Introduction Cell-to-cell communication is essential for the generation of the diverse tissues and organs of the body in multicellular organisms (Gurdon, 1992). One class of molecules important for such interactions are secreted signalling molecules that are involved in intercellular signalling processes. Several conserved families of secreted growth and differentiation factors have been identified and it has been shown that assorted combinations of these factors control cell differentiation and morphogenesis in a variety of organisms (for reviews, see Massague, 2000; Taipale and Beachy, 2001; Wodarz and Nusse, 1998; Grapin-Botton and Melton, 2000). The Drosophila gene hedgehog, which encodes a secreted protein, and the genes wingless and decapentaplegic, which encode members of the WNT and TGFβ families of growth factors, respectively, have been studied extensively during early embryogenesis; limb and eye patterning, and gut development (Affolter and Mann, 2001; Bienz, 1997; Dahmann and Basler, 1999; Lawrence and Struhl, 1996; Stringini and Cohen, 1999). In the ectodermal fore- and hindgut of Drosophila, Wingless, Decapentaplegic and Hedgehog have been shown to coordinate morphogenesis in signalling centres that become established at the junctions of the ectodermal and endodermal tissue layers (Hoch and Pankratz, 1996; Pankratz and Hoch, 1995). At the junction of the foregut and midgut, they direct the formation of the proventriculus, a gut-associated organ that mediates food passage in the larva (Campos-Ortega and Hartenstein, 1997; King, 1988; Snodgrass, 1935; Strasburger, 1992; Skaer, 1993). It has remained largely elusive, however, how the signals are transformed into local activation of target genes that induce the morphogenetic movements. Another class of molecules that is involved in the direct communication between neighbouring cells are transmembrane channel proteins that assemble into gap junctions (for reviews, see Kumar and Gilula, 1996; Goodenough et al., 1996). Gap junctions are composed of two hexameric hemichannel subunits that are transported to the plasma membrane, following the secretory pathway for transmembrane proteins (for reviews, see Falk and Gilula, 1994; Zhang et al., 1996; Yeager et al., 1998). If hemichannels of adjacent cells interact, they form a functional dodecameric gap junction channel directly linking cytoplasm of neighbouring cells (Unger et al., 1997). This allows cells to exchange ions and small molecules that can participate in signalling events. Gap junctions are clusters of intercellular channels and may consist of tens or thousands of channels forming so-called plaques in the membranes of cells. It is believed that the permeability of the intercellular channels for ions and small molecules depends on their molecular composition and on the size and charge of the permeant molecules (for reviews, see Kumar and Gilula, 1996; Goodenough et al., 1996). How plaque formation and localisation is controlled during developmental processes is largely unknown. In vertebrates, gap junction channel proteins 1859 In invertebrates, the direct communication of neighbouring cells is mediated by gap junctions, which are composed of oligomers of the innexin family of transmembrane proteins. Studies of the few known innexin mutants in Drosophila and C. elegans have shown that innexin proteins, which are structurally analogous to the connexins in vertebrates, play a major structural role as gap junctional core components in electric signal transmission. We show that Drosophila innexin 2 mutants display a feeding defect that originates from a failure of epithelial cells to migrate and invaginate during proventriculus organogenesis. The proventriculus is a valve-like organ that regulates food passage from the foregut into the midgut. Immunhistological studies indicate that innexin 2 is functionally required to establish a primordial structure of the proventriculus, the keyhole, during the regionalisation of the embryonic foregut tube, which is under the control of Wingless and Hedgehog signalling. Our genetic lack- and gain-of-function studies, and experiments in Drosophila tissue culture cells provide strong evidence that innexin 2 is a target gene of Wingless signalling in the proventricular cells. This is the first evidence, to our knowledge, that an invertebrate gap junction gene controls epithelial tissue and organ morphogenesis in response to the conserved WNT signalling cascade. Key words: Gap junctions, Wingless signalling, innexin 2, Proventriculus, Drosophila Summary The Drosophila gap junction channel gene innexin 2 controls foregut development in response to Wingless signalling Reinhard Bauer*, Corinna Lehmann*, Bernhard Fuss, Franka Eckardt and Michael Hoch † Institut für Zoophysiologie der Universität Bonn, Abt. für Entwicklungsbiologie, Poppelsdorfer Schloss, 53115 Bonn, Germany *These authors contributed equally to this work † Author for correspondence (e-mail: m.hoch@uni-bonn.de) Accepted 12 February 2002 Journal of Cell Science 115, 1859-1867 (2002) © The Company of Biologists Ltd Research Article