Evolution of the potassium channel gene Kcnj13 underlies colour pattern diversification in Danio fish

The genetic basis of morphological variation provides a major topic in evolutionary biology1-6. Colour patterns in fish are among the most diverse of all vertebrates. Species of the genus Danio display strikingly different colour patterns ranging from horizontal stripes, to vertical bars or spots7-10. Stripe formation in zebrafish, Danio rerio, oriented by the horizontal myoseptum, is a self-organizing process based on cell-contact-mediated interactions between three types of chromatophores with a leading role of iridophores11-14. We investigated genes known to regulate chromatophore interactions in zebrafish as candidates that might have evolved to produce a pattern of vertical bars in its sibling species, Danio aesculapii8,10. Using gene editing15-17 we generated several mutants in D. aesculapii that demonstrate a lower complexity in the interactions between chromatophores in this species, as well as a minor role of iridophores in patterning. Complementation tests in interspecific hybrids18,19 identified obelix/Kcnj13, which encodes an inwardly rectifying potassium channel (Kir7.1)20, as a gene evolved between D. rerio and D. aesculapii as well as in two of seven more Danio species tested. Our results demonstrate that the CRISPR/Cas9-system allows straightforward genetic tests also in non-model vertebrates to identify genes that underlie morphological evolution.