Institut fuer Allgemeine Genetik (240), Universitaet Hohenheim, D-70593 Stuttgart, F.R.G., § Dept. of Anatomy, University of Cambridge, Cambridge CB2 3DY, UK, # Institute of Molecular Biology and Biotechnology, FORTH and Dept. of Biology, University of Crete, GR- 711 10 Heraklion
The E(spl) gene complex comprises seven genes [md, mg, mb, m3, m5, m7, m8] which all encode small, basic helix-loop-helix (bHLH) proteins with a high degree of similarity at the structural and transcriptional level. Although none of these genes is essential for fly survival, their concomitant loss causes extreme neural hyperplasia, demonstrating their requirement in neurogenesis. This synthetic deletion phenotype is, therefore, best reconciled with functional redundancy of the individual bHLH gene products. Still, specific functions for the E(spl) genes are to be expected, since the entire gene complex is extremely well conserved during evolution. During neurogenesis, groups of neuro- competent cells are first established by the activity of proneural genes. From these groups, neural precursors are selected whilst all the other cells are prevented from adopting the neural fate by lateral inhibition controlled by the Notch signalling pathway. E(spl) gene expression is turned on as a consequence of Notch signalling in the presumptive non- neural cells, and E(spl) bHLH gene products function to repress proneural activity in these cells. In the present analysis, we focuss on the degree of overlap in the expression patterns of the individual E(spl) bHLH genes in order to determine the potentials for functional diversity. RNA and protein distribution of individual E(spl) genes was monitored in whole mount embryos and imaginal discs, respectively. Expression patterns were compared either of two E(spl) bHLH genes directly or correlated with independent markers. Embryonic E(spl) gene activity can be divided into four phases, maternal contribution (m3, md), early zygotic expression at late blastoderm stage (m5, m7, m8), expression in the neurogenic ectoderm and subsequent expression in other tissues (mesoderm, dorsal ectoderm, brain, etc.; distinct patterns). Postembryonic gene activity correlates well with E(spl) requirement for neural development. However, the expression of the seven genes in imaginal tissues is clearly different in so far, that some of the genes are expressed only in a subset of tissues compared with the others. Taking alltogether, spatio-temporal patterns of expression are quite diverse. This observation is incompatible with a simple hypothesis that expression of all E(spl) bHLH genes is solely directed by Notch signalling. Rather, additional cooperative factors, specific for the individual E(spl) genes, might be required for proper temporal and spatial gene activation. As a consequence of specific gene activation, individual E(spl) bHLH genes might have very different potentials in inhibiting certain proneural genes and thus may have distinct functions in their target tissues during development.