Thorax closure in Drosophila: involvement of Fos and the JNK pathway

Dorsal closure, a morphogenetic movement during Drosophila embryogenesis, i s controlled by the Drosophila JNK pathway, D-Fos and the phosphatase Pucke red (Puc), To identify principles of epithelial closure processes, we studi ed another cell sheet movement that we term thorax closure, the joining of the parts of the wing imaginal discs which give rise to the adult thorax du ring metamorphosis. In thorax closure a special row of margin cells express puc and accumulate prominent actin fibres during midline attachment. Genet ic data indicate a requirement of D-Fos and the JNK pathway for thorax clos ure, and a negative regulatory role of Puc. Furthermore, puc expression co- localises with elevated levels of D-Fos, is reduced in a JNK or D-Fos loss- of-function background and is ectopically induced after JNK activation. Thi s suggests that Puc acts downstream of the JNK pathway and D-Fos to mediate a negative feed-back loop. Therefore, the molecular circuitry required for thorax closure is very similar to the one directing dorsal closure in the embryo, even though the tissues are not related. This finding supports the hypothesis that the mechanism controlling dorsal closure has been co-opted for thorax closure in the evolution of insect metamorphosis and may represe nt a more widely used functional module for tissue closure in other species as well.