Fig. 6. Model for ds, ft and Atro in PP establishment. (A) Model for how ft/Atro (green) and ds (red) activity gradients relate to Factor X gradient (black), which is instructive for PP. ds activity is high at the D and V regions of the disc and reaches its minimum at the DV midline. Ft/Atro promote production of Factor X, and Ds inhibits it, probably by inhibiting Ft. As a result, Factor X production will be highest at the midline and lowest at the edges. (B) Effects on Factor X gradient and ommatidial polarity of a ft or Atro clone. The clone (black ellipse) generates a sink in the Factor X gradient. Outside the clone, on the polar side a new maximum of Factor X is created. (C) Effects on Factor X gradient and ommatidial polarity of a ds clone. ds clone is predicted to produce excess Factor X, altering the slope of the gradient, and reorganizing the ommatidia accordingly. (D) Model for Atro function in PP. Initially, Wg (expressed at the D and V poles), N (active at the D/V midline) and Unpaired (Upd) (secreted centrally) set up Fj expression gradient with its peak at the midline. Wg also promotes ds expression. Ft and Atro are responsible for the production of Factor X, the promotion of the R3 versus the R4 cell fate and the repression of fj transcription. In addition to their requirement in PP, Ft and Atro control separate activities (broken gray arrows).

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