Early immune cell development precedes gastrulation in annual killifish.

During embryogenesis, cell types arise in a predictable order because developmental regulators act sequentially. But how evolutionary changes in morphogenesis reshape the signaling environments that activate these regulators remains unknown. Across vertebrates, primitive myeloid cells emerge from bone morphogenetic protein (BMP)-patterned ventral mesoderm through a conserved regulatory program. Here we show that in the annual killifish, Nothobranchius furzeri , a vertebrate with highly derived embryogenesis, neutrophils emerge prior to gastrulation, before ventral mesoderm has formed. Vascular progenitors arise later from ventral mesoderm, whereas myeloid progenitors are largely absent from this tissue. BMP inhibition abolishes pre-gastrula neutrophil specification, while disruption of Nodal-dependent mesendoderm formation does not. These findings reveal that conserved cell type programs can be redeployed within an altered embryonic architecture.