<em>Araucan</em> regulates butterfly wing iridescence by coordinating scale structure and pigmentation

Butterfly wings exhibit a remarkable diversity of structural iridescent colors, yet the genetic regulation of iridescence remains poorly understood. Here, we show that the Iroquois-complex transcription factor gene araucan plays a role in modulating wing scale iridescence in the common buckeye butterfly, Junonia coenia. Using CRISPR-Cas9 knockouts, we demonstrate that loss of araucan function causes dorsal wing scales to shift from golden-brown to blue iridescence, and eyespot center scales to shift from saturated purple-violet to dull grey-brown. These phenotypes are associated with changes in thickness of the lower lamina in scale cells, a structural feature known to influence thin-film interference, along with reduction of pigmentation in ground scales. Together, these results identify araucan as a single transcription factor that determines coloration by simultaneously regulating both photonic architecture and light-absorbing pigmentation, and show that these effects can be regulated in pattern-specific manner. Our findings provide a framework for linking gene regulation to the spatially coordinated evolution of structural and pigmentary components of color.