DROSOPHILA DISTAL-LESS Distal-less function

Distal-less function Distal-less (Dll), as its name suggests, is required for distal limb development. Drosophila mutants lacking Dll function die as embryos because they lack the rudimentary larval limbs (Cohen and Jurgens, 1989). Viable combinations of Dll alleles of increasing severity can be used to generate a phenotypic series in which weak allelic combinations lead to fusions of the distal leg segments or tarsi (Fig. 1); intermediate combinations result in loss of the tarsal segments; and stronger combinations cause loss of both the tarsi and a medial leg segment, the tibia (Cohen et al., 1989; Dong et al., 2000; Sunkel and Whittle, 1987). Mitotic clonal analysis was used to generate clones of cells null for Dll in the distal leg. Small Dll null clones in the distal leg delaminate from the disc epithelium and form vesicles within the leg (Gorfinkiel et al., 1997; Wu and Cohen, 1999). In addition, when the behavior of cells in the Dll null clones was observed during the larval stages, they were found to segregate from distal imaginal disc epithelium and to migrate towards the presumptive proximal cells (Wu and Cohen, 1999). These results confirm that Dll is required for the specification of distal leg pattern elements, and indicate that Dll regulates the expression of as yet unknown molecules required for differential affinities between proximal and distal cells. The Drosophila antenna is a second appendage in which Dll is required for development of the proximodistal (PD) axis. However, in the antenna, Dll has a second function, that of specifying antennal identity (Fig. 1) (Cohen et al., 1989; Dong et al., 2000; Sato, 1984; Sunkel and Whittle, 1987). This is of particular interest because the antenna is both the ear and nose of the fly, and vertebrate Dlx genes have been implicated in both ear (Acampora et al., 1999; Depew et al., 1999; Solomon and Fritz, 2002) (reviewed by Kraus and Lufkin, 1999) and nose development (Acampora et al., 1999; Akimenko et al., 1994; Depew et al., 1999; Quint et al., 2000; Robinson et al., 1991; Yang et al., 1998; Zhao et al., 1994). It remains to be seen whether Dll and the Dlx genes have similar roles during the development of these organs. Other limb-derived structures in which Dll is required include the mouthparts (Cohen and Jurgens, 1989) and the analia (Gorfinkiel et al., 1999). It is noteworthy that vertebrate Dlx genes have a prominent role in the development of the mandible and maxilla (Depew et al., 2002). Dll is not required for formation of the PD axis of the wing. In addition to functioning during adult appendage development, the Drosophila Dll gene is required for the formation of parts of the peripheral nervous system. In Dll-null animals, the larval antennal, maxillary and labial sense organs do not form, nor do the mechanosensory vestigial larval legs called ‘Keilin’s organs’. Mutations in vertebrate Dlx genes 4371 Development 129, 4371-4386 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV3655