A study of shaggy reveals spatial domains of expression of achaete-scute alleles on the thorax of Drosophila

A study of shaggy mutant clones on the notum reveals that a greater number of cells are diverted into the bristle pathway of differentiation and fewer cells remain to produce the epidermis, shaggy clones differentiate supernumerary microchaetae and macrochaetae but these are found in the correct spatial locations, e.g. clusters of macrochaetae are formed round the position of the extant macrochaetae. The shaggy mutant phenotype requires the functioning of the genes of the achaete-scute (AS-C) complex but a dosage study shows that it is unlikely that the AS-C is overexpressed in shaggy cells. Data are presented that argue, also, for a correct spatial expression of the AS-C in shaggy mutants. A study of clones doubly mutant for shaggy and different achaete and scute alleles is consistent with the hypothesis that the clusters of macrochaetae formed by shaggy represent the restricted spatial domains of expression of the AS-C. The results can be reconciled with the known role for the AS-C, in determining which bristle types differentiate where, and a role for shaggy in the cell interactions, within domains of the AS-C expression, leading to the definition of only one bristle mother cell.     

Transcriptional activation by heterodimers of the achaete-scute and daughterless gene products of Drosophila

The achaete-scute complex (AS-C) and the daughterless (da) genes encode helix-loop-helix proteins which have been shown to interact in vivo and to be required for neurogenesis. We show in vitro that heterodimers of three AS-C products with DA bind DNA strongly, whereas DA homodimers bind weakly and homo or heterocombinations of AS-C products not at all. Proteins unable to dimerize did not bind DNA. Target sequences for the heterodimers were found in the promoters of the hunchback and the achaete genes. Using sequences of the former we show that the DNA binding results obtained in vitro fully correlate with the ability of different combinations to activate the expression of a reporter gene in yeast. Embryos deficient for the lethal of scute gene fail to activate hunchback in some neural lineages in a pattern consistent with the lack of a member of a multigene family.     

Patterning of the Drosophila nervous system: the achaete-scute gene complex.

The genes of the achaete-scute complex (AS-C) confer on cells the ability to become neural precursors. Their expression is restricted to groups of cells, the proneural clusters, which occupy specific positions within the embryo neural anlagen and the larva imaginal discs. Neuroblasts or sensory organ mother cells are born within these clusters. Thus, the patterns of expression of the AS-C genes help to define the topology of the nervous system.     

The EGF receptor and N signalling pathways act antagonistically in Drosophila mesothorax bristle patterning

An early step in the development of the large mesothoracic bristles (macrochaetae) of Drosophila is the expression of the proneural genes of the achaete-scute complex (AS-C) in small groups of cells (proneural clusters) of the wing imaginal disc. This is followed by a much increased accumulation of AS-C proneural proteins in the cell that will give rise to the sensory organ, the SMC (sensory organ mother cell). This accumulation is driven by cis-regulatory sequences, SMC-specific enhancers, that permit self-stimulation of the achaete, scute and asense proneural genes. Negative interactions among the cells of the cluster, triggered by the proneural proteins and mediated by the Notch receptor (lateral inhibition), block this accumulation in most cluster cells, thereby limiting the number of SMCs. Here we show that the proneural proteins trigger, in addition, positive interactions among cells of the cluster that are mediated by the Epidermal growth factor receptor (EGFR) and the Ras/Raf pathway. These interactions, which we denominate 'lateral co-operation', are essential for macrochaetae SMC emergence. Activation of the EGFR/Ras pathway appears to promote proneural gene self-stimulation mediated by the SMC-specific enhancers. Excess EGFR signalling can overrule lateral inhibition and allow adjacent cells to become SMCs and sensory organs. Thus, the EGFR and Notch pathways act antagonistically in notum macrochaetae determination.     

Gene-dose titration analysis in the search of trans-regulatory genes in Drosophila.

We have searched for trans-regulatory genes in two genetic systems in Drosophila, the bithorax complex (BX-C) and the achaete-scute complex (AS-C). Previous genetic evidence suggests that the activation of both BX-C and AS-C, depends on trans-regulatory genes (Polycomb, Pc, in the former and hairy, h, in the latter) acting in a negative type of control. Mutants of these regulatory genes in heterozygous condition have dominant derepression phenotypes in flies with extra doses of the corresponding gene complexes. We have searched for new loci, with similar gene-dose relationships. We have isolated only new alleles (six) of Pc in the BX-C experiment. In the AS-C experiment four h alleles, and 13 alleles of a new locus (extramacrochaetae, emc) have been discovered. Whereas the h locus shows specific interactions upon achaete, the new locus, emc, is specific for the scute part of the AS-C. Statistical analysis suggests that these are the only loci in the genome with those dose-dependent properties in the two systems.     

Regulation of proneural gene expression and cell fate during neuroblast segregation in the Drosophila embryo.

The Drosophila embryonic central nervous system develops from sets of progenitor neuroblasts which segregate from the neuroectoderm during early embryogenesis. Cells within this region can follow either the neural or epidermal developmental pathway, a decision guided by two opposing classes of genes. The proneural genes, including the members of the achaete-scute complex (AS-C), promote neurogenesis, while the neurogenic genes prevent neurogenesis and facilitate epidermal development. To understand the role that proneural gene expression and regulation play in the choice between neurogenesis and epidermogenesis, we examined the temporal and spatial expression pattern of the achaete (ac) regulatory protein in normal and neurogenic mutant embryos. The ac protein is first expressed in a repeating pattern of four ectodermal cell clusters per hemisegment. Even though 5-7 cells initially express ac in each cluster, only one, the neuroblast, continues to express ac. The repression of ac in the remaining cells of the cluster requires zygotic neurogenic gene function. In embryos lacking any one of five genes, the restriction of ac expression to single cells does not occur; instead, all cells of each cluster continue to express ac, enlarge, delaminate and become neuroblasts. It appears that one key function of the neurogenic genes is to silence proneural gene expression within the nonsegregating cells of the initial ectodermal clusters, thereby permitting epidermal development.     

extramacrochaetae, a negative regulator of sensory organ development in Drosophila, defines a new class of helix-loop-helix proteins.

The function of the extramacrochaetae (emc) gene is required to establish the normal spatial pattern of adult sensory organs in Drosophila. emc acts to suppress sensory organ development in certain regions of the body surface, apparently by antagonizing the function of the achaete and scute genes of the achaetescute complex (AS.C). We have found that emc encodes a novel member of the helix-loop-helix (HLH) family of proteins. The emc protein shares the dimerization domain of other HLH proteins but lacks their DNA binding motif. We propose a model in which the emc protein negatively regulates sensory organ determination by forming heterodimers with the HLH proteins encoded by the AS-C and/or daughterless, thereby altering or interfering with their activity.     

Control of neural precursor specification by proneural proteins in the CNS of Drosophila.

Formation of neural precursors in Drosophila is determined by proneural genes. The distinctive pattern of expression of some genes of the achaete-scute complex in the embryonic neuroectoderm has prompted the speculation that they could also function in the specification of neural precursor identity in the CNS. To test this hypothesis, we have analysed the capacity of different proneural proteins to promote the development of a particular CNS precursor, the MP2 precursor. Our results indicate that: (i) all known proneural proteins are similarly able to support the formation of a neural precursor at the position of MP2; (ii) different proneural proteins promote the expression of different characteristics of MP2; and (iii) a totally normal specification of the MP2 fate can only be attained by the proneural genes achaete or scute.     

asense, a member of the Drosophila achaete-scute complex, is a proneural and neural differentiation gene.

The asense (ase) gene of the achaete-scute complex (AS-C) is expressed in the precursors of all adult sensory organs (SOs), the sensory mother cells (SMCs) and in their immediate progeny. Its deletion causes the loss of some SOs and the abnormal differentiation of part of the remaining ones. These defects, which include malformations of the external part of the SOs, duplication of the innervating neuron etc, are enhanced by the haploid condition for the other AS-C genes and are corrected by an ase transgene. We conclude that ase participates, in combination with other members of the AS-C, in implementing the neural program of differentiation of the SMCs. ase also has a proneural function that participates in the singling out of the SMCs that give rise to the recurved bristles of the anterior wing margin. The proneural potential of ase is shown, in addition, by the generation of SOs induced by the generalized expression of an ase gene driven by a hsp70 promoter.     

Model investigation of central regulatory contour of gene net of D. melanogaster macrochaete morphogenesis

Morphogenesis of drosophila macrochaete functioning as mechanoreceptors includes several steps, each of which has their own genetic support described in terms of gene nets. Mechanoreceptor develops from one parental cell (Parental Cell of Sensor Organ-PCSO), the determination of which has a critical role in macrochaete development. The highest content of AS-C proneural proteins with respect to surrounding cells that initiate a neural way of cellular development and by means of it mechanoreceptor morphogenesis is typical for PCSO. The key object of gene net providing PCSO determination consists of gene complex achaete-scute (AS-C). This complex activity is controlled by central regulatory contour (CRC). Besides AS-C, CRC includes the following genes: hairy, senseless (sens), charlatan (chn), scratch (scrt), daughterless (da), extramacrochaete (emc), and groucho (gro). The system of direct relation and feedback and induction and repression relations between CRC components are realized via the coding by these genes proteins. A mathematical model of CRC functioning as a regulator of proneural AS-C protein content in PCSO determining successful passing of the main phase of morphogenesis of D. melanogaster mechanoreceptor is discussed.     

The notch signaling pathway is required to specify muscle progenitor cells in Drosophila.

Organization and function of the Notch signaling pathway in Drosophila are best understood with respect to its role in the process of selection of neural progenitor cells. However, there is evidence that, besides neurogenesis, the Notch signaling pathway is involved in several other developmental processes, one of which is the selection of muscle progenitor cells. Thus, the number of these cells is increased in neurogenic mutants, and it has been proposed that muscle progenitor cells are selected from clusters of equivalent cells expressing genes of the achaete-scute gene complex (AS-C). Here, I present evidence for the participation of additional elements of the Notch signaling pathway in myogenesis. Gal4 mediated expression of a Notch variant, E(spl) and Hairless shows that the selection of muscle progenitor cells obeys principles apparently identical to those acting at the selection of neural progenitor cells.