Lateral inhibition and the development of the sensory bristles of the adult peripheral nervous system of Drosophila

Cells in the neurectoderm of Drosophila face a choice between neural and epidermal fates. On the notum of the adult fly, neural cells differentiate sensory bristles in a precise pattern. Evidence has accumulated that the bristle pattern arises from the spatial distribution of small groups of cells, proneural clusters, from each of which a single bristle will result. One class of genes, which includes the genes of the achaete-scute complex, is responsible for the correct positioning of the proneural clusters. The cells of a proneural cluster constitute an equivalence group, each of them having the potential to become a neural cell. Only one cell, however, will adopt the primary, dominant, neural fate. This cell is selected by means of cellular interactions between the members of the group, since if the dominant cell is removed, one of the remaining, epidermal, cells will switch fates and become neural. The dominant cell therefore prevents the other cells of the group from becoming neural by a phenomenon known as lateral inhibiton. They, then, adopt the secondary, epidermal, fate. A second class of genes, including the gene shaggy and the neurogenic genes mediate this process. There is some evidence that a proneural cluster is composed of a small number of cells, suggesting a contact-based mechanism of communication. The molecular nature of the protein products of the neurogenic genes is consistent with this idea.     

Genetic mechanisms of early neurogenesis inDrosophila melanogaster

The neurogenic ectoderm ofDrosophila melanogaster consists of the ventral neuroectoderm and the procephalic neuroectoderm. It is hypothesized that epidermal and central neural progenitor cells separate from each other in three steps: conference on the neuroectodermal cells the capability of producing neural or epidermal progenies, separation of the two classes of progenitor cells, and specification of particular types of neuroblasts and epidermoblasts. Separation of neuroblasts and epidermoblasts in controlled by proneural and neurogenic genes.Delta andNotch serve as mediators of direct protein-protein interactions. E(spl)-C inhibits neurogenesis, creating epidermal cells. The achaete-scute complex (AS-C) controls the commitment of nonoverlapping populations of neuroblasts and leads the development of neuroectodermal cells as neuroblasts.     

Changes in cell shape in the ventral neuroectoderm of Drosophila melanogaster depend on the activity of the achaete-scute complex genes

In the embryonic ventral neuroectoderm of Drosophila melanogaster the proneural genes achaete, scute, and lethal of scute are expressed in clusters of cells from which the neuroblasts delaminate in a stereotyped orthogonal array. Analyses of the ventral neuroectoderm before and during delamination of the first two populations of neuroblasts show that cells in all regions of proneural gene activity change their form prior to delamination. Furthermore, the form changes in the neuroectodermal cells of embryos lacking the achaete-scute complex, of embryos mutant for the neurogenic gene Delta, and of embryos overexpressing l’sc suggest that these genes are responsible for most of the morphological alterations observed. Received: 20 August 1999 / Accepted: 3 November 1999     

Regulatory interactions during early neurogenesis in Drosophila

During neurogenesis in Drosophila, ectodermal cells are endowed with the capacity to become neuronal precursors. Following their selection, these cells initiate neuronal lineage development and differentiation. The processes of neuronal precursor specification and neuronal lineage development require the activities of several groups of genes functioning in a complex, hierarchical regulatory network. Whereas the proneural genes promote neurogenic potential, neurogenic genes restrict the acquisition of this identity to a subset of ectodermal cells. Following their selection, these cells express the pan neural neuronal precursor genes and a set of neuronal lineage identity genes. While lineage identity genes allow the various lineages to acquire specific identities, neuronal precursor genes presumably regulate functional and developmental characteristics common to all neuronal precursor cells. © 1996 Wiley-Liss, Inc.     

Local function of the Notch gene for embryonic ectodermal pathway choice in Drosophila

Mutations at the Notch locus affect the fate of cells in the neurogenic region of the Drosophila embryo so that epidermal precursors become neuroblasts. We have analyzed the cellular requirements for wild-type Notch gene function by means of genetic mosaics, using a cuticle marker to distinguish hypodermal cell genotype. Cells that were genotypically Notch never gave rise to hypoderm within the neurogenic region of mosaic embryos. Mosaic dividing lines within the neurogenic region juxtapose N+ hypoderm with regions of neural hypertrophy. This autonomous action of Notch in hypodermal cells is consistent with a local function of the protein during neurogenesis. Comparison of clone distribution in Notch mosaics and controls suggests that islands of wild-type hypodermal cells fail to differentiate cuticle.     

The determination of sense organs in Drosophila: effect of the neurogenic mutations in the embryo.

We have examined the early pattern of sensory mother cells in embryos mutant for six different neurogenic loci. Our results show that the neurogenic loci are required to restrict the number of competent cells that will become sensory mother cells, but are not involved in controlling the localization or the position-dependent specification of competent cells. We conclude that these loci are involved in setting up a system of mutual inhibition, which transforms graded differences within the proneural clusters into an all-or-none difference between one cell, which becomes the sense organ progenitor cell, and the other cells, which remain epidermal.     

The extramacrochaetae gene provides information for sensory organ patterning.

The Drosophila adult epidermis displays a stereotyped pattern of bristles and other types of sensory organs (SOs). Its generation requires the proneural achaete (ac) and scute (sc) genes. In the imaginal wing disc, the anlage for most of the thoracic and wing epidermis, their products accumulate in groups of cells, the proneural clusters, whose distribution prefigures the adult pattern of SOs. These proteins then induce the emergence of SO mother cells (SMCs). Here, we show that the extramacrochaetae (emc) gene, an antagonist of the proneural function, is another agent that contributes to SO positioning. In the wing disc, emc is expressed in a complex and evolving pattern. SMCs appear not only within proneural clusters but also within minima of emc expression. When one of these spatial restrictions is eliminated, by ubiquitously expressing ac-sc, SMCs still emerge within minima of emc. When in addition, the other spatial restriction is reduced by decreasing emc expression, many ectopic SMCs emerge in a relatively even spaced and less constant pattern. Thus, the heterogeneous distribution of the emc product is one of the elements that define the positions where SMCs arise. emc probably refines SMC (and SO) positioning by reducing both the size of proneural clusters and the number of cells within clusters that can become SMCs.     

Role of neurogenic genes in establishment of follicle cell fate and oocyte polarity during oogenesis in Drosophila

Oogenesis in Drosophila involves specification of both germ cells and the surrounding somatic follicle cells, as well as the determination of oocyte polarity. We found that two neurogenic genes, Notch and Delta, are required in oogenesis. These genes encode membrane proteins with epidermal growth factor repeats and are essential in the decision of an embryonic ectodermal cell to take on the fate of neuroblast or epidermoblast. In oogenesis, mutation in either gene leads to an excess of posterior follicle cells, a cell fate change reminiscent of the hyperplasia of neuroblasts seen in neurogenic mutant embryos. Furthermore, the Notch mutation in somatic cells causes mislocalization of bicoid in the oocyte. These results suggest that the neurogenic genes Notch and Delta are involved in both follicle cell development and the establishment of anterior-posterior polarity in the oocyte.     

Neurogenesis in the spider: new insights from comparative analysis of morphological processes and gene expression patterns

While there is a detailed understanding of neurogenesis in insects and partially also in crustaceans, little is known about neurogenesis in chelicerates. In the spider Cupiennius salei Keyserling, 1877 (Chelicerata, Arachnida, Araneae) invaginating cell groups arise sequentially and in a stereotyped pattern comparable to the formation of neuroblasts in Drosophila melanogaster Meigen, 1830 (Insecta, Diptera, Cyclorrhapha, Drosophilidae). In addition, functional analysis revealed that in the spider homologues of the D. melanogaster proneural and neurogenic genes control the recruitment and singling out of neural precursors like in D. melanogaster. Although groups of cells, rather than individual cells, are singled out from the spider neuroectoderm which can thus not be homologized with the insect neuroblasts, similar genes seem to confer neural identity to the neural precursor cells of the spider. We show here that the pan-neural genes snail and the neural identity gene Krüppel are expressed in neural precursors in a heterogenous spatio-temporal pattern that is comparable to the pattern in D. melanogaster. Our data suggest that the early genetic network involved in recruitment and specification of neural precursors is conserved among insects and chelicerates.     

Competence to develop sensory organs is temporally and spatially regulated in Drosophila epidermal primordia.

The Drosophila adult cuticle displays a stereotyped pattern of sensory organs (SOs). Its deployment requires the expression of the achaete (ac) and scute (sc) genes. Their products confer to cells of epidermal primordia (imaginal discs and histoblasts) the ability to become SO precursors (SOPs). In imaginal discs, ac and sc expression is spatially restricted to cell clusters within which one or a few cells become SOP(s). With the help of ubiquitous sc expression provided at different developmental times by a heat shock-sc (HSSC) chimeric gene, we have analyzed the response of epidermal primordia to the proneural action of the sc product, and have tested whether the patterned distribution of ac/sc products is necessary to position SOs correctly within the epidermis. Each primordium responds to HSSC expression by developing SOs only during a characteristic developmental period. In the absence of the endogenous ac and sc genes, most SOs induced by HSSC are of the correct type and are located in wild type positions. These results indicate that the capacity of primordia to respond to sc is temporally and spatially regulated, that specification of the type of SO does not depend on ac/sc, and that SO positioning utilizes topological information independent of the spatially restricted distribution of ac/sc products.     

Distribution and function of the lethal of scute gene product during early neurogenesis in Drosophila.

Genes of the achaete-scute complex (ASC) participate in the formation of the central nervous system in the Drosophila embryo. Previous genetic analyses have indicated that lethal of scute (l'sc) is the most important gene of the complex in that process. We have obtained antibodies against the l'sc protein to study the expression of the gene during early neurogenesis. The protein is found in groups of embryonic neuroectodermal cells, analogous to the proneural clusters that precede the appearance of precursors of peripheral sensory organs in imaginal epithelia. The groups appear in different regions of the neuroectoderm, accompanying the three successive waves of neuroblast segregation. Most neuroblasts delaminate from these clusters and express position-specific levels of l'sc protein. No significant differences have been found between the distribution of l'sc RNA and protein. Phenotypic analysis of a l'sc deficiency has shown that the gene is required for neuroblast commitment, although this requirement is less widespread than the domain of l'sc expression, suggesting a high degree of redundancy in the function of genes that participate in the process of neuroblast segregation. The ASC genes have been postulated to play a role in the control of NB identity, revealed by the generation of a defined lineage of identifiable neurons. However, our study in l'sc mutants of the expression of fushi tarazu, engrailed, and even-skipped, used as markers of neuronal identity, has not provided evidence to support this hypothesis.     

Neurogenesis in the water flea Daphnia magna (Crustacea, Branchiopoda) suggests different mechanisms of neuroblast formation in insects and crustaceans

Within euarthropods, the morphological and molecular mechanisms of early nervous system development have been analysed in insects and several representatives of chelicerates and myriapods, while data on crustaceans are fragmentary. Neural stem cells (neuroblasts) generate the nervous system in insects and in higher crustaceans (malacostracans); in the remaining euarthropod groups, the chelicerates (e.g. spiders) and myriapods (e.g. millipedes), neuroblasts are missing. In the latter taxa, groups of neural precursors segregate from the neuroectoderm and directly differentiate into neurons and glial cells. In all euarthropod groups, achaete–scute homologues are required for neuroblast/neural precursor group formation. In the insects Drosophila melanogaster and Tribolium castaneum achaete–scute homologues are initially expressed in clusters of cells (proneural clusters) in the neuroepithelium but expression becomes restricted to the future neuroblast. Subsequently genes such as snail and prospero are expressed in the neuroblasts which are required for asymmetric division and differentiation. In contrast to insects, malacostracan neuroblasts do not segregate into the embryo but remain in the outer neuroepithelium, similar to vertebrate neural stem cells. It has been suggested that neuroblasts are present in another crustacean group, the branchiopods, and that they also remain in the neuroepithelium. This raises the questions how the molecular mechanisms of neuroblast selection have been modified during crustacean and insect evolution and if the segregation or the maintenance of neuroblasts in the neuroepithelium represents the ancestral state. Here we take advantage of the recently published Daphnia pulex (branchiopod) genome and identify genes in Daphnia magna that are known to be required for the selection and asymmetric division of neuroblasts in the fruit fly D. melanogaster. We unambiguously identify neuroblasts in D. magna by molecular marker gene expression and division pattern. We show for the first time that branchiopod neuroblasts divide in the same pattern as insect and malacostracan neuroblasts. Furthermore, in contrast to D. melanogaster, neuroblasts are not selected from proneural clusters in the branchiopod. Snail rather than ASH is the first gene to be expressed in the nascent neuroblasts suggesting that ASH is not required for the selection of neuroblasts as in D. melanogaster. The prolonged expression of ASH in D. magna furthermore suggests that it is involved in the maintenance of the neuroblasts in the neuroepithelium. Based on these and additional data from various representatives of arthropods we conclude that the selection of neural precursors from proneural clusters as well as the segregation of neural precursors represents the ancestral state of neurogenesis in arthropods. We discuss that the derived characters of malacostracans and branchiopods – the absence of neuroblast segregation and proneural clusters – might be used to support or reject the possible groupings of paraphyletic crustaceans.