Genetic analysis of muscle development in Drosophila melanogaster

Single-cell type analysis of wing premotor circuits in the ventral nerve cord of Drosophila melanogaster

To perform most behaviors, animals must send commands from higher-order processing centers in the brain to premotor circuits that reside in ganglia distinct from the brain, such as the mammalian spinal cord or insect ventral nerve cord. How these circuits are functionally organized to generate the great diversity of animal behavior remains unclear. An important first step in unraveling the organization of premotor circuits is to identify their constituent cell types and create tools to monitor and manipulate these with high specificity to assess their function. This is possible in the tractable ventral nerve cord of the fly. To generate such a toolkit, we used a combinatorial genetic technique (split-GAL4) to create 195 sparse driver lines targeting 198 individual cell types in the ventral nerve cord. These included wing and haltere motoneurons, modulatory neurons, and interneurons. Using a combination of behavioral, developmental, and anatomical analyses, we systematically characterized the cell types targeted in our collection. Taken together, the resources and results presented here form a powerful toolkit for future investigations of neural circuits and connectivity of premotor circuits while linking them to behavioral outputs.

Rbfox1 is required for myofibril development and maintaining fiber type-specific isoform expression in Drosophila muscles

Protein isoform transitions confer muscle fibers with distinct properties and are regulated by differential transcription and alternative splicing. RNA-binding Fox protein 1 (Rbfox1) can affect both transcript levels and splicing, and is known to contribute to normal muscle development and physiology in vertebrates, although the detailed mechanisms remain obscure. In this study, we report that Rbfox1 contributes to the generation of adult muscle diversity in Drosophila Rbfox1 is differentially expressed among muscle fiber types, and RNAi knockdown causes a hypercontraction phenotype that leads to behavioral and eclosion defects. Misregulation of fiber type-specific gene and splice isoform expression, notably loss of an indirect flight muscle-specific isoform of Troponin-I that is critical for regulating myosin activity, leads to structural defects. We further show that Rbfox1 directly binds the 3'-UTR of target transcripts, regulates the expression level of myogenic transcription factors myocyte enhancer factor 2 and Salm, and both modulates expression of and genetically interacts with the CELF family RNA-binding protein Bruno1 (Bru1). Rbfox1 and Bru1 co-regulate fiber type-specific alternative splicing of structural genes, indicating that regulatory interactions between FOX and CELF family RNA-binding proteins are conserved in fly muscle. Rbfox1 thus affects muscle development by regulating fiber type-specific splicing and expression dynamics of identity genes and structural proteins.

Genetics of alcohol consumption in Drosophila melanogaster

Individual variation in alcohol consumption in human populations is determined by genetic, environmental, social and cultural factors. In contrast to humans, genetic contributions to complex behavioral phenotypes can be readily dissected in Drosophila, where both the genetic background and environment can be controlled and behaviors quantified through simple high-throughput assays. Here, we measured voluntary consumption of ethanol in ∼3000 individuals of each sex from an advanced intercross population derived from 37 lines of the Drosophila melanogaster Genetic Reference Panel. Extreme quantitative trait loci mapping identified 385 differentially segregating allelic variants located in or near 291 genes at P < 10^-8 . The effects of single nucleotide polymorphisms associated with voluntary ethanol consumption are sex-specific, as found for other alcohol-related phenotypes. To assess causality, we used RNA interference knockdown or P{MiET1} mutants and their corresponding controls and functionally validated 86% of candidate genes in at least one sex. We constructed a genetic network comprised of 23 genes along with a separate trio and a pair of connected genes. Gene ontology analyses showed enrichment of developmental genes, including development of the nervous system. Furthermore, a network of human orthologs showed enrichment for signal transduction processes, protein metabolism and developmental processes, including nervous system development. Our results show that the genetic architecture that underlies variation in voluntary ethanol consumption is sexually dimorphic and partially overlaps with genetic factors that control variation in feeding behavior and alcohol sensitivity. This integrative genetic architecture is rooted in evolutionarily conserved features that can be extrapolated to human genetic interaction networks.

Coordinated development of muscles and tendons of the Drosophila leg

Since Miller's morphological description, the Drosophila leg musculature and its formation has not been revisited. Here, using a set of GFP markers and confocal microscopy, we analyse Drosophila leg muscle development, and describe all the muscles and tendons present in the adult leg. Importantly, we provide for the first time evidence for tendons located internally within leg segments. By visualising muscle and tendon precursors, we demonstrate that leg muscle development is closely associated with the formation of internal tendons. In the third instars discs, in the vicinity of tendon progenitors, some Twist-positive myoblasts start to express the muscle founder cell marker dumbfounded (duf). Slightly later, in the early pupa, epithelial tendon precursors invaginate inside the developing leg segments, giving rise to the internal string-like tendons. The tendon-associated duf-lacZ-expressing muscle founders are distributed along the invaginating tendon precursors and then fuse with surrounding myoblasts to form syncytial myotubes. At mid-pupation, these myotubes grow towards their epithelial insertion sites, apodemes, and form links between internally located tendons and the leg epithelium. This leads to a stereotyped pattern of multifibre muscles that ensures movement of the adult leg.

Mutual exclusion of sensory bristles and tendons on the notum of dipteran flies

BACKGROUND

Genes of the achaete-scute complex encode transcription factors whose activity regulates the development of neural cells. The spatially restricted expression of achaete-scute on the mesonotum of higher flies governs the development and positioning of the large sensory bristles. On the scutum the bristles are arranged into conserved patterns, based on an ancestral arrangement of four longitudinal rows. This pattern appears to date back to the origin of cyclorraphous flies about 100-140 million years ago. The origin of the four-row bauplan, which is independent of body size, and the reasons for its conservation, are not known.

RESULTS

We report that tendons for attachment of the indirect flight muscles are invariably located between the bristle rows of the scutum throughout the Diptera. Tendon development depends on the activity of a transcription factor encoded by the gene stripe. In Drosophila, stripe and achaete-scute have separate expression domains, leading to spatial segregation of tendon precursors and bristle precursors. Furthermore the products of these genes act antagonistically: ectopic sr expression prevents bristle development and ectopic sc expression prevents normal muscle attachment. The product of stripe acts downstream of Achaete-Scute and interferes with the development of bristle precursors.

CONCLUSIONS

The pattern of flight muscles has changed little throughout the Diptera and we argue that the sites of muscle attachment may have constrained the positioning of bristles during the course of evolution. This could account for the pattern of four bristle rows on the scutum.

Delayed behavioural aging and altered mortality in Drosophila beta integrin mutants

The genetic basis for aging is being intensely investigated in a variety of model systems. Much of the focus in Drosophila has been on the molecular-genetic determinants of lifespan, whereas the molecular-genetic basis for age-related functional declines has been less vigorously explored. We evaluated behavioural aging and lifespan in flies harbouring loss-of-function mutations in myospheroid, the gene that encodes betaPS, a beta integrin. Integrins are adhesion molecules that regulate a number of cellular processes and developmental events. Their role in aging, however, has received limited attention. We report here that age-related declines in locomotor activity are ameliorated and that mean lifespan is increased in myospheroid mutants. The delayed functional senescence and altered mortality in myospheroid flies are independent of changes in body size, reproduction or stress resistance. Our data indicate that functional senescence and age-dependent mortality are influenced by beta integrins in Drosophila.

Epidermal tendon cells require Broad Complex function for correct attachment of the indirect flight muscles in Drosophila melanogaster

Drosophila Broad Complex, a primary response gene in the ecdysone cascade, encodes a family of zinc-finger transcription factors essential for metamorphosis. Broad Complex mutations of the rbp complementation group disrupt attachment of the dorsoventral indirect flight muscles during pupal development. We previously demonstrated that isoform BRC-Z1 mediates the muscle attachment function of rbp(+) and is expressed in both developing muscle fibers and their epidermal attachment sites. We now report two complementary studies to determine the cellular site and mode of action of rbp(+) during maturation of the myotendinous junctions of dorsoventral indirect flight muscles. First, genetic mosaics, produced using the paternal loss method, revealed that the muscle attachment phenotype is determined primarily by the genotype of the dorsal epidermis, with the muscle fiber and the ventral epidermis exerting little or no influence. When the dorsal epidermis was mutant, the vast majority of muscles detached or chose ectopic attachment sites, regardless of the muscle genotype. Conversely, wild-type dorsal epidermis could support attachment of mutant muscles. Second, ultrastructural analysis corroborated and extended these results, revealing defective and delayed differentiation of rbp mutant epidermal tendon cells in the dorsal attachment sites. Tendon cell processes, the stress-bearing links between the epidermis and muscle, were reduced in number and showed delayed appearance of microtubule bundles. In contrast, mutant muscle and ventral epidermis resembled the wild type. In conclusion, BRC-Z1 acts in the dorsal epidermis to ensure differentiation of the myotendinous junction. By analogy with the cell-cell interaction essential for embryonic muscle attachment, we propose that BRC-Z1 regulates one or more components of the epidermal response to a signal from the developing muscle.

A direct screen identifies new flight muscle mutants on the Drosophila second chromosome

An ethyl methanesulfonate mutagenesis of Drosophila melanogaster was undertaken, and >3000 mutagenized second chromosomes were generated. More than 800 homozygous viable lines were established, and adults were screened directly under polarized light for muscle defects. A total of 16 mutant strains in which the indirect flight muscles were reduced in volume or disorganized or were otherwise abnormal were identified. These fell into seven recessive and one semidominant complementation groups. Five of these eight complementation groups, including the semidominant mutation, have been mapped using chromosomal deficiencies and meiotic recombination. Two complementation groups mapped close to the Myosin heavy chain gene, but they are shown to be in different loci. Developmental analysis of three mutations showed that two of these are involved in the early stages of adult myogenesis while the other showed late defects. This is the first report of results from a systematic and direct screen for recessive flight muscle defects. This mutant screen identifies genes affecting the flight muscles, which are distinct from those identified when screening for flightlessness.

A screen to identify Drosophila genes required for integrin-mediated adhesion

Drosophila integrins have essential adhesive roles during development, including adhesion between the two wing surfaces. Most position-specific integrin mutations cause lethality, and clones of homozygous mutant cells in the wing do not adhere to the apposing surface, causing blisters. We have used FLP-FRT induced mitotic recombination to generate clones of randomly induced mutations in the F1 generation and screened for mutations that cause wing blisters. This phenotype is highly selective, since only 14 lethal complementation groups were identified in screens of the five major chromosome arms. Of the loci identified, 3 are PS integrin genes, 2 are blistered and bloated, and the remaining 9 appear to be newly characterized loci. All 11 nonintegrin loci are required on both sides of the wing, in contrast to integrin alpha subunit genes. Mutations in 8 loci only disrupt adhesion in the wing, similar to integrin mutations, while mutations in the 3 other loci cause additional wing defects. Mutations in 4 loci, like the strongest integrin mutations, cause a "tail-up" embryonic lethal phenotype, and mutant alleles of 1 of these loci strongly enhance an integrin mutation. Thus several of these loci are good candidates for genes encoding cytoplasmic proteins required for integrin function.

Patterning muscles using organizers: larval muscle templates and adult myoblasts actively interact to pattern the dorsal longitudinal flight muscles of Drosophila

Pattern formation in muscle development is often mediated by special cells called muscle organizers. During metamorphosis in Drosophila, a set of larval muscles function as organizers and provide scaffolding for the development of the dorsal longitudinal flight muscles. These organizers undergo defined morphological changes and dramatically split into templates as adult fibers differentiate during pupation. We have investigated the cellular mechanisms involved in the use of larval fibers as templates. Using molecular markers that label myoblasts and the larval muscles themselves, we show that splitting of the larval muscles is concomitant with invasion by imaginal myoblasts and the onset of differentiation. We show that the Erect wing protein, an early marker of muscle differentiation, is not only expressed in myoblasts just before and after fusion, but also in remnant larval nuclei during muscle differentiation. We also show that interaction between imaginal myoblasts and larval muscles is necessary for transformation of the larval fibers. In the absence of imaginal myoblasts, the earliest steps in metamorphosis, such as the escape of larval muscles from histolysis and changes in their innervation, are normal. However, subsequent events, such as the splitting of these muscles, fail to progress. Finally, we show that in a mutant combination, null for Erect wing function in the mesoderm, the splitting of the larval muscles is aborted. These studies provide a genetic and molecular handle for the understanding of mechanisms underlying the use of muscle organizers in muscle patterning. Since the use of such organizers is a common theme in myogenesis in several organisms, it is likely that many of the processes that we describe are conserved.

A novel alpha integrin subunit associates with betaPS and functions in tissue morphogenesis and movement during Drosophila development

We have identified a novel alpha integrin subunit in Drosophila, that associates with betaPS integrin. We report the temporal expression of the gene encoding this integrin subunit, which we have called alphaPS3, throughout development and the localization of its expression during embryogenesis. AlphaPS3 RNA was localized to tissues undergoing invagination, tissue movement and morphogenesis such as salivary gland, trachea, midgut, dorsal vessel, midline of the ventral nerve cord, amnioserosa and the amnioproctodeal invagination. AlphaPS3 DNA localized to the chromosomal vicinity of scab (scb), previously identified by a failure of dorsal closure. Embryos homozygous for the 119 allele of scb had no detectable alphaPS3 RNA and the 1035 allele of scb contains a Pelement inserted just 5′ of the coding region for the shorter of the gene's two transcripts. Furthermore, mutations in the scb locus exhibit additional defects corresponding to sites of alphaPS3 transcription, including abnormal salivary glands, mislocalization of the pericardial cells and interrupted trachea. Removal of both maternal and zygotic betaPS produced similar defects, indicating that these two integrin subunits associate in vivo and function in the movement and morphogenesis of tissues during development in Drosophila. Phenotypic similarities suggest that laminin A is a potential ligand for this integrin, at least in some tissues.

A novel KH-domain protein mediates cell adhesion processes in Drosophila

Adhesion of cells to one another and to extracellular matrices has major roles in morphogenetic processes during development. One important family of cell adhesion receptors are the integrins, which in Drosophila have crucial functions in at least two adhesion-mediated developmental events: embryonic muscle attachment and adhesion of the wing epithelia. We have cloned and characterized a gene (struthio) that is expressed in embryonic mesodermal and muscle cells, including cardioblasts, and epidermal muscle attachment sites in a pattern that is reminiscent of the expression pattern of the PS integrins. Maternal and zygotic transcripts are produced by this gene and encode similar proteins with two alternative carboxy tails. Both proteins contain identical KH domains, a protein sequence motif that is found in numerous proteins that interact with RNA. The struthio protein is highly homologous in a region including the KH domain to the mouse quaking and C. elegans gld-1 proteins, two developmentally important genes. Somatic homozygous clones of an embryonic lethal mutation in this gene (stru1A122) cause wing blisters and flight impairment, phenotypes which are associated with PS integrin subunit mutations. Thus, the struthio gene encodes a putative RNA-binding protein that appears to regulate some aspects of Drosophila integrin functioning.

Reciprocal signaling between Drosophila epidermal muscle attachment cells and their corresponding muscles

Directed intercellular interactions between distinct cell types underlie the basis for organogenesis during embryonic development. This paper focuses on the establishment of the final somatic muscle pattern in Drosophila, and on the possible cross-talk between the myotubes and the epidermal muscle attachment cells, occurring while both cell types undergo distinct developmental programs. Our findings suggest that the stripe gene is necessary and sufficient to initiate the developmental program of epidermal muscle attachment cells. In stripe mutant embryos, these cells do not differentiate correctly. Ectopic expression of Stripe in various epidermal cells transforms these cells into muscle-attachment cells expressing an array of epidermal muscle attachment cell-specific markers. Moreover, these ectopic epidermal muscle attachment cells are capable of attracting somatic myotubes from a limited distance, providing that the myotube has not yet been attached to or been influenced by a closer wild-type attachment cell. Analysis of the relationships between muscle binding and differentiation of the epidermal muscle attachment cell was performed in mutant embryos in which loss of muscles, or ectopic muscles were induced. This analysis indicated that, although the initial expression of epidermal muscle-attachment cell-specific genes including stripe and groovin is muscle independent, their continuous expression is maintained only in epidermal muscle attachment cells that are connected to muscles. These results suggest that the binding of a somatic muscle to an epidermal muscle attachment cell triggers a signal affecting gene expression in the attachment cell. Taken together, our results suggest the presence of a reciprocal signaling mechanism between the approaching muscles and the epidermal muscle attachment cells. First the epidermal muscle attachment cells signal the myotubes and induce myotube attraction and adhesion to their target cells. Following this binding, the muscle cells send a reciprocal signal to the epidermal muscle attachment cells inducing their terminal differentiation into tendon-like cells.

heartless encodes a fibroblast growth factor receptor (DFR1/DFGF-R2) involved in the directional migration of early mesodermal cells in the Drosophila embryo

After invagination of the mesodermal primordium in the gastrulating Drosophila embryo, the internalized cells migrate in a dorsolateral direction along the overlying ectoderm. This movement generates a stereotyped arrangement of mesodermal cells that is essential for their correct patterning by later position-specific inductive signals. We now report that proper mesodermal cell migration is dependent on the function of a fibroblast growth factor (FGF) receptor encoded by heartless (htl). In htl mutant embryos, the mesoderm forms normally but fails to undergo its usual dorsolateral migration. As a result, cardiac, visceral, and dorsal somatic muscle fates are not induced by Decapentaplegic (Dpp), a transforming growth factor beta family member that is derived from the dorsal ectoderm. Visceral mesoderm can nevertheless be induced by Dpp in the absence of htl function. Ras1 is an important downstream effector of Htl signaling because an activated form of Ras1 partially rescues the htl mutant phenotype. The evolutionary conservation of htl function is suggested by the strikingly similar mesodermal migration and patterning phenotypes associated with FGF receptor mutations in species as diverse as nematode and mouse. These studies establish that Htl signaling provides a vital connection between initial formation of the embryonic mesoderm in Drosophila and subsequent cell-fate specification within this germ layer.

derailed is required for muscle attachment site selection in Drosophila

During development, muscles must form and attach at highly stereotyped positions to allow for coordinated movements. In Drosophila, muscles grow towards and attach to specifically positioned cells within the epidermis. At the molecular level, very little is known about how muscles recognize these attachment sites. The derailed gene encodes a receptor tyrosine kinase family member that is essential for the pathfinding ability of expressing neurons. Here we show that the Drl RTK is also expressed by a small subset of developing embryonic muscles and neighboring epidermal cells during muscle attachment site selection. In drl mutants, these muscles often fail to attach at appropriate locations although their epidermal attachment cells appear unaffected. These results show that, similar to its role in neuronal pathway recognition, the Drl RTK participates in a mechanism required for muscle attachment site selection. The data suggest that both neurons and muscles use common mechanisms to recognize their paths or targets, and that Drl plays an analogous role in both developing systems.

ERECT WING, the Drosophila member of a family of DNA binding proteins is required in imaginal myoblasts for flight muscle development

The erect wing locus of the fruit fly Drosophila melanogaster encodes a protein, EWG, that shares extensive homology with the P3A2 DNA binding protein of sea urchin and a recently identified mammalian transcription factor. Loss-of-function erect wing alleles result in embryonic lethality. Viable alleles of erect wing cause severe abnormalities of the indirect flight muscles. We have analyzed the spatial pattern of erect wing expression in the developing indirect flight muscles during postembryonic development. EWG is detected, 10 hours after puparium formation, in myoblasts that will form the indirect flight muscles. The early events of muscle development are normal in ewg mutants. However, a few hours after the onset of erect wing expression in myoblasts, defects are seen in the developing indirect flight muscles which subsequently degenerate. We present results that show that the normal development of the indirect flight muscles requires erect wing expression in the progenitor myoblasts themselves. Finally, we examine the role of target muscles in the arborization of motor axons by studying the developing innervation to the flight muscle in erect wing mutants. Our study demonstrates, for the first time, a role for a regulatory gene expressed in imaginal myoblasts in Drosophila.

Analysis of the mitochondrial ATP synthase beta-subunit gene in Drosophilidae: structure, transcriptional regulatory features and developmental pattern of expression in Drosophila melanogaster

We have cloned and determined the structure of the gene encoding the H(+)-ATP synthase beta subunit in two distantly related Drosophila species, D. melanogaster and D. virilis. The gene contains three exons that are extremely well conserved at the amino acid level, not only in the region encoding the mature protein but also in that encoding the leader peptide. Primer extension analysis indicates that the 5' untranslated region is extremely short, and reveals the presence of multiple initiation sites of transcription in both Drosophila species. The promoters of D. melanogaster and D. virilis H(+)-ATP synthase beta-subunit genes contain a conserved region surrounding the initiation transcription sites. Nucleotide sequence analysis has revealed the absence of canonical TATA and CCAAT boxes and the presence of several putative regulatory elements in both promoter regions, including GAGA, GATA and Ets binding sites. We have analysed the pattern of gene expression during D. melanogaster development. The mRNA is stored in oocytes, and activation of transcription takes place after 10 h of development. The expression of the nuclear-encoded H(+)-ATP synthase beta subunit is strictly coordinated with the expression of subunits 6 and 8 of the same complex that are encoded in the mitochondrial genome.

Identification of a Drosophila muscle development gene with structural homology to mammalian early growth response transcription factors

In Drosophila, stripe (sr) gene function is required for normal muscle development. Some mutations disrupt embryonic muscle development and are lethal. Other mutations cause total loss of only a single muscle in the adult. Molecular analysis shows that sr encodes a predicted protein containing a zinc finger motif. This motif is homologous to the DNA binding domains encoded by members of the early growth response (egr) gene family. In mammals, expression of egr genes is induced by intercellular signals, and there is evidence for their role in many developmental events. The identification of sr as an egr gene and its pattern of expression suggest that it functions in muscle development via intercellular communication.

Functional relationships between genes of the Shaker gene complex of Drosophila

Different mutations belonging to the HLI and HLII complementation groups of the haplolethal (HL) region of the Shaker complex (ShC) are described. The HLI complementation group includes viable (hdp), recessive lethals [l(1)1614], semidominant lethals [l(1)8384] and dominant lethals [l(1)5051, l(1)9916, l(1)13193], lack-of-function alleles that affect nervous system, cuticle and muscle development. The HLI complementation group encodes troponin I. HLII lack-of-function mutations [l(1)174 and l(1)4058] affect nervous system development. The semidominant lethal HLI mutation l(1)8384 shows differential complementation with other mutations in the ME and HL regions of ShC. Thus, heterozygous combinations of l(1)8384 with ME mutations l(1)162 and l(1)387 are poorly viable. The same phenomenon is observed for heterozygotes of l(1)8384 with HL mutations l(1)1199, l(1)2288 and l(1)3014. These specific interactions indicate the existence of functional relationships among the genetic elements of ShC. The implications for the understanding of the functional organization of ShC are discussed.

Functional interactions between the gene tetanic and the Shaker gene complex of Drosophila

Different phenotypes associated with the tetanic (tta) mutation such as appendage contraction, maternal effect and low viability and fertility are enhanced by one extra dose of the Shaker gene complex (ShC). The tta mutation is lethal with two extra doses of ShC. In addition, tta embryos have a defective nervous system. In this paper, I analyse the interaction between tta and ShC to gain insight into their relationship. Aneuploid analysis suggests that the lethality is due to an interaction of the tta mutation with the maternal effect (ME) region of this gene complex. Mutations in the ME region of ShC partially suppress this interaction. Trans-heterozygous combinations of MEI[l(1)305] and MEIII [l(1)459] mutations causes dominant lethality in a tta background. Trans-heterozygous combinations of an MEII[l(1)1359] mutation with the cited MEI and MEIII mutations are lethal in a tta background. Double mutant combinations and gene dosage experiments, suggest that tta also interacts with the viable (V) region of ShC. These specific genetic interactions indicate that tta and the ME and V regions of ShC are functionally related. These results, together with the previous electrophysiological, molecular and biochemical studies on these mutants suggest an interaction at the protein level. Thus, in the case of the V region, the tta gene product may modulate the activity of the K+ channels encoded in this region. Furthermore, the extreme dosage sensitivity of the interaction between tta and ShC suggests a stoichiometric requirement for the different gene products involved, which might be physically associated and form heteromultimers.

A central role for epidermal segment border cells in the induction of muscle patterning in the Drosophila embryo

The correct patterning of muscles in the Drosophila embryo depends on the migration of developing muscles over the ectoderm and on the attachment of these muscles to specific attachment sites. We investigate the mechanisms that are involved in this process and describe experiments that allow a genetic dissection of the role of the ectoderm in muscle migration and attachment. We show that cells along the segmental border in the ectoderm are used by the developing muscles to reach their attachment sites. These segment border cells are recognized by dissociated myotubes in single suspensions in culture. Thus, developing muscles have properties that allow the specific recognition of the segment border cells and migrate to attach to these cells. The segment border cells are absent in the mutant wingless and naked. In these mutants, the muscles are severely disorganized. We show that this is not a mere consequence of disruption of the epidermis, since, in the mutant patched, where segmental patterning is affected, the segment border cells are present near their normal position; the muscles in this mutant are relatively organized. Similarly, in the mutant lines where ectopic segment border cells are present, the observed muscle derangement correlates well with the ectopic attachment sites that are present. Finally, we have analyzed mutants at the stripe locus and have shown that lethal alleles disrupt muscle organization during embryogenesis. Enhancer-trap alleles of stripe that we have analyzed show reporter gene expression in the segment border cells. Our results indicate a role for the segment border cells in guidance of migrating muscle fibers to their attachment sites.

Functions of the cytoplasmic domain of the beta PS integrin subunit during Drosophila development

Integrins constitute a family of membrane-spanning, heterodimeric proteins that mediate adhesive interactions between cells and surrounding extracellular matrices (or other cells) and participate in signal transduction. We are interested in assessing integrin functions in the context of developing Drosophila melanogaster. This report, using mutants of the beta PS subunit encoded by the myospheroid (mys) locus, analyzes the relationships between integrin protein structure and developmental functions in an intact organism. As a first step in this analysis, we demonstrated the ability of a fragment of wild-type mys genomic DNA, introduced into the germ line in a P-element vector P[mys+], to rescue phenotypes attributed to lack of (or defects in) the endogenous beta PS during several discrete morphogenetic events. We then produced in vitro a series of modifications of the wild-type P[mys+] transposon, which encode beta PS derivatives with mutations within the small and highly conserved cytoplasmic domain. In vivo analysis of these mutant transposons led to the following conclusions. (1) The cytoplasmic tail of beta PS is essential for all developmental functions of the protein that were assayed. (2) An intron at a conserved position in the DNA sequence encoding the cytoplasmic tail is thought to participate in important alternative splicing events in vertebrate beta integrin subunit genes, but is not required for the developmental functions of the mys gene assayed here. (3) Phosphorylation on two conserved tyrosines found in the C terminus of the beta PS cytoplasmic tail is not necessary for the tested developmental functions. (4) Four highly conserved amino acid residues found in the N-terminal portion of the cytoplasmic tail are important but not critical for the developmental functions of beta PS; furthermore, the efficiencies with which these mutant proteins function during different morphogenetic processes vary greatly, strongly suggesting that the cytoplasmic interactions involving PS integrins are developmentally modulated.

Analyses of PS integrin functions during Drosophila development

The Drosophila position-specific (PS) antigens are homologues of the vertebrate integrins, a family of transmembrane proteins that function in cell-matrix and cell-cell adhesion. The common beta subunit of PS integrins (PS beta) is encoded by the lethal(l)myospheroid gene (mys) and is required during wing, eye and muscle development. By expressing PS beta protein at defined developmental periods, we have shown that PS integrins are required throughout pupation, but not earlier, for normal development of wings. In contrast, the key requirement for PS integrins in eye development occurs only in the late pupa. Furthermore, PS integrins are apparently not required for the differentiation of the ommatidial cells; only for their organization. These results are consistent with roles for PS integrins in the interactions between the wing epithelia during the two phases of pupal wing expansion and in maintaining the attachment of a fully formed fenestrated membrane to the basement membrane of the retina. We have also examined the functional significance of alternative splicing of the transcript of the mys gene using P element-mediated transformation to introduce transgenes producing only one of the two spliced forms of PS beta. We find that either form is sufficient to rescue postembryonic mys phenotypes in the wing, eye and muscle but that both of the two splice forms are necessary to rescue the mys embryonic defects. This result indicates a requirement for the alternative splicing of mys during embryogenesis. The location of the alternative exons suggests that the two forms of the PS beta integrin subunit may interact with alternative alpha subunits and/or ligands.

The Drosophila erect wing gene, which is important for both neuronal and muscle development, encodes a protein which is similar to the sea urchin P3A2 DNA binding protein

The erect wing (ewg) locus of Drosophila melanogaster encodes a vital function important for the development of the nervous system and the indirect flight muscles. In order to understand the ewg function at a molecular level, cDNA clones were isolated. Sequence analysis of cDNAs revealed a single open reading frame (ORF) encoding a protein of 733 residues. The translational start for this ORF is a CTG codon. A 225-amino-acid region of this protein is 71% identical to the DNA binding region of the Strongylocentrotus purpuratus P3A2 DNA binding protein. Additionally, the ORF contains large acidic and basic domains characteristic of those in proteins involved in nuclear regulatory functions. Immunoblot analysis using polyclonal anti-EWG antisera generated against a bacterial fusion protein reveals a single, 116-kDa protein present throughout development, beginning at approximately stage 12 of embryogenesis, which is enriched in adult heads and absent from embryos carrying certain ewg alleles. Additionally, we show that EWG is localized specifically to the nuclei of virtually all embryonic neurons. Finally, a minigene consisting of an ewg cDNA under control of the hsp70 promoter can provide the ewg function in transgenic ewg mutant flies.

Integrins hold Drosophila together

The Drosophila position-specific (PS) integrins are members of the integrin family of cell surface receptors and are thought to be receptors for extracellular matrix components. Each PS integrin consists of an alpha subunit, alpha PS1 or alpha PS2, and a beta PS subunit. Mutations in the beta PS subunit and the alpha PS2 subunit have been characterised and reveal that the PS integrins have an essential role in the adhesion of different cell layers to each other. The PS integrins are especially required for the function of the cell-matrix-cell junctions, where the muscles attach to the epidermis and where one surface of the developing wing adheres to the other. These junctions are similar to vertebrate focal adhesions and hemidesmosomes, which also contain integrins. Integrin-mediated cell to cell adhesion via the extracellular matrix provides a way for tissues to adhere to each other without intermingling of their cells.

The Drosophila erect wing gene, which is important for both neuronal and muscle development, encodes a protein which is similar to the sea urchin P3A2 DNA binding protein

The erect wing (ewg) locus of Drosophila melanogaster encodes a vital function important for the development of the nervous system and the indirect flight muscles. In order to understand the ewg function at a molecular level, cDNA clones were isolated. Sequence analysis of cDNAs revealed a single open reading frame (ORF) encoding a protein of 733 residues. The translational start for this ORF is a CTG codon. A 225-amino-acid region of this protein is 71% identical to the DNA binding region of the Strongylocentrotus purpuratus P3A2 DNA binding protein. Additionally, the ORF contains large acidic and basic domains characteristic of those in proteins involved in nuclear regulatory functions. Immunoblot analysis using polyclonal anti-EWG antisera generated against a bacterial fusion protein reveals a single, 116-kDa protein present throughout development, beginning at approximately stage 12 of embryogenesis, which is enriched in adult heads and absent from embryos carrying certain ewg alleles. Additionally, we show that EWG is localized specifically to the nuclei of virtually all embryonic neurons. Finally, a minigene consisting of an ewg cDNA under control of the hsp70 promoter can provide the ewg function in transgenic ewg mutant flies.

Abnormal muscle development in the heldup3 mutant of Drosophila melanogaster is caused by a splicing defect affecting selected troponin I isoforms

The troponin I (TnI) gene of Drosophila melanogaster encodes a family of 10 isoforms resulting from the differential splicing of 13 exons. Four of these exons (6a1, 6a2, 6b1, and 6b2) are mutually exclusive and very similar in sequence. TnI isoforms show qualitative specificity whereby each muscle expresses a selected repertoire of them. In addition, TnI isoforms show quantitative specificity whereby each muscle expresses characteristic amounts of each isoform. In the mutant heldup3, the development of the thoracic muscles DLM, DVM, and TDT is aborted. The mutation consists of a one-nucleotide displacement of the 3' AG splice site at the intron preceding exon 6b1, resulting in the failure to produce all exon 6b1-containing TnI isoforms. These molecular changes in a constituent of the thin filaments cause the selective failure to develop the DLM, DVM, and TDT muscles while having no visible effect on other muscles wherein exon 6b1 expression is minor.

Abnormal muscle development in the heldup3 mutant of Drosophila melanogaster is caused by a splicing defect affecting selected troponin I isoforms

The troponin I (TnI) gene of Drosophila melanogaster encodes a family of 10 isoforms resulting from the differential splicing of 13 exons. Four of these exons (6a1, 6a2, 6b1, and 6b2) are mutually exclusive and very similar in sequence. TnI isoforms show qualitative specificity whereby each muscle expresses a selected repertoire of them. In addition, TnI isoforms show quantitative specificity whereby each muscle expresses characteristic amounts of each isoform. In the mutant heldup3, the development of the thoracic muscles DLM, DVM, and TDT is aborted. The mutation consists of a one-nucleotide displacement of the 3' AG splice site at the intron preceding exon 6b1, resulting in the failure to produce all exon 6b1-containing TnI isoforms. These molecular changes in a constituent of the thin filaments cause the selective failure to develop the DLM, DVM, and TDT muscles while having no visible effect on other muscles wherein exon 6b1 expression is minor.

Two FGF-receptor homologues of Drosophila: one is expressed in mesodermal primordium in early embryos

The fibroblast growth factor (FGF)/receptor system is thought to mediate various developmental events in vertebrates. We examined molecular structures and expression of DFR1 and DFR2, two Drosophila genes closely related to vertebrate FGF-receptor genes. DFR1 and DFR2 proteins contain two and five immunoglobulin-like domains, respectively, in the extracellular region, and a split tyrosine kinase domain in the intracellular region. In early embryos, DFR1 RNA expression, requiring both twist and snail proteins, is specific to mesodermal primordium and invaginated mesodermal cells. At later stages, putative muscle precursor cells and cells in the central nervous system (CNS) express DFR1. DFR2 expression occurs in endodermal precursor cells, CNS midline cells and certain ectodermal cells such as those of trachea and salivary duct. FGF-receptor homologues in Drosophila would thus appear essential for generation of mesodermal and endodermal layers, invaginations of various types of cells, and CNS formation.

Cell lineage of flight muscle fibers in Drosophila: a fate map of the induced shibire phenotype in mosaics

A blastoderm fate map has been prepared for Drosophila, using mosaics of a temperature-sensitive mutation, shibire (shi). The mutation can cause abnormal flight muscle morphology, inducible only by a short heat pulse in early metamorphosis. Thus muscle lineage and development are unperturbed until the heat pulse in the early pupa. The developmental focus of the shi muscle phenotype maps to the ventral thorax at the expected site of thoracic mesoderm, and probably indicates the blastoderm progenitors of the adult flight muscle. The fate map provides greater detail than previously available for the dorsolongitudinal fibers (DLM) of flight muscle, showing wide separation of the fibers of flight muscle. DLM fibers a and b map close together, and far anterior to fibers e and f, which also map together. On a fate map, common developmental focus indicates a common blastoderm origin. Thus, the observed pattern for DLM fibers suggests that the blastoderm progenitors for each of these syncytial fiber pairs (a, b; e, f) include only one or two cells. It follows that there is usually a single genotype within each fiber pair (a, b; e, f), and that this genotype is directly reflected in the fiber phenotype. In a large number of cases, DLM fibers a and b differ in phenotype from other DLM fibers, in parallel with their other differences (e.g., timing of development in pupa, innervation, motor activity). The separation of fate map locations of the developmental focus for DLM fibers within mesoderm suggests that specific fibers of flight muscle may, in normal development, originate in all three thoracic mesodermal parasegments.

Troponin I is encoded in the haplolethal region of the Shaker gene complex of Drosophila

We have analyzed one of the nine complementation groups that constitute the haplolethal (HL) region of the Shaker gene complex (ShC). Five mutations, including a dominant lethal, define this complementation group: HL I. Mutant phenotypes show abnormal embryogenesis with structural defects in the nervous system and aberrant degeneration of specific adult muscles in addition to altered action potentials. HL I encodes a family of proteins with extensive homology to invertebrate troponin I (TnI). Members of this family are brought about by two alternative and two mutually exclusive exons in conjunction with two differential polyadenylation sites. Transcription analysis indicates that some isoforms are adult specific and others are synthesized throughout development, except during early metamorphosis. Certain isoforms of Drosophila TnI are expressed in specific muscles. The specificity of mutant phenotypes suggests a functional role of particular TnI isoforms in the development and the mature activity of muscle and nervous systems.

The mutations previously designated as flightless-I3, flightless-O2 and standby are members of the W-2 lethal complementation group at the base of the X-chromosome of Drosophila melanogaster

By using a well defined panel of chromosomal deficiencies, duplications and lethals, we have mapped three mutations causing flightlessness, flightless-I3, flightless-O2 and standby, to a single lethal complementation group (termed W-2) at the base of the X-chromosome of D. melanogaster. We also show that a fourth flightless mutation, termed grounded, previously mapped near to the base of the X-chromosome, is distal to the cytogenetic interval 18F to 20F. Mutants homozygous for the flightless-I3, flightless-O2 and standby mutations exhibit abnormalities of myofibrillar arrangements in the indirect flight muscles. They have distorted Z-bands and the myofibrils are often displaced from their normal parallel arrangement. These viable flightless mutations are all hypomorphs since the homozygous deficiency of the W-2 X-chromosomal region is lethal to the organism.

A mutation that causes muscle defects also affects catecholamine metabolism in Drosophila

Biochemical analyses, employing HPLC and electrochemical detection, have shown that the mutation adl-1, which causes muscle defects, also induces a temperature-sensitive defect in catecholamine metabolism. The pool sizes of N-acetyldopamine (NADA) and N-beta-alanyldopamine (NBAD) in mutant adults incubated at 29 degrees attain only a fraction, dependent on the length of incubation, of those in mutants incubated at 22 degrees or in controls. The differences are more striking in relevant hemizygotes. Notably, dopamine is unaffected. Concomitant examination of behavior revealed a correlation between decreases in NADA and NBAD and decreases in locomotor function. That these observations suggest a requirement for catecholamine metabolism in muscle function is discussed.