Endothelin signalling in iridophore development and stripe pattern formation of zebrafish
Krauss, J., Frohnhöfer, H.G., Walderich, B., Maischein, H.M., Weiler, C., Irion, U., Nüsslein-Volhard, C.
ABSTRACT
Colour patterns of adult fish are composed of several different types of pigment cells distributing in the skin during juvenile
development. The zebrafish, Danio rerio, displays a striking pattern of dark stripes of melanophores interspersed with light stripes of xanthophores. A third cell
type, silvery iridophores, contributes to both stripes and plays a crucial role in adult pigment pattern formation. Several
mutants deficient in iridophore development display similar adult phenotypes with reduced numbers of melanophores and defects
in stripe formation. This indicates a supporting role of iridophores for melanophore development and maintenance. One of these
mutants, rose (rse), encodes the Endothelin receptor b1a. Here we describe a new mutant in zebrafish, karneol (kar), which has a phenotype similar to weak alleles of rse with a reduction in iridophore numbers and defects of adult pigment patterning. We show that, unlike rse, kar is not required in iridophores. The gene defective in the kar mutant codes for an endothelin-converting enzyme, Ece2, which activates endothelin ligands by proteolytic cleavage. By morpholino-mediated
knockdown, we identify Endothelin 3b (Edn3b) as the ligand for endothelin receptor signalling in larval iridophores. Thus,
Endothelin signalling is involved in iridophore development, proliferation and stripe morphogenesis in larvae as well as adult
zebrafish. In mammals the pathway is required for melanocyte development; therefore, our results indicate a previously unrecognized
close evolutionary relationship between iridophores in zebrafish and melanocytes in mammals.