A novel Hox gene promoter fuels the evolution of adaptive phenotypic plasticity in wing eyespots of satyrid butterflies.

Adaptive phenotypic plasticity allows organisms to display distinct phenotypes in response to variable environments, but little is known about the genomic changes that promote the evolution of plasticity on a macroevolutionary scale. Here, combining tissue-specific transcriptomics, comparative genomics and genome editing, we show that temperature-mediated plasticity in the size of butterfly eyespot wing patterns, a derived seasonal adaptation estimated to have evolved ~60 million years ago at the base of the satyrid clade (~2,700 extant species), is fuelled by the recruitment of a Hox gene Antennapedia (Antp) to eyespot development. In satyrid butterflies, Antp regulates eyespot size in a temperature-dependent manner, increasing plasticity levels. The cooption of Antp to eyespots was driven by the evolution of a novel eyespot-specific promoter in satyrid genomes, which when disrupted in a model satyrid, Bicyclus anynana, reduced plasticity levels. We show that a taxon-specific cis-regulatory innovation in a conserved developmental gene fuelled the evolution of adaptive phenotypic plasticity across a large clade of animals.