ABSTRACT
Axis formation in fish and amphibians typically begins with a prepattern of maternal gene products. Annual killifish embryogenesis, however, challenges prepatterning models as blastomeres disperse and then aggregate to form the germ layers and body axes. We show that huluwa, a prepatterning factor thought to break symmetry by stabilizing ß-catenin, is truncated and inactive in Nothobranchius furzeri. Nuclear ß-catenin is not selectively stabilized on one side of the blastula but accumulates in cells forming the aggregate. Blocking ß-catenin activity or Nodal signaling disrupts aggregate formation and germ layer specification. Nodal signaling coordinates cell migration, establishing an early role for this signaling pathway. These results reveal a surprising departure from established mechanisms of axis formation: Huluwa-mediated prepatterning is dispensable, and ß-catenin and Nodal regulate morphogenesis.
