Lipid metabolism and resource allocation in relation to ecological challenge in the blowfly, Lucilia sericata

In insects, energy is a fundamental but inevitably limited resources, acquired from the diet and stored in the body, largely in the form of lipid, for later use. Insects must partition these resources between competing physiological processes, and this will be influenced by a range of life-history characteristics, and the environment in which they live. The overarching aim of this thesis, therefore, was to consider resource acquisition and its subsequent allocation to growth and reproduction by the blowfly Lucilia sericata, which is both a useful laboratory model species and of applied importance in both livestock husbandry and forensic science. Initially, larval L. sericata, were exposed to different levels of crowding to investigate the impacts of competition and reduced dietary intake on body size and on the lipid available to newly emerged adults (Chapter 3). Larvae reared in more crowded cultures had significantly lower lipid contents and lipid was shown to decline significantly over the course of pupation, leading to smaller adults with lower lipid contents at emergence. Larger females matured significantly greater numbers of eggs and significantly larger eggs, than smaller females. There was no relationship between adult size and longevity. Small females were shown to produce offspring capable of maturing into large adults in the following generation, if larvae were provided with abundant food. The results in this Chapter demonstrate the importance of phenotypic plasticity in body size as an adaptive response to crowding and reduced dietary intake. Subsequent work focused on adult L. sericata. In the experiments described in Chapter 4, females were fed diets differing in protein and carbohydrate (sucrose) content, and the allocation of lipid to reproduction was quantified. When flies were denied access to sucrose, stored lipid then continued to decrease until flies died, usually within four days of emergence. Flies given access to sucrose were able to increase body lipid content, demonstrating that carbohydrate is essential for homeostasis and that it can be used to synthesise lipid. Nevertheless, female flies fed sucrose only were unable to synthesise egg yolk. Only flies provided with protein were able to mature eggs. The results demonstrate the importance of different dietary components for different elements of the life-history of L. sericata, namely survival and reproduction. This theme was extended in Chapter 5, which attempted to isolate the specific components that allow egg production in the protein diet. For this, L. sericata, were fed diets differing in protein formulation (freeze-dried lamb liver, whey powder and soya powder) or amino acid composition. Although female flies provided with one or three amino acids survived, they were unable to synthesise egg yolk. The results show that the mean lipid content and reproductive output varies significantly between different diets, but the key components found in liver that allow egg production could not be identified from this study. Life-history theory suggests that the trade-off between homeostasis, somatic allocation and reproductive allocation will change as insects age. To investigate this issue, in Chapter (6) an analysis of the number and size of eggs matured and the lipid and protein allocation in eggs over time was undertaken in a laboratory investigation. The data show that lipid invested in the ovary, and the number and size of eggs produced declined over time, particularly during the final period of adult life. Finally, in Chapter 7, the findings from the experimental studies are discussed together and the need for future investigations to explore the specific dietary factors that facilitate egg production and the relevance of these physiological processes to field populations, are highlighted.