Planar cell polarity in the Drosophila eye is directed by graded Four-jointed and Dachsous expression.

Planar cell polarity (PCP) occurs when the cells of an epithelium are polarized along a common axis lying in the epithelial plane. During the development of PCP, cells respond to long-range directional signals that specify the axis of polarization. In previous work on the Drosophila eye, we proposed that a crucial step in this process is the establishment of graded expression of the cadherin Dachsous (Ds) and the Golgi-associated protein Four-jointed (Fj). These gradients were proposed to specify the direction of polarization by producing an activity gradient of the cadherin Fat within each ommatidium. In this report, I test and confirm the key predictions of this model by altering the patterns of Fj, Ds and Fat expression. It is shown that the gradients of Fj and Ds expression provide partially redundant positional information essential for specifying the polarization axis. I further demonstrate that reversing the Fj and Ds gradients can lead to reversal of the axis of polarization. Finally, it is shown that an ectopic gradient of Fat expression can re-orient PCP in the eye. In contrast to the eye, the endogenous gradients of Fj and Ds expression do not play a major role in directing PCP in the wing. Thus, this study reveals that the two tissues use different strategies to orient their PCP.