1. Department of Biochemistry and Biophysics, University of California, San Francisco, California 94143, USA
2. †Present address: Department of Biology, Cell and Molecular Biology Group, San Francisco State University, San Francisco, California 94132, USA

Correspondence to: Thomas B. Kornberg¹ Correspondence and requests for materials should be addressed to T.B.K. (Email: tkornberg@biochem.ucsf.edu).

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- (TGF-) 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^{1, 2, 3}. Wingless (Wg) is produced at the D/V organizer and embodies its activity^{4, 5}. The mechanisms that distribute Dpp and Wg are not known, but proposed mechanisms include extracellular diffusion⁶, successive transfers between neighbouring cells^{7, 8}, vesicle-mediated movement⁹, and direct transfer via cytonemes¹⁰. 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.

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