Spatial, temporal and Notch determination of terminal selector expression controls neuronal cell fate in the Drosophila optic lobe.

In the medulla of the Drosophila optic lobe, the identity of each neuronal type is specified in progenitors and newborn neurons via the integration of temporal, spatial and Notch-driven patterning mechanisms. This identity is maintained in differentiating and adult neurons by the continuous expression of neuronal type-specific combinations of transcription factors called terminal selectors, which are thought to control all neuronal type-specific features. How the patterning mechanisms establish terminal selector expression is unknown. Here we have used single-cell mRNA sequencing to characterize the spatial origins of medulla neurons. Combined with our previous characterization of their temporal and Notch origins, this allowed us to identify correlations between patterning information, terminal selector expression and neuronal features. Our results suggest that different subsets of the patterning information accessible to a given neuronal type control the expression of each of its terminal selectors and modules of terminal features, including neurotransmitter identity.