Early neurogenesis in the spider is characterised by a stereotyped pattern of sequential recruitment of neural cells from the neuroectoderm, comparable with neuroblast formation in Drosophila

A decision between neural and epidermal fate takes place in the ventral neuroectoderm of Drosophila. At the beginning of neurogenesis, groups of neuroectodermal cells acquire the competence to adopt a neural fate. This process depends on the presence of proneural gene products in the neuroectodermal cells (Jimenez and Campos-Ortega, 1979; Jimenez and Campos-Ortega, 1990; Cabrera et al., 1987; Chitnis et al., 1995; Haddon et al., 1998; Chitnis and Kintner, 1996). In Drosophila,proneural genes are expressed in groups of four to five cells – so-called proneural clusters. One cell from each proneural cluster is singled out to become a neuroblast, while the entry into the neural pathway is blocked in the remaining cells and proneural gene expression is downregulated (Cabrera et al., 1987; Romani et al., 1987; Skeath and Carroll, 1992). Analysis of neurogenic mutants in Drosophilashowed that the transmembrane proteins Delta and Notch restrict proneural gene expression and thus the neural fate (Marin-Bermudo et al., 1995). Binding of the ligand Delta to the Notch receptor leads to proteolytic cleavages that release the intracellular domain of Notch (Struhl and Adachi, 1998; Kidd et al., 1998; Schroeter et al., 1998; Lecourtois and Schweisguth, 1998). This intracellular domain enters the nucleus and acts as a coactivator for the transcription factor Supressor of Hairless. An important target of the activators is the Enhancer of split gene complex. The gene products are bHLH proteins that, when expressed in response to Notch signal transduction, inhibit the transcription of proneural genes (Ligoxygakis et al., 1998; Nakao and Campos-Ortega, 1996). As Delta is positively r gulated by the proneural genes, Notch signalling leads to a downregulation of Delta. This feedback loop which links production of the ligand to activation of the receptor promotes amplification of small differences between the cells of a proneural cluster in their potential to adopt a neural fate, allowing a single cell to arise in a proneural cluster which inhibits its neighbours but is not inhibited by them (Heitzler and Simpson, 1991; Sternberg, 1993; Chitnis, 1995; Collier et al., 1996; Lewis, 1996). This system functions in a similar manner in vertebrate neurogenesis (de la Pompa et al., 1997; Chitnis and Kintner, 1996; Chitnis et al., 1995; Haddon et al., 1998) suggesting that the singling out of individual cells may be an ancestral feature of neurogenesis that is coupled to the particular way in which Delta/Notch signalling works. However, our previous analysis of spider neurogenesis (Stollewerk et al., 2001) showed that groups of cells rather than single cells are recruited for the neural fate. This raises the question of how such groupspecific recruitment of cells might be achieved and whether Delta/Notch signalling is involved in this process. Therefore, I cloned homologues of Notch and Delta to study their expression and function in spider neurogenesis. 5339 Development 129, 5339-5348 © 2002 The Company of Biologists Ltd doi:10.1242/dev.00109