Insights into the evolution and molecular underpinnings of locust phase polyphenism

Locust phase polyphenism represents a spectacular example of phenotypic plasticity. Locusts exist along a spectrum of two behaviorally defined phenotypes known as solitarious and gregarious phases, which can transform into one another in response to changes in local population density. In this study, we took a comparative approach to understand the evolution of density-dependent behavioral polyphenism and its molecular underpinnings in the Central American locust, Schistocerca piceifrons, and three closely related non-swarming grasshopper species. We found that all four species showed behavioral polyphenism although there was a spectrum from being high polyphenic to only marginally polyphenic. From RNA-seq data, we documented that all four species had density-responsive genes, but the number of such genes was much higher in the locust than other three species. Most of the density-responsive genes were species-specific, but had largely similar functions across species, suggesting that the molecular basis of behavioral polyphenism could be functionally conserved. Finally, we report a potential role of hexamerins in the regulation of locust phase polyphenism.