Dorsal-ventral midline signaling in the developing Drosophila eye

Boundaries between different cell types play key roles in many developmental patterning processes. They can be established by various mechanisms, and signaling between the different cell types can occur in a number of ways. One mechanism of cross-boundary signaling is controlled by the Notch (N)-modifying protein Fringe (Fng). At the Drosophila wing dorsal-ventral (D-V) border, the mechanism by which an Fng⁺-Fng^- interface controls local N activation has been well characterized. A similar N-activating Fng⁺-Fng^- interface has also been described at the D-V border of the fly eye, but the mechanisms that establish and regulate it are different from those in the wing. Here we describe the ventral role of the Sloppy-paired (Slp) transcription factor, and its interactions with dorsally expressed Iroquois (Iro) transcription factors in the regulation of signaling about the Fng⁺-Fng^- interface in the developing eye. The two transcription factors are mutually repressive and initially abut at the D-V midline. However, N signaling at the interface downregulates Slp expression, and a gap opens between the two expression domains in which Serrate (Ser, an N ligand) is upregulated.