Anterior-posterior subdivision and the diversification of the mesoderm in Drosophila

Twist is required for muscle development of the adult legs in Henosepilachna vigintioctopunctata

Although muscle development has been widely studied in Drosophila melanogaster, it was a great challenge to apply to developmental processes of other insect muscles. This study was focused on the functional characterization of a basic helix-loop-helix transcription factor gene twist in an herbivorous ladybird Henosepilachna vigintioctopunctata. Its transcript (Hvtwist) levels were detected in all developmental stages. RNA interference (RNAi)-aided knockdown of Hvtwist at the penultimate larval instar stage impaired pupation, and caused a deformed adult in the legs. The tarsi were malformed and did not support the bodies in an upright position. The climbing ability was impaired. Moreover, around 50% of the impaired adults had a malformed elytrum. In addition, they consumed less foliage and did not lay eggs. A hematoxylin-eosin staining of the leg demonstrated that the tibial extensor (TE) and the tibial flexor (TF) muscles were originated from the femurs while levator and depressor muscles of the tarsus (TL and TD) were located in the tibia in the control adults, in which tarsal segments were devoid of muscles. RNAi treatment specific to Hvtwist expression markedly impaired TE and TF muscles in the femurs, and prevented the development of TL and TD muscles in the tibia. Therefore, our findings demonstrate Twist plays a vital role in the myogenesis in H. vigintioctopunctata adult legs.

Collective Migrations of Drosophila Embryonic Trunk and Caudal Mesoderm-Derived Muscle Precursor Cells

Mesoderm migration in the Drosophila embryo is a highly conserved, complex process that is required for the formation of specialized tissues and organs, including the somatic and visceral musculature. In this FlyBook chapter, we will compare and contrast the specification and migration of cells originating from the trunk and caudal mesoderm. Both cell types engage in collective migrations that enable cells to achieve new positions within developing embryos and form distinct tissues. To start, we will discuss specification and early morphogenetic movements of the presumptive mesoderm, then focus on the coordinate movements of the two subtypes trunk mesoderm and caudal visceral mesoderm, ending with a comparison of these processes including general insights gained through study.

Drosophila as a Genetic Model for Hematopoiesis

In this FlyBook chapter, we present a survey of the current literature on the development of the hematopoietic system in Drosophila The Drosophila blood system consists entirely of cells that function in innate immunity, tissue integrity, wound healing, and various forms of stress response, and are therefore functionally similar to myeloid cells in mammals. The primary cell types are specialized for phagocytic, melanization, and encapsulation functions. As in mammalian systems, multiple sites of hematopoiesis are evident in Drosophila and the mechanisms involved in this process employ many of the same molecular strategies that exemplify blood development in humans. Drosophila blood progenitors respond to internal and external stress by coopting developmental pathways that involve both local and systemic signals. An important goal of these Drosophila studies is to develop the tools and mechanisms critical to further our understanding of human hematopoiesis during homeostasis and dysfunction.

Stepwise wiring of the Drosophila olfactory map requires specific Plexin B levels

The precise assembly of a neural circuit involves many consecutive steps. The conflict between a limited number of wiring molecules and the complexity of the neural network impels each molecule to execute multiple functions at different steps. Here, we examined the cell-type specific distribution of endogenous levels of axon guidance receptor Plexin B (PlexB) in the developing antennal lobe, the first olfactory processing center in Drosophila. We found that different classes of olfactory receptor neurons (ORNs) express PlexB at different levels in two wiring steps – axonal trajectory choice and subsequent target selection. In line with its temporally distinct patterns, the proper levels of PlexB control both steps in succession. Genetic interactions further revealed that the effect of high-level PlexB is antagonized by its canonical partner Sema2b. Thus, PlexB plays a multifaceted role in instructing the assembly of the Drosophila olfactory circuit through temporally-regulated expression patterns and expression level-dependent effects.

Identification of a multipotent Twist2-expressing cell population in the adult heart

Twist transcription factors function as ancestral regulators of mesodermal cell fates in organisms ranging from Drosophila to mammals. Through lineage tracing of Twist2 (Tw2)-expressing cells with tamoxifen-inducible Tw2-CreERT2 and tdTomato (tdTO) reporter mice, we discovered a unique cell population that progressively contributes to cardiomyocytes (CMs), endothelial cells, and fibroblasts in the adult heart. Clonal analysis confirmed the ability of Tw2-derived tdTO⁺ (Tw2-tdTO⁺) cells to form CMs in vitro. Within the adult heart, Tw2-tdTO⁺ CMs accounted for ∼13% of total CMs, the majority of which resulted from fusion of Tw2-tdTO⁺ cells with existing CMs. Tw2-tdTO⁺ cells also contribute to cardiac remodeling after injury. We conclude that Tw2-tdTO⁺ cells participate in lifelong maintenance of cardiac function, at least in part through de novo formation of CMs and fusion with preexisting CMs, as well as in the genesis of other cellular components of the adult heart.

Conserved signaling mechanisms in Drosophila heart development

Signal transduction through multiple distinct pathways regulates and orchestrates the numerous biological processes comprising heart development. This review outlines the roles of the FGFR, EGFR, Wnt, BMP, Notch, Hedgehog, Slit/Robo, and other signaling pathways during four sequential phases of Drosophila cardiogenesis-mesoderm migration, cardiac mesoderm establishment, differentiation of the cardiac mesoderm into distinct cardiac cell types, and morphogenesis of the heart and its lumen based on the proper positioning and cell shape changes of these differentiated cardiac cells-and illustrates how these same cardiogenic roles are conserved in vertebrates. Mechanisms bringing about the regulation and combinatorial integration of these diverse signaling pathways in Drosophila are also described. This synopsis of our present state of knowledge of conserved signaling pathways in Drosophila cardiogenesis and the means by which it was acquired should facilitate our understanding of and investigations into related processes in vertebrates. Developmental Dynamics 246:641-656, 2017. © 2017 Wiley Periodicals, Inc.

The myogenic repressor gene Holes in muscles is a direct transcriptional target of Twist and Tinman in the Drosophila embryonic mesoderm

Understanding the regulatory circuitry controlling myogenesis is critical to understanding developmental mechanisms and developmentally-derived diseases. We analyzed the transcriptional regulation of a Drosophila myogenic repressor gene, Holes in muscles (Him). Previously, Him was shown to inhibit Myocyte enhancer factor-2 (MEF2) activity, and is expressed in myoblasts but not differentiating myotubes. We demonstrate that different phases of Him embryonic expression arises through the actions of different enhancers, and we characterize the enhancer required for its early mesoderm expression. This Him early mesoderm enhancer contains two conserved binding sites for the basic helix-loop-helix regulator Twist, and one binding site for the NK homeodomain protein Tinman. The sites for both proteins are required for enhancer activity in early embryos. Twist and Tinman activate the enhancer in tissue culture assays, and ectopic expression of either factor is sufficient to direct ectopic expression of a Him-lacZ reporter, or of the endogenous Him gene. Moreover, sustained expression of twist in the mesoderm up-regulates mesodermal Him expression in late embryos. Our findings provide a model to define mechanistically how Twist can both promotes myogenesis through direct activation of Mef2, and can place a brake on myogenesis, through direct activation of Him.

Differential and redundant functions of gooseberry and gooseberry neuro in the central nervous system and segmentation of the Drosophila embryo

The gooseberry locus of Drosophila consists of two homologous Pax genes, gooseberry neuro (gsbn) and gooseberry (gsb). Originally characterized by genetics as a single segment-polarity gene, its role in segmentation has been enigmatic, as only deficiencies uncovering both genes showed a strong segmentation phenotype while mutants of gsb did not. To solve this conundrum and assay for differential roles of gsbn and gsb, we have obtained by homologous recombination for the first time null mutants of either gene as well as a deficiency inactivating only gsbn and gsb. Our analysis shows that (i) gsbn null mutants are subviable while all surviving males and most females are sterile; (ii) gsb and gsbn share overlapping functions in segmentation and the CNS, in which gsbn largely, but not completely depends on the transcriptional activation by the product of gsb; (iii) as a consequence, in the absence of gsbn, gsb becomes haploinsufficient for its function in the CNS, and gsbn(-/-)gsb(-/+) mutants die as larvae. Such mutants display defects in the proper specification of the SNa branch of the segmental nerve, which appears intact in gsbn(-/-) mutants. Lineage analysis in the embryonic CNS showed that gsbn is expressed in the entire lineage derived from NB5-4, which generates 4 or 5 motoneurons whose axons are part of the SNa branch and all of which except one also express BarH1. Analysis of gsbn(-/-)gsb(-/+) clones originating from NB5-4 further suggests that gsb and gsbn specify the SNa fate and concomitantly repress the SNc fate in this lineage and that their products activate BarH1 transcription. Specification of the SNa fate by Gsb and Gsbn occurs mainly at the NB and GMC stage. However, the SNa mutant phenotype can be rescued by providing Gsbn as late as at the postmitotic stage. The hierarchical relationship between gsb and gsbn, the haploinsufficiency of gsb in gsbn mutants, and their redundant roles in the epidermis and CNS are discussed. A model is proposed how selection for both genes occurred after their duplication during evolution.

Diversification of muscle types in Drosophila: upstream and downstream of identity genes

Understanding gene regulatory pathways underlying diversification of cell types during development is one of the major challenges in developmental biology. Progressive specification of mesodermal lineages that are at the origin of body wall muscles in Drosophila embryos has been extensively studied during past years, providing an attractive framework for dissecting cell type diversification processes. In particular, it has been found that muscle founder cells that are at the origin of individual muscles display specific expression of transcription factors that control diversification of muscle types. These factors, encoded by genes collectively called muscle identity genes, are activated in discrete subsets of muscle founders. As a result, each founder cell is thought to carry a unique combinatorial code of identity gene expression. Considering this, to define temporally and spatially restricted expression of identity genes, a set of coordinated upstream regulatory inputs is required. But also, to realize the identity program and to form specific muscle types with distinct properties, an efficient battery of downstream identity gene targets needs to be activated. Here we review how the specificity of expression and action of muscle identity genes is acquired.

Variation in mesoderm specification across Drosophilids is compensated by different rates of myoblast fusion during body wall musculature development

Background

It has been shown that species separated by relatively short evolutionary distances may have extreme variations in egg size and shape. Those variations are expected to modify the polarized morphogenetic gradients that pattern the dorso-ventral axis of embryos. Currently, little is known about the effects of scaling over the embryonic architecture of organisms. We began examining this problem by asking if changes in embryo size in closely related species of Drosophila modify all three dorso-ventral germ layers or only particular layers, and whether or not tissue patterning would be affected at later stages.

Principal Findings

Here we report that changes in scale affect predominantly the mesodermal layer at early stages, while the neuroectoderm remains constant across the species studied. Next, we examined the fate of somatic myoblast precursor cells that derive from the mesoderm to test whether the assembly of the larval body wall musculature would be affected by the variation in mesoderm specification. Our results show that in all four species analyzed, the stereotyped organization of the body wall musculature is not disrupted and remains the same as in D. melanogaster. Instead, the excess or shortage of myoblast precursors is compensated by the formation of individual muscle fibers containing more or less fused myoblasts.

Conclusions

Our data suggest that changes in embryonic scaling often lead to expansions or retractions of the mesodermal domain across Drosophila species. At later stages, two compensatory cellular mechanisms assure the formation of a highly stereotyped larval somatic musculature: an invariable selection of 30 muscle founder cells per hemisegment, which seed the formation of a complete array of muscle fibers, and a variable rate in myoblast fusion that modifies the number of myoblasts that fuse to individual muscle fibers.

The convergence of Notch and MAPK signaling specifies the blood progenitor fate in the Drosophila mesoderm

Blood progenitors arise from a pool of pluripotential cells ("hemangioblasts") within the Drosophila embryonic mesoderm. The fact that the cardiogenic mesoderm consists of only a small number of highly stereotypically patterned cells that can be queried individually regarding their gene expression in normal and mutant embryos is one of the significant advantages that Drosophila offers to dissect the mechanism specifying the fate of these cells. We show in this paper that the expression of the Notch ligand Delta (Dl) reveals segmentally reiterated mesodermal clusters ("cardiogenic clusters") that constitute the cardiogenic mesoderm. These clusters give rise to cardioblasts, blood progenitors and nephrocytes. Cardioblasts emerging from the cardiogenic clusters accumulate high levels of Dl, which is required to prevent more cells from adopting the cardioblast fate. In embryos lacking Dl function, all cells of the cardiogenic clusters become cardioblasts, and blood progenitors are lacking. Concomitant activation of the Mitogen Activated Protein Kinase (MAPK) pathway by Epidermal Growth Factor Receptor (EGFR) and Fibroblast Growth Factor Receptor (FGFR) is required for the specification and maintenance of the cardiogenic mesoderm; in addition, the spatially restricted localization of some of the FGFR ligands may be instrumental in controlling the spatial restriction of the Dl ligand to presumptive cardioblasts.

Drosophila models of cardiac disease

The fruit fly Drosophila melanogaster has emerged as a useful model for cardiac diseases, both developmental abnormalities and adult functional impairment. Using the tools of both classical and molecular genetics, the study of the developing fly heart has been instrumental in identifying the major signaling events of cardiac field formation, cardiomyocyte specification, and the formation of the functioning heart tube. The larval stage of fly cardiac development has become an important model system for testing isolated preparations of living hearts for the effects of biological and pharmacological compounds on cardiac activity. Meanwhile, the recent development of effective techniques to study adult cardiac performance in the fly has opened new uses for the Drosophila model system. The fly system is now being used to study long-term alterations in adult performance caused by factors such as diet, exercise, and normal aging. The fly is a unique and valuable system for the study of such complex, long-term interactions, as it is the only invertebrate genetic model system with a working heart developmentally homologous to the vertebrate heart. Thus, the fly model combines the advantages of invertebrate genetics (such as large populations, facile molecular genetic techniques, and short lifespan) with physiological measurement techniques that allow meaningful comparisons with data from vertebrate model systems. As such, the fly model is well situated to make important contributions to the understanding of complicated interactions between environmental factors and genetics in the long-term regulation of cardiac performance.

Antagonistic function of Lmd and Zfh1 fine tunes cell fate decisions in the Twi and Tin positive mesoderm of Drosophila melanogaster

In this study we show that cell fate decisions in the dorsal and lateral mesoderm of Drosophila melanogaster depend on the antagonistic action of the Gli-like transcription factor Lame duck (Lmd) and the zinc finger homeodomain factor Zfh1. Lmd expression leads to the reduction of Zfh1 positive cell types, thereby restricting the number of Odd-skipped (Odd) positive and Tinman (Tin) positive pericardial cells in the dorsal mesoderm. In more lateral regions, ectopic activation of Zfh1 or loss of Lmd leads to an excess of adult muscle precursor (AMP) like cells. We also observed that Lmd is co-expressed with Tin in the early dorsal mesoderm and leads to a reduction of Tin expression in cells destined to become dorsal fusion competent myoblasts (FCMs). In the absence of Lmd function, these cells remain Tin positive and develop as Tin positive pericardial cells although they do not express Zfh1. We show further that Tin repression and pericardial restriction in the dorsal mesoderm facilitated by Lmd is instructed by a late Decapentaplegic (Dpp) signal that is abolished in embryos carrying the disk region mutation dpp(d6).

A Drosophila Smyd4 homologue is a muscle-specific transcriptional modulator involved in development

Background

SET and MYND domain (Smyd) proteins are involved in the transcriptional regulation of cellular proliferation and development in vertebrates. However, the in vivo functions and mechanisms by which these proteins act are poorly understood.

Methodology/Principal Findings

We have used biochemical and genetic approaches to study the role of a Smyd protein in Drosophila. We identified eleven Drosophila genes that encode Smyd proteins. CG14122 encodes a Smyd4 homologue that we have named dSmyd4. dSmyd4 repressed transcription and recruited class I histone deacetylases (HDACs). A region of dSmyd4 including the MYND domain interacted directly with ∼150 amino acids at the N-termini of dHDAC1 and dHDAC3. dSmyd4 interacts selectively with Ebi, a component of the dHDAC3/SMRTER co-repressor complex. During embryogenesis dSmyd4 was expressed throughout the mesoderm, with highest levels in the somatic musculature. Muscle-specific RNAi against dSmyd4 resulted in depletion of the protein and lead to severe lethality. Eclosion is the final moulting stage of Drosophila development when adult flies escape from the pupal case. 80% of dSmyd4 knockdown flies were not able to eclose, resulting in late pupal lethality. However, many aspects of eclosion were still able to occur normally, indicating that dSmyd4 is likely to be involved in the development or function of adult muscle.

Conclusions/Significance

Repression of transcription by dSmyd4 and the involvement of this protein in development suggests that aspects of Smyd protein function are conserved between vertebrates and invertebrates.

Genetic control of muscle development: learning from Drosophila

Muscle development involves a complex sequence of time and spatially regulated cellular events leading to the formation of highly specialised syncytial muscle cells displaying a common feature, the capacity of contraction. Analyses of mechanisms controlling muscle development reveals that the main steps of muscle formation including myogenic determination, diversification of muscle precursors, myoblast fusion and terminal differentiation involve the actions of evolutionarily conserved genes. Thus dissecting the genetic control of muscle development in simple model organisms appears to be an attractive way to get insights into core genetic cascade that orchestrate myogenesis. In this respect, particularly insightful have been data generated using Drosophila as a model system. Notably, the interplay between intrinsic and extrinsic cues that determine the early myogenic decisions leading to the specification of muscle progenitors and those controlling myoblasts fusion are much better characterised in Drosophila than in vertebrate species. Also, adult Drosophila myogenesis, which leads to the formation of vertebrate-like multi-fibre muscles, emerges as a particularly well-adapted system to study normal and aberrant muscle development.

Muscle stem cells and model systems for their investigation

Stem cells are characterized by their clonal ability both to generate differentiated progeny and to undergo self-renewal. Studies of adult mammalian organs have revealed stem cells in practically every tissue. In the adult skeletal muscle, satellite cells are the primary muscle stem cells, responsible for postnatal muscle growth, hypertrophy, and regeneration. In the past decade, several molecular markers have been found that identify satellite cells in quiescent and activated states. However, despite their prime importance, surprisingly little is known about the biology of satellite cells, as their analysis was for a long time hampered by a lack of genetically amenable experimental models where their properties can be dissected. Here, we review how the embryonic origin of satellite cells was discovered using chick and mouse model systems and discuss how cells from other sources can contribute to muscle regeneration. We present evidence for evolutionarily conserved properties of muscle stem cells and their identification in lower vertebrates and in the fruit fly. In Drosophila, muscle stem cells called adult muscle precursors (AMP) can be identified in embryos and in larvae by persistent expression of a myogenic basic helix-loop-helix factor Twist. AMP cells play a crucial role in the Drosophila life cycle, allowing de novo formation and regeneration of adult musculature during metamorphosis. Based on the premise that AMPs represent satellite-like cells of the fruit fly, important insight into the biology of vertebrate muscle stem cells can be gained from genetic analysis in Drosophila.

Daughterless dictates Twist activity in a context-dependent manner during somatic myogenesis

Somatic myogenesis in Drosophila relies on the reiterative activity of the basic helix-loop-helix transcriptional regulator, Twist (Twi). How Twi directs multiple cell fate decisions over the course of mesoderm and muscle development is unclear. Previous work has shown that Twi is regulated by its dimerization partner: Twi homodimers activate genes necessary for somatic myogenesis, whereas Twi/Daughterless (Da) heterodimers lead to the repression of these genes. Here, we examine the nature of Twi/Da heterodimer repressive activity. Analysis of the Da protein structure revealed a Da repression (REP) domain, which is required for Twi/Da-mediated repression of myogenic genes, such as Dmef2, both in tissue culture and in vivo. This domain is crucial for the allocation of mesodermal cells to distinct fates, such as heart, gut and body wall muscle. By contrast, the REP domain is not required in vivo during later stages of myogenesis, even though Twi activity is required for muscles to achieve their final pattern and morphology. Taken together, we present evidence that the repressive activity of the Twi/Da dimer is dependent on the Da REP domain and that the activity of the REP domain is sensitive to tissue context and developmental timing.

3D analysis of founder cell and fusion competent myoblast arrangements outlines a new model of myoblast fusion

Formation of the Drosophila larval body wall muscles requires the specification, coordinated cellular behaviors and fusion of two cell types: Founder Cells (FCs) that control the identity of the individual muscle and Fusion Competent Myoblasts (FCMs) that provide mass. These two cell types come together to control the final size, shape and attachment of individual muscles. However, the spatial arrangement of these cells over time, the sequence of fusion events and the contribution of these cellular relationships to the fusion process have not been addressed. We analyzed the three-dimensional arrangements of FCs and FCMs over the course of myoblast fusion and assayed whether these issues impact the process of myoblast fusion. We examined the timing of the fusion process by analyzing the fusion profile of individual muscles in wild type and fusion mutants. We showed that there are two temporal phases of myoblast fusion in wild type embryos. Limited fusion events occur during the first 3 h of fusion, while the majority of fusion events occur in the remaining 2.5 h. Altogether, our data have led us to propose a new model of myoblast fusion where the frequency of myoblast fusion events may be influenced by the spatial arrangements of FCs and FCMs.

A core transcriptional network for early mesoderm development in Drosophila melanogaster

Embryogenesis is controlled by large gene-regulatory networks, which generate spatially and temporally refined patterns of gene expression. Here, we report the characteristics of the regulatory network orchestrating early mesodermal development in the fruitfly Drosophila, where the transcription factor Twist is both necessary and sufficient to drive development. Through the integration of chromatin immunoprecipitation followed by microarray analysis (ChIP-on-chip) experiments during discrete time periods with computational approaches, we identified >2000 Twist-bound cis-regulatory modules (CRMs) and almost 500 direct target genes. Unexpectedly, Twist regulates an almost complete cassette of genes required for cell proliferation in addition to genes essential for morophogenesis and cell migration. Twist targets almost 25% of all annotated Drosophila transcription factors, which may represent the entire set of regulators necessary for the early development of this system. By combining in vivo binding data from Twist, Mef2, Tinman, and Dorsal we have constructed an initial transcriptional network of early mesoderm development. The network topology reveals extensive combinatorial binding, feed-forward regulation, and complex logical outputs as prevalent features. In addition to binary activation and repression, we suggest that Twist binds to almost all mesodermal CRMs to provide the competence to integrate inputs from more specialized transcription factors.

Investigating divergent mechanisms of mesoderm development in arthropods: the expression ofPh-twist andPh-mef2 inParhyale hawaiensis

The evolution of mesoderm was important for the development of complex body plans as well as key organ systems. Genetic and molecular studies in the fruitfly, Drosophila melanogaster, have provided the majority of information concerning mesoderm development in arthropods. In Drosophila, twist is necessary for the specification and correct morphogenesis of mesoderm and myocyte enhancing factor 2 (mef2) is involved downstream of twist to activate muscle differentiation. In Drosophila, mesoderm is defined by positional cues in the blastoderm embryo, while in another arthropod group, the amphipod crustaceans, cell lineage plays a greater role in defining the mesoderm. It is not known how different mechanistic strategies such as positional information vs. cell-lineage-dependent development affect the timing and use of gene networks. Here we describe the development of the mesoderm in a malacostracan crustacean, Parhyale hawaiensis, and characterize the expression of Parhyale twist and mef2 orthologues. In Parhyale, the mesoderm of the post-mandibular segments arises mainly through the asymmetric division of mesoteloblasts as the germband elongates. Ph-twist expression is seen in a subset of segmental mesoderm during germband development, but not during early cleavages when the specific mesodermal cell lineages first arise. ph-mef2 expression starts after the segmental mesoderm begins to proliferate and persists in developing musculature. While the association of these genes with mesoderm differentiation appears to be conserved across the animal kingdom, the timing of expression and relationship with different mechanisms of mesoderm development may give us greater insight into the ancestral use of these genes during mesoderm differentiation.

Parcas, a regulator of non-receptor tyrosine kinase signaling, acts during anterior-posterior patterning and somatic muscle development in Drosophila melanogaster

We have isolated parcas (pcs) in a screen to identify novel regulators of muscle morphogenesis. Pcs is expressed in the ovary and oocyte during oogenesis and again in the embryo, specifically in the developing mesoderm, throughout muscle development. pcs is first required in the ovary during oogenesis for patterning and segmentation of the early Drosophila embryo due primarily to its role in the regulation of Oskar (Osk) levels. In addition to the general patterning defects observed in embryos lacking maternal contribution of pcs, these embryos show defects in Wingless (Wg) expression, causing losses of Wg-dependent cell types within the affected segment. pcs activity is required again later during embryogenesis in the developing mesoderm for muscle development. Loss and gain of function studies demonstrate that pcs is necessary at distinct times for muscle specification and morphogenesis. Pcs is predicted to be a novel regulator of non-receptor tyrosine kinase (NRTK) signaling. We have identified one target of Pcs regulation, the Drosophila Tec kinase Btk29A. While Btk29A appears to be regulated by Pcs during its early role in patterning and segmentation, it does not appear to be a major target of Pcs regulation during muscle development. We propose that Pcs fulfils its distinct roles during development by the regulation of multiple NRTKs.

D-six4 plays a key role in patterning cell identities deriving from the Drosophila mesoderm

Patterning of the Drosophila embryonic mesoderm requires the regulation of cell type-specific factors in response to dorsoventral and anteroposterior axis information. For the dorsoventral axis, the homeodomain gene, tinman, is a key patterning mediator for dorsal mesodermal fates like the heart. However, equivalent mediators for more ventral fates are unknown. We show that D-six4, which encodes a Six family transcription factor, is required for the appropriate development of most cell types deriving from the non-dorsal mesoderm - the fat body, somatic cells of the gonad, and a specific subset of somatic muscles. Misexpression analysis suggests that D-Six4 and its likely cofactor, Eyes absent, are sufficient to impose these fates on other mesodermal cells. At stage 10, the mesodermal expression patterns of D-six4 and tin are complementary, being restricted to the dorsal and non-dorsal regions respectively. Our data suggest that D-six4 is a key mesodermal patterning mediator at this stage that regulates a variety of cell-type-specific factors and hence plays an equivalent role to tin. At stage 9, however, D-six4 and tin are both expressed pan-mesodermally. At this stage, tin function is required for full D-six4 expression. This may explain the known requirement for tin in some non-dorsal cell types.

Subdivision and developmental fate of the head mesoderm in Drosophila melanogaster

In this paper, we define temporal and spatial subdivisions of the embryonic head mesoderm and describe the fate of the main lineages derived from this tissue. During gastrulation, only a fraction of the head mesoderm (primary head mesoderm; PHM) invaginates as the anterior part of the ventral furrow. The PHM can be subdivided into four linearly arranged domains, based on the expression of different combinations of genetic markers (tinman, heartless, snail, serpent, mef-2, zfh-1). The anterior domain (PHMA) produces a variety of cell types, among them the neuroendocrine gland (corpus cardiacum). PHMB, forming much of the "T-bar" of the ventral furrow, migrates anteriorly and dorsally and gives rise to the dorsal pharyngeal musculature. PHMC is located behind the T-bar and forms part of the anterior endoderm, besides contributing to hemocytes. The most posterior domain, PHMD, belongs to the anterior gnathal segments and gives rise to a few somatic muscles, but also to hemocytes. The procephalic region flanking the ventral furrow also contributes to head mesoderm (secondary head mesoderm, SHM) that segregates from the surface after the ventral furrow has invaginated, indicating that gastrulation in the procephalon is much more protracted than in the trunk. We distinguish between an early SHM (eSHM) that is located on either side of the anterior endoderm and is the major source of hemocytes, including crystal cells. The eSHM is followed by the late SHM (lSHM), which consists of an anterior and posterior component (lSHMa, lSHMp). The lSHMa, flanking the stomodeum anteriorly and laterally, produces the visceral musculature of the esophagus, as well as a population of tinman-positive cells that we interpret as a rudimentary cephalic aorta ("cephalic vascular rudiment"). The lSHM contributes hemocytes, as well as the nephrocytes forming the subesophageal body, also called garland cells.

Loss of imprinting of Igf2 alters intestinal maturation and tumorigenesis in mice

Loss of imprinting (LOI) of the insulin-like growth factor II gene (IGF2) is an epigenetic alteration that results in a modest increase in IGF2 expression, and it is present in the normal colonic mucosa of about 30% of patients with colorectal cancer. To investigate its role in intestinal tumorigenesis, we created a mouse model of Igf2 LOI by crossing female H19+/- mice with male Apc+/Min mice. Mice with LOI developed twice as many intestinal tumors as did control littermates. Notably, these mice also showed a shift toward a less differentiated normal intestinal epithelium, reflected by an increase in crypt length and increased staining with progenitor cell markers. A similar shift in differentiation was seen in the normal colonic mucosa of humans with LOI. Thus, altered maturation of nonneoplastic tissue may be one mechanism by which epigenetic changes affect cancer risk.

Two functionally identical modular enhancers in Drosophila troponin T gene establish the correct protein levels in different muscle types

The control of muscle-specific expression is one of the principal mechanisms by which diversity is generated among muscle types. In an attempt to elucidate the regulatory mechanisms that control fiber diversity in any given muscle, we have focused our attention on the transcriptional regulation of the Drosophila Troponin T gene. Two, nonredundant, functionally identical, enhancer-like elements activate Troponin T transcription independently in all major muscles of the embryo and larvae as well as in adult somatic and visceral muscles. Here, we propose that the differential but concerted interaction of these two elements underlies the mechanism by which a particular muscle-type establish the correct levels of Troponin T expression, adapting these levels to their specific needs. This mechanism is not exclusive to the Troponin T gene, but is also relevant to the muscle-specific Troponin I gene. In conjunction with in vivo transgenic studies, an in silico analysis of the Troponin T enhancer-like sequences revealed that both these elements are organized in a modular manner. Extending this analysis to the Troponin I and Tropomyosin regulatory elements, the two other components of the muscle-regulatory complex, we have discovered a similar modular organization of phylogenetically conserved domains.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Developmental genomic approaches in model organisms

Functional genomics technologies can help decipher how information encoded in the genome is translated into morphology, physiology, and behavior during the development of complex organisms. A number of researchers have begun to apply DNA microarrays and other functional genomics approaches to study development. Here we review recent studies that take the first steps toward relating genome-wide information to developmental events, we discuss recent genomics approaches taken in animal model systems used to study human disease, and we outline methods that may be useful for constructing genome-wide maps of developmental processes.

The patterns of wingless, decapentaplegic, and tinman position the Drosophila heart

Two secreted signaling molecules, wingless (wg) and decapentaplegic (dpp), are required to specify the heart in Drosophila. wg and dpp are also required to specify other cell types within the mesoderm and in many other regions of the embryo. Because the spatial patterns of wg and dpp are dynamic, different populations of mesodermal cells are exposed to different combinations of wg and/or dpp at different times. To determine whether the patterns of wg and dpp expression provide unique positional information for the specification of heart precursors, we altered these patterns. Our data suggest that wg and dpp contribute progressively to the elaboration of the expression pattern of the mesoderm-specific homeobox-containing gene tinman (tin), and that the overlap of wg and dpp at an early stage (9) as well as at a later stage (11) in the presence of tin-expressing cells directs cardiac-specific differentiation. Furthermore, ectopic tin expression in the ectoderm at wg/dpp intersects (the primordia of the thoracic imaginal disks) also leads to cardiac-specific differentiation, suggesting that tin confers mesoderm-specificity to the wg/dpp response. We conclude that ectopic heart can be generated by altering the patterns of wg and dpp within the tin-expressing mesoderm, or by ectopic induction of tin within the wg- and dpp-expressing ectoderm.

Muscle differentiation: how two cells become one

A key feature of myogenesis is the fusion of myoblasts to form multinucleate myotubes. Recent work in Drosophila has uncovered a collection of genes that operate at different stages of this process. Some interactions between them have been described that begin to define links from outside the cell via the plasma membrane to the cytoskeleton. Future studies will establish the extent to which the molecular mechanisms of myoblast fusion are conserved between Drosophila and other animals, as found in other aspects of myogenesis.

Patterns of gene expression during Drosophila mesoderm development

The transcription factor Twist initiates Drosophila mesoderm development, resulting in the formation of heart, somatic muscle, and other cell types. Using a Drosophila embryo sorter, we isolated enough homozygous twist mutant embryos to perform DNA microarray experiments. Transcription profiles of twist loss-of-function embryos, embryos with ubiquitous twist expression, and wild-type embryos were compared at different developmental stages. The results implicate hundreds of genes, many with vertebrate homologs, in stage-specific processes in mesoderm development. One such gene, gleeful, related to the vertebrate Gli genes, is essential for somatic muscle development and sufficient to cause neural cells to express a muscle marker.

Characterization and mutant analysis of the Drosophila sema 5c gene

Class V semaphorins are transmembrane glycoproteins characterised by the presence of thrombospondin type I (Tsp) repeats linked to their extracellular semaphorin domain. Sema 5C is the only class V semaphorin found in Drosophila. Dsema 5C RNA is maternally provided and its embryonic expression is prominent in the mesoderm and muscle attachment sites. Here, we show that DSema 5C exists in two protein isoforms as a result of alternative splicing and that both protein and RNA have similar expression patterns. Using a combination of various molecular markers, we show that the DSema 5C protein becomes enriched in mesodermal cells that would normally give rise to fat body and visceral structures. In late embryos, DSema 5C is expressed in segment boundary cells that would constitute subsets of muscle attachment sites. Both RNA and protein are excluded from the somatic precursors and the mature muscles. The expression data suggest DSema 5C localised to the epidermal component of muscle attachment sites. Mutations in Dsema 5C were isolated from a P-element excision screen and by blotting analysis. The Dsema 5C mutants are homozygous viable and show no obvious embryonic phenotypes, suggesting that the maternal and zygotic components of Dsema 5C are not essential for fly development.

serpent, a GATA-like transcription factor gene, induces fat-cell development in Drosophila melanogaster

The GATA-like transcription factor gene serpent is necessary for embryonic fat-cell differentiation in Drosophila (Sam, S., Leise, W. and Hoshizaki, D. K. (1996) Mech. Dev. 60, 197–205) and has been proposed to function in a cell-fate choice between fat cell and somatic gonadal precursors (Moore, L. A., Broihier, H. T., Van Doren, M. and Lehmann, R. (1998) Development 125, 837–44; Riechmann, V., Irion, U., Wilson, R., Grosskortenhaus, R. and Leptin, M. (1997) Development 124, 2915–22). Here, we report that deregulated expression of serpent in the mesoderm induces the formation of ectopic fat cells and prevents the migration and coalescence of the somatic gonadal precursors. The ectopic fat cells do not arise from hyperproliferation of the primary fat-cell clusters but they do associate with the endogenous fat cells to form a fat body that is expanded in both the dorsal/ventral and anterior/posterior axes. Misexpression of serpent also affects the differentiation of muscle cells. Few body-wall muscle precursors are specified and there is a loss of most body-wall muscle fibers. The precursors of the visceral mesoderm are also absent and concomitantly the visceral muscle is absent. We suggest that the ectopic fat cells might originate from cells that have the potential, but do not normally, differentiate into fat cells or from cells that have acquired a fat-cell fate. In light of our results, we discuss the role of serpent in fat-cell specification and in cell fate choices.

Control of Drosophila paramyosin/miniparamyosin gene expression. Differential regulatory mechanisms for muscle-specific transcription

To define the transcriptional mechanisms contributing to stage- and tissue-specific expression of muscle genes, we performed transgenic analysis of Drosophila paramyosin gene regulation. This gene has two promoters, one for paramyosin and one for miniparamyosin, which are active in partially overlapping domains. Regions between -0.9 and -1.7 kilobases upstream of each initiation site contribute to the temporal and spatial expression patterns. By comparing the Drosophila melanogaster and Drosophila virilis promoters, conserved binding sites were found for known myogenic factors, including one MEF2 site and three E boxes. In contrast with previous data, our experiments with the paramyosin promoter indicate that the MEF2 site is essential but not sufficient for proper paramyosin gene transcription. Mutations in the three E boxes, on the other hand, do not produce any effect in embryonic/larval muscles. Thus MEF2 site- and E box-binding proteins can play different roles in the regulation of different muscle-specific genes. For the miniparamyosin promoters, several conserved sequences were shown to correspond to functionally important regions. Our data further show that the two promoters work independently. Even when both promoters are active in the same muscle fiber, the transcription driven by one of the promoters is not affected by transcription driven by the other.

Spatially and temporally regulated expression of myosin heavy chain alternative exons during Drosophila embryogenesis

We used alternative exon-specific probes to determine the accumulation of transcripts encoding myosin heavy chain (MHC) isoforms in Drosophila melanogaster embryos. Six isoforms accumulate in body wall muscles. Transverse (external) muscles express a different major form than intermediate and internal muscles, suggesting different physiological properties. Cardioblasts express one of the somatic muscle transcripts; visceral muscles express at least two transcript types. The pharyngeal muscle accumulates a unique Mhc transcript, suggesting unique contractile abilities. Mhc transcription begins in stage 12 in visceral and somatic muscles, but as late as stage 15 in cardioblasts. This is the first study of myosin isoform localization during insect embryogenesis, and forms the basis for transgenic and biochemical experiments designed to determine how MHC domains regulate muscle physiology.

Hindgut visceral mesoderm requires an ectodermal template for normal development in Drosophila

During Drosophila embryogenesis, the development of the midgut endoderm depends on interactions with the overlying visceral mesoderm. Here we show that the development of the hindgut also depends on cellular interactions, in this case between the inner ectoderm and outer visceral mesoderm. In this section of the gut, the ectoderm is essential for the proper specification and differentiation of the mesoderm, whereas the mesoderm is not required for the normal development of the ectoderm. Wingless and the fibroblast growth factor receptor Heartless act over sequential but interdependent phases of hindgut visceral mesoderm development. Wingless is required to establish the primordium and to enhance Heartless expression. Later, Heartless is required to promote the proper differentiation of the hindgut visceral mesoderm itself.

Wingless effects mesoderm patterning and ectoderm segmentation events via induction of its downstream target sloppy paired

Inactivation of either the secreted protein Wingless (Wg) or the forkhead domain transcription factor Sloppy Paired (Slp) has been shown to produce similar effects in the developing Drosophila embryo. In the ectoderm, both gene products are required for the formation of the segmental portions marked by naked cuticle. In the mesoderm, Wg and Slp activities are crucial for the suppression of bagpipe (bap), and hence visceral mesoderm formation, and the promotion of somatic muscle and heart formation within the anterior portion of each parasegment. In this report, we show that, during these developmental processes, wg and slp act in a common pathway in which slp serves as a direct target of Wg signals that mediates Wg effects in both germ layers. We present evidence that the induction of slp by Wg involves binding of the Wg effector Pangolin (Drosophila Lef-1/TCF) to multiple binding sites within a Wg-responsive enhancer that is located in 5′ flanking regions of the slp1 gene. Based upon our genetic and molecular analysis, we conclude that Wg signaling induces striped expression of Slp in the mesoderm. Mesodermal Slp is then sufficient to abrogate the induction of bagpipe by Dpp/Tinman, which explains the periodic arrangement of trunk visceral mesoderm primordia in wild type embryos. Conversely, mesodermal Slp is positively required, although not sufficient, for the specification of somatic muscle and heart progenitors. We propose that Wg-induced slp provides striped mesodermal domains with the competence to respond to subsequent slp-independent Wg signals that induce somatic muscle and heart progenitors. We also propose that in wg-expressing ectodermal cells, slp is an integral component in an autocrine feedback loop of Wg signaling.

Characterization of a novel subset of cardiac cells and their progenitors in the Drosophila embryo

The Drosophila heart is a simple organ composed of two major cell types: cardioblasts, which form the simple contractile tube of the heart, and pericardial cells, which flank the cardioblasts. A complete understanding of Drosophila heart development requires the identification of all cell types that comprise the heart and the elucidation of the cellular and genetic mechanisms that regulate the development of these cells. Here, we report the identification of a new population of heart cells: the Odd skipped-positive pericardial cells (Odd-pericardial cells). We have used descriptive, lineage tracing and genetic assays to clarify the cellular and genetic mechanisms that control the development of Odd-pericardial cells. Odd skipped marks a population of four pericardial cells per hemisegment that are distinct from previously identified heart cells. We demonstrate that within a hemisegment, Odd-pericardial cells develop from three heart progenitors and that these heart progenitors arise in multiple anteroposterior locations within the dorsal mesoderm. Two of these progenitors divide asymmetrically such that each produces a two-cell mixed-lineage clone of one Odd-pericardial cell and one cardioblast. The third progenitor divides symmetrically to produce two Odd-pericardial cells. All remaining cardioblasts in a hemisegment arise from two cardioblast progenitors each of which produces two cardioblasts. Furthermore, we demonstrate that numb and sanpodo mediate the asymmetric divisions of the two mixed-lineage heart progenitors noted above.

The alternative migratory pathways of the Drosophila tracheal cells are associated with distinct subsets of mesodermal cells

The Drosophila tracheal system is a model for the study of the mechanisms that guide cell migration. The general conclusion from many studies is that migration of tracheal cells relies on directional cues provided by nearby cells. However, very little is known about which paths are followed by the migrating tracheal cells and what kind of interactions they establish to move in the appropriate direction. Here we analyze how tracheal cells migrate relative to their surroundings and which tissues participate in tracheal cell migration. We find that cells in different branches exploit different strategies for their migration; while some migrate through preexisting grooves, others make their way through homogeneous cell populations. We also find that alternative migratory pathways of tracheal cells are associated with distinct subsets of mesodermal cells and propose a model for the allocation of groups of tracheal cells to different branches. These results show how adjacent tissues influence morphogenesis of the tracheal system and offer a model for understanding how organ formation is determined by its genetic program and by the surrounding topological constraints.

The Drosophila PAR domain protein 1 (Pdp1) gene encodes multiple differentially expressed mRNAs and proteins through the use of multiple enhancers and promoters

Transcription factors are often expressed at several times and in multiple tissues during development and regulate diverse sets of downstream target genes by varying their combinatorial interactions with other transcription factors. The Drosophila Tropomyosin I (TmI) gene is regulated by a complex of proteins within the enhancer that synergistically interacts with MEF2 to activate TmI transcription as muscle cells fuse and differentiate. One of the components of this complex is PDP1 (PAR domain protein 1), a basic leucine zipper transcription factor that is highly homologous to three vertebrate genes that are members of the PAR domain subfamily. We have isolated and describe here the structure of the Pdp1 gene. The Pdp1 gene is complex, containing at least four transcriptional start sites and producing at least six different mRNAs and PDP1 isoforms. Five of the PDP1 isoforms differ by the substitution or insertion of amino acids at or near the N-terminal of the protein. At least three of these alternately spliced transcripts are differentially expressed in different tissues of the developing embryo in which PDP1 expression is correlated with the differentiation of different cell types. A sixth isoform is produced by splicing out part of the PAR and basic DNA binding domains, and DNA binding and transient transfection experiments suggest that it functions as a dominant negative inhibitor of transcription. Furthermore, two enhancers have been identified within the gene that express in the somatic mesodermal precursors to body wall muscles and fat body and together direct expression in other tissues that closely mimics that of the endogenous gene. These results show that Pdp1 is widely expressed, including in muscle, fat, and gut precursors, and is likely involved in the transcriptional control of different developmental pathways through the use of differentially expressed PDP1 isoforms. Furthermore, the similarities between Pdp1 and the other PAR domain genes suggest that Pdp1 is the homologue of the vertebrate genes.

Migration of the Drosophila primordial midgut cells requires coordination of diverse PS integrin functions

Cell migration during embryogenesis involves two populations of cells: the migrating cells and the underlying cells that provide the substratum for migration. The formation of the Drosophila larval midgut involves the migration of the primordial midgut cells along a visceral mesoderm substratum. We show that integrin adhesion receptors are required in both populations of cells for normal rates of migration. In the absence of the PS integrins, the visceral mesoderm is disorganised, the primordial midgut cells do not display their normal motile appearance and their migration is delayed by 2 hours. Removing PS integrin function from the visceral mesoderm alone results in visceral mesoderm disorganization, but only causes a modest delay in migration and does not affect the appearance of the migrating cells. Removing PS integrin function from the migrating cells causes as severe a delay in migration as the complete loss of PS integrin function. The functions of PS1 and PS2 are specific in the two tissues, endoderm and mesoderm, since they cannot substitute for each other. In addition there is a partial redundancy in the function of the two PS integrins expressed in the endoderm, PS1 (alphaPS1betaPS) and PS3 (alphaPS3betaPS), since loss of just one alpha subunit in the midgut results in either a modest delay (alphaPS1) or no effect (alphaPS3). We have also examined the roles of small GTPases in promoting migration of the primordial midgut cells. We find that dominant negative (N17) versions of Rac and Cdc42 cause a very similar defect in migration as loss of integrins, while those of Rho and Ras have no effect. Thus integrins are involved in mediating migration by creating an optimal substratum for adhesion, adhering to that substratum and possibly by activating Rac and Cdc42.

Controls in patterning and diversification of somatic muscles during Drosophila embryogenesis

Recent genetic studies in Drosophila have provided important insights into the pathways determining the formation and diversification of body wall muscles. These pathways control a progressive subdivision of the mesoderm, ultimately leading to the specification of individual cells, the muscle founders, which are endowed with genetic programs capable of generating distinct muscle fibers. A network of activities of transcriptional regulators, signaling pathways, and lineage genes is beginning to emerge which controls successive steps of this muscle patterning and differentiation process.

The heterotrimeric protein Go is required for the formation of heart epithelium in Drosophila

The gene encoding the alpha subunit of the Drosophila Go protein is expressed early in embryogenesis in the precursor cells of the heart tube, of the visceral muscles, and of the nervous system. This early expression coincides with the onset of the mesenchymal-epithelial transition to which are subjected the cardial cells and the precursor cells of the visceral musculature. This gene constitutes an appropriate marker to follow this transition. In addition, a detailed analysis of its expression suggests that the cardioblasts originate from two subpopulations of cells in each parasegment of the dorsal mesoderm that might depend on the wingless and hedgehog signaling pathways for both their determination and specification. In the nervous system, the expression of Goalpha shortly precedes the beginning of axonogenesis. Mutants produced in the Goalpha gene harbor abnormalities in the three tissues in which the gene is expressed. In particular, the heart does not form properly and interruptions in the heart epithelium are repeatedly observed, henceforth the brokenheart (bkh) name. Furthermore, in the bkh mutant embryos, the epithelial polarity of cardial cells was not acquired (or maintained) in various places of the cardiac tube. We predict that bkh might be involved in vesicular traffic of membrane proteins that is responsible for the acquisition of polarity.

Drosophila mef2 expression during mesoderm development is controlled by a complex array of cis-acting regulatory modules

The function of the Drosophila mef2 gene, a member of the MADS box supergene family of transcription factors, is critical for terminal differentiation of the three major muscle cell types, namely somatic, visceral, and cardiac. During embryogenesis, mef2 undergoes multiple phases of expression, which are characterized by initial broad mesodermal expression, followed by restricted expression in the dorsal mesoderm, specific expression in muscle progenitors, and sustained expression in the differentiated musculatures. In this study, evidence is presented that temporally and spatially specific mef2 expression is controlled by a complex array of cis-acting regulatory modules that are responsive to different genetic signals. Functional testing of approximately 12 kb of 5' flanking region of the mef2 gene showed that the initial widespread mesodermal expression is achieved through a 280-bp twist-dependent enhancer. The subsequent dorsal mesoderm-restricted mef2 expression is mediated through a 460-bp dpp-responsive regulatory module, which involves the function of the Smad4 homolog Medea and contains several binding sites for Medea and Mad. The analysis also showed that regulated mef2 expression in the caudal and trunk visceral mesoderm, which give rise to longitudinal and circular gut musculatures, respectively, is under the control of distinct enhancer elements. In addition, mef2 expression in the cardioblasts of the heart is dependent upon at least two distinct enhancers, which are active at different periods during embryogenesis. Moreover, multiple regulatory elements are differentially activated for specific expression in presumptive muscle founders, prefusion myoblasts, and differentiated muscle fibers. Taken together, the presented data suggest that specific expression of the mef2 gene in myogenic lineages in the Drosophila embryo is the result of multiple genetic inputs that act in an additive manner upon distinct enhancers in the 5' flanking region.

Combinatorial signaling codes for the progressive determination of cell fates in the Drosophila embryonic mesoderm

Mesodermal progenitors arise in the Drosophila embryo from discrete clusters of lethal of scute (l'sc)-expressing cells. Using both genetic loss-of-function and targeted ectopic expression approaches, we demonstrate here that individual progenitors are specified by the sequential deployment of unique combinations of intercellular signals. Initially, the intersection between the Wingless (Wg) and Decapentaplegic (Dpp) expression domains demarcate an ectodermal prepattern that is imprinted on the adjacent mesoderm in the form of a L'sc precluster. All mesodermal cells within this precluster are competent to respond to a subsequent instructive signal mediated by two receptor tyrosine kinases (RTKs), the Drosophila epidermal growth factor receptor (DER) and the Heartless (Htl) fibroblast growth factor receptor. By monitoring the expression of the diphosphorylated form of mitogen-associated protein kinase (MAPK), we found that these RTKs are activated in small clusters of cells within the original competence domain. Each cluster represents an equivalence group because all members initially resemble progenitors in their expression of both L'sc and mesodermal identity genes. Thus, localized RTK activity induces the formation of mesodermal equivalence groups. The RTKs remain active in the single progenitor that emerges from each cluster under the subsequent inhibitory influence of the neurogenic genes. Moreover, DER and Htl are differentially involved in the specification of particular progenitors. We conclude that distinct cellular identity codes are generated by the combinatorial activities of Wg, Dpp, EGF, and FGF signals in the progressive determination of embryonic mesodermal cells.

A role for nautilus in the differentiation of muscle precursors

In the Drosophila embryo, nautilus is expressed in a subset of muscle precursors and differentiated fibers and is capable of inducing muscle-specific transcription, as well as myogenic transformation. In this study, we examine the consequences of nautilus loss-of-function on the development of the somatic musculature. Genetic and molecular characterization of two overlapping deficiencies, Df(3R)nau-9 and Df(3R)nau-11a4, revealed that both of these deficiencies remove the nautilus gene without affecting a common lethal complementation group. Individuals transheterozygous for these deficiencies survive to adulthood, indicating that nautilus is not an essential gene. These embryos are, however, missing a subset of muscle fibers, providing evidence that (1) some muscle loss can be tolerated throughout larval development and (2) nautilus does play a role in muscle development. Examination of muscle precursors in these embryos revealed that nautilus is not required for the formation of muscle precursors, but rather plays a role in their differentiation into mature muscle fibers. Thus, we suggest that nautilus functions in a subset of muscle precursors to implement their specific differentiation programs.

Signalling by the Drosophila epidermal growth factor receptor is required for the specification and diversification of embryonic muscle progenitors

Muscle development initiates in the Drosophila embryo with the segregation of single progenitor cells, from which a complete set of myofibres arises. Each progenitor is assigned a unique fate, characterized by the expression of particular identity genes. We now demonstrate that the Drosophila epidermal growth factor receptor provides an inductive signal for the specification of a large subset of muscle progenitors. In the absence of the receptor or its ligand, SPITZ, specific progenitors fail to segregate. The resulting unspecified mesodermal cells undergo programmed cell death. In contrast, receptor hyperactivation generates supernumerary progenitors, as well as the duplication of at least one SPITZ-dependent myofibre. The development of individual muscles is differentially sensitive to variations in the level of signalling by the epidermal growth factor receptor. Such graded myogenic effects can be influenced by alterations in the functions of Star and rhomboid. In addition, muscle patterning is dependent on the generation of a spatially restricted, activating SPITZ signal, a process that may rely on the localized mesodermal expression of RHOMBOID. Thus, the epidermal growth factor receptor contributes both to muscle progenitor specification and to the diversification of muscle identities.