Comparative analysis of phenotypic plasticity sheds light on the evolution and molecular underpinnings of locust phase polyphenism

Locusts exhibit one of nature’s most spectacular examples of complex
phenotypic plasticity, in which changes in density cause solitary and
cryptic individuals to transform into gregarious and conspicuous locusts
forming large migrating swarms. We investigated how these coordinated
alternative phenotypes might have evolved by studying the Central American
locust and three closely related non-swarming grasshoppers in a
comparative framework. By experimentally isolating and crowding during
nymphal development, we induced density-dependent phenotypic plasticity
and quantified the resulting behavioural, morphological, and molecular
reaction norms. All four species exhibited clear plasticity, but the
individual reaction norms varied among species and showed different
magnitudes. Transcriptomic responses were species-specific, but
density-responsive genes were functionally similar across species. There
were modules of co-expressed genes that were highly correlated with
plastic reaction norms, revealing a potential molecular basis of
density-dependent phenotypic plasticity. These findings collectively
highlight the importance of studying multiple reaction norms from a
comparative perspective.