Antennapedia regulates metallic silver wing scale development and cell shape in Bicyclus anynana butterflies

Butterfly wing scale cells can develop very intricate cuticular nanostructures that interact with light to produce structural colors such as silver, but the genetic basis of such nanostructures is mostly unexplored. Here, we address the genetic basis of metallic silver scale development by leveraging existing crispants in the butterfly Bicyclus anynana, where knockouts of five genes - apterous A, Ultrabithorax, doublesex, Antennapedia and optix - either led to ectopic gains or losses of silver scales. Most wildtype silver scales had low amounts of pigmentation and exhibited a common ultrastructural modification for metallic broadband reflectance, i.e., an undulatory air layer enclosed by an upper and lower lamina. Crispant brown scales differed from wildtype silver scales via the loss of the continuous upper lamina, increased lower lamina thickness, and increased pigmentation. The reverse was seen when brown scales became silver. On the forewings, we identified Antennapedia as a high-level selector gene, acting through doublesex to induce silver scale development in males and having a novel, post-embryonic role in the determination of ridge and crossrib orientation and overall scale cell shape in both sexes. We propose that apterous A and Ultrabithorax repress Antennapedia on the dorsal forewings and ventral hindwings, respectively, thereby repressing silver scale development, whereas apterous A activates the same GRN on the dorsal hindwings, promoting silver scales.