Neuralized functions cell autonomously to regulate Drosophila sense organ development
2000; Springer Nature; Volume: 19; Issue: 17 Linguagem: Inglês
10.1093/emboj/19.17.4827
ISSN1460-2075
Autores Tópico(s)Invertebrate Immune Response Mechanisms
ResumoArticle1 September 2000free access Neuralized functions cell autonomously to regulate Drosophila sense organ development Edward Yeh Edward Yeh Program in Developmental Biology, The Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8 Canada Department of Zoology, University of Toronto, Toronto, Ontario, Canada Collaborative Program in Developmental Biology, University of Toronto, Toronto, Ontario, Canada Search for more papers by this author Lily Zhou Lily Zhou Program in Developmental Biology, The Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8 Canada Search for more papers by this author Nicholas Rudzik Nicholas Rudzik Program in Developmental Biology, The Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8 Canada Department of Zoology, University of Toronto, Toronto, Ontario, Canada Search for more papers by this author Gabrielle L. Boulianne Corresponding Author Gabrielle L. Boulianne Program in Developmental Biology, The Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8 Canada Department of Zoology, University of Toronto, Toronto, Ontario, Canada Collaborative Program in Developmental Biology, University of Toronto, Toronto, Ontario, Canada Department of Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, Canada Search for more papers by this author Edward Yeh Edward Yeh Program in Developmental Biology, The Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8 Canada Department of Zoology, University of Toronto, Toronto, Ontario, Canada Collaborative Program in Developmental Biology, University of Toronto, Toronto, Ontario, Canada Search for more papers by this author Lily Zhou Lily Zhou Program in Developmental Biology, The Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8 Canada Search for more papers by this author Nicholas Rudzik Nicholas Rudzik Program in Developmental Biology, The Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8 Canada Department of Zoology, University of Toronto, Toronto, Ontario, Canada Search for more papers by this author Gabrielle L. Boulianne Corresponding Author Gabrielle L. Boulianne Program in Developmental Biology, The Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8 Canada Department of Zoology, University of Toronto, Toronto, Ontario, Canada Collaborative Program in Developmental Biology, University of Toronto, Toronto, Ontario, Canada Department of Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, Canada Search for more papers by this author Author Information Edward Yeh1,2,3, Lily Zhou1, Nicholas Rudzik1,2 and Gabrielle L. Boulianne 1,2,3,4 1Program in Developmental Biology, The Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, M5G 1X8 Canada 2Department of Zoology, University of Toronto, Toronto, Ontario, Canada 3Collaborative Program in Developmental Biology, University of Toronto, Toronto, Ontario, Canada 4Department of Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, Canada *Corresponding author. E-mail: [email protected] The EMBO Journal (2000)19:4827-4837https://doi.org/10.1093/emboj/19.17.4827 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Neurogenic genes, including Notch and Delta, are thought to play important roles in regulating cell–cell interactions required for Drosophila sense organ development. To define the requirement of the neurogenic gene neuralized (neu) in this process, two independent neu alleles were used to generate mutant clones. We find that neu is required for determination of cell fates within the proneural cluster and that cells mutant for neu autonomously adopt neural fates when adjacent to wild-type cells. Furthermore, neu is required within the sense organ lineage to determine the fates of daughter cells and accessory cells. To gain insight into the mechanism by which neu functions, we used the GAL4/UAS system to express wild-type and epitope-tagged neu constructs. We show that Neu protein is localized primarily at the plasma membrane. We propose that the function of neu in sense organ development is to affect the ability of cells to receive Notch-Delta signals and thus modulate neurogenic activity that allows for the specification of non-neuronal cell fates in the sense organ. Introduction The process of sense organ (SO) formation in Drosophila is well characterized at the genetic and cellular levels and provides an ideal model to study the role of cell lineage and cell–cell interactions during development (Hartenstein and Posakony, 1989; Huang et al., 1991; Posakony, 1994). Bristle SOs are comprised of four cell types—tormogen (socket), tricogen (shaft), thecogen (sheath) and neuron—that arise from a single sense organ precursor (SOP). A fifth cell, the soma sheath cell or glia cell, is associated with each SO. SOP determination occurs within an equivalence group called a proneural cluster and requires the action of proneural group genes (Garcia-Bellido, 1979; Ghysen and O'Kane, 1989; Simpson, 1990). After primary SOP determination, all other cells within the proneural cluster are prevented from becoming SOPs through a process of lateral or mutual inhibition, and these other cells adopt an epidermal cell fate (Ghysen et al., 1993). The primary SOP divides asymmetrically to produce two secondary SOPs called pIIa and pIIb. pIIa will divide to produce the shaft and socket cell (Hartenstein and Posakony, 1989), while pIIb founds a lineage that produces the neuron and sheath cell. Recently, it has been demonstrated that the pIIb cell divides first to produce a glial cell and a daughter cell named pIIIb. The pIIIb cell divides to produce the neuron and sheath, indicating that the SOP lineage yields all five cells associated with a mature SO (Gho et al., 1999). Although the SOP undergoes a stereotypical pattern of cell division and cell lineage is important in cell fate determination, cell–cell interactions have also been shown to play an important role in the determination of SOP and daughter cell fates (Hartenstein and Posakony, 1990). Many of the cell–cell interactions that are important in SO development are thought to be regulated by neurogenic genes. Neurogenic genes, including Notch (N), Delta (Dl), big brain (bib), mastermind (mam) and neuralized (neu), were first identified as embryonic recessive lethal mutations that cause hyperplasia of the embryonic nervous system at the expense of epidermal tissue (Lehmann et al., 1983). The best characterized members of the group, N and Dl, function as a receptor and ligand, respectively. Other members of the group are believed to play a role in generating the signal or propagating the signal. Besides providing the signalling pathway that is believed to be responsible for lateral or mutual inhibition within proneural fields, it is thought that neurogenic genes function together as a genetic cassette to regulate cell–cell interactions important for cell fate decisions in a variety of tissues during development (Ruohola-Baker et al., 1994). The role of neurogenic genes in SO development was demonstrated by generating mutant clones of N or Dl cells during development (Dietrich and Campos-Ortega, 1984). These studies revealed that mutant clones of N or Dl exhibit specific defects in bristle development. In Dl clones, these defects included tufting (supernumerary SO bristles), whereas in N clones both tufting and balding (absence of bristles) were observed. The variability observed in N clones appeared to be due to spatial differences within the notum; some regions yielded the tufted phenotype while other regions produced only the balding phenotype. A detailed analysis of mutant clones of N, Dl and shaggy (sgg), another neurogenic gene, using adult epidermal markers confirmed the involvement of these genes in SO development (Heitzler and Simpson, 1991). Analysis of the genotypes of bristles found at the boundaries between mutant and wild-type cells revealed that N is required autonomously for receiving the neurogenic signal that prevents cells within the proneural cluster from adopting the SOP fate. The same type of analysis revealed that Dl is required non-autonomously to produce the signal that allows epidermal cell specification. shaggy was found to be required cell autonomously to send and receive the neurogenic signal. Taken together, these results provided evidence that N and Dl function in signalling as a receptor and ligand and that sgg probably plays a role in a feedback-based regulatory mechanism (Heitzler and Simpson, 1991). Temperature-sensitive alleles have been used to elaborate further the role of N and Dl in SO development (Hartenstein and Posakony, 1990; Parks and Muskavitch, 1993). These studies have shown that loss of N function prior to or during the determination of the primary SOP causes supernumerary primary SOPs to form. These extra SOPs develop normally and produce bristle tufts. When N loss-of-function is induced, subsequent to primary SOP determination and during the division and differentiation of the accessory cells, all the cells in the SOP lineage are transformed into neurons, resulting in a bald phenotype. Further analysis revealed that N is required at every step of the SO lineage; proper determination of the pIIa and pIIb fates, as well as the accessory cell fates, requires N signalling. These results explain the apparent spatially dependent phenotypes caused by N mutant clones and suggest that chaetae do not develop synchronously. This approach also revealed similar requirements for Dl during SOP development. The neurogenic gene neu has also been implicated in SO development. neu, like N and Dl, was first identified by means of loss-of-function mutations that cause hyperplasia of the central and peripheral nervous system at the expense of the epidermis (Lehmann et al., 1983). Although neu interacts genetically with other neurogenic genes, its role within this pathway remains unclear. The function of Neu protein is also unknown. The amino acid sequence of Neu suggests that it might encode a nuclear protein with a putative nuclear localization signal, helix–turn–helix domain and a C3HC4 Zn-finger ('RING') domain at the C-terminus (Boulianne et al., 1991; Price et al., 1993). However, the subcellular distribution of Neu has yet to be determined. Homologues of neu have been identified in other species including human (Nakamura et al., 1998), mouse (Moschonas, 1998) and Caenorhabditis elegans (Wilson et al., 1994) suggesting that the function of neu in N–Dl signalling has been conserved. Comparison of these sequences reveals that the RING finger domain is present within all homologues. However, the putative nuclear localization signal and helix–turn–helix domains have not been well conserved. In situ hybridization studies have shown that neu is broadly expressed during early embryogenesis, but becomes restricted to the ventral neurogenic region and eventually to neuroblasts during neuroblast determination. During the third larval instar stage, neu is expressed in SOP cells that will give rise to macrochaetae on the adult notum (Boulianne et al., 1991). Dietrich and Campos-Ortega (1984) carried out mosaic analysis to determine the role of neu in SO development and found that neu mutant clones gave rise to a balding phenotype, which, in contrast to N, is spatially independent. However, these studies did not reveal the cellular nature of these defects. Furthermore, in the absence of appropriate markers, these studies could not establish whether neu was required autonomously or non-autonomously during SO development. To characterize further the role of neu in the neurogenic signalling pathway we have studied its function during SO development by generating mutant clones using the FLP-FRT system (Golic and Lindquist, 1989). Using two independent neu alleles we find that neu is required for epidermal cell fate determination within the proneural cluster. neu mutant clones that overlap proneural regions exhibit supernumerary determination of SOPs. Analysis of the bristle genotypes found at clonal boundaries reveals that neu functions cell autonomously in receiving the signal that prevents SOP determination. Also, loss of neu function produces phenotypes similar to those seen in loss-of-function mutants for N and Dl, indicating that it is required for proper determination of cell fates in the SO lineage. To gain insight into where neu functions in the neurogenic pathway, we examined the localization of Neu within the cell. Using the GAL4/UAS system (Brand and Perrimon, 1993) to express wild-type and epitope-tagged neu constructs during development, we show that both constructs are able to partially rescue the embryonic neurogenic phenotype caused by a mutation in neu and that Neu localizes to the plasma membrane. We propose that the function of neu during SO development is to modulate the efficacy of neurogenic signalling within the proneural cluster by affecting the ability of cells to receive or propagate signals through N and Dl. Results Loss of neu function affects cell fate decisions during sense organ development To assay the effects of loss of neu during SO development we generated mutant clones using FLP/FRT-mediated somatic recombination (Golic and Lindquist, 1989; Xu and Harrison, 1994). For these purposes, two alleles of neu were recombined onto third chromosome arms containing FRT sequences at 82B. neuA101 is a hypomorphic allele resulting from the insertion of a lacZ enhancer trap into the upstream regulatory region of the neu locus (Bellen et al., 1989), while neu1F65 is an amorphic ethylmethane sulfonate-induced allele (Brand and Campos-Ortega, 1988; de la Concha et al., 1988). Both alleles produce severe hyperplasia of the embryonic nervous system leading to lethality and both fail to complement any other known neu allele. Flies heterozygous for neuA101 and carrying a source of FLP (e22cGAL4, UAS-FLP; FRT82B, πM/FRT82B, neuA101) displayed a bristle tufting phenotype affecting both macrochaetae and microchaetae. The severity of the phenotype ranged from duplicated bristles to tufts containing several bristles (Figure 1). Supernumerary macrochaetae and microchaeate were always found in characteristically normal positions. With the exception of extreme cases of microchaetae tufting observed only at the anterior-most part of the notum (Figure 1C), regions between bristles and bristle tufts did not appear to be affected. This suggests that neu functions to prevent cells from adopting SOP fates within the proneural cluster. Figure 1.Scanning electron micrographs of neuA101 clones in adult eyes and nota. (A and D) SEMs of wild-type nota. (B, C, E and F) Nota from flies in which neuA101 clones have been induced. The bristle phenotype observed is tufting of both microchaetae (arrowhead) and macrochaetae (arrows). Tufts appear in locations of normal bristle formation (B) and are separated by epidermal cells (E, asterisks), although severe tufts can occur at the anterior of the notum (C). The majority of bristles within tufts contain a socket associated with each shaft (F). neuA101 clones can also cause eye defects. (G) Repetitive uniform organization of ommatidia and regular spacing of sensory bristles in a wild-type eye. (H) Some of the defects seen in eyes in which neu clones have been induced including irregularly shaped ommatidia, and mispositioned, missing or extra sensory bristles. Severe scarring can also be observed (I). Download figure Download PowerPoint The effects of neuA101 clones were not limited to bristles on the notum. Tufting could also be observed with adult head sensilla surrounding the eye and ocelli. In addition, bristle sensilla throughout the body, including the dorsal and ventral abdomen, also appeared to form tufts. As was observed for macrochaetae, these tufts always occurred in the location where normal bristles are formed. neuA101 clones also gave rise to defects in the adult eye. The severity of the phenotype ranged from ectopic interommatidial bristles and aberrant ommatidial size to scarring (Figure 1H and I) and defective photoreceptor development (data not shown). In addition, defects in wing development, including irregular wing margin sensory bristle formation and ectopic wing vein formation, were observed (data not shown). Tufting caused by neuA101 mutant clones is the result of supernumerary SOP determination To determine whether the supernumerary bristles (i.e. tufting) that we observe in mutant clones are due to commitment to the SOP fate, we took advantage of the fact that the neu mutant allele neuA101 is a lacZ enhancer trap line in the neu locus that can be used as a marker of SOP determination. Previous studies have demonstrated that neuA101 expression can be detected within third larval instar wing imaginal discs in primary SOPs that give rise to macrochaetae on the notum and sensory bristles along the wing margin (Huang et al., 1991). As development proceeds, expression of neuA101 can also be detected in secondary SOPs as well as the accessory cells that are associated with each primary SOP. neuA101 is similarly expressed in SOPs on the pupal notum that will give rise to microchaetae. Since the appearance and differentiation of each macrochaete SOP is well documented, it is possible to examine the fate of each SOP at particular developmental time points. For example, the primary SOPs that will give rise to bristles along the adult wing margin are determined during late third larval instar, but do not divide until 5–10 h after puparium formation (APF) (Huang et al., 1991). Therefore, any supernumerary β-gal positive cells along the wing margin that are observed during third larval instar development are most likely primary SOPs rather than secondary SOPs. Using the πMyc marker to identify neuA101 clones, we found that supernumerary SOPs arose from neuA101 cells (Figure 2). Since supernumerary SOPs are not observed in ectopic locations in the wing disc this suggests that neu functions normally in the proneural cluster to determine epidermal cell fates. Figure 2.Bristle tufts in neuA101 clones are caused by supernumerary sensory organ precursors. (A) SOP expression pattern of the lacZ enhancer trap, which causes the mutation A101. Wing discs in (B) and (C) have been double stained with anti-myc (red) to identify the neuA101 clone and anti-β-gal (green) to identify SOPs. When a mutant clone (absence of myc staining) overlaps a proneural region, supernumerary SOPs (arrows) are determined. SOPs do not arise in ectopic locations, suggesting that neu functions within proneural clusters to properly determine cell fates. Download figure Download PowerPoint To ask whether the supernumerary primary SOPs arose from determination of excess primary SOPs within the proneural cluster or from abnormal proliferation of the primary SOPs, BrdU labelling experiments were performed. As described earlier, wild-type primary SOPs at the wing margin remain mitotically quiescent until after pupariation. Thus, they do not incorporate BrdU during third instar larval development (Figure 3A). Similarly, the supernumerary SOPs that developed along the wing margin in neuA101 mutant clones did not incorporate BrdU (Figure 3B, inset). Using an antibody against a phosphorylated form of histone H3 as a marker of mitosis (Hendzel et al., 1997) further reveals that supernumary SOPs are not actively dividing (Figure 3D). These two results demonstrate that the increase in primary SOPs was not a result of aberrant mitotic events. Figure 3.Supernumerary SOPs are the result of changes in cell fate and not aberrant mitotis. (A) In wild-type third larval instar wing discs, SOPs (green) along the wing margin do not divide under after puparium formation and thus do not incorporate BrdU (red) during larval development. (B) Supernumerary SOPs caused by neuA101 clones also do not incorporate BrdU, indicating that extra SOPs (arrow, inset) do not arise from abnormal mitosis. (C) Similarly, wild-type SOPs (red) do not stain with an antibody to phospho-histone H3 (green), indicating that they are mitotically quiescent. (D) Supernumerary SOPs caused by neuA101 clones do not stain with anti-phospho-histone H3, indicating that they are not mitotically active. Download figure Download PowerPoint neu is also required for pIIa/pIIb and accessory cell fate determination neu mutant clones were also generated using neu1F65. In this case, we found that mutant clones gave rise to a balding phenotype characterized by the absence of chaetae on the adult notum (Figure 4B). This is consistent with a role for neu in the determination of both pIIa/pIIb and accessory cell fates. In N and Dl mutant clones, loss-of-function during secondary SOP and accessory cell fate determination causes cells to assume a neuronal cell fate (Hartenstein and Posakony, 1990). To determine whether neu1F65 clones give rise to similar alterations in secondary SOP and accessory cell fates, pupal nota (24 h APF) were dissected and stained with the neuronal marker 22C10. In wild-type notum at this stage, 22C10 expression is detected in a single neuron comprising each individual sense organ (as identified by the double axon processes; Figure 4C). In contrast, 22C10 expression in pupal nota from neu1F65 clones revealed clusters of 22C10-positive cells (Figure 4D) that all displayed the double axon processes associated with neuronal differentiation. The presence of more than four 22C10 positive cells in some clusters further demonstrates that supernumerary primary SOPs are determined in neu mutant clones and that the descendants of these mutant SOPs differentiate to assume neuronal cell fates. Taken together, these data demonstrate that neu affects multiple cell fate decisions required for the proper development of sense organs. Like N and Dl, neu is required for proper determination of the pIIa cell fate, and is also required for determination of the thecogen cell fate in the pIIIb lineage. Figure 4.The amorphic allele, neu1F65, causes a bald cuticle phenotype. (A) Notum of a wild-type fly. (B) When clones of neu1F65 are generated, bristles do not form causing a bald cuticle (arrow). Epidermal cells appear to form normally. The bald phenotype of neu1F65 clones is caused by transformation of the SOP accessory cell fates into neuronal fates. (C) Expression of a neuronal marker 22C10 in wild-type pupal nota 24 APF. Neurons can also be unambiguously identified by their double axon projections. (D) Clusters of cells expressing 22C10 (some containing more than four positive staining cells) can be detected in nota (arrowheads) in which clones have been generated, indicating that multiple primary SOPs are determined and that each SOP divides and differentiates into the neuronal cell fate. Download figure Download PowerPoint neu functions cell autonomously in sense organ development Using mosaic analysis, we have demonstrated a role for neu in sense organ development. To determine whether neu is required autonomously in this process, we generated neu mutant clones that were genetically marked with y in a y+ background. Somatic recombination was induced by heat-shocking flies of the genotype y w hsflp; FRT 82B, neuA101/FRT 82B, y+Sb πM during late embryogenesis. We found that the supernumerary bristles were non-Sb and y− (Figure 5A and B) demonstrating that they arose from neuA101 cells; mixtures of wild-type and mutant bristles were never observed. Figure 5.neu functions cell autonomously in cell fate decisions. (A and B) Supernumerary bristles are genetically homozygous mutant as y+ bristles are never seen within tufts. Arrowhead indicates supernumerary macrochaetae, arrow indicates supernumerary microchaetae. Adult neu clones identified using pwn confirm the cell autonomous nature of neu function. (C) The majority of bristles observed at clonal boundaries are mutant (arrow) for neuA101. (D) A smaller fraction of bristles at boundaries are wild type (arrowhead). Also, single bristles that are neuA101 (arrow) can exist at clonal boundaries (D). Download figure Download PowerPoint To ascertain the ability of mutant cells to send or receive the signal that prevents neural determination, and thus delineate autonomous versus non-autonomous neu function, adult clones were examined using the epidermal hair marker pawn (pwn), which can be used to identify clonal boundaries on the adult cuticle. Since pwn affects bristle morphology (producing truncated bristles), mutant cells can be identified as well. If mutant bristles can be found at the clonal boundary, and these are unaffected by neighbouring wild-type cells, then neu must be required autonomously to inhibit neuronal cell fates. In contrast, if wild-type bristles are found at the boundary next to neu mutant cells, then neu must act non-autonomously within cells since they fail to suppress neighbouring cells from becoming SOs. We found that mutant bristles exist at clonal boundaries next to wild-type cells more frequently than wild-type bristles next to mutant cells (81 versus 19%, respectively, n = 382). Furthermore, mutant bristles at the clonal boundary were observed as either single bristles or tufts (Figure 5C and D). Thus, neu mutant cells are affected in their ability to receive the signal that prevents neural determination and they form SOs at clonal borders despite the presence of wild-type cells. The ability of mutant cells to send a signal does not appear to be affected as mixtures of wild-type and mutant bristles were not observed. Taken together, these results clearly demonstrate that neu functions cell autonomously during SOP determination to specify epidermal fates in Drosophila. Neu is associated with the plasma membrane To understand how neu could function in the signalling process that allows for epidermal cell fate determination, we examined the expression pattern of neu during SOP determination using in situ hybridization techniques on staged third larval instar wing imaginal discs. neu was undetectable in proneural clusters prior to SOP determination (Figure 6A and B). The first detectable neu expression occurs within SOPs in wing discs of late third larval instars as previously described (Figure 6C; Boulianne et al., 1991). neu expression was also examined within the notum at 24 h APF where its expression was found to be associated with the neuron of each SO cluster (Figure 6D). At this stage, all the accessory cells of each SO have been determined and the neuron can be identified based on its shape. Figure 6.Expression of neu during SOP development. Detection of neu using in situ hybridization reveals expression within SOPs in the third instar imaginal disc. Early (A) and mid (B)-larval wing discs reveal no expression of neu in proneural regions, and weak expression in emerging SOPs. Late (C) third instar wing discs show expression of neu within emerging SOPs. (D) In situ analysis of pupal nota (24 APF) reveals neu expression within the neuron (characterized by dual processes of the axons) of each future SO. Download figure Download PowerPoint To determine where Neu protein functions within the cell, we then generated transgenic lines that expressed wild-type or myc epitope-tagged neu constructs in the vector pUAST. To ensure that the myc tag did not disrupt Neu function, the ability to rescue the neu1F65 mutant allele with both the wild-type and myc-tagged construct was compared. The neu1F65 allele produces a severe neurogenic phenotype characterized by hyperplasia of the central and peripheral nervous system (Figure 7B), and complete lack of ventral cuticle. Using a ptc-GAL4 line to drive expression, we found that both constructs were equally able to partially rescue the neurogenic phenotype (Figure 7C and D) and restore ventral cuticle (data not shown). This indicates that the myc epitope does not disrupt the Neu protein and that the fusion protein is functionally wild-type. In addition to being able to rescue neuIF65 embryonic phenotypes, we found that ectopic expression of either tagged or untagged neu constructs yielded identical adult phenotypes characterized by missing macrochaetae and incomplete wing vein formation (Figure 8). Figure 7.Wild-type and myc-tagged neu constructs can rescue the neu1F65 neurogenic phenotype. (A) A wild-type embryo stained with anti-HRP at late stages of embryogenesis displays an organized central nervous system characterized by its fasicles. (B) A neurogenic neu1F65 embryo displays severe hyperplasia of the central nervous system. Fasicles do not form and the central nervous system is greatly expanded. A neurogenic neu1F65 embryo that contains ptc-GAL4-driven expression of wild-type neu (C) or myc-tagged neu (D) has a partially rescued nervous system. The central nervous system is not as expanded and is more organized, with partial fasicles forming. Download figure Download PowerPoint Figure 8.Ectopic expression of neu causes adult phenotypes. (A) Wild-type macrochaetae assume stereotyped locations on the notum (white arrows). (B) Ectopic expression of neu using A78 causes missing macrochaet
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