INTRODUCTION Neuroblasts, the progenitor cells of the central nervous system (CNS) in insects, delaminate from the embryonic neuroecto- derm of Drosophila in a stereotyped manner that involves several waves of segregation (Hartenstein and Campos-Ortega, 1984; Doe, 1992). It has been shown, at least for thirteen neu- roblasts that segregate in each hemisegment during the first two waves (S1 and S2), that their formation is initiated by the acquisition of neural potential by small groups of ectodermal cells, called proneural clusters (see Campos-Ortega, 1993, for a recent review). Proneural clusters in the embryonic neuroec- toderm are defined by the expression of achaete (ac), scute (sc) and lethal of scute (l’sc), genes of the achaete-scute complex (AS-C) (Cabrera et al., 1987; Romani et al., 1987; Martín- Bermudo et al., 1991; Skeath and Carroll, 1992; Ruiz-Gómez and Ghysen, 1993). During segregation, each neuroblast accu- mulates the highest levels of AS-C proteins, whereas in the remaining cells of the proneural cluster these proteins gradually dissapear. This pattern of expression and the analysis of mutant phenotypes indicates that the function of the AS-C genes is to confer upon cells of the clusters the capacity to become neuroblasts (reviewed by Campuzano and Modolell, 1992; Campos-Ortega, 1993). The restriction of the neural fate to a single cell of a proneural cluster is mediated by neurogenic genes, through an inhibitory process that involves intercellular communication (reviewed by Ghysen et al., 1993). According to Cabrera (1990, 1992), the molecular outcome of the inhibitory process would be the accumulation of an active, unphosphorylated form of the AS-C proteins exclusively in the presumptive neuroblasts. This contrasts with another report which suggests that the control of the AS-C proteins is largely transcriptional (Skeath and Carroll, 1992). It has been proposed that neurogenic genes constitute a func- tional cassette that mediates cell interactions in many devel- opmental processes (Ruohola et al., 1991). One such process could be the specification of the mesectoderm in the early embryo, for the analysis of mesectoderm-specific gene expression in different mutants has lead to the suggestion that mesectoderm specification requires an inductive signal from the mesoderm (see Nambu et al., 1993, and references therein). Additional evidence for the induction derives from the ectopic transplantation of mesodermal cells, which results in the expression in the surrounding ectoderm of single-minded (sim), a master regulatory gene for mesectodermal development (Leptin and Roth, 1994; Nambu et al., 1991). In this paper, we first attempt to clarify the discrepancies existing in the control of proneural gene expression by neuro- 219 Development 121, 219-224 (1995) Printed in Great Britain © The Company of Biologists Limited 1995 The development of the central nervous system in the Drosophila embryo is initiated by the acquisition of neural potential by clusters of ectodermal cells, promoted by the activity of proneural genes. Proneural gene function is antagonized by neurogenic genes, resulting in the realiza- tion of the neural potential in a single cell per cluster. To analyse the relationship between proneural and neuro- genic genes, we have studied, in specific proneural clusters and neuroblasts of wild-type and neurogenic mutants embryos, the expression at the RNA and protein levels of lethal of scute, the most important known proneural gene in central neurogenesis. We find that the restriction of lethal of scute expression that accompanies the restriction of the neural potential to the delaminating neuroblast is regulated at the transcriptional level by neurogenic genes. These genes, however, do not control the size of proneural clusters. Morover, available antibodies do not provide evidence for an hypothetical posttranscriptional regula- tion of proneural proteins by neurogenic genes. We also find that neurogenic genes are required for the specifica- tion of the mesectoderm. This has been shown for neural- ized and Notch, and could also be the case for Delta and for the Enhancer of split gene complex. Neurogenic genes would control at the transcriptional level the repression of proneural genes and the activation of single-minded in the anlage of the mesectoderm. Key words: Drosophila, neurogenic genes, lethal of scute, neurogenesis, single-minded, mesectoderm specification SUMMARY Neurogenic genes control gene expression at the transcriptional level in early neurogenesis and in mesectoderm specification María D. Martín-Bermudo* ,† , Ana Carmena* and Fernando Jiménez ‡ Centro de Biología Molecular ‘Severo Ochoa’ (CSIC-UAM), Universidad Autónoma de Madrid, 28049 Madrid, Spain *Both authors contributed equally † Present address: Wellcome/CRC Institute, Cambridge CB2 1QR, UK ‡ Author for correspondence