Planar polarity is evident in the coordinated orientation of ommatidia in the Drosophila eye. This process requires that the R3 photoreceptor precursor of each ommatidium have a higher level of Frizzled signaling than its neighboring R4 precursor. We show that two cadherin superfamily members, Fat and Dachsous, and the transmembrane/secreted protein Four-jointed play important roles in this process. Our data support a model in which the bias of Frizzled signaling between the R3/R4 precursors results from higher Fat function in the precursor cell closer to the equator, which becomes R3. We also provide evidence that positional information regulating Fat action is provided by graded expression of Dachsous across the eye and the action of Four-jointed, which is expressed in an opposing expression gradient and appears to modulate Dachsous function.
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