in the ratio of macronutrients (protein and carbohydrate) and their calorie content to cover a large region of nutrient space. Caterpillars were then handled or injected with either live or dead Xenorhabdus nematophila bacterial cells. The expression of nine genes (5 immune, 4 non-immune) was measured 20 h post immune challenge. For two of the immune genes (PPO and Lysozyme) we also measured the relevant functional immune response in the haemolymph. Gene expression and functional immune responses were then mapped against nutritional intake. The expression of all immune genes was up-regulated by injection with dead bacteria, but only those in the IMD pathway (Moricin and Relish) were substantially up-regulated by both dead and live bacterial challenge. Functional immune responses increased with the protein content of the diet but the expression of immune genes was much less predictable. Our results indicate that diet does play an important role in the ability of an animal to mount an adequate immune response, with the availability of protein being the most important predictor of the functional (physiological) immune response. Importantly, however, immune gene expression responds quite di

Ecosystems consist of pairwise relationships that determine the characteristics of communities. Host-pathogen relationships are unique in ecological communities since hosts can also be considered as an ecosystem in which commensal microorganisms, parasites and host tissues compete for resources. At all ecosystem levels, resource availability produces bottom-up effects that can determine the outcome of competitive interactions. Nutrients are key resources due to their ability to influence life-history. Theories such as nutritional geometry provide a framework for the investigation of nutrient interactions with life-history traits. Insect models are common due to their economic importance as pests as well as some similarities with human biology, hence much of what is known about nutrition and immunity comes from insects. Using the Egyptian cotton leafworm, Spodoptera littoralis (Lepidoptera: Noctuidae) and its natural pathogen Xenorhabdus nematophilus (Enterobacteriaceae) as a model system, this thesis investigates the influence of host nutrition on pathogen virulence and host resistance/tolerance. Findings confirmed the role of dietary protein intake on S. littoralis resistance against pathogens (Chapter 2) and indicated possible carbohydrate-use by the caterpillar to tolerate infection (Chapter 3). Dietary macronutrient intake altered the proportion of key nutrient groups in the haemolymph as well as concentrations of individual nutrients (Chapter 4). Both in vitro and in vivo, X. nematophilus exploited host carbohydrates for growth, indicating direct competition for host resources (Chapters 3 & 5). In vitro assays also revealed direct inhibition of bacterial growth by host haemolymph protein (Chapter 5). Altogether these results allude to a greater significance for the self-medication behaviour displayed by S. littoralis during infection.

Nutrition is vital to health and the availability of resources has long been acknowledged as a key factor in the ability to fight off parasites, as investing in the immune system is costly. Resources have typically been considered as something of a “black box”, with the quantity of available food being used as a proxy for resource limitation. However, food is a complex mixture of macro- and micronutrients, the precise balance of which determines an animal’s fitness. To unpick the effects of the balance and amount of nutrients on an animal’s ability to mount an immune response, we used the Geometric Framework for Nutrition (GFN). Spodoptera littoralis caterpillars were assigned to one of 25 diets that varied in the ratio of macronutrients (protein and carbohydrate) and their energy density to cover a large region of nutrient space. Caterpillars were then handled or injected with either live or dead Xenorhabdus nematophila bacterial cells. The expression of 9 genes (6 immune, 3 non-immune) was measured 24 h post immune challenge. For two of the immune genes (PPO and Lysozyme) we also measured the relevant functional immune response in the blood. Gene expression and functional immune responses were then mapped against nutritional intake. The expression of all immune genes was upregulated by injection with dead bacteria, but only those in the IMD pathway (Moricin and Relish) were substantially upregulated by both dead and live bacterial challenge. Functional immune responses increased with the protein content of the diet but the expression of immune genes was much less predictable. Nonetheless, immune gene expression did predict the relevant functional immune response, but the relationship between the two varied over nutrient space, such that you could infer functional activity from gene expression only when protein availability was not limiting. Our results indicate that diet does play an important role in the ability of an animal to mount an adequate immune response

Ecosystems consist of pairwise relationships that determine the characteristics of communities. Host-pathogen relationships are unique in ecological communities since hosts can also be considered as an ecosystem in which commensal microorganisms, parasites and host tissues compete for resources. At all ecosystem levels, resource availability produces bottom-up effects that can determine the outcome of competitive interactions. Nutrients are key resources due to their ability to influence life-history. Theories such as nutritional geometry provide a framework for the investigation of nutrient interactions with life-history traits. Insect models are common due to their economic importance as pests as well as some similarities with human biology, hence much of what is known about nutrition and immunity comes from insects. Using the Egyptian cotton leafworm, Spodoptera littoralis (Lepidoptera: Noctuidae) and its natural pathogen Xenorhabdus nematophilus (Enterobacteriaceae) as a model system, this thesis investigates the influence of host nutrition on pathogen virulence and host resistance/tolerance. Findings confirmed the role of dietary protein intake on S. littoralis resistance against pathogens (Chapter 2) and indicated possible carbohydrate-use by the caterpillar to tolerate infection (Chapter 3). Dietary macronutrient intake altered the proportion of key nutrient groups in the haemolymph as well as concentrations of individual nutrients (Chapter 4). Both in vitro and in vivo, X. nematophilus exploited host carbohydrates for growth, indicating direct competition for host resources (Chapters 3 & 5). In vitro assays also revealed direct inhibition of bacterial growth by host haemolymph protein (Chapter 5). Altogether these results allude to a greater significance for the self-medication behaviour displayed by S. littoralis during infection.