Spatially restricted expression of pipe in the Drosophila egg chamber defines embryonic dorsal-ventral polarity

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Knockout of ovary serine protease Leads to Ovary Deformation and Female Sterility in the Asian Corn Borer, Ostrinia furnacalis

Oogenesis in insects is a carefully orchestrated process, facilitating the formation of female gametes, which is regulated by multiple extrinsic and intrinsic factors, including ovary serine protease (Osp). As a member of the serine protease family, Osp is a homolog of Nudel, a maternally required protease defining embryonic dorsoventral polarity in Drosophila. In this study, we used CRISPR/Cas9-mediated mutagenesis to functionally characterize Osp in the Asian corn borer, Ostrinia furnacalis, a devastating maize pest throughout Asia and Australia. Building on previous knowledge, we hypothesized that knockout of Osp would disrupt embryonic development in O. furnacalis females. To examine this overarching hypothesis, we (1) cloned and characterized Osp from O. furnacalis, (2) designed target sites on exons 1 and 4 to construct a CRISPR/Cas9 mutagenesis system, and (3) documented phenotypic impacts among O. furnacalis Osp mutants. As a result, we (1) examined the temporal-spatial expression profiles of OfOsp, which has an open reading frame of 5648 bp in length and encodes a protein of 1873 amino acids; (2) established O. furnacalis Osp mutants; and (3) documented recessive, female-specific sterility among OfOspF mutants, including absent or deformed oviducts and reduced fertility in female but not male mutants. Overall, the combined results support our initial hypothesis that Osp is required for embryonic development, specifically ovarian maturation, in O. furnacalis females. Given its substantial impacts on female sterility, Osp provides a potential target for the Sterile Insect Technique (SIT) to manage Lepidoptera pests in general and the species complex Ostrinia in particular.

Polarity and axis formation in the Drosophila female germ line

By the time a Drosophila egg is laid, both major body axes have already been defined and it contains all the nutrients needed to develop into a free-living larva in 24 h. By contrast, it takes almost a week to make an egg from a female germline stem cell, during the complex process of oogenesis. This review will discuss key symmetry-breaking steps in Drosophila oogenesis that lead to the polarisation of both body axes: the asymmetric divisions of the germline stem cells; the selection of the oocyte from the 16-cell germline cyst; the positioning of the oocyte at the posterior of the cyst; Gurken signalling from the oocyte to polarise the anterior-posterior axis of the somatic follicle cell epithelium around the developing germline cyst; the signalling back from the posterior follicle cells to polarise the anterior-posterior axis of the oocyte; and the migration of the oocyte nucleus that specifies the dorsal-ventral axis. Since each event creates the preconditions for the next, I will focus on the mechanisms that drive these symmetry-breaking steps, how they are linked and the outstanding questions that remain to be answered.

Expression and Function of Toll Pathway Components in the Early Development of the Wasp Nasonia vitripennis

The Toll signaling pathway is the main source of embryonic DV polarity in the fly Drosophila melanogaster. This pathway appears to have been co-opted from an ancestral innate immunity system within the insects and has been deployed in different ways among insect taxa. Here we report the expression and function of homologs of the important components of the D. melanogaster Toll pathway in the wasp Nasonia vitripennis. We found homologs for all the components; many components had one or more additional paralogs in the wasp relative the fly. We also found significant deviations in expression patterns of N. vitripennis homologs. Finally, we provide some preliminary functional analyses of the N. vitripennis homologs, where we find a mixture of conservation and divergence of function.

The Toll gene in Drosophila pattern formation

Toll-like receptors (TLRs) play a crucial role in innate immunity in animals. Their discovery was rewarded a Nobel Prize to Jules Hoffmann and Bruce Beutler in 2011. The name Toll stems from a Drosophila mutant that was isolated in 1980 by Eric Wieschaus and myself as a byproduct of our screen for segmentation genes in Drosophila for which we received the Nobel Prize in 1995. It was named Toll due to its amazing dominant phenotype displayed in embryos from Toll/+ females. The analysis of Toll by Kathryn Anderson in my laboratory in Tübingen and subsequently in her own laboratory in Berkeley singled out Toll as a central component of the complex pathway regulating dorsoventral polarity and pattern of the Drosophila embryo. The Drosophila Toll story provides a striking example for the value of curiosity-driven research in providing fundamental insights that later gain strong impact on applied medical research.

Striking parallels between dorsoventral patterning in Drosophila and Gryllus reveal a complex evolutionary history behind a model gene regulatory network

Dorsoventral pattering relies on Toll and BMP signalling in all insects studied so far, with variations in the relative contributions of both pathways. Drosophila and the beetle Tribolium share extensive dependence on Toll, while representatives of more distantly related lineages like the wasp Nasonia and bug Oncopeltus rely more strongly on BMP signalling. Here, we show that in the cricket Gryllus bimaculatus, an evolutionarily distant outgroup, Toll has, like in Drosophila, a direct patterning role for the ventral half of the embryo. In addition, Toll polarises BMP signalling, although this does not involve the conserved BMP inhibitor Sog/Chordin. Finally, Toll activation relies on ovarian patterning mechanisms with striking similarity to Drosophila. Our data suggest two surprising hypotheses: (1) that Toll's patterning function in Gryllus and Drosophila is the result of convergent evolution or (2) a Drosophila-like system arose early in insect evolution and was extensively altered in multiple independent lineages.

The Physical Mechanisms of Drosophila Gastrulation: Mesoderm and Endoderm Invagination

A critical juncture in early development is the partitioning of cells that will adopt different fates into three germ layers: the ectoderm, the mesoderm, and the endoderm. This step is achieved through the internalization of specified cells from the outermost surface layer, through a process called gastrulation. In Drosophila, gastrulation is achieved through cell shape changes (i.e., apical constriction) that change tissue curvature and lead to the folding of a surface epithelium. Folding of embryonic tissue results in mesoderm and endoderm invagination, not as individual cells, but as collective tissue units. The tractability of Drosophila as a model system is best exemplified by how much we know about Drosophila gastrulation, from the signals that pattern the embryo to the molecular components that generate force, and how these components are organized to promote cell and tissue shape changes. For mesoderm invagination, graded signaling by the morphogen, Spätzle, sets up a gradient in transcriptional activity that leads to the expression of a secreted ligand (Folded gastrulation) and a transmembrane protein (T48). Together with the GPCR Mist, which is expressed in the mesoderm, and the GPCR Smog, which is expressed uniformly, these signals activate heterotrimeric G-protein and small Rho-family G-protein signaling to promote apical contractility and changes in cell and tissue shape. A notable feature of this signaling pathway is its intricate organization in both space and time. At the cellular level, signaling components and the cytoskeleton exhibit striking polarity, not only along the apical-basal cell axis, but also within the apical domain. Furthermore, gene expression controls a highly choreographed chain of events, the dynamics of which are critical for primordium invagination; it does not simply throw the cytoskeletal "on" switch. Finally, studies of Drosophila gastrulation have provided insight into how global tissue mechanics and movements are intertwined as multiple tissues simultaneously change shape. Overall, these studies have contributed to the view that cells respond to forces that propagate over great distances, demonstrating that cellular decisions, and, ultimately, tissue shape changes, proceed by integrating cues across an entire embryo.

Formation, interpretation, and regulation of the Drosophila Dorsal/NF-κB gradient

The morphogen gradient of the transcription factor Dorsal in the early Drosophila embryo has become one of the most widely studied tissue patterning systems. Dorsal is a Drosophila homolog of mammalian NF-κB and patterns the dorsal-ventral axis of the blastoderm embryo into several tissue types by spatially regulating upwards of 100 zygotic genes. Recent studies using fluorescence microscopy and live imaging have quantified the Dorsal gradient and its target genes, which has paved the way for mechanistic modeling of the gradient. In this review, we describe the mechanisms behind the initiation of the Dorsal gradient and its regulation of target genes. The main focus of the review is a discussion of quantitative and computational studies of the Dl gradient system, including regulation of the Dl gradient. We conclude with a discussion of potential future directions.

Setting up for gastrulation: D. melanogaster

Drosophila melanogaster embryos develop initially as a syncytium of totipotent nuclei and subsequently, once cellularized, undergo morphogenetic movements associated with gastrulation to generate the three somatic germ layers of the embryo: mesoderm, ectoderm, and endoderm. In this chapter, we focus on the first phase of gastrulation in Drosophila involving patterning of early embryos when cells differentiate their gene expression programs. This patterning process requires coordination of multiple developmental processes including genome reprogramming at the maternal-to-zygotic transition, combinatorial action of transcription factors to support distinct gene expression, and dynamic feedback between this genetic patterning by transcription factors and changes in cell morphology. We discuss the gene regulatory programs acting during patterning to specify the three germ layers, which involve the regulation of spatiotemporal gene expression coupled to physical tissue morphogenesis.

Genetic Screens to Analyze Pattern Formation of Egg and Embryo in Drosophila: A Personal History

In Drosophila development, the axes of the egg and future embryo are established during oogenesis. To learn about the underlying genetic and molecular pathways that lead to axis formation, I conducted a large-scale genetic screen at the beginning of my independent career. This led to the eventual understanding that both anterior-posterior and dorsal-ventral pattern information is transmitted from the oocyte to the surrounding follicle cells and in turn from the follicle cells back to the oocyte. How I came to conduct this screen and what further insights were gained by studying the mutants isolated in the screen are the topics of this autobiographical article.

Signaling between somatic follicle cells and the germline patterns the egg and embryo of Drosophila

In Drosophila, specification of the embryonic body axes requires signaling between the germline and the somatic follicle cells. These signaling events are necessary to properly localize embryonic patterning determinants in the egg or eggshell during oogenesis. There are three maternal patterning systems that specify the anterior-posterior axis, and one that establishes the dorsal-ventral axis. We will first review oogenesis, focusing on the establishment of the oocyte and nurse cells and patterning of the follicle cells into different subpopulations. We then describe how two coordinated signaling events between the oocyte and follicle cells establish polarity of the oocyte and localize the anterior determinant bicoid, the posterior determinant oskar, and Gurken/epidermal growth factor (EGF), which breaks symmetry to initiate dorsal-ventral axis establishment. Next, we review how dorsal-ventral asymmetry of the follicle cells is transmitted to the embryo. This process also involves Gurken-EGF receptor (EGFR) signaling between the oocyte and follicle cells, leading to ventrally-restricted expression of the sulfotransferase Pipe. These events promote the ventral processing of Spaetzle, a ligand for Toll, which ultimately sets up the embryonic dorsal-ventral axis. We then describe the activation of the terminal patterning system by specialized polar follicle cells. Finally, we present open questions regarding soma-germline signaling during Drosophila oogenesis required for cell identity and embryonic axis formation.

A facilitated diffusion mechanism establishes the Drosophila Dorsal gradient

The transcription factor NF-κB plays an important role in the immune system, apoptosis and inflammation. Dorsal, a Drosophila homolog of NF-κB, patterns the dorsal-ventral axis in the blastoderm embryo. During this stage, Dorsal is sequestered outside the nucleus by the IκB homolog Cactus. Toll signaling on the ventral side breaks the Dorsal/Cactus complex, allowing Dorsal to enter the nucleus to regulate target genes. Fluorescent data show that Dorsal accumulates on the ventral side of the syncytial blastoderm. Here, we use modeling and experimental studies to show that this accumulation is caused by facilitated diffusion, or shuttling, of the Dorsal/Cactus complex. We also show that active Toll receptors are limiting in wild-type embryos, which is a key factor in explaining global Dorsal gradient formation. Our results suggest that shuttling is necessary for viability of embryos from mothers with compromised dorsal levels. Therefore, Cactus not only has the primary role of regulating Dorsal nuclear import, but also has a secondary role in shuttling. Given that this mechanism has been found in other, independent, systems, we suggest that it might be more prevalent than previously thought.

A WntD-Dependent Integral Feedback Loop Attenuates Variability in Drosophila Toll Signaling

Patterning by morphogen gradients relies on the capacity to generate reproducible distribution profiles. Morphogen spread depends on kinetic parameters, including diffusion and degradation rates, which vary between embryos, raising the question of how variability is controlled. We examined this in the context of Toll-dependent dorsoventral (DV) patterning of the Drosophila embryo. We find that low embryo-to-embryo variability in DV patterning relies on wntD, a Toll-target gene expressed initially at the posterior pole. WntD protein is secreted and disperses in the extracellular milieu, associates with its receptor Frizzled4, and inhibits the Toll pathway by blocking the Toll extracellular domain. Mathematical modeling predicts that WntD accumulates until the Toll gradient narrows to its desired spread, and we support this feedback experimentally. This circuit exemplifies a broadly applicable induction-contraction mechanism, which reduces patterning variability through a restricted morphogen-dependent expression of a secreted diffusible inhibitor.

Maternal control of the Drosophila dorsal-ventral body axis

The pathway that generates the dorsal-ventral (DV) axis of the Drosophila embryo has been the subject of intense investigation over the previous three decades. The initial asymmetric signal originates during oogenesis by the movement of the oocyte nucleus to an anterior corner of the oocyte, which establishes DV polarity within the follicle through signaling between Gurken, the Drosophila Transforming Growth Factor (TGF)-α homologue secreted from the oocyte, and the Drosophila Epidermal Growth Factor Receptor (EGFR) that is expressed by the follicular epithelium cells that envelop the oocyte. Follicle cells that are not exposed to Gurken follow a ventral fate and express Pipe, a sulfotransferase that enzymatically modifies components of the inner vitelline membrane layer of the eggshell, thereby transferring DV spatial information from the follicle to the egg. These ventrally sulfated eggshell proteins comprise a localized cue that directs the ventrally restricted formation of the active Spätzle ligand within the perivitelline space between the eggshell and the embryonic membrane. Spätzle activates Toll, a transmembrane receptor in the embryonic membrane. Transmission of the Toll signal into the embryo leads to the formation of a ventral-to-dorsal gradient of the transcription factor Dorsal within the nuclei of the syncytial blastoderm stage embryo. Dorsal controls the spatially specific expression of a large constellation of zygotic target genes, the Dorsal gene regulatory network, along the embryonic DV circumference. This article reviews classic studies and integrates them with the details of more recent work that has advanced our understanding of the complex pathway that establishes Drosophila embryo DV polarity. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Components of the dorsal-ventral pathway also contribute to anterior-posterior patterning in honeybee embryos (Apis mellifera)

Background

A key early step in embryogenesis is the establishment of the major body axes; the dorsal-ventral (DV) and anterior-posterior (AP) axes. Determination of these axes in some insects requires the function of different sets of signalling pathways for each axis. Patterning across the DV axis requires interaction between the Toll and Dpp/TGF-β pathways, whereas patterning across the AP axis requires gradients of bicoid/orthodenticle proteins and the actions of a hierarchy of gene transcription factors. We examined the expression and function of Toll and Dpp signalling during honeybee embryogenesis to assess to the role of these genes in DV patterning.

Results

Pathway components that are required for dorsal specification in Drosophila are expressed in an AP-restricted pattern in the honeybee embryo, including Dpp and its receptor Tkv. Components of the Toll pathway are expressed in a more conserved pattern along the ventral axis of the embryo. Late-stage embryos from RNA interference (RNAi) knockdown of Toll and Dpp pathways had both DV and AP patterning defects, confirmed by staining with Am-sna, Am-zen, Am-eve, and Am-twi at earlier stages. We also identified two orthologues of dorsal in the honeybee genome, with one being expressed during embryogenesis and having a minor role in axis patterning, as determined by RNAi and the other expressed during oogenesis.

Conclusions

We found that early acting pathways (Toll and Dpp) are involved not only in DV patterning but also AP patterning in honeybee embryogenesis. Changes to the expression patterns and function of these genes may reflect evolutionary changes in the placement of the extra-embryonic membranes during embryogenesis with respect to the AP and DV axes.

asunder is required for dynein localization and dorsal fate determination during Drosophila oogenesis

We previously showed that asunder (asun) is a critical regulator of dynein localization during Drosophila spermatogenesis. Because the expression of asun is much higher in Drosophila ovaries and early embryos than in testes, we herein sought to determine whether ASUN plays roles in oogenesis and/or embryogenesis. We characterized the female germline phenotypes of flies homozygous for a null allele of asun (asun(d93)). We find that asun(d93) females lay very few eggs and contain smaller ovaries with a highly disorganized arrangement of ovarioles in comparison to wild-type females. asun(d93) ovaries also contain a significant number of egg chambers with structural defects. A majority of the eggs laid by asun(d93) females are ventralized to varying degrees, from mild to severe; this ventralization phenotype may be secondary to defective localization of gurken transcripts, a dynein-regulated step, within asun(d93) oocytes. We find that dynein localization is aberrant in asun(d93) oocytes, indicating that ASUN is required for this process in both male and female germ cells. In addition to the loss of gurken mRNA localization, asun(d93) ovaries exhibit defects in other dynein-mediated processes such as migration of nurse cell centrosomes into the oocyte during the early mitotic divisions, maintenance of the oocyte nucleus in the anterior-dorsal region of the oocyte in late-stage egg chambers, and coupling between the oocyte nucleus and centrosomes. Taken together, our data indicate that asun is a critical regulator of dynein localization and dynein-mediated processes during Drosophila oogenesis.

Role of sulphate in development

Sulphate contributes to numerous processes in mammalian physiology, particularly during development. Sulphotransferases mediate the sulphate conjugation (sulphonation) of numerous compounds, including steroids, glycosaminoglycans, proteins, neurotransmitters and xenobiotics, transforming their biological activities. Importantly, the ratio of sulphonated to unconjugated molecules plays a significant physiological role in many of the molecular events that regulate mammalian growth and development. In humans, the fetus is unable to generate its own sulphate and therefore relies on sulphate being supplied from maternal circulation via the placenta. To meet the gestational needs of the growing fetus, maternal blood sulphate concentrations double from mid-gestation. Maternal hyposulphataemia has been linked to fetal sulphate deficiency and late gestational fetal loss in mice. Disorders of sulphonation have also been linked to a number of developmental disorders in humans, including skeletal dysplasias and premature adrenarche. While recognised as an important nutrient in mammalian physiology, sulphate is largely unappreciated in clinical settings. In part, this may be due to technical challenges in measuring sulphate with standard pathology equipment and hence the limited findings of perturbed sulphate homoeostasis affecting human health. This review article is aimed at highlighting the importance of sulphate in mammalian development, with basic science research being translated through animal models and linkage to human disorders.

Response to the dorsal anterior gradient of EGFR signaling in Drosophila oogenesis is prepatterned by earlier posterior EGFR activation

Spatially restricted epidermal growth factor receptor (EGFR) activity plays a central role in patterning the follicular epithelium of the Drosophila ovary. In midoogenesis, localized EGFR activation is achieved by the graded dorsal anterior localization of its ligand, Gurken. Graded EGFR activity determines multiple dorsal anterior fates along the dorsal-ventral axis but cannot explain the sharp posterior limit of this domain. Here, we show that posterior follicle cells express the T-box transcription factors Midline and H15, which render cells unable to adopt a dorsal anterior fate in response to EGFR activation. The posterior expression of Midline and H15 is itself induced in early oogenesis by posteriorly localized EGFR signaling, defining a feedback loop in which early induction of Mid and H15 confers a molecular memory that fundamentally alters the outcome of later EGFR signaling. Spatial regulation of the EGFR pathway thus occurs both through localization of the ligand and through localized regulation of the cellular response.

The Drosophila GOLPH3 homolog regulates the biosynthesis of heparan sulfate proteoglycans by modulating the retrograde trafficking of exostosins

The exostosin (EXT) genes encode glycosyltransferases required for glycosaminoglycan chain polymerization in the biosynthesis of heparan sulfate proteoglycans (HSPGs). Mutations in the tumor suppressor genes EXT1 and EXT2 disturb HSPG biosynthesis and cause multiple osteochondroma (MO). How EXT1 and EXT2 traffic within the Golgi complex is not clear. Here, we show that Rotini (Rti), the Drosophila GOLPH3, regulates the retrograde trafficking of EXTs. A reduction in Rti shifts the steady-state distribution of EXTs to the trans-Golgi. These accumulated EXTs tend to be degraded and their re-entrance towards the route for polymerizing GAG chains is disengaged. Conversely, EXTs are mislocalized towards the transitional endoplasmic reticulum/cis-Golgi when Rti is overexpressed. Both loss of function and overexpression of rti result in incomplete HSPGs and perturb Hedgehog signaling. Consistent with Drosophila, GOLPH3 modulates the dynamic retention and protein stability of EXT1/2 in mammalian species. Our data demonstrate that GOLPH3 modulates the activities of EXTs, thus implicating a putative role for GOLPH3 in the formation of MO.

Localized serine protease activity and the establishment of Drosophila embryonic dorsoventral polarity

Drosophila embryo dorsoventral polarity is established by a maternally encoded signal transduction pathway in which three sequentially acting serine proteases, Gastrulation Defective, Snake and Easter, generate the ligand that activates the Toll receptor on the ventral side of the embryo. The spatial regulation of this pathway depends upon ventrally restricted expression of the Pipe sulfotransferase in the ovarian follicle during egg formation. Several recent observations have advanced our understanding of the mechanism regulating the spatially restricted activation of Toll. First, several protein components of the vitelline membrane layer of the eggshell have been determined to be targets of Pipe-mediated sulfation. Second, the processing of Easter by Snake has been identified as the first Pipe-dependent, ventrally-restricted processing event in the pathway. Finally, Gastrulation Defective has been shown to undergo Pipe-dependent, ventral localization within the perivitelline space and to facilitate Snake-mediated processing of Easter. Together, these observations suggest that Gastrulation Defective, localized on the interior ventral surface of the eggshell in association with Pipe-sulfated eggshell proteins, recruits and mediates an interaction between Snake and Easter. This event leads to ventrally-restricted processing and activation of Easter and consequently, localized formation of the Toll ligand, and Toll activation.

A genome-wide screen indicates correlation between differentiation and expression of metabolism related genes

Differentiated tissues may be considered as materials with distinct properties. The differentiation program of a given tissue ensures that it acquires material properties commensurate with its function. It may be hypothesized that some of these properties are acquired through production of tissue-specific metabolites synthesized by metabolic enzymes. To establish correlation between metabolism and organogenesis we have carried out a genome-wide expression study of metabolism related genes by RNA in-situ hybridization. 23% of the metabolism related genes studied are expressed in a tissue-restricted but not tissue-exclusive manner. We have conducted the screen on whole mount chicken (Gallus gallus) embryos from four distinct developmental stages to correlate dynamic changes in expression patterns of metabolic enzymes with spatio-temporally unique developmental events. Our data strongly suggests that unique combinations of metabolism related genes, and not specific metabolic pathways, are upregulated during differentiation. Further, expression of metabolism related genes in well established signaling centers that regulate different aspects of morphogenesis indicates developmental roles of some of the metabolism related genes. The database of tissue-restricted expression patterns of metabolism related genes, generated in this study, should serve as a resource for systematic identification of these genes with tissue-specific functions during development. Finally, comprehensive understanding of differentiation is not possible unless the downstream genes of a differentiation cascade are identified. We propose, metabolic enzymes constitute a significant portion of these downstream target genes. Thus our study should help elucidate different aspects of tissue differentiation.

Creating gradients by morphogen shuttling

Morphogen gradients are used to pattern a field of cells according to variations in the concentration of a signaling molecule. Typically, the morphogen emanates from a confined group of cells. During early embryogenesis, however, the ability to define a restricted source for morphogen production is limited. Thus, various early patterning systems rely on a broadly expressed morphogen that generates an activation gradient within its expression domain. Computational and experimental work has shed light on how a sharp and robust gradient can be established under those situations, leading to a mechanism termed 'morphogen shuttling'. This mechanism relies on an extracellular shuttling molecule that forms an inert, highly diffusible complex with the morphogen. Morphogen release from the complex following cleavage of the shuttling molecule by an extracellular protease leads to the accumulation of free ligand at the center of its expression domain and a graded activation of the developmental pathway that decreases significantly even within the morphogen-expression domain.

Self-organized shuttling: generating sharp dorsoventral polarity in the early Drosophila embryo

Morphogen gradients pattern tissues and organs during development. When morphogen production is spatially restricted, diffusion and degradation are sufficient to generate sharp concentration gradients. It is less clear how sharp gradients can arise within the source of a broadly expressed morphogen. A recent solution relies on localized production of an inhibitor outside the domain of morphogen production, which effectively redistributes (shuttles) and concentrates the morphogen within its expression domain. Here, we study how a sharp gradient is established without a localized inhibitor, focusing on early dorsoventral patterning of the Drosophila embryo, where an active ligand and its inhibitor are concomitantly generated in a broad ventral domain. Using theory and experiments, we show that a sharp Toll activation gradient is produced through "self-organized shuttling," which dynamically relocalizes inhibitor production to lateral regions, followed by inhibitor-dependent ventral shuttling of the activating ligand Spätzle. Shuttling may represent a general paradigm for patterning early embryos.

EGFR-dependent downregulation of Capicua and the establishment of Drosophila dorsoventral polarity

Dorsoventral (DV) axis formation in Drosophila begins during oogenesis through the graded activation of the EGF receptor (EGFR)-Ras-MAPK signaling pathway in the follicle cell layer of the egg chamber. EGFR signaling, which is higher in dorsal follicle cells, represses expression of the sulfotransferase-encoding gene pipe, thereby delimiting a ventral domain of Pipe activity that is critical for the subsequent induction of ventral embryonic fates. We have characterized the transcriptional circuit that links EGFR signaling to pipe repression: in dorsal follicle cells, the homeodomain transcription factor Mirror (Mirr), which is induced by EGFR signaling, directly represses pipe transcription, whereas in ventral follicle cells, the HMG-box protein Capicua (Cic) supports pipe expression by repressing mirr. Although Cic is under negative post-transcriptional regulation by Ras-MAPK signaling in different contexts, the relevance of this mechanism for the interpretation of the EGFR signal during DV pattern formation remains unclear. Here, we consider a model where EGFR-mediated downregulation of Cic modulates the spatial distribution of Mirr protein in lateral follicle cells, thereby contributing to define the position at which the pipe expression border is formed.

A ventrally localized protease in the Drosophila egg controls embryo dorsoventral polarity

Drosophila embryo dorsoventral (DV) polarity is defined by serine protease activity in the perivitelline space (PVS) between the embryonic membrane and the inner layer of the eggshell. Gastrulation Defective (GD) cleaves and activates Snake (Snk). Activated Snk cleaves and activates Easter (Ea), exclusively on the ventral side of the embryo. Activated Ea then processes Spätzle (Spz) into the activating ligand for Toll, a transmembrane receptor that is distributed throughout the embryonic plasma membrane. Ventral activation of Toll depends upon the activity of the Pipe sulfotransferase in the ventral region of the follicular epithelium that surrounds the developing oocyte. Pipe transfers sulfate residues to several protein components of the inner vitelline membrane layer of the eggshell. Here we show that GD protein becomes localized in the ventral PVS in a Pipe-dependent process. Moreover, ventrally concentrated GD acts to promote the cleavage of Ea by Snk through an extracatalytic mechanism that is distinct from GD's proteolytic activation of Snk. Together, these observations illuminate the mechanism through which spatially restricted sulfotransferase activity in the developing egg chamber leads to localization of serine protease activity and ultimately to spatially specific activation of the Toll receptor in the Drosophila embryo.

Mirror represses pipe expression in follicle cells to initiate dorsoventral axis formation in Drosophila

Dorsoventral (DV) axis formation in Drosophila begins with selective activation of EGFR, a receptor tyrosine kinase (RTK), in dorsal-anterior (DA) ovarian follicle cells. A critical event regulated by EGFR signaling is the repression of the sulfotransferase-encoding gene pipe in dorsal follicle cells, but how this occurs remains unclear. Here we show that Mirror (Mirr), a homeodomain transcription factor induced by EGFR signaling in DA follicle cells, directly represses pipe expression by binding to a conserved element in the pipe regulatory region. In addition, we find that the HMG-box protein Capicua (Cic) supports pipe expression in ventral follicle cells by repressing Mirr in this region. Interestingly, this role of Cic resembles its function in regulating anteroposterior (AP) body patterning, where Cic supports gap gene expression in central regions of the embryo by repressing Tailless, a repressor induced by RTK signaling at the embryonic poles. Thus, related RTK-Cic repressor circuits regulate the early stages of Drosophila DV and AP body axis formation.

Transcriptional interpretation of the EGF receptor signaling gradient

Epidermal growth factor receptor (EGFR) controls a wide range of developmental events, from body axes specification in insects to cardiac development in humans. During Drosophila oogenesis, a gradient of EGFR activation patterns the follicular epithelium. Multiple transcriptional targets of EGFR in this tissue have been identified, but their regulatory elements are essentially unknown. We report the regulatory elements of broad (br) and pipe (pip), two important targets of EGFR signaling in Drosophila oogenesis. br is expressed in a complex pattern that prefigures the formation of respiratory eggshell appendages. We found that this pattern is generated by dynamic activities of two regulatory elements, which display different responses to Pointed, Capicua, and Mirror, transcription factors involved in the EGFR-mediated gene expression. One of these elements is active in a pattern similar to pip, a gene repressed by EGFR and essential for establishing the dorsoventral polarity of the embryo. We demonstrate that this similarity of expression depends on a common sequence motif that binds Mirror in vitro and is essential for transcriptional repression in vivo.

Molecular mechanisms of EGF signaling-dependent regulation of pipe, a gene crucial for dorsoventral axis formation in Drosophila

During Drosophila oogenesis the expression of the sulfotransferase Pipe in ventral follicle cells is crucial for dorsoventral axis formation. Pipe modifies proteins that are incorporated in the ventral eggshell and activate Toll signaling which in turn initiates embryonic dorsoventral patterning. Ventral pipe expression is the result of an oocyte-derived EGF signal which down-regulates pipe in dorsal follicle cells. The analysis of mutant follicle cell clones reveals that none of the transcription factors known to act downstream of EGF signaling in Drosophila is required or sufficient for pipe regulation. However, the pipe cis-regulatory region harbors a 31-bp element which is essential for pipe repression, and ovarian extracts contain a protein that binds this element. Thus, EGF signaling does not act by down-regulating an activator of pipe as previously suggested but rather by activating a repressor. Surprisingly, this repressor acts independent of the common co-repressors Groucho or CtBP.

Pattern formation by receptor tyrosine kinases: analysis of the Gurken gradient in Drosophila oogenesis

Spatial patterns of cell differentiation in developing tissues can be controlled by receptor tyrosine kinase (RTK) signaling gradients, which may form when locally secreted ligands activate uniformly expressed receptors. Graded activation of RTKs can span multiple cell diameters, giving rise to spatiotemporal patterns of signaling through the Extracellular Signal Regulated/Mitogen Activated Protein Kinase (ERK/MAPK), which connects receptor activation to multiple aspects of tissue morphogenesis. This general mechanism has been identified in numerous developmental contexts, from body axis specification in insects to patterning of the mammalian neocortex. We review recent quantitative studies of this mechanism in Drosophila oogenesis, an established genetic model of signaling through the Epidermal Growth Factor Receptor (EGFR), a highly conserved RTK.

Sulfate in fetal development

Sulfate (SO(4)(2-)) is an important nutrient for human growth and development, and is obtained from the diet and the intra-cellular metabolism of sulfur-containing amino acids, including methionine and cysteine. During pregnancy, fetal tissues have a limited capacity to produce sulfate, and rely on sulfate obtained from the maternal circulation. Sulfate enters and exits placental and fetal cells via transporters on the plasma membrane, which maintain a sufficient intracellular supply of sulfate and its universal sulfonate donor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) for sulfate conjugation (sulfonation) reactions to function effectively. Sulfotransferases mediate sulfonation of numerous endogenous compounds, including proteins and steroids, which biotransforms their biological activities. In addition, sulfonation of proteoglycans is important for maintaining normal structure and development of tissues, as shown for reduced sulfonation of cartilage proteoglycans that leads to developmental dwarfism disorders and four different osteochondrodysplasias (diastrophic dysplasia, atelosteogenesis type II, achondrogenesis type IB and multiple epiphyseal dysplasia). The removal of sulfate via sulfatases is an important step in proteoglycan degradation, and defects in several sulfatases are linked to perturbed fetal bone development, including mesomelia-synostoses syndrome and chondrodysplasia punctata 1. In recent years, interest in sulfate and its role in developmental biology has expanded following the characterisation of sulfate transporters, sulfotransferases and sulfatases and their involvement in fetal growth. This review will focus on the physiological roles of sulfate in fetal development, with links to human and animal pathophysiologies.

The evolution of dorsal-ventral patterning mechanisms in insects

The gene regulatory network (GRN) underpinning dorsal-ventral (DV) patterning of the Drosophila embryo is among the most thoroughly understood GRNs, making it an ideal system for comparative studies seeking to understand the evolution of development. With the emergence of widely applicable techniques for testing gene function, species with sequenced genomes, and multiple tractable species with diverse developmental modes, a phylogenetically broad and molecularly deep understanding of the evolution of DV axis formation in insects is feasible. Here, we review recent progress made in this field, compare our emerging molecular understanding to classical embryological experiments, and suggest future directions of inquiry.

Identification of chondroitin/dermatan sulfotransferases in the protochordate, Ciona intestinalis

Sulfated glycosaminoglycans are important components of connective tissues. The pattern of sulfation is important for their biological functions. Ascidians, the closest relatives of vertebrates, have a simple chordate body plan. In the present study, we identified an almost complete set of genes encoding proteins homologous to chondroitin/dermatan sulfotransferases in the genome of the ascidian Ciona intestinalis. We found eight genes encoding 4-O-sulfotransferases, eight genes encoding 6-O-sulfotransferases, and three genes encoding uronyl 2-O-sulfotransferases. The number of sulfotransferase genes was unexpectedly large, considering that ascidians do not have a well-developed endoskeleton. In addition, most of the genes within each sub-family seemed to have arisen by gene duplication events that occurred in the ascidian lineage after divergence from the main chordate lineage. This suggests that a unique pattern of sulfation independently developed during ascidian evolution. Some of the genes identified in the present study showed tissue-specific expression in the epidermis, notochord, muscle, and central nervous system. Region-specific expression in the epidermis was also observed. The present study provides useful information for further comparative and functional analyses of sulfotransferases and proteoglycans in chordate embryos.

Graded dorsal and differential gene regulation in the Drosophila embryo

A gradient of Dorsal activity patterns the dorsoventral (DV) axis of the early Drosophila melanogaster embryo by controlling the expression of genes that delineate presumptive mesoderm, neuroectoderm, and dorsal ectoderm. The availability of the Drosophila melanogaster genome sequence has accelerated the study of embryonic DV patterning, enabling the use of systems-level approaches. As a result, our understanding of Dorsal-dependent gene regulation has expanded to encompass a collection of more than 50 genes and 30 cis-regulatory sequences. This information, which has been integrated into a spatiotemporal atlas of gene regulatory interactions, comprises one of the best-understood networks controlling any developmental process to date. In this article, we focus on how Dorsal controls differential gene expression and how recent studies have expanded our understanding of Drosophila embryonic development from the cis-regulatory level to that controlling morphogenesis of the embryo.

Drosophila vitelline membrane assembly: a critical role for an evolutionarily conserved cysteine in the "VM domain" of sV23

The vitelline membrane (VM), the oocyte proximal layer of the Drosophila eggshell, contains four major proteins (VMPs) that possess a highly conserved "VM domain" which includes three precisely spaced, evolutionarily conserved, cysteines (CX⁷CX⁸C). Focusing on sV23, this study showed that the three cysteines are not functionally equivalent. While substitution mutations at the first (C123S) or third (C140S) cysteines were tolerated, females with a substitution at the second position (C131S) were sterile. Fractionation studies showed that sV23 incorporates into a large disulfide linked network well after its secretion ceases, suggesting that post-depositional mechanisms are in place to restrict disulfide bond formation until late oogenesis, when the oocyte no longer experiences large volume increases. Affinity chromatography utilizing histidine tagged sV23 alleles revealed small sV23 disulfide linked complexes during the early stages of eggshell formation that included other VMPs, namely sV17 and Vml. The early presence but late loss of these associations in an sV23 double cysteine mutant suggests that reorganization of disulfide bonds may underlie the regulated growth of disulfide linked networks in the vitelline membrane. Found within the context of a putative thioredoxin active site (CXXS) C131, the critical cysteine in sV23, may play an important enzymatic role in isomerizing intermolecular disulfide bonds during eggshell assembly.

Pipe-dependent ventral processing of Easter by Snake is the defining step in Drosophila embryo DV axis formation

The establishment of Drosophila embryonic dorsal-ventral (DV) polarity relies on serine proteolytic activity in the perivitelline space between the embryonic membrane and the eggshell. Gastrulation Defective cleaves and activates Snake, which processes and activates Easter, which cleaves Spätzle to form the activating ligand for the Toll receptor. Ventral restriction of ligand formation depends on the Pipe sulfotransferase, which is expressed in ventral cells of the follicular epithelium surrounding the developing oocyte. Pipe modifies components of the developing eggshell to produce a ventral cue embedded in the vitelline membrane. This ventral cue is believed to promote one or more of the proteolysis steps in the perivitelline space. By examining the processing of transgenic, tagged versions of the perivitelline proteins during DV patterning, we find that the proteolysis of Easter by Snake is the first Pipe-dependent step and therefore the key ventrally restricted event in the protease cascade. We also find that Snake and Easter associate together in a complex in both wild-type and pipe mutant-derived embryos. This observation suggests a mechanism in which the sulfated target of Pipe promotes a productive interaction between Snake and Easter, perhaps by facilitating conformational changes in a complex containing the two proteins.

EGF signaling and the origin of axial polarity among the insects

The eggs of insects are unusual in that they often have bilateral symmetry when they are laid, indicating that both anterior-posterior (AP) and dorsal-ventral (DV) symmetries are broken during oogenesis. The molecular basis of this process is well understood in Drosophila melanogaster, in which symmetry breaking events for both axes depend on the asymmetric position of the oocyte nucleus and on germline-soma signaling mediated by the Tgf alpha-like epidermal growth factor (EGF) ligand Gurken. Germline-soma signaling interactions centered around the oocyte nucleus have been proposed in other insect species, but the molecular nature of these interactions has not been elucidated. We have examined the behavior of the oocyte nucleus and the function of EGF signaling components in the ovaries of the wasp Nasonia vitripennis, the beetle Tribolium castaneum, and the cricket Gryllus bimaculatus. We have found that EGF signaling has broadly conserved roles in mediating the encapsulation of oocytes by the somatic follicle cell layer, in establishing polarity of the egg chambers, and in setting up the DV axis of the embryo. These results provide insights into the evolutionary origins of the unique strategy employed by insects to establish embryonic axial polarity during oogenesis.

Sulfatase activities towards the regulation of cell metabolism and signaling in mammals

In higher vertebrates, sulfatases belong to a conserved family of enzymes that are involved in the regulation of cell metabolism and in developmental cell signaling. They cleave the sulfate from sulfate esters contained in hormones, proteins, and complex macromolecules. A highly conserved cysteine in their active site is post-translationally converted into formylglycine by the formylglycine-generating enzyme encoded by SUMF1 (sulfatase modifying factor 1). This post-translational modification activates all sulfatases. Sulfatases are extensively glycosylated proteins and some of them follow trafficking pathways through cells, being secreted and taken up by distant cells. Many proteoglycans, glycoproteins, and glycolipids contain sulfated carbohydrates, which are sulfatase substrates. Indeed, sulfatases operate as decoding factors for a large amount of biological information contained in the structures of the sulfated sugar chains that are covalently linked to proteins and lipids. Modifications to these sulfate groups have pivotal roles in modulating specific signaling pathways and cell metabolism in mammals.

The PAPS transporter PST-1 is required for heparan sulfation and is essential for viability and neural development in C. elegans

Sulfations of sugars, such as heparan sulfates (HS), or tyrosines require the universal sulfate donor 3'-phospho-adenosine-5'-phosphosulfate (PAPS) to be transported from the cytosol into the Golgi. Metazoan genomes encode two putative PAPS transporters (PAPST1 and PAPST2), which have been shown in vitro to preferentially transport PAPS across membranes. We have identified the C. elegans orthologs of PAPST1 and PAPST2 and named them pst-1 and pst-2, respectively. We show that pst-1 is essential for viability in C. elegans, functions non-redundantly with pst-2, and can act non-autonomously to mediate essential functions. Additionally, pst-1 is required for specific aspects of nervous system development rather than for formation of the major neuronal ganglia or fascicles. Neuronal defects correlate with reduced complexity of HS modification patterns, as measured by direct biochemical analysis. Our results suggest that pst-1 functions in metazoans to establish the complex HS modification patterns that are required for the development of neuronal connectivity.

Distinct functional specificities are associated with protein isoforms encoded by the Drosophila dorsal-ventral patterning gene pipe

Spatially regulated transcription of the pipe gene in ventral cells of the Drosophila ovary follicle cell epithelium is a key event that specifies progeny embryo dorsal-ventral (DV) polarity. pipe encodes ten putative protein isoforms, all of which exhibit similarity to vertebrate glycosaminoglycan-modifying enzymes. Expression of one of the isoforms, Pipe-ST2, in follicle cells has previously been shown to be essential for DV patterning. pipe is also expressed in the embryonic salivary gland and its expression there is required for normal viability. Here, we show that in addition to Pipe-ST2, seven of the other Pipe isoforms are expressed in the ovary, whereas all Pipe isoforms are abundantly expressed in the embryo. Of the ten isoforms, only Pipe-ST2 can restore ventral and lateral pattern elements to the progeny of otherwise pipe-null mutant females. By contrast, three Pipe isoforms, but not Pipe-ST2, support the production of a novel pipe-dependent epitope present in the embryonic salivary gland. These data indicate that differences in functional specificity, and presumably enzymatic specificity, are associated with several of the Pipe isoforms. In addition, we show that uniform expression of the Pipe-ST2 isoform in the follicle cell layer of females otherwise lacking pipe expression leads to the formation of embryos with a DV axis that is appropriately oriented with respect to the intrinsic polarity of the eggshell. This suggests the existence of a second mechanism that polarizes the Drosophila embryo, in addition to the ventrally restricted transcription of the pipe gene.

Combinatorial patterning mechanisms in the Drosophila embryo

The classical concept of the morphogen gradient proposes that small differences in the levels of a signalling molecule or transcription factor are responsible for producing a continuous spectrum of distinctive cellular identities across a naïve field of cells. In this review, we discuss how the Dorsal gradient controls the dorsal-ventral patterning of the early Drosophila embryo. This gradient extends from the ventral midline of the embryo into dorso-lateral regions, encompassing a cross-sectional field of approximately 20 cells. There is no evidence that these cells acquire distinctive identities due to subtle changes in the nuclear concentrations of the Dorsal protein. Rather, a variety of evidence suggests that the Dorsal gradient generates just three primary thresholds of gene activity. High levels activate gene expression in the presumptive mesoderm, while intermediate and low levels activate gene expression in the ventral and dorsal neurogenic ectoderm, respectively. We discuss how these primary readouts of the gradient establish localized domains of cell signalling, which work in a combinatorial manner with transcriptional networks to produce complex patterns of gene expression and tissue differentiation.

Gastrulation defective protease interacts with anionic components of the Drosophila ovary extracellular matrix

The Drosophila proteases Gastrulation Defective and Snake function in embryonic polarity establishment and bind heparin, a surrogate for anionic species present in the extracellular matrix. Here we demonstrate binding of GD, but not Snake, to anionic species that appear to be tightly associated with a highly purified eggshell matrix.

Sulfation of eggshell components by Pipe defines dorsal-ventral polarity in the Drosophila embryo

Drosophila embryonic dorsal-ventral (DV) polarity is controlled by a group of sequentially acting serine proteases located in the fluid-filled perivitelline space between the embryonic membrane and the eggshell, which generate the ligand for the Toll receptor on the ventral side of the embryo. Spatial control of the protease cascade relies on the Pipe sulfotransferase, a fly homolog of vertebrate glycosaminoglycan-modifying enzymes, which is expressed in ventral cells of the follicular epithelium surrounding the developing oocyte. Here we show that the vitelline membrane-like (VML) protein undergoes Pipe-dependent sulfation and, consistent with a role in conveying positional information from the egg chamber to the embryo, becomes incorporated into the eggshell at a position corresponding to the location of the follicle cells from which it was secreted. Although VML influences embryonic DV pattern in a sensitized genetic background, VML is not essential for DV axis formation, suggesting that there is redundancy in the composition of the Pipe enzymatic target. Correspondingly, we find that additional structural components of the vitelline membrane undergo Pipe-dependent sulfation. In identifying the elusive targets of Pipe, this work points to the vitelline membrane as the source of signals that generate the Drosophila DV axis.

Glycosphingolipids control the extracellular gradient of the Drosophila EGFR ligand Gurken

Glycosphingolipids (GSLs) are present in all eukaryotic membranes and are implicated in neuropathologies and tumor progression in humans. Nevertheless, their in vivo functions remain poorly understood in vertebrates, partly owing to redundancy in the enzymes elongating their sugar chains. In Drosophila, a single GSL biosynthetic pathway is present that relies on the activity of the Egghead and Brainiac glycosyltransferases. Mutations in these two enzymes abolish GSL elongation and yield oogenesis defects, providing a unique model system in which to study GSL roles in signaling in vivo. Here, we use egghead and brainiac mutants to show that GSLs are necessary for full activation of the EGFR pathway during oogenesis in a time-dependent manner. In contrast to results from in vitro studies, we find that GSLs are required in cells producing the TGFalpha-like ligand Gurken, but not in EGFR-expressing cells. Strikingly, we find that GSLs are not essential for Gurken trafficking and secretion. However, we characterize for the first time the extracellular Gurken gradient and show that GSLs affect its formation by controlling Gurken planar transport in the extracellular space. This work presents the first in vivo evidence that GSLs act in trans to regulate the EGFR pathway and shows that extracellular EGFR ligand distribution is tightly controlled by GSLs. Our study assigns a novel role for GSLs in morphogen diffusion, possibly through regulation of their conformation.

A combinatorial code for pattern formation in Drosophila oogenesis

Two-dimensional patterning of the follicular epithelium in Drosophila oogenesis is required for the formation of three-dimensional eggshell structures. Our analysis of a large number of published gene expression patterns in the follicle cells suggests that they follow a simple combinatorial code based on six spatial building blocks and the operations of union, difference, intersection, and addition. The building blocks are related to the distribution of inductive signals, provided by the highly conserved epidermal growth factor receptor and bone morphogenetic protein signaling pathways. We demonstrate the validity of the code by testing it against a set of patterns obtained in a large-scale transcriptional profiling experiment. Using the proposed code, we distinguish 36 distinct patterns for 81 genes expressed in the follicular epithelium and characterize their joint dynamics over four stages of oogenesis. The proposed combinatorial framework allows systematic analysis of the diversity and dynamics of two-dimensional transcriptional patterns and guides future studies of gene regulation.

Distinct heparan sulfate compositions in wild-type and pipe-mutant eggshell matrix

Spatial information embedded in the extracellular matrix establishes the dorsoventral polarity of the Drosophila embryo through the ventral activity of a serine protease cascade. Pipe is a Golgi-localized protein responsible for generating this spatial information during oogenesis through sulfation of unknown glycans. Although Pipe has sequence homology to glycosaminoglycan 2-O-sulfotransferases, its activity and authentic substrates have not been demonstrated and genetic evidence has argued against a role for glycosaminoglycans in dorsoventral polarity establishment. Here, direct examination of matrix glycosaminoglycans demonstrates that pipe-mutant matrix shows decreased tri-sulfated heparan sulfate compared to wild-type matrix, with correspondingly increased 2-O-sulfated heparan sulfate. Chondroitin sulfate was not detected in this matrix. These results suggest that Pipe promotes 6-O- and/or N-sulfation of heparan sulfate but is not required for heparan sulfate 2-O-sulfation. We discuss the possible significance of these unexpected findings and how they might be reconciled with the genetic data.