Cell-cell communication at anterior/posterior compartment borders in Drosophila involves Hedgehog (Hh), a protein secreted by posterior cells, and Cubitus interruptus (Ci), a protein in the Hh response pathway in anterior cells. Although Ci is thought to have roles as a transcription factor repressing hh expression and activating target genes, it localizes in the cytoplasm of anterior cells. We report here the identification of a domain that tethers Ci in the cytoplasm and show that in some anterior cells, Ci is cleaved to generate a form that lacks the tethering domain. This form translocates to the nucleus where it represses hh and other target genes. Hh inhibits proteolysis of Ci, and we suggest that this inhibition leads to the observed patterns of expression of key target genes at the compartment border.

The anterior/posterior (A/P) and dorsal/ventral (D/V) compartment borders that subdivide the wing imaginal discs of Drosophila third instar larvae are each associated with a developmental organizer. Decapentaplegic (Dpp), a member of the transforming growth factor-beta (TGF-beta) superfamily, embodies the activity of the A/P organizer. It is produced at the A/P organizer and distributes in a gradient of decreasing concentration to regulate target genes, functioning non-autonomously to regulate growth and patterning of both the anterior and posterior compartments. Wingless (Wg) is produced at the D/V organizer and embodies its activity. The mechanisms that distribute Dpp and Wg are not known, but proposed mechanisms include extracellular diffusion, successive transfers between neighbouring cells, vesicle-mediated movement, and direct transfer via cytonemes. Cytonemes are actin-based filopodial extensions that have been found to orient towards the A/P organizer from outlying cells. Here we show that in the wing disc, cytonemes orient towards both the A/P and D/V organizers, and that their presence and orientation correlates with Dpp signalling. We also show that the Dpp receptor, Thickveins (Tkv), is present in punctae that move along cytonemes. These observations are consistent with a role for cytonemes in signal transduction.

Wing imaginal disc cells in Drosophila develop by using information received from a signaling center associated with the anterior/posterior compartment border. We show here that disc cells have thin, actin-based extensions (cytonemes) that project to this signaling center. Cytonemes can be induced when cells from the lateral flanks of a wing disc are cultured next to cells from the A/P border or next to a source of fibroblast growth factor. Mouse limb bud cells also grow projections during a brief culture period, indicating that cytonemes are an attribute of both vertebrate and invertebrate cells. We suggest that cytonemes may be responsible for some forms of long-range cell-cell communication.

We constructed green fluorescent protein (GFP)-expressing balancer chromosomes for each of the three major chromosomes of Drosophila melanogaster. Expression of GFP in these chromosomes is driven indirectly by a Kruppel (Kr) promoter, via the yeast GAL4-UAS regulatory system. GFP fluorescence can be seen in embryos as early as the germ band extension stage, and can also be seen in larvae, pupae, and adults. We show the patterns of GFP expression of these balancers and demonstrate the use of the balancers to identify homozygous progeny.