Intrasegmental patterning in the Drosophila embryo is regulated by cell-cell communication. One of the signaling pathways that operates to specify positional information throughout the segment is mediated by the wingless (wg) protein, which is the homolog of the proto-oncogene Wnt-1. The early role of wg is to stabilize engrailed (en) expression by initiating a phase of en autoregulation in the adjacent more posterior cells. Here, we report that the segment polarity gene zeste-white 3 (zw3; also known as shaggy) acts as a repressor of en autoregulation. Genetic epistasis experiments indicate that wg signaling operates by inactivating the zw3 repression of en autoactivation. In addition, we demonstrate that zw3 encodes the Drosophila homolog of mammalian glycogen synthase kinase-3.

The metameric pattern of the Drosophila embryo is regulated by a combination of maternal and zygotic genes. The segment-polarity class of genes are required for the correct patterning within each segmental unit. Mutations in any one of these genes results in deletions and duplications of parts of each segment. The segment-polarity genes act coordinately by means of local cellular interactions to assign and maintain an identity for each cell in the segment, and to establish segment boundaries. Here we describe the molecular characterization of a novel segment-polarity gene, zeste-white3 (zw3). Embryos derived from germ lines that are homozygous for zw3 mutations (zw3 embryos) have phenotypes similar to embryos that are mutant for the segment-polarity gene naked (nkd). These embryos lack most of the ventral denticles, which are differentiated structures derived from the most anterior region of each segment. We have isolated the zw3 gene and compared the structure of one maternal and one zygotic transcript encoded by the gene. The zw3 gene is unique among the segment-polarity genes so far characterized, in that it encodes proteins that have homology to serine-threonine protein kinases. This indicates that zw3 may play a part in a signal transduction pathway involved in the establishment of cell identity within each embryonic segment.

Lack of both maternal and zygotic gene activity at the zeste-white 3 (zw3) locus causes severe developmental transformations. Embryos derived from germ cells that lack zw3+ gene activity die during embryogenesis and have a phenotype that is similar to that of embryos mutant for the segment polarity gene naked (nkd). In both nkd and germ line clone-derived zw3 embryos the pattern elements derived from the anterior-most part of each segment, the denticle belts, are deleted. Similar abnormal patterns of the zygotically expressed genes engrailed and Ultrabithorax are detected in both mutants, suggesting that the two genes are involved in the same developmental process. Additionally, the induction of clones of zw3 mutant cells in imaginal discs causes homeotic transformations of noninnervated hair cells into innervated sensory bristles. The multiple roles of zw3 during development and its possible interactions with the zygotic gene nkd are discussed.