In Drosophila, the Jun-N-terminal Kinase-(JNK) signaling pathway is required for epithelial cell shape changes during dorsal closure of the embryo. In the absence of JNK pathway activity, as in the DJNKK/hemipterous (hep)

Epithelial morphogenesis is a fundamental and widespread process controlling cell, tissue and organ development, hence the ultimate shape and organization of metazoans. The ability of fields of cells to translate spatial and temporal information into appropriate morphogenetic activity is crucial for the making of a functional organism. In Drosophila, several genes have been identified that are important for correct morphogenesis, including both structural and regulatory (signaling) activities. For example, during mid-embryogenesis, the dorsal part of the embryo is progressively covered by the lateral ectodermal cells undergoing dorsal closure (DC): these epithelial cells elongate dorsally and move in concert toward the dorsal midline where they fuse at the end of the process (Martinez Arias, 1993; Noselli, 1998). It was recently shown that the Jun-N-terminal Kinase (JNK) pathway plays an essential role in this process, by controlling specific gene activity in a single row of cells leading the migrating ectoderm (Glise et al., 1995; Glise and Noselli, 1997; Harden et al., 1996, 1995, 1999; Hou et al., 1997; Kockel et al., 1997; Liu et al., 1999; Lu and Settleman, 1999; Martin-Blanco et al., 1998; Ricos et al., 1999; Riesgo-Escovar and Hafen, 1997a,b, 1996; Sluss and Davis, 1997; Sluss et al., 1996; Su et al., 1998; Zeitlinger et al., 1997). In JNK pathway mutant embryos, these leading edge (LE) cells lose both their identity and ability to move dorsally. Thus, a regional defect can affect the ability of a larger field of ectodermal cells to undergo proper morphogenesis. A current model proposes that the LE cells may play an organizer role for morphogenesis of the whole ectoderm through the coupling of JNK and dpp signaling pathways (for review, see Goberdhan and Wilson, 1998; Ip and Davis, 1998; Noselli, 1998; Noselli and Agnes, 1999). We are interested in the signaling mechanisms governing the morphogenesis of epithelia, and the way these are modulated in order to generate morphogenetic diversity. One way to study this is to compare related processes that are controlled by the same pathway in a single organism. The identification of the putative flexible parts would permit an understanding of how a single pathway may control apparently different morphogenetic processes and thus contribute to diversity. In this context, we investigated the role of hep and the JNK pathway in metamorphosis. Drosophila is an holometabolous insect, i.e., the adult form (imago) derives from complete metamorphosis during the pupal stage. The transition from larval to adult body shape is 5453 Development 126, 5453-5462 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV5358