Analysis of the locomotor circuit formation in zebrafish spinal cord

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The p2 progenitor domain in the ventral spinal cord gives rise to two interneuron subtypes, V2a and V2b. The mechanism as to how V2a and V2b neurons arise from a single progenitor domain was unknown. We showed in this study that p2 progenitors divide once to produce V2a/V2b neuron pairs, indicating that V2a and V2b neurons are generated by the asymmetric division of pair-producing progenitor cells. We also showed that Delta-Notch interactions between sister cells play a critical role in the final outcome of these asymmetric divisions. This mechanism for determining cell fate is similar to asymmetric divisions that occur during Drosophila neurogenesis, where ganglion mother cells divide once to produce distinct neurons. However, unlike in Drosophila, the divisional axes of p2 progenitors in zebrafish were not fixed. We report that the terminal division of pair-producing progenitor cells in vertebrate neurogenesis can reproducibly produce two distinct neurons through a mechanism that may not depend on the orientation of the division axis.