Conservation and flexibility in the gene regulatory landscape of Heliconiine butterfly wings

Background Many traits evolve by cis-regulatory modification, by which changes to non-coding sequences affect the binding affinity for available transcription factors and thus modify the expression profile of genes. Multiple examples of cis-regulatory evolution have been described at pattern switch genes responsible for butterfly wing pattern polymorphism, including in the diverse neotropical genus Heliconius, but the identities of the factors that can regulate these switch genes have not been identified. Results We investigated the spatial transcriptomic landscape across the wings of three closely related butterfly species, two of which have a convergently-evolved, co-mimetic pattern, the other having a divergent pattern. We identified candidate factors for regulating the expression of wing patterning genes, including transcription factors with a conserved expression profile in all three species, and others, including both transcription factors and Wnt pathway genes, with markedly different profiles in each of the three species. We verified the conserved expression profile of the transcription factor homothorax by immunofluorescence, and showed that its expression profile strongly correlates with that of the selector gene optix in butterflies with the Amazonian forewing pattern element 9dennis9. Conclusions Here we show that, in addition to factors with conserved expression profiles like homothorax, there are also a variety of transcription factors and signaling pathway components that appear to vary in their expression profiles between closely related butterfly species, highlighting the importance of genome-wide regulatory evolution between species.