Specification of Drosophila aCC motoneuron identity by a genetic cascade involving even-skipped, grain and zfh1

During nervous system development, combinatorial codes of regulators act to specify different neuronal subclasses. However, within any given subclass, there exists a further refinement, apparent in Drosophila and C. elegans at single-cell resolution. The mechanisms that act to specify final and unique neuronal cell fates are still unclear. In the Drosophila embryo, one well-studied motoneuron subclass, the intersegmental motor nerve (ISN), consists of seven unique motoneurons. Specification of the ISN subclass is dependent upon both even-skipped (eve) and the zfh1 zinc-finger homeobox gene. We find that ISN motoneurons also express the GATA transcription factor Grain, and grn mutants display motor axon pathfinding defects. Although these three regulators are expressed by all ISN motoneurons, these genes act in an evegrnzfh1 genetic cascade unique to one of the ISN motoneurons, the aCC. Our results demonstrate that the specification of a unique neuron, within a given subclass, can be governed by a unique regulatory cascade of subclass determinants.