The effects of density-dependent polyphenism on circadian biology of the desert locust Schistocerca gregaria

Locusts demonstrate phenotypic plasticity in behaviour, morphology and physiology, driven by population density changes. At low densities, locusts become ‘solitarious’, are cryptic in behaviour and appearance, and avoid other locusts. At high densities, locusts become ‘gregarious’, are conspicuously coloured, primarily day-active, and aggregate with conspecifics. In this thesis I investigate the endogenous circadian clock, its mechanisms and its outputs. I investigated the timing of both hatching and emergence of eggs from each phenotype. Eggs from gregarious parents hatched earlier than those from solitarious parents but the larvae emerged from the substratum later. I propose that the avoidance response of solitarious animals is expressed in hatchlings and encourages them to escape the egg pod. This represents the first investigation of both hatching and emergence in the desert locust. Subsequently I investigate electroretinogram (ERG) and behavioural responses to visual stimuli. The behavioural response differed, with solitarious animals less likely to hide than gregarious animals and more likely to startle. The amplitude of hiding response was modulated in a diurnal pattern in solitarious but not gregarious animals. I demonstrate diurnal and circadian rhythmicity in the ERG response, showing that the free-running circadian clock expresses a shorter period in gregarious locusts. I hypothesis that this is due to the continuous social interactions that gregarious individuals experience. Finally, I used molecular tools to identify key circadian clock genes and their expression patterns under diurnal conditions. I describe differences in patterns of gene expression between gregarious and solitarious animals with significantly greater depth and accuracy than previous work. I indentify diurnally rhythmic patterns of expression in genes that account for 15% of the transcriptome. This work provides the foundation for future molecular work on S. gregaria, both in terms of differential and rhythmic expression, but also in identifying genes of interest and enabling structural characterisation of the resulting proteins.