Department of Cell and Developmental Biology

University College London

Title: Tissue architecture and feedback from newly induced neurons set neuronal number in the Drosophila visual system

Abstract: Neural circuit formation and function require that diverse neurons are specified in appropriate numbers relative to each other. Known strategies that control neuronal numbers involve regulating either the number of progeny produced or the number of progeny to survive. We used the Drosophila visual system to probe how neuronal numbers are set. Photoreceptors from each unit eye or ommatidium achieve retinotopy with the lamina, the photoreceptor target field, by inducing a lamina unit (column) for each ommatidium. Photoreceptor-Hedgehog (Hh) specifies lamina precursor cells (LPCs) and promotes their assembly into columns. Although each column initially consists of 6 LPCs, only 5 differentiate into neurons, one for each of the L-neuron types (L1-L5), while the ‘extra’ LPC is eliminated by apoptosis. We uncovered that differential Hh signalling levels along the length of columns restrict LPC potential, such that both the ‘extra’ LPC and its neighbour, fated to be an L5, are restricted to L5 identity. We show that a glial population called the outer chiasm giant glia (xgO), which sit below the lamina secrete multiple ligands to induce L5 differentiation. The LPCs in closest proximity to xgO receive signals first and differentiate into L5s. These newly induced L5 neurons then negatively feedback to prevent glial signals from reaching the ‘extra’ LPCs. Thus, an intricate interplay between glial signals, tissue architecture and feedback from newly induced neurons determines which and, therefore, how many neurons are induced.

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