Optimized tools for multicolor stochastic labeling reveal diverse stereotyped cell arrangements in the fly visual system

Significance Nervous systems contain vast numbers of neurons with diverse shapes and complex spatial relationships. We describe new genetic tools for the efficient visualization by light microscopy of individual neurons and their relative positions in Drosophila. The application of these methods to the visual system revealed an unexpected diversity of cell-type–specific arrangements of neuronal processes within a single brain region. This wide range of stereotyped cell arrangements provides distinct circuit elements for processing visual information and implies the existence of a surprisingly large number of genetic programs that produce these arrangements during development. We describe the development and application of methods for high-throughput neuroanatomy in Drosophila using light microscopy. These tools enable efficient multicolor stochastic labeling of neurons at both low and high densities. Expression of multiple membrane-targeted and distinct epitope-tagged proteins is controlled both by a transcriptional driver and by stochastic, recombinase-mediated excision of transcription-terminating cassettes. This MultiColor FlpOut (MCFO) approach can be used to reveal cell shapes and relative cell positions and to track the progeny of precursor cells through development. Using two different recombinases, the number of cells labeled and the number of color combinations observed in those cells can be controlled separately. We demonstrate the utility of MCFO in a detailed study of diversity and variability of Distal medulla (Dm) neurons, multicolumnar local interneurons in the adult visual system. Similar to many brain regions, the medulla has a repetitive columnar structure that supports parallel information processing together with orthogonal layers of cell processes that enable communication between columns. We find that, within a medulla layer, processes of the cells of a given Dm neuron type form distinct patterns that reflect both the morphology of individual cells and the relative positions of their arbors. These stereotyped cell arrangements differ between cell types and can even differ for the processes of the same cell type in different medulla layers. This unexpected diversity of coverage patterns provides multiple independent ways of integrating visual information across the retinotopic columns and implies the existence of multiple developmental mechanisms that generate these distinct patterns.