The neurochemical basis of locust behavioural phase change: a pharmacological, behavioural and molecular approach

Locusts undergo dramatic changes in behaviour, morphology and physiology in response to changing population densities, leading to the formation of devastating swarms. Of these forms of phenotypic plasticity, known collectively as phase polyphenism, behavioural plasticity is the most rapid. Shy, cryptic solitarious phase locusts can change their behaviour within 4 h of exposure to conspecifics to resemble that of locusts in the active, swarming gregarious phase (“gregarise”). In this thesis, I use a multidisciplinary approach to investigate the neurochemical mechanisms underpinning behavioural phase polyphenism in the locust. In Chapters 3 and 4, my investigations using an established behavioural assay, High-Performance Liquid Chromatography and pharmacology found no evidence supporting published claims that the neuromodulator 5-hydroxytryptamine in the thoracic ganglia is required for gregarisation. My work indicates a need for further re-investigation of the roles of monoamines in locust phase polyphenism. In Chapter 5, I ran locusts through repeated behavioural assays to reproduce an unexpected phase-specific behavioural change observed in Chapter 4, a potentially novel phase-specific familiarity response to repeated behavioural observations. Solitarious locusts increased their exploratory activity after repeated observations at 2 h intervals, but this effect was not present for longer intervals nor in a different paradigm. I suggest methodological refinements to examine the origins of this potential phase-specific familiarity response. In Chapter 6, I used an assembled transcriptome to identify locust orthologues of four immediate-early genes widely used to create neuronal “activity maps” in other species, in an attempt to apply this to S. gregaria during gregarisation. Quantitative Polymerase Chain Reaction analyses found no activity-related changes in transcription of these genes in solitarious locusts crowded for 30 min compared with uncrowded controls. My work suggests no involvement of these genes in the early stages of gregarisation. This paves the way for optimisation of this novel tool in the locust.