The gene cortex controls scale colour identity in Heliconius

Abstract

The wing patterns of butterflies are an excellent system with which to study phenotypic evolution. The incredibly diverse patterns are generated from an array of pigmented scales on a largely two-dimensional surface, resulting in a visibly tractable system for studying the evolution of pigmentation. In Heliconius butterflies, much of this diversity is controlled by a few genes of large effect that regulate pattern switches between races and species across a large mimetic radiation. One of these genes – cortex - has been repeatedly mapped in association with colour pattern evolution in both Heliconius and other Lepidoptera, but we lack functional data supporting its role in modulating wing patterns. Here we carried out CRISPR knock-outs in multiple Heliconius species and show that cortex is a major determinant of scale cell identity. Mutant wing clones lacking cortex showed shifts in colour identity, with melanic and red scales acquiring a yellow or white state. These homeotic transformations include changes in both pigmentation and scale ultrastructure, suggesting that cortex acts during early stages of scale cell fate specification rather than during the deployment of effector genes. In addition, mutant clones were observed across the entire wing surface, contrasting with other known Heliconius mimicry loci that act in specific patterns. Cortex is known as a cell-cycle regulator that modulates mitotic entry in Drosophila, and we found the Cortex protein to accumulate in the nuclei of the polyploid scale building cells of the butterfly wing epithelium, speculatively suggesting a connection between scale cell endocycling and colour identity. In summary, and while its molecular mode of action remains mysterious, we conclude that cortex played key roles in the diversification of lepidopteran wing patterns in part due to its switch-like effects in scale identity across the entire wing surface.