The extracellular protein factor Decapentaplegic (Dpp) is critical for proper wing development in Drosophila (Figure Q21–3A). It is normally expressed in a narrow stripe in the middle of the wing, along the anterior–posterior boundary. Flies that are defective for Dpp form stunted “wings” (Figure Q21–3B). If an additional copy of the gene is placed under control of a promoter that is active in the anterior part of the wing, or in the posterior part of the wing, a large mass of wing tissue composed of normal looking cells is produced at the site of Dpp expression (Figure Q21–3C and D). Does Dpp stimulate cell division, cell growth, or both? How can you tell?

Figure Q21-3

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(A) (B) anterior posterior (C) (D)