E-cadherin is a member of the classical cadherin family and is known to be involved in cell-cell adhesion and the adhesion-dependent morphogenesis of various tissues. We isolated a zebrafish mutant (cdh1(rk3)) that has a mutation in the e-cadherin/cdh1 gene. The mutation rk3 is a hypomorphic allele, and the homozygous mutant embryos displayed variable phenotypes in gastrulation and tissue morphogenesis. The most severely affected embryos displayed epiboly delay, decreased convergence and extension movements, and the dissociation of cells from the embryos, resulting in early embryonic lethality. The less severely affected embryos survived through the pharyngula stage and showed flattened anterior neural tissue, abnormal positioning and morphology of the hatching gland, scattered trigeminal ganglia, and aberrant axon bundles from the trigeminal ganglia. Maternal-zygotic cdh1(rk3) embryos displayed epiboly arrest during gastrulation, in which the enveloping layer (EVL) and the yolk syncytial layer but not the deep cells (DC) completed epiboly. A similar phenotype was observed in embryos that received antisense morpholino oligonucleotides (cdh1MO) against E-cadherin, and in zebrafish epiboly mutants. Complementation analysis with the zebrafish epiboly mutant weg suggested that cdh1(rk3) is allelic to half baked/weg. Immunohistochemistry with an anti-beta-catenin antibody and electron microscopy revealed that adhesion between the DCs and the EVL was mostly disrupted but the adhesion between DCs was relatively unaffected in the MZcdh1(rk3) mutant and cdh1 morphant embryos. These data suggest that E-cadherin-mediated cell adhesion between the DC and EVL plays a role in the epiboly movement in zebrafish.

The zebrafish mutant ogon (also called mercedes and short tail) displays ventralized phenotypes similar to the chordino (dino) mutant, in which the gene for the Bmp antagonist Chordin is mutated. We isolated the gene responsible for ogon by a positional cloning strategy and found that the ogon locus encodes a zebrafish homolog of Secreted Frizzled(Sizzled), which has sequence similarity to a Wnt receptor, Frizzled. Unlike other secreted Frizzled-related proteins (sFrps) and the Wnt inhibitor Dickkopf1, the misexpression of Ogon/Sizzled dorsalized, but did not anteriorize, the embryos, suggesting a role for Ogon/Sizzled in Bmp inhibition. Ogon/Sizzled did not inhibit a Wnt8-dependent transcription in the zebrafish embryo. ogon/sizzled was expressed on the ventral side from the late blastula through the gastrula stages. The ventral ogon/sizzled expression in the gastrula stage was reduced or absent in the swirl/bmp2b mutants but expanded in the chordinomutants. Misexpression of ogon/sizzled did not dorsalize the chordino mutants, suggesting that Ogon/Sizzled required Chordin protein for dorsalization and Bmp inhibition. These data indicate that Ogon/Sizzled functions as a negative regulator of Bmp signaling and reveal a novel role for a sFrp in dorsoventral patterning.

Animal body color is generated primarily by neural crest-derived pigment cells in the skin. Mammals and birds have only melanocytes on the surface of their bodies; however, fish have a variety of pigment cell types or chromatophores, including melanophores, xanthophores, and iridophores. The medaka has a unique chromatophore type called the leucophore. The genetic basis of chromatophore diversity remains poorly understood. Here, we report that three loci in medaka, namely, leucophore free (lf), lf-2, and white leucophore (wl), which affect leucophore and xanthophore differentiation, encode solute carrier family 2, member 15b (slc2a15b), paired box gene 7a (pax7a), and solute carrier family 2 facilitated glucose transporter, member 11b (slc2a11b), respectively. Because lf-2, a loss-of-function mutant for pax7a, causes defects in the formation of xanthophore and leucophore precursor cells, pax7a is critical for the development of the chromatophores. This genetic evidence implies that leucophores are similar to xanthophores, although it was previously thought that leucophores were related to iridophores, as these chromatophores have purine-dependent light reflection. Our identification of slc2a15b and slc2a11b as genes critical for the differentiation of leucophores and xanthophores in medaka led to a further finding that the existence of these two genes in the genome coincides with the presence of xanthophores in nonmammalian vertebrates: birds have yellow-pigmented irises with xanthophore-like intracellular organelles. Our findings provide clues for revealing diverse evolutionary mechanisms of pigment cell formation in animals.