Spatially restricted activity of a Drosophila lipid phosphatase guides migrating germ cells

Regulation of cellular and systemic sphingolipid homeostasis

One hundred and fifty years ago, Johann Thudichum described sphingolipids as unusual "Sphinx-like" lipids from the brain. Today, we know that thousands of sphingolipid molecules mediate many essential functions in embryonic development and normal physiology. In addition, sphingolipid metabolism and signalling pathways are dysregulated in a wide range of pathologies, and therapeutic agents that target sphingolipids are now used to treat several human diseases. However, our understanding of sphingolipid regulation at cellular and organismal levels and their functions in developmental, physiological and pathological settings is rudimentary. In this Review, we discuss recent advances in sphingolipid pathways in different organelles, how secreted sphingolipid mediators modulate physiology and disease, progress in sphingolipid-targeted therapeutic and diagnostic research, and the trans-cellular sphingolipid metabolic networks between microbiota and mammals. Advances in sphingolipid biology have led to a deeper understanding of mammalian physiology and may lead to progress in the management of many diseases.

The journey of a generation: advances and promises in the study of primordial germ cell migration

The germline provides the genetic and non-genetic information that passes from one generation to the next. Given this important role in species propagation, egg and sperm precursors, called primordial germ cells (PGCs), are one of the first cell types specified during embryogenesis. In fact, PGCs form well before the bipotential somatic gonad is specified. This common feature of germline development necessitates that PGCs migrate through many tissues to reach the somatic gonad. During their journey, PGCs must respond to select environmental cues while ignoring others in a dynamically developing embryo. The complex multi-tissue, combinatorial nature of PGC migration is an excellent model for understanding how cells navigate complex environments in vivo. Here, we discuss recent findings on the migratory path, the somatic cells that shepherd PGCs, the guidance cues somatic cells provide, and the PGC response to these cues to reach the gonad and establish the germline pool for future generations. We end by discussing the fate of wayward PGCs that fail to reach the gonad in diverse species. Collectively, this field is poised to yield important insights into emerging reproductive technologies.

Astrocyte growth is driven by the Tre1/S1pr1 phospholipid-binding G protein-coupled receptor

Astrocytes play crucial roles in regulating neural circuit function by forming a dense network of synapse-associated membrane specializations, but signaling pathways regulating astrocyte morphogenesis remain poorly defined. Here, we show the Drosophila lipid-binding G protein-coupled receptor (GPCR) Tre1 is required for astrocytes to establish their intricate morphology in vivo. The lipid phosphate phosphatases Wunen/Wunen2 also regulate astrocyte morphology and, via Tre1, mediate astrocyte-astrocyte competition for growth-promoting lipids. Loss of s1pr1, the functional analog of Tre1 in zebrafish, disrupts astrocyte process elaboration, and live imaging and pharmacology demonstrate that S1pr1 balances proper astrocyte process extension/retraction dynamics during growth. Loss of Tre1 in flies or S1pr1 in zebrafish results in defects in simple assays of motor behavior. Tre1 and S1pr1 are thus potent evolutionarily conserved regulators of the elaboration of astrocyte morphological complexity and, ultimately, astrocyte control of behavior.

Disruptions in cell fate decisions and transformed enteroendocrine cells drive intestinal tumorigenesis in Drosophila

Most epithelial tissues are maintained by stem cells that produce the different cell lineages required for proper tissue function. Constant communication between different cell types ensures precise regulation of stem cell behavior and cell fate decisions. These cell-cell interactions are often disrupted during tumorigenesis, but mechanisms by which they are co-opted to support tumor growth in different genetic contexts are poorly understood. Here, we introduce PromoterSwitch, a genetic platform we established to generate large, transformed clones derived from individual adult Drosophila intestinal stem/progenitor cells. We show that cancer-driving genetic alterations representing common colon tumor genome landscapes disrupt cell fate decisions within transformed tissue and result in the emergence of abnormal cell fates. We also show that transformed enteroendocrine cells, a differentiated, hormone-secreting cell lineage, support tumor growth by regulating intestinal stem cell proliferation through multiple genotype-dependent mechanisms, which represent potential vulnerabilities that could be exploited for therapy.

Hedgehog signaling guides migration of primordial germ cells to the Drosophila somatic gonad

In addition to inducing nonautonomous specification of cell fate in both Drosophila and vertebrates, the Hedgehog pathway guides cell migration in a variety of different tissues. Although its role in axon guidance in the vertebrate nervous system is widely recognized, its role in guiding the migratory path of primordial germ cells (PGCs) from the outside surface of the Drosophila embryo through the midgut and mesoderm to the SGPs (somatic gonadal precursors) has been controversial. Here we present new experiments demonstrating (1) that Hh produced by mesodermal cells guides PGC migration, (2) that HMG CoenzymeA reductase (Hmgcr) potentiates guidance signals emanating from the SGPs, functioning upstream of hh and of 2 Hh pathway genes important for Hh-containing cytonemes, and (3) that factors required in Hh receiving cells in other contexts function in PGCs to help direct migration toward the SGPs. We also compare the data reported by 4 different laboratories that have studied the role of the Hh pathway in guiding PGC migration.

Beyond energy and growth: the role of metabolism in developmental signaling, cell behavior and diapause

Metabolism is crucial for development through supporting cell growth, energy production, establishing cell identity, developmental signaling and pattern formation. In many model systems, development occurs alongside metabolic transitions as cells differentiate and specialize in metabolism that supports new functions. Some cells exhibit metabolic flexibility to circumvent mutations or aberrant signaling, whereas other cell types require specific nutrients for developmental progress. Metabolic gradients and protein modifications enable pattern formation and cell communication. On an organism level, inadequate nutrients or stress can limit germ cell maturation, implantation and maturity through diapause, which slows metabolic activities until embryonic activation under improved environmental conditions.

A targeted genetic modifier screen in Drosophila uncovers vulnerabilities in a genetically complex model of colon cancer

Received on 16 January 2023; accepted on 21 February 2023Kinases are key regulators of cellular signal transduction pathways. Many diseases, including cancer, are associated with global alterations in protein phosphorylation networks. As a result, kinases are frequent targets of drug discovery efforts. However, target identification and assessment, a critical step in targeted drug discovery that involves identifying essential genetic mediators of disease phenotypes, can be challenging in complex, heterogeneous diseases like cancer, where multiple concurrent genomic alterations are common. Drosophila is a particularly useful genetic model system to identify novel regulators of biological processes through unbiased genetic screens. Here, we report 2 classic genetic modifier screens focusing on the Drosophila kinome to identify kinase regulators in 2 different backgrounds: KRAS TP53 PTEN APC, a multigenic cancer model that targets 4 genes recurrently mutated in human colon tumors and KRAS alone, a simpler model that targets one of the most frequently altered pathways in cancer. These screens identified hits unique to each model and one shared by both, emphasizing the importance of capturing the genetic complexity of human tumor genome landscapes in experimental models. Our follow-up analysis of 2 hits from the KRAS-only screen suggests that classical genetic modifier screens in heterozygous mutant backgrounds that result in a modest, nonlethal reduction in candidate gene activity in the context of a whole animal-a key goal of systemic drug treatment-may be a particularly useful approach to identify the most rate-limiting genetic vulnerabilities in disease models as ideal candidate drug targets.

Imaginal disk growth factors are Drosophila chitinase-like proteins with roles in morphogenesis and CO2 response

Chitinase-like proteins (CLPs) are members of the family 18 glycosyl hydrolases, which include chitinases and the enzymatically inactive CLPs. A mutation in the enzyme's catalytic site, conserved in vertebrates and invertebrates, allowed CLPs to evolve independently with functions that do not require chitinase activity. CLPs normally function during inflammatory responses, wound healing, and host defense, but when they persist at excessive levels at sites of chronic inflammation and in tissue-remodeling disorders, they correlate positively with disease progression and poor prognosis. Little is known, however, about their physiological function. Drosophila melanogaster has 6 CLPs, termed Imaginal disk growth factors (Idgfs), encoded by Idgf1, Idgf2, Idgf3, Idgf4, Idgf5, and Idgf6. In this study, we developed tools to facilitate characterization of the physiological roles of the Idgfs by deleting each of the Idgf genes using the CRISPR/Cas9 system and assessing loss-of-function phenotypes. Using null lines, we showed that loss of function for all 6 Idgf proteins significantly lowers viability and fertility. We also showed that Idgfs play roles in epithelial morphogenesis, maintaining proper epithelial architecture and cell shape, regulating E-cadherin and cortical actin, and remarkably, protecting these tissues against CO2 exposure. Defining the normal molecular mechanisms of CLPs is a key to understanding how deviations tip the balance from a physiological to a pathological state.

Plpp3, a novel regulator of pluripotency exit and endodermal differentiation of mouse embryonic stem cells

In recent decades, study of the actions of bioactive lipids such as lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) has increased since they are involved in regulating many processes, including self-renewal of embryonic stem cells, embryo development and cancer. Phospholipid phosphatase type 3 (PLPP3) has been shown to be a key player in regulating the balance of these lipids and, in consequence, their signaling. Different lines of evidence suggest that PLPP3 could play a role in endoderm development. To approach this hypothesis, we used mouse embryonic stem cells (ESC) as a model to study Plpp3 function in self-renewal and the transition towards differentiation. We found that lack of PLPP3 mainly affects endoderm formation during differentiation of suspension-formed embryoid bodies. PLPP3-deficient ESC strongly decrease the amount of FOXA2-expressing cells and fail to properly downregulate the expression of pluripotency factors when subjected to an endoderm-directed differentiation protocol. Impaired endoderm differentiation correlated with a transient reduction in nuclear localization of YAP1. These phenotypes were rescued by transiently restoring the expression of catalytically active hPLPP3. In conclusion, PLPP3 plays a role in downregulating pluripotency-associated factors and in endodermal differentiation. PLPP3 regulates proper lipid/YAP1 signaling required for endodermal differentiation.

Wun2-mediated integrin recycling promotes apoptotic cell clearance in Drosophila melanogaster

Apoptotic cell (AC) clearance is a complex process in which phagocytes recognize, engulf, and digest ACs during organismal development and tissue homeostasis. Impaired efferocytosis results in developmental defects and autoimmune diseases. In the current study, we performed RNA-sequencing to systematically identify regulators involved in the phagocytosis of ACs by Drosophila melanogaster macrophage-like S2 cells, followed by targeted RNA interference screening. Wunen2 (Wun2), a homolog of mammalian lipid phosphate phosphatase (LPP), was deemed as required for efferocytosis both in vitro and in vivo. However, efferocytosis was independent of Wun2 phosphatase activity. Proteomic analysis further revealed that Rab11 and its effector Rip11 are interaction partners of Wun2. Therefore, Wun2 collaborates with Rip11 and Rab11 to mediate efficient recycling of the phagocytic receptor βν integrin subunit to the plasma membrane. The loss of Wun2 results in the routing of βv integrin subunit (Itgbn) into lysosomes, leading to its degradation. The deficiency of βv integrin subunit on the cell surface leads to aberrant and disorganized actin cytoskeleton, thereby influencing the formation of macrophage pseudopodia toward ACs and thus failure to engulf them. The findings of this study provide insights that clarify how phagocytes coordinate AC signals and adopt a precise mechanism for the maintenance of engulfment receptors at their cell membrane surface to regulate efferocytosis.

The great small organisms of developmental genetics: Caenorhabditis elegans and Drosophila melanogaster

Experimental embryologists working at the turn of the 19th century suggested fundamental mechanisms of development, such as localized cytoplasmic determinants and tissue induction. However, the molecular basis underlying these processes proved intractable for a long time, despite concerted efforts in many developmental systems to isolate factors with a biological role. That road block was overcome by combining developmental biology with genetics. This powerful approach used unbiased genome-wide screens to isolate mutants with developmental defects and to thereby identify genes encoding key determinants and regulatory pathways that govern development. Two small invertebrates were the pioneers: the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. Their modes of development differ in many ways, but the two together led the way to unraveling the molecular mechanisms of many fundamental developmental processes. The discovery of the grand homologies between key players in development throughout the animal kingdom underscored the usefulness of studying these small invertebrate models for animal development and even human disease. We describe developmental genetics in Drosophila and C. elegans up to the rise of genomics at the beginning of the 21st Century. Finally, we discuss themes that emerge from the histories of such distinct organisms and prospects of this approach for the future.

CG11426 gene product negatively regulates glial population size in the Drosophila eye imaginal disc

Glial cells play essential roles in the nervous system. Although glial populations are tightly regulated, the mechanisms regulating the population size remain poorly understood. Since Drosophila glial cells are similar to the human counterparts in their functions and shapes, rendering them an excellent model system to understand the human glia biology. Lipid phosphate phosphatases (LPPs) are important for regulating bioactive lipids. In Drosophila, there are three known LPP-encoding genes: wunen, wunen-2, and lazaro. The wunens are important for germ cell migration and survival and septate junction formation during tracheal development. Lazaro is involved in phototransduction. In the present study, we characterized a novel Drosophila LPP-encoding gene, CG11426. Suppression of CG11426 increased glial cell number in the eye imaginal disc during larval development, while ectopic CG11426 expression decreased it. Both types of mutation also caused defects in axon projection to the optic lobe in larval eye-brain complexes. Moreover, CG11426 promoted apoptosis via inhibiting ERK signaling in the eye imaginal disc. Taken together, these findings demonstrated that CG11426 gene product negatively regulates ERK signaling to promote apoptosis for proper maintenance of the glial population in the developing eye disc.

Downregulation of glial genes involved in synaptic function mitigates Huntington's disease pathogenesis

Most research on neurodegenerative diseases has focused on neurons, yet glia help form and maintain the synapses whose loss is so prominent in these conditions. To investigate the contributions of glia to Huntington's disease (HD), we profiled the gene expression alterations of Drosophila expressing human mutant Huntingtin (mHTT) in either glia or neurons and compared these changes to what is observed in HD human and HD mice striata. A large portion of conserved genes are concordantly dysregulated across the three species; we tested these genes in a high-throughput behavioral assay and found that downregulation of genes involved in synapse assembly mitigated pathogenesis and behavioral deficits. To our surprise, reducing dNRXN3 function in glia was sufficient to improve the phenotype of flies expressing mHTT in neurons, suggesting that mHTT's toxic effects in glia ramify throughout the brain. This supports a model in which dampening synaptic function is protective because it attenuates the excitotoxicity that characterizes HD.

When a neuron dies, through injury or disease, the body loses all communication that passes through it. The brain compensates by rerouting the flow of information through other neurons in the network. Eventually, if the loss of neurons becomes too great, compensation becomes impossible. This process happens in Alzheimer's, Parkinson's, and Huntington's disease. In the case of Huntington's disease, the cause is mutation to a single gene known as huntingtin. The mutation is present in every cell in the body but causes particular damage to parts of the brain involved in mood, thinking and movement.

Neurons and other cells respond to mutations in the huntingtin gene by turning the activities of other genes up or down, but it is not clear whether all of these changes contribute to the damage seen in Huntington's disease. In fact, it is possible that some of the changes are a result of the brain trying to protect itself. So far, most research on this subject has focused on neurons because the huntingtin gene plays a role in maintaining healthy neuronal connections. But, given that all cells carry the mutated gene, it is likely that other cells are also involved. The glia are a diverse group of cells that support the brain, providing care and sustenance to neurons. These cells have a known role in maintaining the connections between neurons and may also have play a role in either causing or correcting the damage seen in Huntington's disease.

The aim of Onur et al. was to find out which genes are affected by having a mutant huntingtin gene in neurons or glia, and whether severity of Huntington’s disease improved or worsened when the activity of these genes changed. First, Onur et al. identified genes affected by mutant huntingtin by comparing healthy human brains to the brains of people with Huntington's disease. Repeating the same comparison in mice and fruit flies identified genes affected in the same way across all three species, revealing that, in Huntington's disease, the brain dials down glial cell genes involved in maintaining neuronal connections.

To find out how these changes in gene activity affect disease severity and progression, Onur et al. manipulated the activity of each of the genes they had identified in fruit flies that carried mutant versions of huntingtin either in neurons, in glial cells or in both cell types. They then filmed the flies to see the effects of the manipulation on movement behaviors, which are affected by Huntington’s disease. This revealed that purposely lowering the activity of the glial genes involved in maintaining connections between neurons improved the symptoms of the disease, but only in flies who had mutant huntingtin in their glial cells. This indicates that the drop in activity of these genes observed in Huntington’s disease is the brain trying to protect itself.

This work suggests that it is important to include glial cells in studies of neurological disorders. It also highlights the fact that changes in gene expression as a result of a disease are not always bad. Many alterations are compensatory, and try to either make up for or protect cells affected by the disease. Therefore, it may be important to consider whether drugs designed to treat a condition by changing levels of gene activity might undo some of the body's natural protection. Working out which changes drive disease and which changes are protective will be essential for designing effective treatments.

DNase II mediates a parthanatos-like developmental cell death pathway in Drosophila primordial germ cells

During Drosophila embryonic development, cell death eliminates 30% of the primordial germ cells (PGCs). Inhibiting apoptosis does not prevent PGC death, suggesting a divergence from the conventional apoptotic program. Here, we demonstrate that PGCs normally activate an intrinsic alternative cell death (ACD) pathway mediated by DNase II release from lysosomes, leading to nuclear translocation and subsequent DNA double-strand breaks (DSBs). DSBs activate the DNA damage-sensing enzyme, Poly(ADP-ribose) (PAR) polymerase-1 (PARP-1) and the ATR/Chk1 branch of the DNA damage response. PARP-1 and DNase II engage in a positive feedback amplification loop mediated by the release of PAR polymers from the nucleus and the nuclear accumulation of DNase II in an AIF- and CypA-dependent manner, ultimately resulting in PGC death. Given the anatomical and molecular similarities with an ACD pathway called parthanatos, these findings reveal a parthanatos-like cell death pathway active during Drosophila development.

Cis-acting variation is common across regulatory layers but is often buffered during embryonic development

Precise patterns of gene expression are driven by interactions between transcription factors, regulatory DNA sequences, and chromatin. How DNA mutations affecting any one of these regulatory "layers" are buffered or propagated to gene expression remains unclear. To address this, we quantified allele-specific changes in chromatin accessibility, histone modifications, and gene expression in F1 embryos generated from eight Drosophila crosses at three embryonic stages, yielding a comprehensive data set of 240 samples spanning multiple regulatory layers. Genetic variation (allelic imbalance) impacts gene expression more frequently than chromatin features, with metabolic and environmental response genes being most often affected. Allelic imbalance in cis-regulatory elements (enhancers) is common and highly heritable, yet its functional impact does not generally propagate to gene expression. When it does, genetic variation impacts RNA levels through two alternative mechanisms involving either H3K4me3 or chromatin accessibility and H3K27ac. Changes in RNA are more predictive of variation in H3K4me3 than vice versa, suggesting a role for H3K4me3 downstream from transcription. The impact of a substantial proportion of genetic variation is consistent across embryonic stages, with 50% of allelic imbalanced features at one stage being also imbalanced at subsequent developmental stages. Crucially, buffering, as well as the magnitude and evolutionary impact of genetic variants, is influenced by regulatory complexity (i.e., number of enhancers regulating a gene), with transcription factors being most robust to cis-acting, but most influenced by trans-acting, variation.

Evidence that ABC transporter-mediated autocrine export of an eicosanoid signaling molecule enhances germ cell chemotaxis in the colonial tunicate Botryllus schlosseri

The colonial ascidian Botryllus schlosseri regenerates the germline during repeated cycles of asexual reproduction. Germline stem cells (GSCs) circulate in the blood and migrate to new germline niches as they develop and this homing process is directed by a Sphigosine-1-Phosphate (S1P) gradient. Here, we find that inhibition of ABC transporter activity reduces migration of GSCs towards low concentrations of S1P in vitro In addition, inhibiting phospholipase A2 (PLA2) or lipoxygenase (Lox) blocks chemotaxis towards low concentrations of S1P. These effects can be rescued by addition of the 12-Lox product 12-S-HETE. Blocking ABC transporter, PLA2 or 12-Lox activity also inhibits homing of germ cells in vivo Using a live-imaging chemotaxis assay in a 3D matrix, we show that a shallow gradient of 12-S-HETE enhances chemotaxis towards low concentrations of S1P and stimulates motility. A potential homolog of the human receptor for 12-S-HETE, gpr31, is expressed on GSCs and differentiating vasa⁺ germ cells. These results suggest that 12-S-HETE might be an autocrine signaling molecule exported by ABC transporters that enhances chemotaxis in GSCs migrating towards low concentrations of S1P.

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.

Germ cell migration-Evolutionary issues and current understanding

In many organisms, primordial germ cells (PGCs) are specified at a different location than where the gonad forms, meaning that PGCs must migrate toward the gonad within the early developing embryo. Following species-specific paths, PGCs can be passively carried by surrounding tissues and also perform active migration. When PGCs actively migrate through and along a variety of embryonic structures in different organisms, they adopt an ancestral robust migration mode termed "amoeboid motility", which allows cells to migrate within diverse environments. In this review, we discuss the possible significance of the PGC migration process in facilitating the evolution of animal body shape. In addition, we summarize the latest findings relevant for the molecular and cellular mechanisms controlling the movement and the directed migration of PGCs in different species.

Hmgcr promotes a long-range signal to attract Drosophila germ cells independently of Hedgehog

During development, many cell types migrate along stereotyped routes determined through deployment of cell surface or secreted guidance molecules. Although we know the identity of many of these molecules, the distances over which they natively operate can be difficult to determine. Here, we have quantified the range of an attractive signal for the migration of Drosophila germ cells. Their migration is guided by an attractive signal generated by the expression of genes in the 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (Hmgcr) pathway, and by a repulsive signal generated by the expression of Wunens. We demonstrate that the attractive signal downstream of Hmgcr is cell-contact independent and acts at long range, the extent of which depends on Hmgcr levels. This range would be sufficient to reach all of the germ cells for their entire migration. Furthermore, Hmgcr-mediated attraction does not require Wunens but can operate simultaneously with Wunen-mediated repulsion. Finally, several papers posit Hedgehog (Hh) as being the germ cell attractant downstream of H mgcr Here, we provide evidence that this is not the case.

Site-specific transgenesis of the Drosophila melanogaster Y-chromosome using CRISPR/Cas9

Despite the importance of Y-chromosomes in evolution and sex determination, their heterochromatic, repeat-rich nature makes them difficult to sequence (due, in part, to ambiguities in sequence alignment and assembly) and to genetically manipulate. Therefore, they generally remain poorly understood. For example, the Drosophila melanogaster Y-chromosome, one of the most extensively studied Y-chromosomes, is widely heterochromatic and composed mainly of highly repetitive sequences, with only a handful of expressed genes scattered throughout its length. Efforts to insert transgenes on this chromosome have thus far relied on either random insertion of transposons (sometimes harbouring 'landing sites' for subsequent integrations) with limited success or on chromosomal translocations, thereby limiting the types of Y-chromosome-related questions that could be explored. Here, we describe a versatile approach to site-specifically insert transgenes on the Y-chromosome in D. melanogaster via CRISPR/Cas9-mediated homology-directed repair. We demonstrate the ability to insert, and detect expression from, fluorescently marked transgenes at two specific locations on the Y-chromosome, and we utilize these marked Y-chromosomes to detect and quantify rare chromosomal nondisjunction effects. Finally, we discuss how this Y-docking technique could be adapted to other insects to aid in the development of genetic control technologies for the management of insect disease vectors and pests.

RNA Interference Screening for Genes Regulating Drosophila Muscle Morphogenesis

RNA interference (RNAi) is the method of choice to systematically test for gene function in an intact organism. The model organism Drosophila has the advantage that RNAi is cell autonomous, meaning it does not spread from one cell to the next. Hence, RNAi can be performed in a tissue-specific manner by expressing short or long inverted repeat constructs (hairpins) designed to target mRNAs from one specific target gene. This achieves tissue-specific knock-down of a target gene of choice. Here, we detail the methodology to test gene function in Drosophila muscle tissue by expressing hairpins in a muscle-specific manner using the GAL4-UAS system. We further discuss the systematic RNAi resource collections available which also permit large scale screens in a muscle-specific manner. The full power of such screens is revealed by combination of high-throughput assays followed by detailed morphological assays. Together, this chapter should be a practical guide to enable the reader to either test a few candidate genes, or large gene sets for particular functions in Drosophila muscle tissue and provide first insights into the biological process the gene might be important for in muscle.

Patterns of chromatin accessibility along the anterior-posterior axis in the early Drosophila embryo

As the Drosophila embryo transitions from the use of maternal RNAs to zygotic transcription, domains of open chromatin, with relatively low nucleosome density and specific histone marks, are established at promoters and enhancers involved in patterned embryonic transcription. However it remains unclear how regions of activity are established during early embryogenesis, and if they are the product of spatially restricted or ubiquitous processes. To shed light on this question, we probed chromatin accessibility across the anterior-posterior axis (A-P) of early Drosophila melanogaster embryos by applying a transposon based assay for chromatin accessibility (ATAC-seq) to anterior and posterior halves of hand-dissected, cellular blastoderm embryos. We find that genome-wide chromatin accessibility is highly similar between the two halves, with regions that manifest significant accessibility in one half of the embryo almost always accessible in the other half, even for promoters that are active in exclusively one half of the embryo. These data support previous studies that show that chromatin accessibility is not a direct result of activity, and point to a role for ubiquitous factors or processes in establishing chromatin accessibility at promoters in the early embryo. However, in concordance with similar works, we find that at enhancers active exclusively in one half of the embryo, we observe a significant skew towards greater accessibility in the region of their activity, highlighting the role of patterning factors such as Bicoid in this process.

The migrations of Drosophila muscle founders and primordial germ cells are interdependent

Caudal visceral mesoderm (CVM) cells migrate from posterior to anterior of the Drosophila embryo as two bilateral streams of cells to support the specification of longitudinal muscles along the midgut. To accomplish this long-distance migration, CVM cells receive input from their environment, but little is known about how this collective cell migration is regulated. In a screen we found that wunen mutants exhibit CVM cell migration defects. Wunens are lipid phosphate phosphatases known to regulate the directional migration of primordial germ cells (PGCs). PGC and CVM cell types interact while PGCs are en route to the somatic gonadal mesoderm, and previous studies have shown that CVM impacts PGC migration. In turn, we found here that CVM cells exhibit an affinity for PGCs, localizing to the position of PGCs whether mislocalized or trapped in the endoderm. In the absence of PGCs, CVM cells exhibit subtle changes, including more cohesive movement of the migrating collective, and an increased number of longitudinal muscles is found at anterior sections of the larval midgut. These data demonstrate that PGC and CVM cell migrations are interdependent and suggest that distinct migrating cell types can coordinately influence each other to promote effective cell migration during development.

C-X-C chemokine receptor type 4 (CXCR4) is a key receptor for chicken primordial germ cell migration

In mammals, germ cells originate outside of the developing gonads and follow a unique migration pattern through the embryonic tissue toward the genital ridges. Many studies have attempted to identify critical receptors and factors involved in germ cell migration. However, relatively few reports exist on germ cell receptors and chemokines that are involved in germ cell migration in avian species. In the present study, we investigated the specific migratory function of C-X-C chemokine receptor type 4 (CXCR4) in chicken primordial germ cells (PGCs). We induced loss-of-function via a frameshift mutation in the CXCR4 gene in chicken PGCs using clustered regularly interspaced short palindromic repeat-CRISPR-associated protein 9 (CRISPR/Cas9) genome editing. The migratory capacity of CXCR4 knockout PGCs was significantly reduced in vivo after transplantation into recipient embryos. However, CXCR4-expressing somatic cell lines, such as chicken DT40 and DF1, failed to migrate into the developing gonads, suggesting that another key factor(s) is necessary for targeting and settlement of PGCs into the genital ridges. In conclusion, we show that CXCR4 plays a critical role in the migration of chicken germ cells.

Domain-specific control of germ cell polarity and migration by multifunction Tre1 GPCR

Migrating cells encounter directional cues to reach their destinations, often using G protein–coupled receptors (GPCRs) to interpret such cues. LeBlanc and Lehmann show that two highly conserved domains in the GPCR Tre1 mediate distinct migratory responses in germ cells via separate signaling pathways, one regulating cell polarization and the other directional migration.

The migration of primordial germ cells (PGCs) from their place of origin to the embryonic gonad is an essential reproductive feature in many animal species. In Drosophila melanogaster, a single G protein–coupled receptor, Trapped in endoderm 1 (Tre1), mediates germ cell polarization at the onset of active migration and directs subsequent migration of PGCs through the midgut primordium. How these different aspects of cell behavior are coordinated through a single receptor is not known. We demonstrate that two highly conserved domains, the E/N/DRY and NPxxY motifs, have overlapping and unique functions in Tre1. The Tre1-NRY domain via G protein signaling is required for reading and responding to guidance and survival cues controlled by the lipid phosphate phosphatases Wunen and Wunen2. In contrast, the Tre1-NPIIY domain has a separate role in Rho1- and E-cadherin–mediated polarization at the initiation stage independent of G protein signaling. We propose that this bifurcation of the Tre1 G protein–coupled receptor signaling response via G protein–dependent and independent branches enables distinct spatiotemporal regulation of germ cell migration.

Cells on the move: Modulation of guidance cues during germ cell migration

In Drosophila melanogaster the progenitors of the germ-line stem cells, the primordial germ cells (PGCs) are formed on the outside surface of the early embryo, while the somatic gonadal precursor cells (SGPs) are specified during mid-embryogenesis. To form the primitive embryonic gonad, the PGCs travel from outside of the embryo, across the mid-gut and then migrate through the mesoderm to the SGPs. The migratory path of PGCs is dictated by a series of attractive and repulsive cues. Studies in our laboratory have shown that one of the key chemoattractants is the Hedgehog (Hh) ligand. Although, Hh is expressed in other cell types, the long-distance transmission of this ligand is specifically potentiated in the SGPs by the hmgcr isoprenoid biosynthetic pathway. The distant transmission of the Hh ligand is gated by restricting expression of hmgcr to the SGPs. This is particularly relevant in light of the recent findings that an ABC transporter, mdr49 also acts in a mesoderm specific manner to release the germ cell attractant. Our studies have demonstrated that mdr49 functions in hh signaling likely via its role in the transport of cholesterol. Given the importance of cholesterol in the processing and long distance transmission of the Hh ligand, this observation has opened up an exciting avenue concerning the possible role of components of the sterol transport machinery in PGC migration.

Quantitative Differences in a Single Maternal Factor Determine Survival Probabilities among Drosophila Germ Cells

Germ cell death occurs in many species [1-3] and has been proposed as a mechanism by which the fittest, strongest, or least damaged germ cells are selected for transmission to the next generation. However, little is known about how the choice is made between germ cell survival and death. Here, we focus on the mechanisms that regulate germ cell survival during embryonic development in Drosophila. We find that the decision to die is a germ cell-intrinsic process linked to quantitative differences in germ plasm inheritance, such that higher germ plasm inheritance correlates with higher primordial germ cell (PGC) survival probability. We demonstrate that the maternal factor lipid phosphate phosphatase Wunen-2 (Wun2) regulates PGC survival in a dose-dependent manner. Since wun2 mRNA levels correlate with the levels of other maternal determinants at the single-cell level, we propose that Wun2 is used as a readout of the overall germ plasm quantity, such that only PGCs with the highest germ plasm quantity survive. Furthermore, we demonstrate that Wun2 and p53, another regulator of PGC survival, have opposite yet independent effects on PGC survival. Since p53 regulates cell death upon DNA damage and various cellular stresses, we hypothesize that together they ensure selection of the PGCs with highest germ plasm quantity and least cellular damage.

Drosophila melanogaster as a Model of Muscle Degeneration Disorders

Drosophila melanogaster provides a powerful platform with which researchers can dissect complex genetic questions and biochemical pathways relevant to a vast array of human diseases and disorders. Of particular interest, much work has been done with flies to elucidate the molecular mechanisms underlying muscle degeneration diseases. The fly is particularly useful for modeling muscle degeneration disorders because there are no identified satellite muscle cells to repair adult muscle following injury. This allows for the identification of endogenous processes of muscle degeneration as discrete events, distinguishable from phenotypes due to the lack of stem cell-based regeneration. In this review, we will discuss the ways in which the fruit fly provides a powerful platform with which to study human muscle degeneration disorders.

Finding their way: themes in germ cell migration

Embryonic germ cell migration is a vital component of the germline lifecycle. The translocation of germ cells from the place of origin to the developing somatic gonad involves several processes including passive movements with underlying tissues, transepithelial migration, cell adhesion dynamics, the establishment of environmental guidance cues and the ability to sustain directed migration. How germ cells accomplish these feats in established model organisms will be discussed in this review, with a focus on recent discoveries and themes conserved across species.

Repulsive cues combined with physical barriers and cell-cell adhesion determine progenitor cell positioning during organogenesis

The precise positioning of organ progenitor cells constitutes an essential, yet poorly understood step during organogenesis. Using primordial germ cells that participate in gonad formation, we present the developmental mechanisms maintaining a motile progenitor cell population at the site where the organ develops. Employing high-resolution live-cell microscopy, we find that repulsive cues coupled with physical barriers confine the cells to the correct bilateral positions. This analysis revealed that cell polarity changes on interaction with the physical barrier and that the establishment of compact clusters involves increased cell–cell interaction time. Using particle-based simulations, we demonstrate the role of reflecting barriers, from which cells turn away on contact, and the importance of proper cell–cell adhesion level for maintaining the tight cell clusters and their correct positioning at the target region. The combination of these developmental and cellular mechanisms prevents organ fusion, controls organ positioning and is thus critical for its proper function.

The precise positioning of organ progenitor cells is essential for organ development and function. Here the authors use live imaging and mathematical modelling to show that the confinement of a motile progenitor cell population results from coupled physical barriers and cell-cell interactions.

A Guide to Genome-Wide In Vivo RNAi Applications in Drosophila

RNAi technologies enable the testing of gene function in a cell-type- and stage-specific manner in Drosophila. The development of genome-wide RNAi libraries has allowed expansion of this approach to the genome scale and supports identification of most genes required for a given process in a cell type of choice. However, a large-scale RNAi approach also harbors many potential pitfalls that can complicate interpretation of the results. Here, we summarize published screens and provide a guide on how to optimally plan and perform a large-scale, in vivo RNAi screen. We highlight the importance of assay design and give suggestions on how to optimize the assay conditions by testing positive and negative control genes. These genes are used to estimate false-negative and false-positive rates of the screen data. We discuss the planning and logistics of a large-scale screen in detail and suggest bioinformatics platforms to identify and select gene groups of interest for secondary assays. Finally, we review various options to confirm RNAi knock-down specificity and thus identify high confidence genes for more detailed case-by-case studies in the future.

Multipotent versus differentiated cell fate selection in the developing Drosophila airways

Developmental potentials of cells are tightly controlled at multiple levels. The embryonic Drosophila airway tree is roughly subdivided into two types of cells with distinct developmental potentials: a proximally located group of multipotent adult precursor cells (P-fate) and a distally located population of more differentiated cells (D-fate). We show that the GATA-family transcription factor (TF) Grain promotes the P-fate and the POU-homeobox TF Ventral veinless (Vvl/Drifter/U-turned) stimulates the D-fate. Hedgehog and receptor tyrosine kinase (RTK) signaling cooperate with Vvl to drive the D-fate at the expense of the P-fate while negative regulators of either of these signaling pathways ensure P-fate specification. Local concentrations of Decapentaplegic/BMP, Wingless/Wnt, and Hedgehog signals differentially regulate the expression of D-factors and P-factors to transform an equipotent primordial field into a concentric pattern of radially different morphogenetic potentials, which gradually gives rise to the distal-proximal organization of distinct cell types in the mature airway.

DOI:http://dx.doi.org/10.7554/eLife.09646.001

Many organs are composed of tubes of different sizes, shapes and patterns that transport vital substances from one site to another. In the fruit fly species Drosophila melanogaster, oxygen is transported by a tubular network, which divides into finer tubes that allow the oxygen to reach every part of the body.

Different parts of the fruit fly’s airways develop from different groups of tracheal precursor cells. P-fate cells form the most 'proximal' tubes (which are found next to the outer layer of the fly). These cells are 'multipotent' stem cells, and have the ability to specialize into many different types of cells during metamorphosis. The more 'distal' branches that emerge from the proximal tubes develop from D-fate cells. These are cells that generally acquire a narrower range of cell identities.

By performing a genetic analysis of fruit fly embryos, Matsuda et al. have now identified several proteins and signaling molecules that control whether tracheal precursor cells become D-fate or P-fate cells. For example, several signaling pathways work with a protein called Ventral veinless to cause D-fate cells to develop instead of P-fate cells. However, molecules that prevent signaling occurring via these pathways help P-fate cells to form. Different amounts of the molecules that either promote or hinder these signaling processes are present in different parts of the fly embryo; this helps the airways of the fly to develop in the correct pattern.

This work provides a comprehensive view of how cell types with different developmental potentials are positioned in a complex tubular network. This sets a basis for future studies addressing how the respiratory organs – and indeed the entire organism – are sustained.

DOI:http://dx.doi.org/10.7554/eLife.09646.002

Cellular and molecular mechanisms of single and collective cell migrations in Drosophila: themes and variations

The process of cell migration is essential throughout life, driving embryonic morphogenesis and ensuring homeostasis in adults. Defects in cell migration are a major cause of human disease, with excessive migration causing autoimmune diseases and cancer metastasis, whereas reduced capacity for migration leads to birth defects and immunodeficiencies. Myriad studies in vitro have established a consensus view that cell migrations require cell polarization, Rho GTPase-mediated cytoskeletal rearrangements, and myosin-mediated contractility. However, in vivo studies later revealed a more complex picture, including the discovery that cells migrate not only as single units but also as clusters, strands, and sheets. In particular, the role of E-Cadherin in cell motility appears to be more complex than previously appreciated. Here, we discuss recent advances achieved by combining the plethora of genetic tools available to the Drosophila geneticist with live imaging and biophysical techniques. Finally, we discuss the emerging themes such studies have revealed and ponder the puzzles that remain to be solved.

The extracellular protease stl functions to inhibit migration of v'ch1 sensory neuron during Drosophila embryogenesis

Proper migration of cells through the dense and complex extracellular matrix (ECM) requires constant restructuring of the ECM to allow cells to move forward in a smooth manner. This restructuring can occur through the action of extracellular enzymes. Among these extracellular enzymes is the ADAMTS (A Disintegrin And Metalloprotease with ThromboSpondin repeats) family of secreted extracellular proteases. Drosophila stl encodes an ADAMTS protease expressed in and around the peripheral nervous system (PNS) during embryogenesis. The absence of stl displayed one specific neuron, the v'ch1 sensory neuron, migrating to its target sooner than in wild type. During normal development, the v'ch1 sensory neuron migrates dorsally at the same time it is extending an axon ventrally toward the CNS. Surprisingly, in the absence of stl, the v'ch1 neuron migrated further dorsally as compared to the wild type at stage 15, but did not migrate past its correct target at stage 16, suggesting a novel role for this extracellular protease in inhibiting migration of this neuron past a certain point.

Cloning and characterization of three Eimeria tenella lipid phosphate phosphatases

Although lipid phosphate phosphatases (LPPs) play an important role in cellular signaling in addition to lipid biosynthesis, little is thus far known about parasite LPPs. In this study, we characterized three Eimeria tenella cDNA clones encoding LPP named EtLPP1, EtLPP2 and EtLPP3. Key structural features previously described in LPPs, including the three conserved domains proposed as catalytic sites, a single conserved N-glycosylation site, and putative transmembrane domains were discovered in the three resulting EtLPP amino acid sequences. Expression of His6-tagged EtLPP1, -2, and -3 in HEK293 cells produced immunoreactive proteins with variable molecular sizes, suggesting the presence of multiple forms of each of the three EtLPPs. The two faster-migrating protein bands below each of the three EtLPP proteins were found to be very similar to the porcine 35-kDa LPP enzyme in their molecular size and the extent of their N-glycosylation, suggesting that the three EtLPPs are partially N-glycosylated. Kinetic analyses of the activity of the three enzymes against PA, LPA, C1P and S1P showed that Km values for each of the substrates were (in μM) 284, 46, 28, and 22 for EtLPP1; 369, 179, 237, and 52 for EtLPP2; and 355, 83, and 260 for EtLPP3. However, EtLPP3 showed negligible activity on S1P. These results confirmed that the three EtLPPs have broad substrate specificity. The results also indicated that despite structural similarities, the three EtLPPs may play distinct functions through their different models of substrate preference. Furthermore, particularly high expression levels of the three EtLPP genes were detected in the sporozoite stage of the E. tenella life cycle (p<0.001), suggesting that their encoded proteins might play an important biological function in the sporozoite stage.

The intersection of the extrinsic hedgehog and WNT/wingless signals with the intrinsic Hox code underpins branching pattern and tube shape diversity in the drosophila airways

The tubular networks of the Drosophila respiratory system and our vasculature show distinct branching patterns and tube shapes in different body regions. These local variations are crucial for organ function and organismal fitness. Organotypic patterns and tube geometries in branched networks are typically controlled by variations of extrinsic signaling but the impact of intrinsic factors on branch patterns and shapes is not well explored. Here, we show that the intersection of extrinsic hedgehog(hh) and WNT/wingless (wg) signaling with the tube-intrinsic Hox code of distinct segments specifies the tube pattern and shape of the Drosophila airways. In the cephalic part of the airways, hh signaling induces expression of the transcription factor (TF) knirps (kni) in the anterior dorsal trunk (DTa1). kni represses the expression of another TF spalt major (salm), making DTa1 a narrow and long tube. In DTa branches of more posterior metameres, Bithorax Complex (BX-C) Hox genes autonomously divert hh signaling from inducing kni, thereby allowing DTa branches to develop as salm-dependent thick and short tubes. Moreover, the differential expression of BX-C genes is partly responsible for the anterior-to-posterior gradual increase of the DT tube diameter through regulating the expression level of Salm, a transcriptional target of WNT/wg signaling. Thus, our results highlight how tube intrinsic differential competence can diversify tube morphology without changing availabilities of extrinsic factors.

Tubes are common structural elements of many internal organs, facilitating fluid flow and material exchange. To meet the local needs of diverse tissues, the branching patterns and tube shapes vary regionally. Diametric tapering and specialized branch targeting to the brain represent two common examples of variations with organismal benefits in the Drosophila airways and our vascular system. Several extrinsic signals instruct tube diversifications but the impact of intrinsic factors remains underexplored. Here, we show that the local, tube-intrinsic Hox code instructs the pattern and shape of the dorsal trunk (DT), the main Drosophila airway. In the cephalic part (DT1), where Bithorax Complex (BX-C) Hox genes are not expressed, the extrinsic Hedgehog signal is epistatic to WNT/Wingless signals. Hedgehog instructs anterior DT1 cells to take a long and narrow tube fate targeting the brain. In more posterior metameres, BX-C genes make the extrinsic WNT/Wingless signals epistatic over Hedgehog. There, WNT/Wingless instruct all DT cells to take the thick and short tube fate. Moreover, BX-C genes modulate the outputs of WNT/wingless signaling, making the DT tubes thicker in more posterior metameres. We provide a model for how intrinsic factors modify extrinsic signaling to control regional tube morphologies in a network.

Barrier mechanisms in the Drosophila blood-brain barrier

The invertebrate blood-brain barrier (BBB) field is growing at a rapid pace and, in recent years, studies have shown a physiologic and molecular complexity that has begun to rival its vertebrate counterpart. Novel mechanisms of paracellular barrier maintenance through G-protein coupled receptor signaling were the first demonstrations of the complex adaptive mechanisms of barrier physiology. Building upon this work, the integrity of the invertebrate BBB has recently been shown to require coordinated function of all layers of the compound barrier structure, analogous to signaling between the layers of the vertebrate neurovascular unit. These findings strengthen the notion that many BBB mechanisms are conserved between vertebrates and invertebrates, and suggest that novel findings in invertebrate model organisms will have a significant impact on the understanding of vertebrate BBB functions. In this vein, important roles in coordinating localized and systemic signaling to dictate organism development and growth are beginning to show how the BBB can govern whole animal physiologies. This includes novel functions of BBB gap junctions in orchestrating synchronized neuroblast proliferation, and of BBB secreted antagonists of insulin receptor signaling. These advancements and others are pushing the field forward in exciting new directions. In this review, we provide a synopsis of invertebrate BBB anatomy and physiology, with a focus on insights from the past 5 years, and highlight important areas for future study.

Drosophila heart cell movement to the midline occurs through both cell autonomous migration and dorsal closure

The Drosophila heart is a linear organ formed by the movement of bilaterally specified progenitor cells to the midline and adherence of contralateral heart cells. This movement occurs through the attachment of heart cells to the overlying ectoderm which is undergoing dorsal closure. Therefore heart cells are thought to move to the midline passively. Through live imaging experiments and analysis of mutants that affect the speed of dorsal closure we show that heart cells in Drosophila are autonomously migratory and part of their movement to the midline is independent of the ectoderm. This means that heart formation in flies is more similar to that in vertebrates than previously thought. We also show that defects in dorsal closure can result in failure of the amnioserosa to properly degenerate, which can physically hinder joining of contralateral heart cells leading to a broken heart phenotype.

Using MARCM to study Drosophila brain development

Mosaic analysis with a repressible cell marker (MARCM) generates positively labeled, wild-type or mutant mitotic clones by unequally distributing a repressor of a cell lineage marker, originally tubP-driven GAL80 repressing the GAL4/UAS system. Variations of the technique include labeling of both sister clones (twin spot MARCM), the simultaneous use of two different drivers within the same clone (dual MARCM), as well as the use of different repressible transcription systems (Q-MARCM). MARCM can be combined with any UAS-based construct, such as localized GFP fusions to visualize subcellular compartments, genes for rescue and ectopic expression, and modifiers of neural activity. A related technique, the twin spot generator, generates positively labeled clones without the use of a repressor, thus minimizing the lag time between clone induction and appearance of label. The present protocol provides a detailed description of a standard MARCM analysis of brain development that includes generation of MARCM stocks and crosses, induction of clones, brain dissection at various stages of development, immunohistochemistry, and confocal microscopy, and can be modified for similar experiments involving mitotic clones.

Opposing dopaminergic and GABAergic neurons control the duration and persistence of copulation in Drosophila

Behavioral persistence is a major factor in determining when and under which circumstances animals will terminate their current activity and transition into more profitable, appropriate, or urgent behavior. We show that, for the first 5 min of copulation in Drosophila, stressful stimuli do not interrupt mating, whereas 10 min later, even minor perturbations are sufficient to terminate copulation. This decline in persistence occurs as the probability of successful mating increases and is promoted by approximately eight sexually dimorphic, GABAergic interneurons of the male abdominal ganglion. When these interneurons were silenced, persistence increased and males copulated far longer than required for successful mating. When these interneurons were stimulated, persistence decreased and copulations were shortened. In contrast, dopaminergic neurons of the ventral nerve cord promote copulation persistence and extend copulation duration. Thus, copulation duration in Drosophila is a product of gradually declining persistence controlled by opposing neuronal populations using conserved neurotransmission systems.

Quantifying the range of a lipid phosphate signal in vivo

Quantitative information about the range of influence of extracellular signaling molecules is critical for understanding their effects, but is difficult to determine in the complex and dynamic three-dimensional environment of a living embryo. Drosophila germ cells migrate during embryogenesis and use spatial information provided by expression of lipid phosphate phosphatases called Wunens to reach the somatic gonad. However, whether guidance requires cell contact or involves a diffusible signal is not known. We ectopically expressed Wunens in various segmentally repeated ectodermal and parasegmental patterns in embryos otherwise null for Wunens and used germ cell behavior to show that the signal is diffusible and to define its range. We correlated this back to the wild-type scenario and found that the germ cell migratory path can be primarily accounted for by Wunen expression. This approach provides the first quantitative information of the effective range of a lipid phosphate in vivo and has implications for the migration of other cell types that respond to lipid phosphates.

Variation in meiotic recombination frequencies between allelic transgenes inserted at different sites in the Drosophila melanogaster genome

Meiotic crossovers are distributed nonrandomly across the genome. Classic studies in Drosophila suggest that the position of a gene along a chromosome arm can affect the outcome of the recombination process, with proximity to the centromere being associated with lower crossing over. To examine this phenomenon molecularly, we developed an assay that measures meiotic crossovers and noncrossover gene conversions between allelic transgenes inserted into different genomic positions. To facilitate collecting a large number of virgin females, we developed a useful genetic system that kills males and undesired classes of females. We found that the recombination frequency at a site in the middle of the X chromosome, where crossovers are normally frequent, was similar to the frequency at the centromere-proximal end of the euchromatin, where crossovers are normally infrequent. In contrast, we recovered no recombinants--crossovers or noncrossovers--at a site on chromosome 4 and at a site toward the distal end of the X chromosome. These results suggest that local sequence or chromatin features have a stronger impact on recombination rates in this transgene assay than position along the chromosome arm.

The union of somatic gonad precursors and primordial germ cells during Caenorhabditis elegans embryogenesis

Somatic gonadal niche cells control the survival, differentiation, and proliferation of germline stem cells. The establishment of this niche-stem cell relationship is critical, and yet the precursors to these two cell types are often born at a distance from one another. The simple Caenorhabditis elegans gonadal primordium, which contains two somatic gonad precursors (SGPs) and two primordial germ cells (PGCs), provides an accessible model for determining how stem cell and niche cell precursors first assemble during development. To visualize the morphogenetic events that lead to formation of the gonadal primordium, we generated transgenic strains to label the cell membranes of the SGPs and PGCs and captured time-lapse movies as the gonadal primordium formed. We identify three distinct phases of SGP behavior: posterior migration along the endoderm towards the PGCs, extension of a single long projection around the adjacent PGC, and a dramatic wrapping over the PGC surfaces. We show that the endoderm and PGCs are dispensable for SGP posterior migration and initiation of projections. However, both tissues are required for the final positioning of the SGPs and the morphology of their projections, and PGCs are absolutely required for SGP wrapping behaviors. Finally, we demonstrate that the basement membrane component laminin, which localizes adjacent to the developing gonadal primordium, is required to prevent the SGPs from over-extending past the PGCs. Our findings provide a foundation for understanding the cellular and molecular regulation of the establishment of a niche-stem cell relationship.

Genetic elevation of sphingosine 1-phosphate suppresses dystrophic muscle phenotypes in Drosophila

Duchenne muscular dystrophy is a lethal genetic disease characterized by the loss of muscle integrity and function over time. Using Drosophila, we show that dystrophic muscle phenotypes can be significantly suppressed by a reduction of wunen, a homolog of lipid phosphate phosphatase 3, which in higher animals can dephosphorylate a range of phospholipids. Our suppression analyses include assessing the localization of Projectin protein, a titin homolog, in sarcomeres as well as muscle morphology and functional movement assays. We hypothesize that wunen-based suppression is through the elevation of the bioactive lipid Sphingosine 1-phosphate (S1P), which promotes cell proliferation and differentiation in many tissues, including muscle. We confirm the role of S1P in suppression by genetically altering S1P levels via reduction of S1P lyase (Sply) and by upregulating the serine palmitoyl-CoA transferase catalytic subunit gene lace, the first gene in the de novo sphingolipid biosynthetic pathway and find that these manipulations also reduce muscle degeneration. Furthermore, we show that reduction of spinster (which encodes a major facilitator family transporter, homologs of which in higher animals have been shown to transport S1P) can also suppress dystrophic muscle degeneration. Finally, administration to adult flies of pharmacological agents reported to elevate S1P signaling significantly suppresses dystrophic muscle phenotypes. Our data suggest that localized intracellular S1P elevation promotes the suppression of muscle wasting in flies.

Identification of a suppressor of retinal degeneration in Drosophila photoreceptors

During sensory transduction, Drosophila photoreceptors experience substantial increases in intracellular Ca(2+) levels ([Ca(2+)](i)). Nevertheless in a number of mutants associated with excessive Ca(2+) influx through transient receptor potential (TRP) channels, Drosophila photoreceptors undergo loss of normal cellular structure manifest as a retinal degeneration. However, the molecular mechanisms that underpin this degeneration process remain unclear. The authors previously isolated a mutant, su(40), that is able to suppress the retinal degeneration seen in photoreceptors from loss-of-function alleles of rdgA that are known to have constitutively active TRP channels. Here the authors report the genetic mapping of su(40) as well the isolation of additional alleles of su(40). Studies of su(40) as well as these new alleles should facilitate the understanding of the mechanisms by which excessive Ca(2+) influx results in retinal degeneration.

An assay to detect in vivo Y chromosome loss in Drosophila wing disc cells

Loss of the Y chromosome in Drosophila has no impact on cell viability and therefore allows us to assay the impact of environmental agents and genetic alterations on chromosomal loss. To detect in vivo chromosome loss in cells of the developing Drosophila wing primordia, we first engineered a Y chromosome with an attP docking site. By making use of the ΦC31 integrase system, we site-specifically integrated a genomic transgene encompassing the multiple wing hair (mwh) locus into this attP site, leading to a mwh⁺Y chromosome. This chromosome fully rescues the mwh mutant phenotype, an excellent recessive wing cell marker mutation. Loss of this mwh⁺Y chromosome in wing primordial cells then leads to manifestation of the mwh mutant phenotype in mwh-homozygous cells. The forming mwh clones permit us to quantify the effect of agents and genetic alterations by assaying frequency and size of the mwh mosaic spots. To illustrate the use of the mwh⁺Y loss system, the effects of four known mutagens (X-rays, colchicine, ethyl methanesulfonate, and formaldehyde) and two genetic conditions (loss- and gain-of-function lodestar mutant alleles) are documented. The procedure is simple, sensitive, and inexpensive.

vasa is expressed in somatic cells of the embryonic gonad in a sex-specific manner in Drosophila melanogaster

Vasa is a DEAD box helicase expressed in the Drosophila germline at all stages of development. vasa homologs are found widely in animals and vasa has become the gene of choice in identifying germ cells. I now show that Drosophila vasa expression is not restricted to the germline but is also expressed in a somatic lineage, the embryonic somatic gonadal precursor cells. This expression is sexually dimorphic, being maintained specifically in males, and is regulated post-transcriptionally. Although somatic Vasa expression is not required for gonad coalescence, these data support the notion that Vasa is not solely a germline factor.