Valentino, P. | Search bioRxiv

bioRxiv2024

High-throughput identification of the spatial origins of Drosophila optic lobe neurons using single-cell mRNA-sequencing

The medulla is the part of the Drosophila optic lobe with the greatest neuronal diversity, in which 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 expression of neuronal type-specific combinations of terminal selectors, which are transcription factors expressed continuously during development and in the adult that are thought to control all neuronal type-specific gene expression. However, how the patterning mechanisms establish terminal selector expression is unknown. We have previously characterized the temporal and Notch origin of medulla neurons. Here we have used single-cell mRNA-sequencing to characterize their spatial origins and identified two new spatial subdomains. Together, this makes the medulla the first complex brain structure for which the patterning mechanisms specifying the identity of each neuronal type are known. This knowledge 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 of modules of terminal features, including neurotransmitter identity. Therefore, the evolution of new neuronal types could rely on the acquisition of modules of neuronal features pre-determined by their developmental origin.

developmental biology↗

bioRxiv2022

Concurrent temporal patterning of neural stem cells in the fly visual system

The temporal and spatial patterning of neural stem cells is a powerful mechanism by which to generate neural diversity in both vertebrate and invertebrate brains. In the Drosophila optic lobe, the neuroblasts (NBs) that generate the [~]120 neuronal cell types of the medulla are patterned by independent temporal and spatial inputs. In the temporal axis, a cascade of twelve transcription factors (TFs) are expressed in medulla NBs as they age. In the spatial axis, the neuroepithelium from which these NBs are generated is sub-divided into eight compartments by the expression of five additional TFs. Distinct neuronal types are generated by NBs based on their spatio-temporal address. Here, we describe a third major patterning axis that further diversifies neuronal fates in the medulla. We show that the symmetrically dividing neuroepithelial cells from which the medulla NBs are generated are temporally patterned by opposing gradients of the Imp and Syp RNA-binding proteins. Imp and Syp regulate the expression of a set of TFs in the neuroepithelium to confer NBs from the same spatio-temporal address with unique identities based on the developmental stage they are generated. We show that Imp and Syp differentially pattern NBs in the Vsx1-Hth spatio-temporal birth window to generate seven distinct neuronal cell types (Li2, TmY17, TmY15, Tm23, Pm3a, Pm3b and TmY12) in successive developmental windows. We further demonstrate that the birthdate of these neurons correlates with their final position in the adult cortex, resulting in unanticipated specializations of the retinotopic circuit in the anterior-posterior axis of the visual system. The concurrent temporal patterning of symmetrically and asymmetrically dividing neural stem cells thus acts as a powerful mechanism to couple the generation of neural diversity with circuit patterning.

developmental biology↗