The Netrin receptor Frazzled is required in the target for establishment of retinal projections in the Drosophila visual system

Retinal axons in Drosophila make precise topographic connections with their target cells in the optic lobe, Here we investigate the role of the Netrin s and their receptor Frazzled in the establishment of retinal projections. We find that the Netrins, although expressed in the target, are not require d for retinal projections. Surprisingly, Frazzled, found on both retinal fi bers and target cells, is required in the target for attracting retinal fib ers, while playing at best a redundant role in the retinal fibers themselve s; this finding demonstrates that target attraction is necessary for topogr aphic map formation. Finally, we show that Frazzled is not required for the differentiation of cells in the target. Our data suggest that Frazzled doe s not function as a Netrin receptor in attracting retinal fibers to the tar get; nor does it seem to act as a homotypic cell adhesion molecule. We favo r the possibility that Frazzled in the target interacts with a component on the surface of retinal fibers, possibly another Netrin receptor.