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Abstract

The skin pattern of zebrafish is formed by three types of chromatophores: black melanophores, yellow xanthophores, and iridescent iridophores. To create the characteristic horizontal stripes, chomatophores interact with each other and the surrounding tissues. Although a number of regulators of the stripe formation, pigmentation, and pigment cell development have been described, the underlying mechanisms of these processes remain elusive and controversial. Here we present the water and glycerol channel Aquaporin 3a as a novel regulator of stripe formation in adult zebrafish. Chemical mutagenesis screens identified four dominant mutations of mau gene, coding for Aqp3a. All of them carry amino acid substitutions in conserved positions which are, according to bioinformatic prediction, at the pore surface of aquaporin. All four mutations lead to a similar phenotype: although all the chromatophore types are present in the skin, the mutants display breakages in stripes. In addition, they have short fins, with fewer normally sized fin ray segments. Both phenotypes are not present in larvae, but appear during metamorphosis. Experiments in Xenopus oocytes show that all four mutant alleles of mau are deficient in water and glycerol transport. Interestingly, three of them partially or completely retained in the ER, when expressed in mammalian cells and overexpressed in zebrafish embryos. Cell transplantations indicate that mutations in mau interfere with melanophore behaviour rather than with functioning of xanthophores or iridophores. However, given the strong expression of of mau in the epidermis, it is suggested, that the tissue environment is one of the main players in the chromatophore organization. Several gap junction and ion channel proteins have been associated with the patterning of zebrafish skin and regulation of fin size. This is the first report of the involvement of water and glycerol channels in these processes, which offers a new dimension to our view on interactions regulating chromatophore patterning, and fin development.