Induction of the endoderm in Drosophila

The endoderm of Drosophila develops into the larval midgut epithelium which is subdivided into sections of distinct and highly differentiated cell types. This subdivision cannot occur autonomously since the endodermal cells, due to their developmental history, do not possess intrinsic knowledge of their position along the body axis. Instead, endodermal subdivision is achieved through induction by the visceral mesoderm. This induction depends on Ultrabithorax, a homeotic gene expressed in a central section of the visceral mesoderm. Ultrabithorax stimulates expression of decapentaplegic in the same, and expression of wingless in a neighbouring section. The extracellular protein products of decapentaplegic and wingless signal to adhering endodermal cells to induce expression of the homeotic gene labial. During this induction, wingless acts at true signalling thresholds to specify two different cell types in the larval gut.