The Drosophila optic lobes receive retinotopic inputs from photoreceptors specialized in motion vision (lamina), or color and polarized light vision (medulla). At least 100 types of neurons in the optic lobes process these inputs for extracting visual information. They are organized in 800 columns corresponding to the 800 unit eyes in the retina (ommatidia). How is this variety of neurons generated and how is retinotopy established?
Neural stem cells that produce neurons in the medulla, the main part of the optic lobes, sequentially express six transcription factors in a temporal manner. Different neurons emerge in each temporal window, therefore generating a series of 800 neurons of each type: These 'Uni-columnar neurons' are generated throughout the neuroepithelium and have a 1:1 stoichiometry with the photoreceptors that innervate the medulla. The less numerous 'multi-columnar' neurons that have larger receptor fields and are present at a lower stoichiometry with photoreceptors emerge from the same neural stem cells but differ in distinct regions of the medulla neuroepithelium. In spite of their restricted origins, these neurons still contribute to the entire retinotopic map through dispersion of their cell bodies. Therefore, the generation of 80 cell types involves the integration of temporal and spatial patterning that preserves retinotopy of neurons present at different stoichiometry.
Once generated in the larva, neurons acquire their adult functional characteristics in response to combinations of transcription factors that are expressed in the 100 types of optic lobe adult neurons. To identify these transcription factors, we usedsingle-cell mRNA sequencing to identify the transcriptome of >80,000 individual cells. We were able to group these transcriptomes into 90 clusters, 60 of which could be assigned to a known cell type. To identify the transcription factors responsible for inducing each of their specific terminal differentiation features, we generated a ‘random forest’ model and we could show that one transcription factors was required in many, but not all, cholinergic (Apterous) or glutamatergic neurons (Traffic-jam). In fact, the same terminal characters are often regulated by different transcription factors in different cell types, arguing for extensive phenotypic convergence. These data provide a deep understanding of the developmental and functional specification of a complex brain structure.
