A novel temporal identity window generates alternating Eve/Nkx6 motor neuron subtypes in a single progenitor lineage.

BACKGROUND

Spatial patterning specifies neural progenitor identity, with further diversity generated by temporal patterning within individual progenitor lineages. In vertebrates, these mechanisms generate "cardinal classes" of neurons that share a transcription factor identity and common morphology. In Drosophila, two cardinal classes are Even-skipped (Eve) motor neurons projecting to dorsal longitudinal muscles, and Nkx6 motor neurons projecting to ventral oblique muscles. Cross-repressive interactions prevent stable double-positive motor neurons. The Drosophila neuroblast 7-1 (NB7-1) lineage uses a temporal transcription factor cascade to generate five distinct Eve motor neurons; the origin and development of Nkx6 motor neurons remains unclear.

METHODS

We use a neuroblast specific Gal4 line, sparse labelling and molecular markers to identify an Nkx6 VO motor neuron produced by the NB7-1 lineage. We use lineage analysis to birth-date the VO motor neuron to the Kr Pdm neuroblast temporal identity window. We use gain- and loss-of-function strategies to test the role of Kr Pdm temporal identity and the Nkx6 transcription factor in specifying VO neuron identity.

RESULTS

Lineage analysis identifies an Nkx6 neuron born from the Kr Pdm temporal identity window in the NB7-1 lineage, resulting in alternation of cardinal motor neuron subtypes within this lineage (Eve>Nkx6 > Eve). Co-overexpression of Kr/Pdm generates ectopic VO motor neurons within the NB7-1 lineage - the first evidence that this TTF combination specifies neuronal identity. Moreover, the Kr/Pdm combination promotes Nkx6 expression, which itself is necessary and sufficient for motor neuron targeting to ventral oblique muscles, thereby revealing a molecular specification pathway from temporal patterning to cardinal transcription factor expression to motor neuron target selection.

CONCLUSIONS

We show that one neuroblast lineage generates interleaved cardinal motor neurons fates; that the Kr/Pdm TTFs form a novel temporal identity window that promotes expression of Nkx6; and that the Kr/Pdm > Nkx6 pathway is necessary and sufficient to promote VO motor neuron targeting to the correct ventral muscle group.