620A Poster - 08. Patterning, morphogenesis and organogenesis
Thursday April 07, 2:00 PM - 4:00 PM

Authors:
Charlotte Wit ^1,2; Melinda Kehribar ^1,2; Robin Hiesinger ²

Affiliations:
1) equal contribution; 2) Freie Universität Berlin

Keywords:
z. other (Visual map); a. axon guidance

The Drosophila visual map is a highly patterned synaptic brain region formed by photoreceptor axon terminals in the lamina. We investigated the spatiotemporal organization and the underlying molecular determinants that ensure early pattern formation of this visual map. Unit eyes (ommatidia) and photoreceptor neurons (R1-R6) differentiate in a temporal wave in the developing eye disc; axon outgrowth follows the same wave pattern through the optic stalk that connects the eye disc and the lamina. Photoreceptors bundles are organized in two ways: first, each ommatidium preserves the rotational organization of R1-2-3-4-5-6 neighboring each other (intra-bundle organization); second, R1-R6 bundles preserve their relative positions to each other through the optic stalk and into the lamina (inter-bundle organization). Both types of bundle organization require the cell adhesion molecule Sidekick to preserve the pattern originating in the eye disc. Following the well-characterized differentiation order in the eye disc (R2/5, followed by R3/4, followed by R1/6), the R2 and R5 axon arrive first in the lamina, where they form an 'equator-blind' and initially orthogonal pattern. The adhesion G-protein coupled receptor Flamingo/Starry Night (Fmi) is strongly expressed in the early arrivals and its protein localization preserves the equator-blind and orthogonal scaffold throughout visual map formation. Loss of fmi in R2 and R5, but not in R3,4,5,6 leads to a disruption of the early pattern. The function of fmi does not seem to require canonical planar cell polarity signaling, based on frizzled and frizzled2 loss of function studies and the absence of their localization in the lamina. Mirror-symmetry around the equator is subsequently established through the positioning of R3/4 and R1/6 on opposite sides of the R2/5 axis, which again requires Sidekick. Loss of either fmi or sidekick in several neighboring cells disrupts early patterning as described above, while loss of either protein in individual photoreceptors had no discernible effects on the overall pattern. We conclude that early visual map patterning is a multi-layered, self-organizing process that is largely robust to single-cell perturbation.