Role of the gooseberry gene in Drosophila embryos: maintenance of wingless expression by a wingless--gooseberry autoregulatory loop.

During Drosophila embryogenesis, segment polarity genes, such as engrailed (en), wingless (wg) and gooseberry (gsb) show complex interactions that provide positional information along the antero-posterior axis within each segment. Little is known about the specific role of each of these genes in this pattern determining process. Here we demonstrate that the main function of gsb, which encodes a transcription factor containing a paired-domain and a prd-type homeodomain, is the maintenance of wg expression by a wg-gsb autoregulatory loop after 6 h of development. The function of wg, the homologue of the murine Wnt-1 gene, is to specify the denticle pattern by repressing a default state of ubiquitous denticle formation in the ventral epidermis. This repression of denticles by the wg signal is different from the wingless signalling pathways that activate gsb or en. Mutual activations involving gsb, wg and en show temporal asymmetries that lead to their different mutant phenotypes. A general model is proposed for the generation of morphogenetic fields by self-propagating autoregulatory loops.