Compartment (development) - A/P Boundary

A/P Boundary

In 1970, by means of clonal analysis, the Anterior-Posterior boundary was identified. The founder cells, found at the border between parasegments 4 and 5 of embryo, are already determined at the early blastoderm stage and defined into the two populations they will generate by stripes of the engrailed gene. The selector gene, engrailed (en), is a key determinant in boundary formation between the anterior and posterior compartments. As the wing imaginal disc expands, posterior, but not anterior cells will express engrailed and maintain this expression state as they expand and form the disc. Engrailed mutant clones of posterior origin will gain anterior affinity and move towards the anterior compartment and intermix with those cells. Within the posterior compartment these clones will sort out and form an ectopicborder where they meet other posterior cells. Similarly, a clone of anterior cells expressing engrailed will gain posterior identity and create an ectopic boundary where the clone meets other anterior cells in this compartment. In addition,to its cell autonomous role in specifying posterior compartment identity, engrailed also has a non-cell autonomous function in the general growth and patterning of the wing disc, through the activation of signaling pathways such as Hedgehog (Hh) and Decapentaplegic (Dpp). The presence of engrailed in the posterior cells leads to the secretion of the short-range inducer Hh which can cross over to the anterior compartment to activate the long-range morphogen, Dpp. Cells in the posterior compartment produce Hh, but only anterior cells can transduce the signal. Optomotor-blind (omb) is involved in the transcriptional response of Dpp, which is only required in the anterior cells to interpret Hh signaling for boundary formation and maintenance. In addition, Cubitus interruptus (Ci), the signal transducer of the Hh signal, is expressed throughout the anterior compartment, particularly in anterior border cells. In posterior cells engrailed prevents the expression of Ci, such it is only expressed in anterior cells and hence only these cells can respond to Hh signaling by up-regulating the expression of dpp. Loss of engrailed function in posterior cells, results in anterior transformation, where Hh expression is decreased and dpp, ci and patched (ptc) is increased, resulting in the formation of a new A/P boundary, suggesting that en positively regulates hh, while negatively regulating ci, ptc and dpp.