Phase Polyphenism in Locusts: Mechanisms, Population, Consequences, Adaptive Significance and Evolution

Authors: SIMPSON, STEPHEN - UNIVERSITY OF SYDNEY; Sword, Gregory

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 1/1/2009
Publication Date: 1/1/2009
Citation: Simpson, S.J., Sword, G.A. 2009. Phase Polyphenism in Locusts: Mechanisms, Population, Consequences, Adaptive Significance and Evolution. In: Whitman, D.W., and Ananthakrishnan, T.N., editors. Phenotypic Plasticity of Insects: Mechanisms and Consequences. Science Publishers Inc., Plymouth. p. 147-190.

Interpretive Summary: Locusts express a remarkable form of phenotypic plasticity called phase polyphenism in which local population density affects the expression of a variety of behavioural, physiological, and morphological traits. Behaviour is the most labile of the various density-dependent phenotypic changes and provides positive feedbacks that drive the process at a population level. Locusts avoid each other in the low-density 'solitarious' phase, but they actively aggregate in the high-density 'gregarious' phase. Individuals change behavioural phase rapidly, in a matter of hours. Behavioural phase change is mediated primarily by direct physical contact among locusts, with the major site of mechanosensory input being the hind legs - a finding that has opened the possibility to study phase change as a model system for neuronal plasticity. Behavioural phase state is also transmitted epigenetically across generations. Females can manipulate the phenotypes of their developing offspring to an extent that reflects both maternal and paternal experience of crowding. This maternal effect is mediated during oviposition by a chemical agent introduced into the foam surrounding the eggs. Laboratory and field experiments in conjunction with individual-based computer models have elucidated the relationship between individual behaviour, population responses, and the spatial distribution and chemical quality of resources within the local environment. Whether a local population of solitarious phase locusts will gregarize, and hence potentially seed larger scale outbreaks, depends critically on the fine-scale distribution and quality of resources. A consequence of fluctuating population densities is that locusts are intermittently exposed to periods of increased disease and predation risk. Recent insights into the expression of density-dependent anti-predator and pathogen resistance strategies indicate that a number of traits involved in locust phase change are specific adaptations to these risks.

Technical Abstract: Locusts express a remarkable form of phenotypic plasticity called phase polyphenism in which local population density affects the expression of a variety of behavioural, physiological, and morphological traits. Behaviour is the most labile of the various density-dependent phenotypic changes and provides positive feedbacks that drive the process at a population level. Locusts avoid each other in the low-density 'solitarious' phase, but they actively aggregate in the high-density 'gregarious' phase. Individuals change behavioural phase rapidly, in a matter of hours. Behavioural phase change is mediated primarily by direct physical contact among locusts, with the major site of mechanosensory input being the hind legs - a finding that has opened the possibility to study phase change as a model system for neuronal plasticity. Behavioural phase state is also transmitted epigenetically across generations. Females can manipulate the phenotypes of their developing offspring to an extent that reflects both maternal and paternal experience of crowding. This maternal effect is mediated during oviposition by a chemical agent introduced into the foam surrounding the eggs. Laboratory and field experiments in conjunction with individual-based computer models have elucidated the relationship between individual behaviour, population responses, and the spatial distribution and chemical quality of resources within the local environment. Whether a local population of solitarious phase locusts will gregarize, and hence potentially seed larger scale outbreaks, depends critically on the fine-scale distribution and quality of resources. A consequence of fluctuating population densities is that locusts are intermittently exposed to periods of increased disease and predation risk. Recent insights into the expression of density-dependent anti-predator and pathogen resistance strategies indicate that a number of traits involved in locust phase change are specific adaptations to these risks.