INTRODUCTION The wing imaginal disc of Drosophila gives rise to the mesonotum, the major portion of the fly notum, and its associated appendage, the wing (Bryant, 1978). The wing is composed of a more distal portion, called the wing blade, and a more proximal region, called the wing hinge (Fig. 1A). The mesonotum, wing hinge and wing blade are derived from different positions along the proximo-distal (P-D) axis of the wing disc (Fig. 1B). Although much is known about the control of wing blade development (see below), relatively little is known about the formation of the wing hinge, or how cells within the wing imaginal disc are allocated to form either the mesonotum, wing hinge or wing blade. To provide answers to these questions, the mechanisms that govern positional information along the P-D axis in the wing must be investigated. Wing development requires wingless (wg), which encodes a member of the Wnt family of secreted proteins (reviewed in Klingensmith and Nusse, 1994). In the absence of wg function, neither the wing blade nor hinge are formed, and a duplication of the notum results (Sharma and Chopra, 1976; Morata and Lawrence, 1977). wg is first expressed in the wing imaginal disc during second instar in a small ventral-anterior patch of cells (Couso et al., 1993; Williams et al., 1993; Ng et al., 1996). At about this same time in wing disc development, a cell lineage restriction arises in wing discs that separates the dorsal compartment (D) from the ventral compartment (V) (Garcia- Bellido et al., 1973). This lineage restriction is due to the expression of the LIM homeobox gene apterous in the dorsal compartment (Diaz-Benjumea and Cohen, 1993; Blair et al., 1994). The D/V boundary divides the wing blade, hinge and notum regions of the wing disc (Fig. 1B). The D/V boundary plays an important role in wing development by providing a source of both short- and long- range signals. The Notch signaling pathway is activated on both sides of the D/V boundary and, in the wing blade, activates expression of wg at the D/V boundary (Diaz-Benjumea and Cohen, 1995; Kim et al., 1996; Neumann and Cohen, 1996b; Klein and Martinez-Arias, 1998). Both Notch and wg activate the gene vestigial (vg) which, together with its cofactor encoded by scalloped (sd), is essential for the growth of the wing blade (Williams et al., 1991, 1994; Kim et al., 1996; Neumann and Cohen, 1997; Halder et al., 1998; Simmonds et al., 1998; Varadarajan and VijayRaghavan, 1999). Importantly, the ways in which Notch and wg activate vg are very different. Notch activates vg in cells very close to the D/V boundary, both within and outside the wing blade, by activating an enhancer called the vg-boundary enhancer (vgBE) (Williams et al., 1994; Kim et al., 1996) (Fig. 1B). In contrast, wg, secreted from cells at the D/V boundary, induces vg expression at a distance from the wg-secreting cells, thereby providing positional information along the P-D axis within the wing blade (Zecca et al., 1996; Neumann and Cohen, 1997). The result is a 1499 Development 127, 1499-1508 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV7776 The Drosophila wing imaginal disc gives rise to three body parts along the proximo-distal (P-D) axis: the wing blade, the wing hinge and the mesonotum. Development of the wing blade initiates along part of the dorsal/ventral (D/V) compartment boundary and requires input from both the Notch and wingless (wg) signal transduction pathways. In the wing blade, wg activates the gene vestigial (vg), which is required for the wing blade to grow. wg is also required for hinge development, but wg does not activate vg in the hinge, raising the question of what target genes are activated by wg to generate hinge structures. Here we show that wg activates the gene homothorax (hth) in the hinge and that hth is necessary for hinge development. Further, we demonstrate that hth also limits where along the D/V compartment boundary wing blade development can initiate, thus helping to define the size and position of the wing blade within the disc epithelium. We also show that the gene teashirt (tsh), which is coexpressed with hth throughout most of wing disc development, collaborates with hth to repress vg and block wing blade development. Our results suggest that tsh and hth block wing blade development by repressing some of the activities of the Notch pathway at the D/V compartment boundary. Key words: Proximal-distal axis, Wing development, Wing hinge, homothorax, wingless, Notch, vestigial, Drosophila, extradenticle SUMMARY A dual role for homothorax in inhibiting wing blade development and specifying proximal wing identities in Drosophila Fernando Casares and Richard S. Mann* Department of Biochemistry and Molecular Biophysics, Columbia University, 701 West 168th Street, HHSC 1108, New York, NY 10032 USA *Author for correspondence (e-mail: rsm10@columbia.edu) Accepted 18 January; published on WWW 7 March 2000