Mechanisms of behavioural variance and plasticity in the desert locust

Individual behavioural variability is widespread and well-studied, but evidence of underlying mechanisms remains scarce. The desert locust has evolved the capacity to express two very distinct eco-phenotypes; a lone-living, cryptic ‘solitarious phase’ and a swarming ‘gregarious phase’ that aggregates into very large and dense groups, in response to contrasting selection pressures that change rapidly. In this thesis, I use this ‘phase polyphenism’ to provide much-needed empirical evidence of evolutionary and proximate mechanisms which underpin individual behavioural variability. In Chapter 1, I review between- and within-individual behavioural variability and discuss implications of variability and plasticity for selection and evolution. I suggest how the molecular chaperone Hsp90, which has been shown to reduce morphological variability, may also regulate both behavioural variability and plasticity in the form of polyphenisms.

In Chapter 2, I demonstrate that gregarious locusts show reduced between- and withinindividual behavioural variability in a novel locomotor hesitation assay. Analyses of an archive of locomotor tracks from a different behavioural assay validate this finding. This provides empirical evidence supporting the hypothesis that group-living animals express lower behavioural variability than lone-living animals.

In Chapter 3, I inhibit Hsp90 activity and expression using pharmacology and RNAi and show that this increases within- and between-individual behavioural variability in locomotor hesitation. This provides the first evidence that Hsp90 reduces behavioural variability analogous to its role in morphology.

In Chapter 4, immuno-dot blot analysis of dissected central nervous systems reveals Hsp90 protein expression is comparable between the two phases and does not change during phase transition. Additionally, inhibition of Hsp90 prior to phase transition does not affect behavioural phase state. This indicates that Hsp90 is not involved in canalising behaviour in locust phase polyphenism.

In Chapter 5, I synthesize these findings, suggest avenues for future research, and discuss mechanistic and evolutionary insights gained from my work.