Population morphology implies a common developmental blueprint for Drosophila motion detectors

Quantitative analysis of neuron morphology is essential in order to develop our understanding of circuit organisation and development. The recent acquisition of whole-brain electron microscopy-based (EM) reconstructions of the Drosophila melanogaster nervous system now provide the resolution needed to examine morphology at scale. Utilising these data, together with new computational tools, we extract and analyse the dendrites of all T4 and T5 neurons within one hemisphere (n \~ 6000).T4 and T5 neurons are the first uniquely direction-selective neurons in the visual pathway, and are classified into four subtypes (a,b,c, and d). Each subtype encodes one of four cardinal motion directions (up, down, forwards, backwards). The dendrites of these neurons form in two distinct neuropils, the Medulla (T4) and the Lobula (T5), and are asymmetrically oriented in a direction inverse to the direction of motion which they encode. However, their densely overlapping and compact arbours has made rigorous morphological quantification challenging. The presence of differences beyond their characteristic orientation, both between T4 and T5, as well as within subtypes, has remained poorly understood.Our analysis reveals a high degree of structural similarity across both types and subtypes. Particularly, measures of geometry and graph topology show only minor variation, with no consistent separation between T4 and T5, or their subtypes.These results indicate that, despite forming in different neuropils, and serving distinct motion directions, T4 and T5 dendrites follow closely aligned morphological patterns. This suggests that their arborization may be governed by shared developmental constraints and mechanisms.