Abstract

Butterflies have evolved different color patterns on their dorsal and ventral wing surfaces to serve different signaling functions, yet the developmental mechanisms controlling surface-specific patterning are still unknown. Here, we mutate both copies of the transcription factor apterous in Bicyclus anynana butterflies using CRISPR/Cas9 and show that apterous A functions both as a repressor and modifier of ventral wing color patterns, as well as a promoter of dorsal sexual ornaments in males. We propose that the surface-specific diversification of wing patterns in butterflies proceeded via the co-option of apterous A into various gene regulatory networks involved in the differentiation of discrete wing traits. Further, interactions between apterous and sex-specific factors such as doublesex may have contributed to the origin of sexually dimorphic surface-specific patterns. Finally, we discuss the evolution of eyespot pattern diversity in the family Nymphalidae within the context of developmental constraints due to apterous regulation.\n\nSignificance statementButterflies have evolved different wing patterns on their dorsal and ventral wing surfaces that serve different signaling functions. We identify the transcription factor, apterous A, as a key regulator of this surface-specific differentiation in butterflies. We also show a role for apterous A in restricting the developmental origin of a novel trait, eyespots, to just the ventral wing surface. Dorsal-ventral differentiation of tissues is not just restricted to butterfly wings but occurs in many other organs and organisms from arthropods to humans. Thus, we believe that our work will be of interest to a diverse group of biologists and layman alike interested in the role of development in shaping biodiversity.