Butterfly wing patterns provide an important model system for studying the interplay among ecological, developmental, and genetic factors in the evolution of complex morphological traits. Dozens of genes have been implicated in wing pattern development thanks to a combination of comparative expression and, more recently, knockout studies (1–4). Interestingly, however, mapping and association work has highlighted only a small subset of these genes that seem to play a causative role in wing pattern adaptation in nature: optix, WntA, cortex, and doublesex (5–10). These genes are particularly compelling for two reasons. First, they have all been genetically associated with local adaptation in multiple populations and/or species, and are thus characterized as “adaptive hotspot” genes that repeatedly drive morphological evolution across different lineages (11, 12). Second, based on detailed crossing and expression studies, we infer that these genes behave as complex trait regulators, with different alleles associated with different spatial expression domains that determine highly varied and complex color patterns, not simply the presence or absence of individual features. Although there is strong interest in these genes for these reasons, their specific developmental roles and the depth of conservation of their color patterning functions remain unclear. Here we present a comparative functional analysis of the optix gene in butterflies. This gene is linked to adaptive geographic variation of red ommochrome color patterns in the genus Heliconius, although its actual function remained unconfirmed before the present study (5, 13). optix is also interesting because it is expressed in association with nonpigmentation wing traits in various species, including morphologically derived wing conjugation scales, suggesting that it may have multiple regulatory roles in both wing scale coloration and structure (5, 14). In the present work, we used Cas9-mediated targeted deletion of optix to test its color patterning function in four species of nymphalid butterflies. Not only did we confirm deeply conserved roles for optix in coordinating pigmentation and scale morphology in all species surveyed, but we were surprised to find that this gene simultaneously regulates blue structural iridescence in some butterflies. Importantly, this coordinated regulation of pigmentation and iridescence strongly phenocopies wing patterns seen in other distantly related species, leading us to hypothesize that optix may have played a role in wing pattern evolution in many different butterfly lineages. create a concept map using the above introduction and the following terms: Cas9-mediated targeted deletion wing conjugation scales wing pattern development knockout studies red ommochrome color pattern optix blue structural iridescence adaptive hotspot adaptive geographic variation