Summary The role of buttonhead and Sp 1 in the development of the ventral imaginal discs of Drosophila

One principal question in developmental biology is how the body of multicellular organisms is genetically subdivided. In Drosophila there is a great deal of information about the subdivision along the anteroposterior axis (AP) body axis: the successive activities of maternal and segmentation genes (reviewed by Lawrence and Morata, 1994) generate a number of equivalent body units that form the parasegmental trunk. The various Hox genes become active in specific positions of the parasegmental trunk and confer different identities, thus generating the morphological diversity in the AP axis. Within the domains defined by the activity of the Hox genes there are other genes that discriminate the identity of more restricted parts. For example, pannier (pnr) appears to be responsible for distinguishing between medial and lateral regions of all thoracic and abdominal segments (Calleja et al., 2000; Herranz and Morata, 2001). The gene apterous(ap) distinguishes between the dorsal and ventral regions of the wing appendages (Díaz-Benjumea and Cohen, 1993), and Distal-less (Dll) specifies the growth and identity of ventral appendages (Cohen and Jurgens, 1989; Gorfinkiel et al., 1997). A major morphological distinction in the embryonic, larval and adult body is that between dorsal and ventral regions. There is evidence that the dorsal and ventral adult structures of the thorax share a common lineage in early development (Steiner, 1976; Wieschaus and Gehring, 1976; Lawrence and Morata, 1977), but by late embryogenesis the dorsal and ventral primordia have different lineages. It is not clear whether this restriction corresponds to a genuine compartment segregation or whether it is the result of the physical separation of the two primordia, which can be observed during embryogenesis (Goto and Hayashi, 1997; Kubota et al., 2000). Nonetheless there is a clear difference between dorsal and ventral patterns, which is also reflected in the activity of different developmental genes. There are identity conferring genes which are restricted to either the dorsal or the ventral regions, such as pnr, Dll or vestigial (vg) (reviewed by Mann and Morata, 2000). We have developed a method (Calleja et al., 1996) which allows the visualisation of gene expression patterns in living flies. It is a modification of the Gal4/UAS procedure of Brand and Perrimon (Brand and Perrimon, 1993): mobilisation of the pGalw element yields a collection of insertion lines, which are tested using an UAS-y construct. Individual adult flies showing dark (y+) patches of interest were used to establish new Gal4 lines. We have used this method for a systematic screen of genes showing restricted expression in adult flies. Among those, the line MD808 was found to confer y+ expression in ventral adult derivatives, proboscis, antennae, legs and genitalia, suggesting the gene driving its expression may have a general function in the formation of these structures. We report a functional characterisation of the gene driving MD808 expression. It turned out to be uttonhead (btd), which encodes a zinc-finger transcription factor (Wimmer et al., The related genes buttonhead (btd) and Drosophila Sp1(the Drosophilahomologue of the human SP1gene) encode zincfinger transcription factors known to play a developmental role in the formation of the Drosophilahead segments and the mechanosensory larval organs. We report a novel function of btd and Sp1: they induce the formation and are required for the growth of the ventral imaginal discs. They act as activators of the headcase (hdc) and Distal-less (Dll) genes, which allocate the cells of the disc primordia. The requirement for btd and Sp1 persists during the development of ventral discs: inactivation by RNA interference results in a strong reduction of the size of legs and antennae. Ectopic expression of btd in the dorsal imaginal discs (eyes, wings and halteres) results in the formation of the corresponding ventral structures (antennae and legs). However, these structures are not patterned by the morphogenetic signals present in the dorsal discs; the cells expressing btd generate their own signalling system, including the establishment of a sharp boundary of engrailedexpression, and the local activation of the winglessand decapentaplegicgenes. Thus, the Btd product has the capacity to induce the activity of the entire genetic network necessary for ventral imaginal discs development. We propose that this property is a reflection of the initial function of the btd/Sp1genes that consists of establishing the fate of the ventral disc primordia and determining their pattern and growth.