Homotypic dendritic interactions constrain growth and receptor distribution in Drosophila T4 neurons without affecting orientation or function.

Abstract

Direction-selective T4 neurons are among the most abundant cells in the Drosophila visual system. Although arranged retinotopically, their dendrites do not exhibit classical tiling. Instead, the four T4 subtypes are present once within each of the 750 columns in the fly optic lobe, with their dendrites spanning seven to nine neighboring columns. This results in a dense mesh of overlapping neural processes. Such deviation from classical tiling raises the question of whether homotypic interactions among highly intermingled dendrites such as T4s still contribute to shaping of their dendritic architecture. To address this, we developed Flp2Rescue, a genetic tool that ablates most T4 neurons while stochastically rescuing solitary ones. Our findings reveal that solitary T4 dendrites exhibit significant enlargement, indicating that homotypic interactions normally constrain the dendritic size of T4 neurons. Despite this enlargement, solitary T4 dendrites preserve their main subtype-specific orientations. In the enlarged dendrites we also found a higher number of glutamatergic receptors exhibiting a broader distribution along the dendrite. Surprisingly, these changes do not alter the functional identity of the neurons, meaning solitary T4 neurons continue to respond selectively to motion in their preferred direction.