The zebrafish tailbud contains two independent populations of midline progenitor cells that maintain long-term germ layer plasticity and differentiate based on local signaling cues

Abstract

Vertebrate body axis formation depends on a population of bipotential cells along the posterior wall of the tailbud that make a germ layer decision after gastrulation to form spinal cord and mesoderm. Despite exhibiting germ layer plasticity, these bipotential neuromesodermal tailbud cells never give rise to midline tissues of the notochord, floor plate, and dorsal endoderm, raising the question of whether midline tissues also arise from basal posterior progenitors after gastrulation. Using zebrafish we show that local posterior signals specify germ layer fate in two different basal tailbud midline progenitor populations. Wnt signaling induces notochord within a population of notochord / floor plate bipotential cells, and does so through negative transcriptional regulation of the sox2 transcription factor. Notch signaling, which is required for hypochord induction during gastrulation, continues to act in the tailbud to specify hypochord from a notochord / hypochord bipotential cell population. Our results lend strong support to a continuous allocation model of midline tissue formation in zebrafish. Additionally, the genetic evidence of two independent posterior notochord progenitor pools provides an embryological basis for zebrafish and mouse bifurcated notochord phenotypes, and the rare human congenital split notochord syndrome (SNS). Finally, we demonstrate developmental equivalency between different tailbud progenitor cell populations. Ectopic expression of mesogenin1, a master regulator of paraxial mesoderm fate, is sufficient to transfate midline progenitors from a notochord to a somite fate after gastrulation. Midline progenitor cells also adopt a somite fate if they are transplanted into the bipotential progenitors that normally give rise to somites. Taken together, our results indicate that the entire non-epidermal posterior body is derived from discrete, basal tailbud cell populations. These cells remain receptive to extracellular cues after gastrulation and continue to make basic germ layer decisions.