Figure 9.25. Model for the transcription of the segment polarity genes engrailed (en) and wingless (wg).

Figure 9.25

Model for the transcription of the segment polarity genes engrailed (en) and wingless (wg). (A) The expression of wg and en is initiated by pair-rule genes. The en gene is expressed when the cells contain high concentrations of either Even-skipped or Fushi tarazu proteins. The wg gene is transcribed when neither eve or ftz genes are active, but a third gene (probably odd-paired) is expressed. (B) The continued expression of wg and en is maintained by interactions between the Engrailed- and Wingless-expressing cells. Wingless protein is secreted and diffuses to the surrounding cells. In those cells competent to express Engrailed (having Eve or Ftz proteins), Wingless protein is bound by the Frizzled receptor, which enables the activation of the en gene. (Armadillo is the Drosophila name for β-catenin.) Engrailed protein activates the transcription of the hedgehog gene and also activates its own (en) gene transcription. Hedgehog protein diffuses from these cells and binds to the Patched protein. This binding prevents the Patched protein from inhibiting signaling by the Smoothened protein. That signal enables the transcription of the wg gene and the subsequent secretion of the Wingless protein.

From: The Origins of Anterior-Posterior Polarity

Cover of Developmental Biology
Developmental Biology. 6th edition.
Gilbert SF.
Sunderland (MA): Sinauer Associates; 2000.
Copyright © 2000, Sinauer Associates.

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