Leg Imaginal Discs Research Articles

Shaping Leg Muscles in Drosophila: Role of ladybird, a Conserved Regulator of Appendicular Myogenesis

Legs are locomotor appendages used by a variety of evolutionarily distant vertebrates and invertebrates. The primary biological leg function, locomotion, requires the formation of a specialised appendicular musculature. Here we report evidence that ladybird, an orthologue of the Lbx1 gene recognised as a hallmark of appendicular myogenesis in vertebrates, is expressed in leg myoblasts, and regulates the shape, ultrastructure and functional properties of leg muscles in Drosophila. ladybird expression is progressively activated in myoblasts associated with the imaginal leg disc and precedes that of the founder cell marker dumbfounded. The RNAi-mediated attenuation of ladybird expression alters properties of developing myotubes, impairing their ability to grow and interact with the internal tendons and epithelial attachment sites. It also affects sarcomeric ultrastructure, resulting in reduced leg muscle performance and impaired mobility in surviving flies. The over-expression of ladybird also results in an abnormal pattern of dorsally located leg muscles, indicating different requirements for ladybird in dorsal versus ventral muscles. This differential effect is consistent with the higher level of Ladybird in ventrally located myoblasts and with positive ladybird regulation by extrinsic Wingless signalling from the ventral epithelium. In addition, ladybird expression correlates with that of FGF receptor Heartless and the read-out of FGF signalling downstream of FGF. FGF signals regulate the number of leg disc associated myoblasts and are able to accelerate myogenic differentiation by activating ladybird, leading to ectopic muscle fibre formation. A key role for ladybird in leg myogenesis is further supported by its capacity to repress vestigial and to down-regulate the vestigial-governed flight muscle developmental programme. Thus in Drosophila like in vertebrates, appendicular muscles develop from a specialised pool of myoblasts expressing ladybird/Lbx1. The ladybird/Lbx1 gene family appears as a part of an ancient genetic circuitry determining leg-specific properties of myoblasts and making an appendage adapted for locomotion.

How functions in leg development during Drosophila metamorphosis

The Drosophila how gene encodes a KH RNA binding protein with strong similarity to GLD-1 from nematodes and QK1 from mice. Here, we investigate the function of how during metamorphosis. We show that how RNA and protein are present in a variety of tissues, and phenotypic analyses of how mutants reveal multiple lethal phases and defects during metamorphosis. In addition to previously reported abnormalities in muscle and wing development, how mutants exhibit defects in leg development. how mutant leg imaginal discs undergo cell shape changes associated with elongation, but are oriented improperly, do not evert normally, and often remain incased in peripodial epithelium longer than normal. Consequently, how mutants exhibit short, crooked legs. Our findings suggest that how functions in interactions between imaginal epithelium, peripodial epithelium, and larval epidermal cells during imaginal disc eversion.

Mutational Analysis of Stubble-stubbloid Gene Structure and Function in Drosophila Leg and Bristle Morphogenesis

The Stubble-stubbloid (Sb-sbd) gene is required for ecdysone-regulated epithelial morphogenesis of imaginal tissues during Drosophila metamorphosis. Mutations in Sb-sbd are associated with defects in apical cell shape changes critical for the evagination of the leg imaginal disc and with defects in assembly and extension of parallel actin bundles in growing mechanosensory bristles. The Sb-sbd gene encodes a type II transmembrane serine protease (TTSP). Here we use a Sb-sbd transgenic construct to rescue both bristle and leg morphogenesis defects in Sb-sbd mutations. Molecular characterization of Sb-sbd mutations and rescue experiments with wild-type and modified Sb-sbd transgenic constructs show that the protease domain is required for both leg and bristle functions. Truncated proteins that express the noncatalytic domains without the protease have dominant effects in bristles but not in legs. Leg morphogenesis, but not bristle growth, is sensitive to Sb-sbd overexpression. Antibody localization of the Sb-sbd protein shows apical expression in elongating legs. Sb-sbd protein is found in the base and shaft in budding bristles and then concentrates at the growing tip when bristles are elongating rapidly. We propose a model whereby Sb-sbd helps coordinate proteolytic modification of extracellular matrix attachments with cytoskeletal changes in both legs and bristles.

Palmitoylation of the EGFR Ligand Spitz by Rasp Increases Spitz Activity by Restricting Its Diffusion

Lipid modifications such as palmitoylation or myristoylation target intracellular proteins to cell membranes. Secreted ligands of the Hedgehog and Wnt families are also palmitoylated; this modification, which requires the related transmembrane acyltransferases Rasp and Porcupine, can enhance their secretion, transport, or activity. We show here that rasp is also essential for the developmental functions of Spitz, a ligand for the Drosophila epidermal growth factor receptor (EGFR). In cultured cells, Rasp promotes palmitate addition to the N-terminal cysteine residue of Spitz, and this cysteine is required for Spitz activity in vivo. Palmitoylation reduces Spitz secretion and enhances its plasma membrane association, but does not alter its ability to activate the EGFR in vitro. In vivo, overexpressed unpalmitoylated Spitz has an increased range of action but reduced activity. These data suggest a role for palmitoylation in restricting Spitz diffusion, allowing its local concentration to reach the threshold required for biological function.

Distal‐less functions in subdividing the Drosophila thoracic limb primordium

The thoracic limb primordium of Drosophila melanogaster is a useful experimental model in which to study how unique tissue types are specified from multipotent founder cell populations. The second thoracic segment limb primordium gives rise to three structures: the wing imaginal disc, the leg imaginal disc, and a larval mechanosensory structure called Keilin's organ. We report that most of the limb primordium arises within neurogenic ectoderm and demonstrate that the neural and imaginal components of the primordium have distinct developmental potentials. We also provide the first analysis of the genetic pathways that subdivide the progenitor cell population into uniquely imaginal and neural identities. In particular, we demonstrate that the imaginal gene escargot represses Keilin's organ fate and that Keilin's organ is specified by Distal-less in conjunction with the downstream achaete-scute complex. This specification involves both the activation of the neural genes cut and couch potato and the repression of escargot. In the absence of achaete-scute complex function, cells adopt mixed identities and subsequently die. We propose that central cells of the primordium previously thought to contribute to the distal leg are Keilin's organ precursors, while both proximal and distal leg precursors are located more peripherally and within the escargot domain.

Patterning of the branched head appendages in Schistocerca americana and Tribolium castaneum

Much of our understanding of arthropod limb development comes from studies on the leg imaginal disc of Drosophila melanogaster. The fly limb is a relatively simple unbranched (uniramous) structure extending out from the body wall. The molecular basis for this outgrowth involves the overlap of two signaling molecules, Decapentaplegic (Dpp) and Wingless (Wg), to create a single domain of distal outgrowth, clearly depicted by the expression of the Distal-less gene (Dll). The expression of wg and dpp during the development of other arthropod thoracic limbs indicates that these pathways might be conserved across arthropods for uniramous limb development. The appendages of crustaceans and the gnathal appendages of insects, however, exhibit a diverse array of morphologies, ranging from those with no distal elements, such as the mandible, to appendages with multiple distal elements. Examples of the latter group include branched appendages or those that possess multiple lobes; such complex morphologies are seen for many crustacean limbs as well as the maxillary and labial appendages of many insects. It is unclear how, if at all, the known patterning genes for making a uniramous limb might be deployed to generate these diverse appendage forms. Experiments in Drosophila have shown that by forcing ectopic overlaps of Wg and Dpp signaling it is possible to generate artificially branched legs. To test whether naturally branched appendages form in a similar manner, we detailed the expression patterns of wg, dpp, and Dll in the development of the branched gnathal appendages of the grasshopper, Schistocerca americana, and the flour beetle, Tribolium castaneum. We find that the branches of the gnathal appendages are not specified through the redeployment of the Wg-Dpp system for distal outgrowth, but our comparative studies do suggest a role for Dpp in forming furrows between tissues.

CHROMOSOMES OF THE CARIBBEAN FRUIT FLY (DIPTERA: TEPHRITIDAE)

Abstract Larval tissues of Anastrepha suspensa (Loew) (Diptera: Tephritidae) were examined to determine the optimal tissue and stage for chromosomal preparations and to determine the karyotype. Tissues were dissected in saline, stained in 2% aceto-orcein for 45 minutes, and squashed on a coverglass by thumb pressure. The compound eye imaginal discs from 6-day-old larvae yielded the best preparations of dividing cells. Mitotic figures also can be obtained from larval brain tissue, ventral nerve cord, and leg imaginal discs. In larvae 6 days old, many cells in the tissues examined were dividing. Cell division appears to be synchronized in the different tissues examined, with most cells in interphase or dividing at the same time during all instars. The male is heterogametic (XY) and the female is homogametic (XX). The chromosome number is 12 (10 autosomes + XX or XY). There are 3 pairs of subtelocentric and 2 pairs of submetacentric chromosomes. The X chromosome is subtelocentric and the Y chromosome is subm...

Opposing inputs by Hedgehog and Brinker define a stripe of hairy expression in the Drosophila leg imaginal disc.

The sensory organs of the Drosophila adult leg provide a simple model system with which to investigate pattern-forming mechanisms. In the leg, a group of small mechanosensory bristles is organized into a series of longitudinal rows, a pattern that depends on periodic expression of the hairy gene (h) and the proneural genes achaete (ac) and scute (sc). Expression of ac in longitudinal stripes in prepupal leg discs defines the positions of the mechanosensory bristle rows. The ac/sc expression domains are delimited by the Hairy repressor, which is itself periodically expressed. In order to gain insight into the molecular mechanisms involved in leg sensory organ patterning, we have analyzed a Hedgehog (Hh)- and Decapentaplegic (Dpp)-responsive enhancer of the h gene, which directs expression of h in a narrow stripe in the dorsal leg imaginal disc (the D-h stripe). Our studies suggest that the domain of D-h expression is defined by the overlap of Hh and high-level Dpp signaling. We find that the D-h enhancer consists of a Hh-responsive activation element (HHRE) and a repression element (REPE), which responds to the transcriptional repressor Brinker (Brk). The HHRE directs expression of h in a broad stripe along the anteroposterior (AP) compartment boundary. HHRE-directed expression is refined along the AP and dorsoventral axes by Brk1, acting through the REPE. In D-h-expressing cells, Dpp signaling is required to block Brk-mediated repression. This study elucidates a molecular mechanism for integration of the Hh and Dpp signals, and identifies a novel function for Brk as a repressor of Hh-target genes.

Genetic interactions between the RhoA and Stubble-stubbloid loci suggest a role for a type II transmembrane serine protease in intracellular signaling during Drosophila imaginal disc morphogenesis.

The Drosophila RhoA (Rho1) GTPase is essential for postembryonic morphogenesis of leg and wing imaginal discs. Mutations in RhoA enhance leg and wing defects associated with mutations in zipper, the gene encoding the heavy chain of nonmuscle myosin II. We demonstrate here that mutations affecting the RhoA signaling pathway also interact genetically with mutations in the Stubble-stubbloid (Sb-sbd) locus that encodes an unusual type II transmembrane serine protease required for normal leg and wing morphogenesis. In addition, a leg malformation phenotype associated with overexpression of Sb-sbd in prepupal leg discs is suppressed when RhoA gene dose is reduced, suggesting that RhoA and Sb-sbd act in a common pathway during leg morphogenesis. We also characterized six mutations identified as enhancers of zipper mutant leg defects. Three of these genes encode known members of the RhoA signaling pathway (RhoA, DRhoGEF2, and zipper). The remaining three enhancer of zipper mutations interact genetically with both RhoA and Sb-sbd mutations, suggesting that they encode additional components of the RhoA signaling pathway in imaginal discs. Our results provide evidence that the type II transmembrane serine proteases, a class of proteins linked to human developmental abnormalities and pathology, may be associated with intracellular signaling required for normal development.

Genetic Modifier Screens in Drosophila Demonstrate a Role for Rho1 Signaling in Ecdysone-Triggered Imaginal Disc Morphogenesis

Drosophila adult leg development provides an ideal model system for characterizing the molecular mechanisms of hormone-triggered morphogenesis. A pulse of the steroid hormone ecdysone at the onset of metamorphosis triggers the rapid transformation of a flat leg imaginal disc into an immature adult leg, largely through coordinated changes in cell shape. In an effort to identify links between the ecdysone signal and the cytoskeletal changes required for leg morphogenesis, we performed two large-scale genetic screens for dominant enhancers of the malformed leg phenotype associated with a mutation in the ecdysone-inducible broad early gene (br1). From a screen of >750 independent deficiency and candidate mutation stocks, we identified 17 loci on the autosomes that interact strongly with br1. In a complementary screen of approximately 112,000 F1 progeny of EMS-treated br1 animals, we recovered 26 mutations that enhance the br1 leg phenotype [E(br) mutations]. Rho1, stubbloid, blistered (DSRF), and cytoplasmic Tropomyosin were identified from these screens as br1-interacting genes. Our findings suggest that ecdysone exerts its effects on leg morphogenesis through a Rho1 signaling cascade, a proposal that is supported by genetic interaction studies between the E(br) mutations and mutations in the Rho1 signaling pathway. In addition, several E(br) mutations produce unexpected defects in midembryonic morphogenetic movements. Coupled with recent evidence implicating ecdysone signaling in these embryonic morphogenetic events, our results suggest that a common ecdysone-dependent, Rho1-mediated regulatory pathway controls morphogenesis during the two major transitions in the life cycle, embryogenesis and metamorphosis.

DNA‐binding characteristics of cnidarian Pax‐C and Pax‐B proteins in vivo and in vitro: No simple relationship with the Pax‐6 and Pax‐2/5/8 classes

Cnidarians are the simplest animals in which distinct eyes are present. We have previously suggested that cnidarian Pax-Cam might represent a precursor of the Pax-6 class. Here we show that when expressed in Drosophila imaginal discs, Pax-Cam chimeric proteins containing the C-terminal region of EY were capable of eye induction and driving expression of a reporter gene under the control of a known EY target (the sine oculis gene). Whilst these results are consistent with a Pax-6-like function for Pax-Cam, in band shift experiments we were unable to distinguish the DNA-binding behaviour of the Pax-Cam Paired domain from that of a second Acropora Pax protein, Pax-Bam. The ability of a Pax-Bam/EY chimera to also induce eye formation in leg imaginal discs, together with the in vitro data, cast doubt on previously assumed direct relationships between cnidarian Pax genes and the Pax-6 and Pax-2/5/8 classes of bilateral animals.

Splits ends is a tissue/promoter specific regulator of Wingless signaling.

Wingless directs many developmental processes in Drosophila by regulating expression of specific target genes through a conserved signaling pathway. Although many nuclear factors have been implicated in mediating Wingless-induced transcription, the mechanism of how Wingless regulates different targets in different tissues remains poorly understood. We report here that the split ends gene is required for Wingless signaling in the eye, wing and leg imaginal discs. Expression of a dominant-negative version of split ends resulted in more dramatic reductions in Wingless signaling than split ends-null alleles, suggesting that it may have a redundant partner. However, removal of split ends or expression of the dominant-negative had no effect on several Wingless signaling readouts in the embryo. The expression pattern of Split ends cannot explain this tissue-specific requirement, as the protein is predominantly nuclear and present throughout embryogenesis and larval tissues. Consistent with its nuclear location, the split ends dominant-negative acts downstream of Armadillo stabilization. Our data indicate that Split ends is an important positive regulator of Wingless signaling in larval tissues. However, it has no detectable role in the embryonic Wingless pathway, suggesting that it is a tissue or promoter-specific factor.

The role of Wg signaling in the patterning of embryonic leg primordium in Drosophila

Cellular interaction between the proximal and distal domains of the limb plays key roles in proximal–distal patterning. In Drosophila, these domains are established in the embryonic leg imaginal disc as a proximal domain expressing escargot, surrounding the Distal-less expressing distal domain in a circular pattern. The leg imaginal disc is derived from the limb primordium that also gives rise to the wing imaginal disc. We describe here essential roles of Wingless in patterning the leg imaginal disc. Firstly, Wingless signaling is essential for the recruitment of dorsal–proximal, distal, and ventral–proximal leg cells. Wingless requirement in the proximal leg domain appears to be unique to the embryo, since it was previously shown that Wingless signal transduction is not active in the proximal leg domain in larvae. Secondly, downregulation of Wingless signaling in wing disc is essential for its development, suggesting that Wg activity must be downregulated to separate wing and leg discs. In addition, we provide evidence that Dll restricts expression of a proximal leg-specific gene expression. We propose that those embryo-specific functions of Wingless signaling reflect its multiple roles in restricting competence of ectodermal cells to adopt the fate of thoracic appendages.

Distal antenna and distal antenna related encode nuclear proteins containing pipsqueak motifs involved in antenna development in Drosophila.

Legs and antennae are considered to be homologous appendages. The fundamental patterning mechanisms that organize spatial pattern are conserved, yet appendages with very different morphology develop. A genetic hierarchy for specification of antennal identity has been partly elucidated. We report identification of a novel family of genes with roles in antennal development. The distal antenna (dan) and distal antenna-related (danr) genes encode novel nuclear proteins that are expressed in the presumptive distal antenna, but not in the leg imaginal disc. Ectopic expression of dan or danr causes partial transformation of distal leg structure toward antennal identity. Mutants that remove dan and danr activity cause partial transformation of antenna toward leg identity. Therefore we suggest that dan and danr contribute to differentiation of antenna-specific characteristics. Antenna-specific expression of dan and danr depends on a regulatory hierarchy involving homothorax and Distal-less, as well as cut and spineless. We propose that dan and danr are effector genes that act downstream of these genes to control differentiation of distal antennal structures.

Expression of the drosophila gene disconnected using the UAS/GAL4 system.

The Drosophila disconnected (disco) gene encodes a transcription factor containing two C2H2 zinc-fingers that functions in the development of both embryonic and adult structures. The disco gene was initially identified from a mutation in which adult photoreceptor axons fail to connect with their target cells in the larval optic lobe (Steller et al., 1987). The optic lobes of disco mutants are poorly developed and lack optic lobe neurons, including the lateral neurons. Abnormal circadian rhythmicity has been attributed to the absence or aberrant differentiation of these lateral neurons (Hardin et al., 1992; Helfrich-Forster et al., 1998). Additionally, disco mutants show defects in thoracic and abdominal sensory axon development (Glossop and Shepherd, 1998). More recently, disco was found to have a broader role in embryonic pattern formation, functioning as a cofactor for the Hox proteins Deformed (Dfd) and Sex Combs Reduced (Scr) in the gnathocephalic segments (mandibular, maxillary, and labial); this more extensive role was masked by the partially redundant gene, disco-related (disco-r) (Mahaffey et al., 2001). Lack of both Disco and Disco-r causes embryonic lethality and the absence of some larval mouth structures arising from the gnathocephalic segments. The disco gene is also expressed in the antennal and leg imaginal discs and likely has a role in the development of these structures (Lee et al., 1991). The disco gene is located in region 14B of the X chromosome and consists of two exons separated by a single intron. The first exon is untranslated and comprises the majority of the disco 5 UTR. The second exon contains the complete open reading frame (ORF) encoding the 568 amino acid protein. The two C2H2 zinc finger regions, defining the DNA binding domain, are located between amino acids 89 and 149 (Heilig et al., 1991). As tools for further study of Disco function, we generated three UAS-disco constructs—two encoding wildtype proteins and one producing an altered protein like that encoded by the disco null allele. The UAS-disco wild-type construct was generated by the sequential insertion of two disco fragments into the Drosophila transformation vector pUAST (Brand and Perrimon, 1993). A 1,270-bp BglII-XbaI fragment representing the majority of the disco ORF (a subclone from 542.2; Surdej et al., 1990) was inserted into BglII-XbaI digested pUAST. Next, a 473-bp fragment, with an artificial EcoRI site immediately upstream of the initiation codon, was obtained by an EcoRI-BglII digestion of a 1,276-bp PCR product generated with the following primers: Forward primer, 5 GGA TCC GAATTC ATG GAG CAC ATA ATG 3 ; Reverse Primer, 5 ATG TAG GCA CTG TCG CTA TCC 3 . This fragment was inserted into the EcoRI-BglII sites of the initial BglII-XbaI pUAST construct. The final construct includes a 1,743-bp EcoRI-XbaI insert encoding the entire disco ORF with some 3 UTR. The second wild-type construct (further referred to as UAS-disco) and the mutant construct (further referred to as UAS-disco) were generated by PCR amplification of genomic DNA. A 1,704-bp fragment containing the complete disco ORF was generated using primers containing EcoRI sites: Forward primer, 5 CCC ACC ACA GAATTC ATG GAG CAC 3 ; Reverse Primer, 5 CCA TGG ATC GAATTC TGG ACA TTA 3 . After EcoRI digestion, the resulting PCR product was inserted into an EcoRI digested pUAST. For the UAS-disco construct, Oregon R genomic DNA was used as a template, while homozygous disco genomic DNA was used as a template for the mutant UAS-disco construct. This form of the protein contains a single amino acid change— Cysteine 127 is substituted with a Serine—which disrupts the first zinc finger. Prior analysis indicated that this mutant form of the Disco protein is nonfunctional (Heilig et al., 1991; Mahaffey et al., 2001). The sequence and reading frame of all constructs were verified by DNA sequence analysis. Integration of UAS-disco constructs into the Drosophila genome was carried out using P-element-mediated germ-line transformation essentially as described by Rubin and Spradling (1982). Two UAS-disco lines were obtained—a heterozygous line carrying UAS-disco on the second chromosome balanced over CyO (UAS-disco(II)), and a homozygous line with the insert on the

Coordinated Morphological Changes in Midgut, Imaginal Discs, and Respiratory Trumpets During Metamorphosis of <I>Aedes aegypti</I> (Diptera: Culicidae)

Abstract The study of mosquito metamorphosis has relevance to understanding the mosquitocidal mechanisms of juvenile hormone analogues and may provide information concerning dipteran evolution. This report details the chronology and coordination of morphological changes in imaginal disc growth and differentiation, respiratory trumpet morphogenesis, and midgut remodeling that occur during Aedes aegypti (L.) metamorphosis. These observations were facilitated by the transparent A. aegypti larval cuticle and the relatively synchronous development of individually grown larvae. The results indicate that within 12 h of the last larval-larval ecdysis, leg imaginal discs start to grow and differentiate, pupal respiratory trumpets start their morphogenesis, and midgut remodeling starts in that programmed death of midgut polytene cells and division of midgut diploid cells are initiated. Morphological changes in all of these structures continue throughout the fourth stadium and are concomitant with substantial larval...

Cytogenetic analysis of two ecdysone-regulated puffs 62C and 62E in Drosophila melanogaster

Genetic analysis has been performed to reveal vital genes around two puffs, a late 62C puff and an early-late 62E puff. Their roles in hormonal regulatory mechanisms have been estimated. A locus represented by four lethal mutations has been found in the vicinity of the 62E puff. The mutants display disturbed puffing, which suggests the involvement of this locus in hormonal regulatory mechanisms. In the 62C puff region. 26 mutations have been found that proved to be allelic to mutations in the D-Titin gene. The giant D-Titin gene is essential for the sarcomeric organization of striated muscles. According to the results of in situ hybridization with polytene chromosomes, the D-Titin gene occupies the entire 62C puff. The phenotypic characteristics of the novel mutants suggest that this protein is polyfunctional, and its role is not restricted to processes in the muscular tissue. It may also be involved in the morphogenesis of leg imaginal disks, and it is necessary for condensation and separation of sister chromatids during mitosis. Mutations in the ecdysone-induced BR-C and E74 genes cause disturbances similar to those found in this study. In addition, mutations of these genes can affect the D-Titin gene activity, which suggests that the three genes are involved in similar morphogenetic and myogenetic processes.

Spatial regulation of DELTA expression mediates NOTCH signalling for segmentation of Drosophila legs.

The Notch (N) signalling pathway is recruited for segregation of cell fates in a number of Drosophila tissue types. We show here that N dependent segmentation of Drosophila legs is regulated by a dynamic pattern of expression of its ligand, DELTA (DL). During third larval instar and early stages of pupation, high levels of DL expression is seen in stripes of cells in the leg imaginal discs which later form the proximal borders of leg joints. These domains also displayed heightened Dl enhancer activity. During subsequent stages of pupation, following segmentation of the leg primordium, DL expression becomes uniform throughout these segments barring the joints. We further show that regulatory Dl mutations or mis-expression of DL abolish leg segmentation. Domains of N signalling for segmentation of legs of flies are thus set up by a stringent spatial regulation of expression of its ligand at the segment border. Further, a comparable role of DL in antennal development reveals a common paradigm of DL-N signalling for segmentation of appendages in flies.

Multiple function of poxn gene in larval PNS development and in adult appendage formation of Drosophila.

The gene pox-neuro (poxn), which encodes a transcriptional regulator including a paired domain, specifies the differences between mono-innervated external sensory (m-es) organs and poly-innervated external sensory (p-es) organs in Drosophila. Here, we analyse the function of poxn in the development of the larval peripheral nervous system (PNS) and in other developmental aspects using a loss-of-function mutant of poxn. We observed that, in addition to the transformation of p-es into m-es organs in the mutant embryo, the external structure of the trichome-like sensilla (hairs) misdifferentiates into that of the campaniform-like sensilla (papillae) in the second and third larval instars. We also observed that POXN is expressed in a cell associated with the external structure of the trichome-like sensilla in the first and second instar larvae. These results imply that poxn is required in two distinct steps in the development of the larval PNS: (1) development of the larval p-es organs during embryogenesis and (2) re-formation of larval sensory hairs after each larval moult. In addition to its expression in the developing PNS, POXN is also expressed in concentric domains of the leg and antennal imaginal discs of early third instar larvae, and in the region of the wing disc that will form the wing hinge. The loss of poxn function results in defects of segmentation of the tarsus and antenna and in a distortion in the wing hinge. These results indicate that the poxn gene plays crucial roles in the morphogenesis of the appendages, in addition to its role in the early specification of neuronal identity.

Conserved overlapping and reciprocal expression of msh/Msx1 and apterous/Lhx2 in Drosophila and mice

Here we describe and compare the expression patterns of the murine genes Lhx2 and Msx1 and their Drosophila orthologues apterous (ap) and muscle-segment homeobox (msh). We find that Lhx2 and Msx1 show complementary patterns of expression in most tissues including the neural and cranial epithelium, pituitary gland, olfactory organs, and neural tube; in contrast, Lhx2 and Msx1 are coexpressed in the developing limbs. Strikingly, the spatial relationship between ap and msh expression in Drosophila is very reminiscent of the expression of their murine orthologues. ap and msh show complementary expression in the leg and antennal imaginal discs, brain and ventral ganglion of the central nervous system (CNS), but both are coexpressed in the wing imaginal disc. These observations suggest conservation in the regulation of these genes between Drosophila and mice.