Countershading in zebrafish results from an Asip1 controlled dorsoventral gradient of pigment cell differentiation

13 pages, 8 figures.-- his article is licensed under a Creative Commons Attribution 4.0 International LicenseDorso-ventral (DV) countershading is a highly-conserved pigmentary adaptation in vertebrates. In mammals, spatially regulated expression of agouti-signaling protein (ASIP) generates the difference in shading by driving a switch between the production of chemically-distinct melanins in melanocytes in dorsal and ventral regions. In contrast, fish countershading seemed to result from a patterned DV distribution of differently-coloured cell-types (chromatophores). Despite the cellular differences in the basis for counter-shading, previous observations suggested that Agouti signaling likely played a role in this patterning process in fish. To test the hypotheses that Agouti regulated counter-shading in fish, and that this depended upon spatial regulation of the numbers of each chromatophore type, we engineered asip1 homozygous knockout mutant zebrafish. We show that loss-of-function asip1 mutants lose DV countershading, and that this results from changed numbers of multiple pigment cell-types in the skin and on scales. Our findings identify asip1 as key in the establishment of DV countershading in fish, but show that the cellular mechanism for translating a conserved signaling gradient into a conserved pigmentary phenotype has been radically altered in the course of evolutionThis work was funded by the Spanish Economy and Competitiveness Ministry projects AGL2011-23581, AGL2014-52473R, AGL2017-89648P to JR, and by a BBSRC SWBio DTP Studentship to JO. Partial funding was obtained from AGL2016-74857-C3-3-R to JMCR. L. Cal was supported by pre-doctoral fellowship FPI funded by Spanish Economy and Competitiveness Ministry (AGL2011-23581) and by pre-doctoral fellowship of the Spanish Personnel Research Training Program funded by Spanish Economy and Competitiveness Ministry (EEBB-C-14- 00467). P Suarez-Bregua was supported by a Campus do Mar PhD grant, Xunta de Galicia and AGL2014-52473R project contract.Peer reviewe