Pattern Formation: The link between ovary and embryo

The Drosophila gene nudel may encode a spatially restricted serine protease involved in producing the ligand for the receptor Toll and linking dorsal-ventral polarity in the egg chamber to the developing embryonic axis. One of the important insights to have emerged from the study of early pattern formation in the Drosophila embryo is that the basic embryonic axes have their origins in the polarity established in the egg chamber in which the oocyte develops [1]. During oogenesis, the female uses a signalling cascade mediated by a receptor tyrosine kinase to establish dorsal-ventral polarity in the egg chamber [2]. This polarity is subsequently communicated to the embryo via a signaling pathway similar to the inter-leukin-1/NF-KB pathway used by the mammalian immune system [3]. A gap in our understanding of embryonic dorsal-ventral pattern has been in the integration of these two signalling pathways. How does the fol-licle cell layer impose its dorsal-ventral polarity on the embryo? The product of the recently cloned nudel gene may play a direct role in integrating the two systems, and may do so as a component of the Drosophila egg shell. The Drosophila egg chamber comprises the developing oocyte, its fifteen associated nurse cells and a surrounding epithelium of somatically derived follicle cells (Fig. la). The first morphological manifestations of dorsal-ventral pattern in the egg chamber can be seen during stages 8 to 10 of oogenesis, when the oocyte nucleus assumes a dorsal-anterior location within the oocyte. The outside of the egg is also visibly polarized dorsoventrally, with curved ventral and flattened dorsal surfaces and two appendages located dorsally. The first morphological signs of dorsal-ventral polarity exhibited by the embryo that lies within the polarized shell occur during gastrula-tion, when cells that will go on to become mesoderm invaginate into a furrow on the presumptive ventral side of the embryo. This ventral furrow always forms along the curved ventral surface of the egg shell-thus the polarity of the embryo and the egg shell are coordinated. A group of twelve 'maternal-effect' genes (mutations of which are manifest in eggs or embryos produced by homozygous female flies) termed the 'dorsal group' is required for the establishment of correct embryonic dor-sal-ventral polarity, but not for egg polarity [4]. The Toll gene encodes a transmembrane receptor that is uniformly distributed in the embryonic membrane. Toll is thought to be activated on the ventral side of the embryo by the binding of …