Patterning Imaginal Discs In D Melanogaster

The identity of each disc primordium is specified during embryonic development. This specification relies on the disc hedgehog induces decapentaplegic

engrailed invected

FIGURE 3 Signaling and anterior boundary cells in wing disc of D.

melanogaster.

decapentaplegic signals from boundary

engrailed invected

FIGURE 3 Signaling and anterior boundary cells in wing disc of D.

melanogaster.

specific expression of a small number of genes, encoding largely transcription factors or their cofactors. The loss or misexpression of these genes causes dramatic homeotic transformations of disc identities. The localized expression of these transcription factors is to a large extent inherited from or signaled by the embryonic epidermis, which was subdivided into anterior—posterior and dorsal—ventral domains prior to the formation of the disc primordia.

How are different tissues formed within each disc? Even at late third instar most imaginal disc cells appear morphologically similar; only a few (mostly sensory) elements have begun to differentiate terminally. However, the premetamor-phic imaginal disc is in no sense a tabula rasa, since it has already been subdivided into number of regions. In fact, most disc primordia are subdivided into anterior and posterior lineage "compartments" from the earliest stages of their development. This subdivision is controlled by the posterior-specific expression of the engrailed and invected transcription factors in each disc, which act as a binary switch, controlling the choice between posterior and anterior identities and preventing cells from crossing between compartments. The wing and haltere discs are further subdivided into prospective dorsal and ventral lineage compartments, in this case by the dorsal-specific expression of the apterous transcription factor.

Why have lineage compartments? Cells in different lineage compartments have different signaling capabilities, and this has important developmental consequences. Posterior cells secrete the signaling molecule Hedgehog, but only anterior cells are capable of responding to that signal (Fig. 3). Because Hedgehog diffuses only a short distance into the anterior compartment, it induces the formation of a specialized group of cells just anterior to the compartment boundary. The formation of this group of boundary cells is critical for appendage development because boundary cells in turn secrete several important signals (Fig. 3), including Decapentaplegic (a member of the BMP family of morphogens, generated in the wing, dorsal leg, and dorsal antenna), and Wingless (a member of the Wnt family of morphogens, generated in ventral leg and ventral antenna). Cells outside the boundary region judge their approximate position in the disc by the

Notch activity induces wingless wingless signals from boundary

Dorsal

Ventral

0 0

Post a comment