Figure 1. Ommatidia architecture and the mechanisms of TGFβ pathway signaling.

(A) The six outer photoreceptors R1-R6 all express Rh1 and are involved in motion detection and image formation in dim light. They surround the rhabdomeres of the two inner photoreceptors R7 and R8, which are located on top of each other, thus sharing the same optic path. (B) The transcription factor Ss is expressed in 65% of R7 cells (yellow cells), activating Rh4 and repressing Rh3. Ss inhibits the instructive signal from R7 to R8, allowing for the default phosphorylation and activation of Wts in R8 and subsequent expression of Rh6 and repression of Rh5. Wts makes up one half of a double-negative feedback loop that establishes and maintains R8 subtypes. The other half of the loop, Melt, is expressed in R8s downstream of the other 35% of R7 cells (pale cells). These pale R7 cells lack Ss expression, allowing for expression of Rh3 and instructive signaling to R8 (red arrow). The instructive signal likely activates Melt, which represses Wts, allowing for Rh5 expression. (C) TGFβ superfamily ligands induce oligomerization of Type I and Type II serine-threonine kinase receptors. Binding of the ligand dimer to the Type II receptor initiates its kinase activity, phosphorylating residues on the Type I receptor, which becomes activated. Type I receptors then phosphorylate members of the receptor regulated (R)-SMAD family of transcription factors, allowing them to bind co-SMADs, translocate to the nucleus and activate or repress transcription of downstream target genes. (D) The TGFβ pathway in Drosophila contains both BMP and Activin subfamilies. The BMP subfamily is composed of three ligands, Dpp, Gbb and Scw, two Type I receptors, Tkv and Sax and one R-SMAD, Mad. The Activin subfamily also contains three ligands, dActβ, Daw and Myo, but only one Type I receptor, Babo and one R-SMAD, dSmad2. Both subfamilies share the Type II receptors Punt and Wit as well as the Co-SMAD, Med.