Maternally controlled beta-catenin-mediated signaling is required for organizer formation in the zebrafish

We have identified and characterized a zebrafish recessive maternal effect mutant, ichabod, that results in severe anterior and dorsal defects during early development. The ichabod mutation is almost completely penetrant, but exhibits variable expressivity. All mutant embryos fail to form a normal e mbryonic shield; most fail to form a head and notochord and have excessive development of ventral tail fin tissue and blood. Abnormal dorsal patternin g can first be observed at 3.5 hpf by the lack of nuclear accumulation of b eta-catenin in the dorsal yolk syncytial layer, which also fails to express bozozok/dharma/nieuwkoid and znr2/ndr1/squint. At the onset of gastrulatio n, deficiencies in expression of dorsal markers and expansion of expression of markers of ventral tissues indicate a dramatic alteration of dorsoventr al identity. Injection of beta-catenin RNA markedly dorsalized ichabod embr yos and often completely rescued the phenotype, but no measurable dorsaliza tion was obtained with RNAs encoding upstream Wnt pathway components. In co ntrast, dorsalization was obtained when RNAs encoding either Bozozok/Dharma / Nieuwkoid or Znr2/Ndr1/Squint were injected. Moreover, injection of beta- catenin RNA into ichabod embryos resulted in activation of expression of th ese two genes, which could also activate each other. RNA injection experime nts strongly suggest that the component affected by the ichabod mutation ac ts on a step affecting beta-catenin nuclear localization that is independen t of regulation of beta-catenin stability. This work demonstrates that a ma ternal gene controlling localization of beta-catenin in dorsal nuclei is ne cessary for dorsal yolk syncytial layer gene activity and formation of the organizer in the zebrafish.