A FUNCTIONAL HOMOLOG OF GOOSECOID IN DROSOPHILA

We have cloned a Drosophila homologue (D-gsc) of the vertebrate homeob ox gene goosecoid (gsc). In the Gsc proteins, the pressure for conserv ation has been imposed on the homeodomain, the functional domain of th e protein: sequence homology is limited to the homeodomain (78% identi ty) and to a short stretch of 7 aminoacids also found in other homeopr oteins such as Engrailed, Despite this weak homology, D-gsc is able to mimic gsc function in a Xenopus assay, as shown by its ability to res cue the axis development of a UV-irradiated embryo, Moreover, our data suggest that the position of insect and vertebrate gsc homologues wit hin a regulatory network has also been conserved: D-gsc expression is controlled by decapentaplegic, orthodenticle, sloppy-paired and taille ss whose homologues control gsc expression (for BMP4 and Otx-2), or ar e expressed at the right time and the right place (for XFKH1/Pintallav is and Tlx) to be interacting with gsc during vertebrate development, However, the pattern of D-gsc expression in ectodermal cells of the ne rvous system and foregut cannot easily be reconciled with that of vert ebrate gsc mesodermal expression, suggesting that its precise developm ental function might have diverged, Still, this comparison of domains of expression and functions among Gsc proteins could shed light on a c ommon origin of gut formation and/or on basic cellular processes, The identification of gsc target genes and/or other genes involved in simi lar developmental processes will allow the definition of the precise p hylogenetic relationship among Gsc proteins.