Axis formation in annual killifish: Nodal coordinates morphogenesis in absence of Huluwa prepatterning

Axis formation in fish and amphibians is initiated by a prepattern of maternal gene products in the blastula. The embryogenesis of annual killifish challenges prepatterning models because blastomeres disperse and then re-aggregate to form the germ layers and body axes. This dispersion-aggregation process prompts the question how axis determinants such as Huluwa and germ layer inducers such as Nodal function in annual killifish. Here we show in Nothobranchius furzeri that huluwa , the factor thought to break symmetry by stabilizing β-catenin, is a non-functional pseudogene. Nuclear β-catenin is not selectively stabilized on one side of the blastula but accumulates in cells forming the incipient aggregate. Inhibition of Nodal signaling blocks aggregation and disrupts coordinated cell migration, establishing a novel role for this signaling pathway. These results reveal a surprising departure from classic mechanisms of axis formation: canonical Huluwa-mediated prepatterning is dispensable and Nodal coordinates morphogenesis.