The gene cortex controls mimicry and crypsis in butterflies and moths
Wing colour patterning of multiple species in the butterfly genus Heliconius is controlled by differential expression of the gene cortex, a member of a conserved family of cell cycle regulators. The darkening of the peppered moth Biston betularia, the phenomenon known as industrial melanism, is a textbook example of evolutionary biology in action. However, the genetic background of the black or carbonaria variants has remained unclear. Building on their earlier work that isolated the gene responsible to within a roughly 400-kilobase region containing 13 genes, Ilik Saccheri and colleagues have identified the melanism-causing event as the insertion of a class II transposable element into the first intron of a gene called cortex. Statistical inference indicates that the polymorphism occurred around 1819, when the Industrial Revolution was well under way. In a separate study, Nicola Nadeau et al. report that colour patterning in butterflies of the genus Heliconius is also mediated by expression of the cortex gene, apparently co-opted to control the rate of scale cell development, on which colour depends. Taken together, these two papers suggest that cortex, conserved widely within Lepidoptera, is a major target for natural selection acting on colour and pattern variation. The wing patterns of butterflies and moths (Lepidoptera) are diverse and striking examples of evolutionary diversification by natural selection1,2. Lepidopteran wing colour patterns are a key innovation, consisting of arrays of coloured scales. We still lack a general understanding of how these patterns are controlled and whether this control shows any commonality across the 160,000 moth and 17,000 butterfly species. Here, we use fine-scale mapping with population genomics and gene expression analyses to identify a gene, cortex, that regulates pattern switches in multiple species across the mimetic radiation in Heliconius butterflies. cortex belongs to a fast-evolving subfamily of the otherwise highly conserved fizzy family of cell-cycle regulators3, suggesting that it probably regulates pigmentation patterning by regulating scale cell development. In parallel with findings in the peppered moth (Biston betularia)4, our results suggest that this mechanism is common within Lepidoptera and that cortex has become a major target for natural selection acting on colour and pattern variation in this group of insects.