An identified motoneuron with variable fates in embryonic zebrafish.

Every trunk hemisegment of the zebrafish is innervated by 3 identified primary motoneurons whose development can be observed directly in living embryos. In this paper, we describe another identified neuron that is part of this system. Unlike the other primary motoneurons which are present in all trunk hemisegments, this cell is present in slightly less than half of the trunk hemisegments. Additionally, this cell has at least 2 different fates: it may become a primary motoneuron and arborize in an exclusive muscle territory, or it may die during embryonic development. We have named this cell VaP, for variable primary. We show that the presence of VaP does not affect the early development of the other primary motoneurons in the same hemisegment. Moreover, we show that ablation of both VaP and caudal primary does not alter pathfinding by another identified primary motoneuron.