Evolution of patterning

Developing tissues are patterned in space and time; this enables them to differentiate their cell types and form complex structures to support different body plans. Although space and time are two independent entities, there are many examples of spatial patterns that originate from temporal ones. The most prominent example is the expression of the genes hunchback, Krüppel, pdm, and castor, which are expressed temporally in the neural stem cells of the Drosophila ventral nerve cord and spatially along the anteroposterior axis of the blastoderm stage embryo. In this Viewpoint, we investigate the relationship between space and time in specific examples of spatial and temporal patterns with the aim of gaining insight into the evolutionary history of patterning.

Free volume theory explains the unusual behavior of viscosity in a non-confluent tissue during morphogenesis

A recent experiment on zebrafish blastoderm morphogenesis showed that the viscosity (η) of a non-confluent embryonic tissue grows sharply until a critical cell packing fraction (ϕS). The increase in η up to ϕS is similar to the behavior observed in several glass-forming materials, which suggests that the cell dynamics is sluggish or glass-like. Surprisingly, η is a constant above ϕS. To determine the mechanism of this unusual dependence of η on ϕ, we performed extensive simulations using an agent-based model of a dense non-confluent two-dimensional tissue. We show that polydispersity in the cell size, and the propensity of the cells to deform, results in the saturation of the available free area per cell beyond a critical packing fraction. Saturation in the free space not only explains the viscosity plateau above ϕS but also provides a relationship between equilibrium geometrical packing to the dramatic increase in the relaxation dynamics.

Protocol for extracting live blastoderm cells from embryos of annual killifish

The implementation of in vitro approaches using undifferentiated embryonic cells from annual killifish to complement existing in vivo developmental studies has been hindered by a lack of efficient isolation techniques. Here, we present a protocol to isolate annual killifish blastoderm cells, at the epiboly and early dispersion phase, from embryos. We describe steps for hair removal, embryo cleaning, dechorionation, and cell purification. This protocol may also be used to develop strategies to isolate cells from embryos presenting similar challenges.

Imaging and quantification of apical microvilli in the syncytial blastoderm of Drosophila embryos

The syncytial Drosophila blastoderm embryo contains apical microvilli with filamentous actin that are remodeled during nuclear division cycles 10-13. Here, we describe a protocol for preparing whole embryo samples and capturing images of microvilli using confocal and super-resolution STED microscopy. This protocol enables visualization and quantification of lengths and numbers of microvilli oriented along the imaging plane. We provide information on identifying different nuclear division cycles and examples of quantification from the interphase and metaphase of cycle 12. For complete details on the use and execution of this protocol, please refer to Sherlekar et al. (2020).

Molecular characteristics of the edge cells responsible for expansion of the chick embryo on the vitelline membrane

During early avian development, only a narrow band of cells (the edge cells, also called 'margin of overgrowth') at the rim of the embryo is responsible for blastoderm expansion by crawling over the vitelline membrane (VM) to cover the whole egg yolk in just 4 days (a process called epiboly). Surprisingly, this has not yet been studied in detail. Here we explore the edge cells of the chick embryo using in situ hybridization, immunohistochemistry and live imaging. Morphological and molecular properties reveal that the edge has a distinctive structure, being subdivided into sub-regions, including at least four distinct zones (which we name as leading, trailing, deep and stalk zones). This allows us to study reorganization of the edge region that accompanies reattachment of an explanted blastoderm to the VM. Immunohistochemistry uncovers distinct polarized cellular features resembling the process of collective cell migration described in other systems. Live imaging reveals dynamic lamellipodial and filopodial activity at the leading edge of the outermost cells. Our data provide evidence that edge cells are a distinct tissue. We propose that edge cells may be a useful model system for the study of wound healing and other closure events in epithelial cell sheets.

The normal stages of development for the California valley quail

One challenge in avian embryology is establishing a standard developmental timetable, primarily because eggs incubated for identical durations can vary in developmental progress, even within the same species. For remedy, avian development is classified into distinct stages based on the formation of key morphological structures. Developmental stages exist for a few galliform species, but the literature is lacking a description of normal stages for California valley quail (Callipepla californica). Thus, the objective of this study was to stage and document the morphological and structural development of California valley quail. Over two laying seasons, 390 eggs were incubated at 37.8֯ C in 60% RH for ≤23 days. Eggs were opened every ≤6 hours to document embryonic development, including, blastoderm diameter, anterior angle of nostril to beak tip, and lengths of wing, tarsus, third toe, total beak, total foot, and embryo. California valley quail embryos were staged and compared to domestic chicken (Gallus gallus domesticus), the staging standard for galliformes, as well as Japanese quail (Coturnix japonica), blue-breasted quail (Synoicus chinensis) and northern bobwhite quail (Colinus virginianus). This study produced the first description of the 43 normal stages of development for California valley quail. Compared with other galliformes, the California valley quail has a different number of stages and displays developmental heterochrony in stages 1-24, and morphological and developmental differences in stages 25-hatch. The observed differences emphasize the importance of staging individual avian species instead of relying on poultry animal models or close relatives for developmental reference. This is extremely important in species-specific embryological studies that evaluate critical windows of development or evaluate the impacts of environmental change on avian development. This study also suggests that staging frequencies of ≤6 hours and egg transport protocols should be standardized for future staging studies.

Preformation and epigenesis converge to specify primordial germ cell fate in the early Drosophila embryo

A critical step in animal development is the specification of primordial germ cells (PGCs), the precursors of the germline. Two seemingly mutually exclusive mechanisms are implemented across the animal kingdom: epigenesis and preformation. In epigenesis, PGC specification is non-autonomous and depends on extrinsic signaling pathways. The BMP pathway provides the key PGC specification signals in mammals. Preformation is autonomous and mediated by determinants localized within PGCs. In Drosophila, a classic example of preformation, constituents of the germ plasm localized at the embryonic posterior are thought to be both necessary and sufficient for proper determination of PGCs. Contrary to this longstanding model, here we show that these localized determinants are insufficient by themselves to direct PGC specification in blastoderm stage embryos. Instead, we find that the BMP signaling pathway is required at multiple steps during the specification process and functions in conjunction with components of the germ plasm to orchestrate PGC fate.

Proper specification of primordial germ cells (PGCs) is crucial as PGCs serve as the precursors of germline stem cells. To specify PGC fate, invertebrates rely upon cell autonomous preformation involving maternally deposited germ plasm. In Drosophila melanogaster, to insulate newly formed PGCs from the adverse effects of the cell-cell signaling pathways, germ plasm determinants silence transcription and attenuate the cell cycle. However, our data on the BMP signaling pathway challenge this long-held view of PGC specification and suggest that appropriate specification of embryonic PGCs is sensitive to the BMP ligand, decapentaplegic (dpp), and its cognate receptor, thickveins. We find that PGCs are not only capable of responding to BMP signals from the soma, but also that these signals impact the proper determination of the germ cells. Based on these unanticipated similarities between mammals and flies, we propose a model integrating contribution of both the cell-autonomous (preformation) and non-autonomous (epigenesis) pathways during PGC determination. Consistent with the model, we have observed dominant genetic interactions between, oskar, the maternal determinant of PGC fate, and the BMP pathway ligand dpp.

Coordination of Embryogenesis by the Centrosome in Drosophila melanogaster

The first 3 h of Drosophila melanogaster embryo development are exemplified by rapid nuclear divisions within a large syncytium, transforming the zygote to the cellular blastoderm after 13 successive cleavage divisions. As the syncytial embryo develops, it relies on centrosomes and cytoskeletal dynamics to transport nuclei, maintain uniform nuclear distribution throughout cleavage cycles, ensure generation of germ cells, and coordinate cellularization. For the sake of this review, we classify six early embryo stages that rely on processes coordinated by the centrosome and its regulation of the cytoskeleton. The first stage features migration of one of the female pronuclei toward the male pronucleus following maturation of the first embryonic centrosomes. Two subsequent stages distribute the nuclei first axially and then radially in the embryo. The remaining three stages involve centrosome-actin dynamics that control cortical plasma membrane morphogenesis. In this review, we highlight the dynamics of the centrosome and its role in controlling the six stages that culminate in the cellularization of the blastoderm embryo.

Pluripotency properties of embryonic stem cells isolated from stage X blastoderm of Mazandaran native chicken

The aim of this study was to isolate Embryonic Stem Cells (ESCs) from native chicken and to characterize their pluripotency properties through the cellular and molecular markers. Samples obtained from fertilized eggs from Mazandaran native hens. Cells were isolated from area of pellucida from stage X native hens' blastoderm. Then the cells were cultured on inactivated mouse SNL feeder cells in the presence of LIF, IGF-1, bFGF, CNTF, OSM, SCF, Il-6, and Il-11 growth factors. The native chickens' ESCs colonies were picked up and subsequently passaged. To characterize the cells, they were analyzed for their alkaline phosphatase activity, and also for the expression of SSEA-4, and TRA-1-60 as embryonic-specific markers at the protein level. Furthermore, the expression of pluripotency (cPouV, Sox2, and Nanog) and cell lineage specific (Cvh, Brachyury, and Gata6) gene markers was evaluated at the level of mRNA using quantitative RT-PCR. Isolated cells were passaged repeatedly and successfully up to ten passages. The stemness of embryonic cells has been approved by the activity of the alkaline phosphatase, presence of the SSEA-4, and TRA-1-60 protein, and expression of the molecular marker (cPouV, Nanog, and Sox-2) genes. The spontaneous differentiation of chicken ESCs confirmed the pluripotecy of the cells in differentiation into specialized cell lineages. Our observation showed that ESCs can be isolated successfully from stage X blastoderm of Mazandaran native chickens and these cells maintain their stemness properties during multi-passages in vitro.

[Morphogenetic consequences of partial removal of blastomere cytoplasm during early embryonic development of the loach, Misgurnus fossilis L]

The development of loach embryos is successfully regulated (normalized) after partial removal of the cytoplasm from one blastomere at the two- or four-cell stage or complete removal of one or two blastomeres at the stage of 8-16 cells. Using time-lapse video imaging and morphometric analysis, it has been shown that this regulation is a two-stage process. At the first stage, the ratio between the volumes of the blastodisk and yolk sac is rapidly (within one or two cell cycles) restored almost to the initial level; at the second stage, morphogenesis of the embryo is modified according to its new structural features acquired after the operation. After several rounds of cytokinesis, the cytoplasm remaining in the operated blastomere fuses with the marginal yolk syncytium (periblast),which at the blastula stage forms a distinct extension at the operation site. This extension marks the site of embryonic shield formation. The results of morphometric analysis show that restoration of the initial blastoderm volume in operated embryos leads to a reduction of active tension at the blastoderm--yolk boundary and an increase in the ratio of blastoderm surface to its volume at the moment of epiboly initiation. As a result, the convergence of blastoderm cells to the operation site and the embryonic shield formation begin at a lesser degree of epiboly, compared to the control.

Artificial selection on egg size perturbs early pattern formation in Drosophila melanogaster

Pattern formation in Drosophila embryogenesis has been widely investigated as a developmental and evolutionary model of robustness. To ask whether genetic variation for pattern formation is suppressed in this system, artificial selection for divergent egg size was used to challenge the scaling of even-skipped (eve) pattern formation in mitotic cycle 14 (stage 5) embryos of Drosophila melanogaster. Three-dimensional confocal imaging revealed shifts in the allometry of eve pair-rule stripes along both anterior–posterior (A–P) and dorsoventral (D–V) axes as a correlated response to egg size selection, indicating the availability of genetic variation for this buffered trait. Environmental perturbation was not required for the manifestation of this variation. The number of nuclei at the cellular blastoderm stage also changed in response to selection, with large-egg selected lines having more than 1000 additional nuclei relative to small-egg lines. This increase in nuclear number in larger eggs does not scale with egg size, however, as nuclear density is inversely correlated with egg length. Nuclear density varies along the A–P axis but does not correlate with the shift in eve stripe allometry between the selection treatments. Despite its macroevolutionary conservation, both eve stripe patterning and blastoderm cell number vary genetically both within and between closely related species.

Early embryonic development in the spider Achaearanea tepidariorum: Microinjection verifies that cellularization is complete before the blastoderm stage

The spider Achaearanea tepidariorum is emerging as a non-insect model for studying developmental biology. However, the availability of microinjection into early embryos of this spider has not been reported. We defined the early embryonic stages in A. tepidariorum and applied microinjection to its embryos. During the preblastoderm 16- and 32-nucleus stages, the energids were moving toward the egg periphery. When fluorochrome-conjugated dextran was microinjected into the peripheral region of 16-nucleus stage embryos, it was often incorporated into a single energid and inherited in the progeny without leaking out to surrounding energids. This suggested that 16-nucleus stage embryos consisted of compartments, each containing a single energid. These compartments were considered to be separate cells. Fluorochrome-conjugated dextran could be introduced into single cells of 16- to 128-nucleus stage embryos, allowing us to track cell fate and movement. Injection with mRNA encoding a nuclear localization signal/green fluorescent protein fusion construct demonstrated exogenous expression of the protein in live spider embryos. We propose that use of microinjection will facilitate studies of spider development. Furthermore, these data imply that in contrast to the Drosophila syncytial blastoderm embryo, the cell-based structure of the Achaearanea blastoderm embryo restricts diffusion of cytoplasmic gene products.

The TAGteam DNA motif controls the timing of Drosophila pre-blastoderm transcription

The Drosophila sex-determination switch gene Sex-lethal (Sxl) and the X-chromosome signal element genes (XSEs) that induce the female-specific expression of Sxl are transcribed extremely early in development when most of the genome of this organism is still silent. The DNA sequence CAGGTAG had been implicated in this pre-cellular blastoderm activation of sex-determination genes. A genome-wide computational search, reported here, suggested that CAGGTAG is not specific to early sex-determination genes, since it is over-represented upstream of most genes that are transcribed pre-cellular blastoderm, not just those involved in sex determination. The same search identified similarly over-represented, one-base-pair degenerate sequences as possible functional synonyms of CAGGTAG. We call these heptamers collectively, the TAGteam. Relevance of the TAGteam sequences to pre-cellular blastoderm transcription was established through analysis of TAGteam changes in Sxl, scute (an XSE), and the ;ventral repression element' of the pattern-formation gene zerknüllt. Decreasing the number of TAGteam sites retarded the onset of pre-blastoderm transcription, whereas increasing their number correlated with an advanced onset. Titration of repressors was thought to be the rate-limiting step determining the onset of such early transcription, but this TAGteam dose effect shows that activators must also play an important role in the timing of pre-blastoderm gene expression.

vasa mRNA accumulates at the posterior pole during blastoderm formation in the flour beetle Tribolium castaneum

The correct specification of germ cells during embryogenesis is a fundamental step in life that ensures the existence of the next generation. Although different species display various cellular modes of generating germ cells, the product of the vasa gene proves to be a reliable marker of primordial germ cells in metazoans [Extavour and Akam (2003) 130:5869-5884; Raz (2000) 1:1017]. Here, I report the isolation of the vasa ortholog from the red flour beetle Tribolium castaneum, named Tc-vasa, and describe its sequence, its genomic organisation and its expression pattern during early embryogenesis. Unlike in Drosophila where vasa messenger RNA (mRNA) is ubiquitously distributed in the egg, Tc-vasa mRNA gradually accumulates at the posterior egg pole during blastoderm formation. Shortly before gastrulation, Tc-vasa mRNA marks a group of intra-blastodermal cells at the posterior pole. In the germ rudiment, a ball-like group of vasa-positive cells adheres to the growth zone at the posterior end of the embryo. These vasa-positive cells likely represent the primordial germ cells that have not been described in Tribolium prior to gonad formation. At the beginning of germ growth, a small band of vasa-positive cells starts to migrate along the dorsal side of the growth zone. vasa transcription ceases during further germ band extension. In contrast to Drosophila, Tc-vasa transcripts cannot be detected in the germ cells within the gonadal anlage after segmentation is completed.

High-level expression of single-chain Fv-Fc fusion protein in serum and egg white of genetically manipulated chickens by using a retroviral vector

We report here the generation of transgenic chickens using a retroviral vector for the production of recombinant proteins. It was found that the transgene expression was suppressed when a Moloney murine leukemia virus-based retroviral vector was injected into chicken embryos at the blastodermal stage. When a concentrated viral solution was injected into the heart of developing embryos after 50 to 60 h of incubation, transgene expression was observed throughout the embryo, including the gonads. For practical production, a retroviral vector encoding an expression cassette of antiprion single-chain Fv fused with the Fc region of human immunoglobulin G1 (scFv-Fc) was injected into chicken embryos. The birds that hatched stably produced scFv-Fc in their serum and eggs at high levels (approximately 5.6 mg/ml). We obtained transgenic progeny from a transgenic chicken generated with this procedure. The transgene was stably integrated into the chromosomes of transgenic progeny. The transgenic progeny also expressed scFv-Fc in the serum and eggs.

Culture System for Embryos of Blue-Breasted Quail from the Blastoderm Stage to Hatching

The blue-breasted quail (Coturnix chinensis), the smallest species in the order Galliforms, is a candidate model animal for avian developmental engineering because it is precocious and prolific. This species requires 17 days to hatch and 8 to 9 weeks to mature to an adult body weight of about 50 g, whereas the Japanese quail (Coturnix japonica) requires 16 days to hatch and 6 to 8 weeks to mature to an adult body weight of 100 to 150 g. The early embryo is the most challenging embryonic stage in terms of culture and manipulation for avian biotechnology. We have evaluated various conditions for the culture of blue-breasted quail embryos from the blastoderm stage to hatching. A hatchability rate of 26% (10/39) is among the best of the various culture conditions examined in the present study and the embryo culture system should facilitate advances in avian biotechnology.

The genes orthodenticle and hunchback substitute for bicoid in the beetle Tribolium

In Drosophila, the morphogen Bicoid organizes anterior patterning in a concentration-dependent manner by activating the transcription of target genes such as orthodenticle (otd) and hunchback (hb), and by repressing the translation of caudal. Homologues of the bicoid gene have not been isolated in any organism apart from the higher Dipterans. In fact, head and thorax formation in other insects is poorly understood. To elucidate this process in a short-germband insect, I analysed the function of the conserved genes orthodenticle-1 (otd-1) and hb in the flour beetle Tribolium castaneum. Here I show that, in contrast to Drosophila, Tribolium otd-1 messenger RNA is maternally inherited by the embryo. Reduction of Tribolium otd-1 levels by RNA interference (RNAi) results in headless embryos. This shows that otd-1 is required for anterior patterning in Tribolium. As in Drosophila, Tribolium hb specifies posterior gnathal and thoracic segments. The head, thorax and the anterior abdomen fail to develop in otd-1/hb double-RNAi embryos. This phenotype is similar to that of strong bicoid mutants in Drosophila. I propose that otd-1 and hb are part of an ancestral anterior patterning system.

SCAR is a primary regulator of Arp2/3-dependent morphological events in Drosophila

The Arp2/3 complex and its activators, Scar/WAVE and Wiskott-Aldrich Syndrome protein (WASp), promote actin polymerization in vitro and have been proposed to influence cell shape and motility in vivo. We demonstrate that the Drosophila Scar homologue, SCAR, localizes to actin-rich structures and is required for normal cell morphology in multiple cell types throughout development. In particular, SCAR function is essential for cytoplasmic organization in the blastoderm, axon development in the central nervous system, egg chamber structure during oogenesis, and adult eye morphology. Highly similar developmental requirements are found for subunits of the Arp2/3 complex. In the blastoderm, SCAR and Arp2/3 mutations result in a reduction in the amount of cortical filamentous actin and the disruption of dynamically regulated actin structures. Remarkably, the single Drosophila WASp homologue, Wasp, is largely dispensable for these numerous Arp2/3-dependent functions, whereas SCAR does not contribute to cell fate decisions in which Wasp and Arp2/3 play an essential role. These results identify SCAR as a major component of Arp2/3-dependent cell morphology during Drosophila development and demonstrate that the Arp2/3 complex can govern distinct cell biological events in response to SCAR and Wasp regulation.

Soluble factors and the emergence of chick primordial germ cells in vitro

Previous observations obtained from a culture of blastodermal cells on a mouse fibroblast feeder layer (STO) suggested that STO cells provide a factor or factors that facilitate development of avian primordial germ cells (PGC) from dispersed embryo cells. The purpose of the current study was to test the hypothesis that soluble factors produced by STO cells are responsible, at least in part, in supporting the development of PGC in culture and to examine the effect of stem cell factor (SCF), ciliary neurotrophic factor (CNTF), and basic fibroblast growth factor (bFGF) in the development of PGC in culture. Blastodermal cells on gelatin-coated plastic or on feeder layers of CV-1 cells yielded a small number of PGC. When blastodermal cells were cultured on STO cells, a marked increase in PGC was observed. The addition of STO cell-conditioned medium (STO-CM) to blastodermal cells cultured on gelatin-coated plastic and on feeder layers of CV-1 cells resulted in a significant increase in the number of PGC, indicating that soluble factors produced by STO cells can enhance the development of chicken PGC in culture. Supplementation of blastodermal cells with SCF (100 ng/mL) or CNTF (2 ng/mL) or with CNTF and SCF together resulted in a significant increase in the number of PGC after 48 h of culture on feeder layers of CV-1 cells. However, addition of bFGF (100 ng/mL) did not increase PGC. We concluded from these observations that the culture of blastodermal cells on feeder layers of STO and CV-1 cells can be used as a useful biological system in examining the regulatory factors that govern the ontogeny of the germ cell lineage in the avian embryo.

hairy stripe 7 element mediates activation and repression in response to different domains and levels of Krüppel in the Drosophila embryo

The Drosophila gap gene Krüppel (Kr) encodes a zinc finger-type transcription factor required for controlling the spatial expression of other segmentation genes during early blastoderm stage. Here we show that two independent and transferable repressor domains of Krüppel act to control expression of the pair-rule gene hairy, and that the minimal cis-acting element of hairy stripe7 (h7) mediates either Krüppel-dependent activation or repression in different regions of the blastoderm embryo. The C-terminal region of Krüppel which encompasses the predominant repressor domain is not essential for activation, but is required to fully suppress h7-mediated transcription in response to high levels of Krüppel activity. This domain contains an interaction motif for dCtBP, a homologue of the human co-repressor CtBP. dCtBP activity is, however, dispensable for Krüppel-mediated repression in the embryo since Krüppel-mediated repression functions in the absence of dCtBP. Possible modes of h7-mediated gene regulation in response to the different domains and levels of Krüppel are discussed.

Identification of two discrete peptide: N-glycanases in Oryzias latipes during embryogenesis

Two different types of peptide:N-glycanase (PNGase) were identified in developing embryos of medaka fish ( Oryzias latipes ). Because the optimum pH values for their activities were acidic and neutral, they were designated as acid PNGase M and neutral PNGase M, respectively. The acid PNGase M corresponded to the enzyme that had been partially purified from medaka embryos (Seko,A., Kitajima,K., Inoue,Y. and Inoue,S. (1991) J. Biol. Chem., 266, 22110-22114). The apparent molecular weight of this enzyme was 150 K, and the optimal pH was 3.5-4.0, and the K m for L-hyosophorin was 44 microM. L-Hyosophorin is a cortical alveolus-derived glycononapeptide with a large N-linked glycan chain present in the perivitelline space of the developing embryo. Acid PNGase M was competitively inhibited by a free de-N-glycosylated nonapeptide derived from L-hyosophorin. This enzyme was expressed in ovaries and embryos at all developmental stages after gastrulation, but activity was not detected in embryos at developmental stages between fertilization and gastrula. Several independent lines of evidence suggested that acid PNGase M may be responsible for the unusual accumulation of free N-glycans derived from yolk glycoproteins (Iwasaki,M., Seko,A., Kitajima,K., Inoue,Y. and Inoue,S. (1992) J. Biol. Chem., 267, 24287-24296). In contrast, the neutral PNGase M was expressed in blastoderms from the 4-8 cell stage and in cells up to early gastrula. The general significance of these findings is that they show a developmental stage-dependent expression of the two PNGase activities, and that expression of the neutral PNGase M activity occurs concomitantly with the de-N-glycosylation of L-hyosophorin. These data thus support our conclusion that the neutral PNGase M is responsible for the developmental-stage-related de-N-glycosylation of the L-hyosophorin.

bloated tubules (blot) encodes a Drosophila member of the neurotransmitter transporter family required for organisation of the apical cytocortex

We have identified a novel member of the vertebrate sodium- and chloride-dependent neurotransmitter symporter family from Drosophila melanogaster. This gene, named bloated tubules (blot), shows significant sequence similarity to a subgroup of vertebrate orphan transporters. blot transcripts are maternally supplied and during embryogenesis exhibit a complex and dynamic pattern in a subset of ectodermally derived epithelia, notably in the Malpighian tubules, and in the nervous system. Animals mutant for this gene are larval lethals, in which the Malpighian tubule cells are distended with an enlarged and disorganised apical surface. Embryos lacking the maternal component of blot expression die during early stages of development. They show an inability to form actin filaments in the apical cortex, resulting in impaired syncytial nuclear divisions, severe defects in the organisation of the cortical cytoskeleton, and a failure to cellularise. For the first time, a neurotransmitter transporter-like protein has been implicated in a function outside the nervous system. The isolation of blot thus provides the basis for an analysis of the relationship between the function of this putative transporter and epithelial morphogenesis.

Timing and cell interactions underlying neural induction in the chick embryo

Previous studies on neural induction have identified regionally localized inducing activities, signaling molecules, potential competence factors and various other features of this important, early differentiation event. In this paper, we have developed an improved model system for analyzing neural induction and patterning using transverse blastoderm isolates obtained from gastrulating chick embryos. We use this model to establish the timing of neural specification and the spatial distribution of perinodal cells having organizer activity. We show that a tissue that acts either as an organizer or as an inducer of an organizer is spatially co-localized with the prospective neuroectoderm immediately rostral to the primitive streak in the early gastrula. As the primitive streak elongates, this tissue with organizing activity and the prospective neuroectoderm rostral to the streak separate. Furthermore, we show that up to and through the mid-primitive streak stage (i.e., stage 3c/3+), the prospective neuroectoderm cannot self-differentiate (i.e., express neural markers and acquire neural plate morphology) in isolation from tissue with organizer activity. Signals from the organizer and from other more caudal regions of the primitive streak act on the rostral prospective neuroectoderm and the latter gains potency (i.e., is specified) by the fully elongated primitive streak stage (i.e., stage 3d). Transverse blastoderm isolates containing non-specified, prospective neuroectoderm provide an improved model system for analyzing early signaling events involved in neuraxis initiation and patterning.

Eve and ftz regulate a wide array of genes in blastoderm embryos: the selector homeoproteins directly or indirectly regulate most genes in Drosophila

The selector homeoproteins are a highly conserved group of transcription factors found throughout the Eumetazoa. Previously, the Drosophila selector homeoproteins Eve and Ftz were shown to bind with similar specificities to all genes tested, including four genes chosen because they were thought to be unlikely targets of Eve and Ftz. Here, we demonstrate that the expression of these four unexpected targets is controlled by Eve and probably by the other selector homeoproteins as well. A correlation is observed between the level of DNA binding and the degree to which gene expression is regulated by Eve. Suspecting that the selector homeoproteins may affect many more genes than previously thought, we have characterized the expression of randomly selected genes at different stages of embryogenesis. At cellular blastoderm, 25–50% of genes whose transcription can be monitored are regulated by both Eve and Ftz. In late embryogenesis, 87% of genes are directly or indirectly controlled by most or all selector homeoproteins. We argue that this broad control of gene expression is essential to coordinate morphogenesis. Our results raise the possibility that each selector homeoprotein may directly regulate the expression of most genes.

Conservation of read-through transcription of the Drosophila serendipity genes during evolution is gratuitous

The Drosophila melanogaster serendipity (sry) genes sry alpha and sry beta, which code for unrelated proteins and functions, are transcribed both as monocistronic (sry alpha, srv beta) and dicistronic (sry beta-sry alpha) polyadenylated transcripts, the three types of transcripts being differentially expressed throughout development. We show here that, while the sry gene cluster is conserved in two other distantly related Drosophila species, sry beta-sry alpha dicistronic transcription is observed in one of them, Drosophila pseudoobscura but not in the other, Drosophila virilis, indicating that this mode of transcription is not subject to selection during evolution. Sequence comparison of the intergenic sry beta-sry alpha region suggests that sry, beta-sry alpha read-through transcription results from interference between initiation of transcription at the sry alpha promoter and 3' processing of the upstream sry beta transcript. From these and previous analyses, we propose that read-through transcription at the sry locus reflects its clustered gene organisation but is functionally gratuitous. This conclusion is supported by the autonomous rate of evolutionary sequence divergence shown by each individual sry gene.

Binding of trithorax and Polycomb proteins to the bithorax complex: dynamic changes during early Drosophila embryogenesis

In Drosophila, the maintenance of developmentally important transcription patterns is controlled at the level of chromatin structure. The Polycomb group (PcG) and trithorax group (trxG) genes encode proteins involved in chromatin remodelling. PcG genes have been proposed to act by packaging transcriptional repressed chromosomal domains into condensed heterochromatin-like structures. Some of the trxG proteins characterized so far are members of chromatin opening complexes (e.g. SWI/SNF and GAGA/NURF) which facilitate binding of transcription factors and components of the basal transcriptional machinery. Genetic and biochemical data suggest that these two groups of regulatory factors may act through a common set of DNA elements. In the present study, we have investigated the binding of Trithorax (TRX) and Polycomb (PC) protein in the bithorax complex (BX-C) during embryogenesis. In addition, we have identified the minimal fragments from the Ultrabithorax (Ubx) regulatory region that are capable of recruiting TRX to chromosomal sites containing them. Comparative analysis of the binding of the two proteins shows that TRX and PC bind target sequences (PcG-regulated elements, PREs) by cellular blastoderm, when BX-C transcription begins. At the same stage, TRX but not PC is strongly associated with core promoters. Later, at germ band extension, the time of derepression in Polycomb mutants, PC binding is also detected outside core PREs and additionally binds to the fragments containing promoters.

De novo induction of the organizer and formation of the primitive streak in an experimental model of notochord reconstitution in avian embryos

We have developed a model system for analyzing reconstitution of the notochord using cultured blastoderm isolates lacking Hensen's node and the primitive streak. Despite lacking normal notochordal precursor cells, the notochord still forms in these isolates during the 36 hours in culture. Reconstitution of the notochord involves an inducer, which acts upon a responder, thereby inducing a reconstituted notochord. To better understand the mechanism of notochord reconstitution, we asked whether formation of the notochord in the model system was preceded by reconstitution of Hensen's node, the organizer of the avian neuraxis. Our results show not only that a functional organizer is reconstituted, but that this organizer is induced from the responder. First, fate mapping reveals that the responder forms a density, morphologically similar to Hensen's node, during the first 10–12 hours in culture, and that this density expresses typical markers of Hensen's node. Second, the density, when fate mapped or when labeled and transplanted in place of Hensen's node, forms typical derivatives of Hensen's node such as endoderm, notochord and the floor plate of the neural tube. Third, the density, when transplanted to an ectopic site, induces a secondary neuraxis, identical to that induced by Hensen's node. And fourth, the density acts as a suppressor of notochord reconstitution, as does Hensen's node, when transplanted to other blastoderm isolates. Our results also reveal that the medial edge of the isolate forms a reconstituted primitive streak, which gives rise to the normal derivatives of the definitive primitive streak along its rostrocaudal extent and which expresses typical streak markers. Finally, our results demonstrate that the notochordal inducer also induces the reconstituted Hensen's node and, therefore, acts like a Nieuwkoop Center. These findings increase our understanding of the mechanism of notochord reconstitution, provide new information and a novel model system for studying the induction of the organizer and reveal the potential of the epiblast to regulate its cell fate and patterns of gene expression during late gastrula/early neurula stage in higher vertebrates.

Embryonic expression and characterization of a Ptx1 homolog in Drosophila

We describe the molecular characterization of the paired-type homeobox gene D-Ptx1 of Drosophila, a close homolog of the mouse pituitary homeobox gene Ptx1 and the unc-30 gene of C. elegans, characterized by a lysine residue at position 9 of the third alpha-helix of the homeodomain. D-Ptx1 is expressed at various restricted locations throughout embryogenesis. Initial expression of D-Ptx1 in the posterior-most region of the blastoderm embryo is controlled by fork head activity in response to the activated Ras/Raf signaling pathway. During later stages of embryonic development. D-Ptx1 transcripts and protein accumulate in the posterior portion of the midgut, in the developing Malpighian tubules, in a subset of ventral somatic muscles, and in neural cells. Phenotypic analysis of gain-of-function and lack-of-function mutant embryos show that the D-Ptx1 gene is not involved in morphologically apparent differentiation processes. We conclude that D-Ptx1 is more likely to control physiological cell functions than pattern formation during Drosophila embryogenesis.

buttonhead does not contribute to a combinatorial code proposed for Drosophila head development

The Drosophila gap-like segmentation genes orthodenticle, empty spiracles and buttonhead (btd) are expressed and required in overlapping domains in the head region of the blastoderm stage embryo. Their expression domains correspond to two or three segment anlagen that fail to develop in each mutant. It has been proposed that these overlapping expression domains mediate head metamerization and could generate a combinatorial code to specify segment identity. To test this model, we developed a system for targeted gene expression in the early embryo, based on region specific promoters and the flp-out system. Misexpression of btd in the anterior half of the blastoderm embryo directed by the hunchback proximal promoter rescues the btd mutant head phenotype to wild-type. This indicates that, while btd activity is required for the formation of specific head segments, its ectopic expression does not disturb head development. We conclude that the spatial limits of btd expression are not instructive for metamerization of the head region and that btd activity does not contribute to a combinatorial code for specification of segment identity.

Pattern formation in janus-mutant zebrafish embryos

Mechanisms that underlie the formation of the vertebrate body appear to be highly conserved between amphibia and teleosts. For teleosts, however, mesoderm induction and the establishment of dorsoventral polarity are poorly understood. In this study, we present an analysis of early pattern formation in the zebrafish maternal-effect mutation janus. This mutation frequently results in a separation of the cleavage stage blastoderm into two halves that undergo separate development until fusion occurs at the end of gastrulation. Here, we employ janus-mutant embryos to analyze the mechanisms of mesoderm formation and ventral specification in a teleost. Analysis of the expression of the panmesodermal marker no tail in janus-mutant embryos indicates that mesoderm induction depends on a marginal position. In an analysis of ventral specification, we show that the early expression of the ventral marker GATA-2 is confined to the area on both blastodermal halves opposite the dorsal shield region. Since, in janus-mutant embryos, the dorsal position is random with respect to the division plane bisecting the two blastodermal halves, a variety of dorsoventral asymmetries arise within individual embryos. In one constellation, the dorsal position is localized to the plane of bisection and two ventral positions develop at opposite ends of the blastodermal halves. Hence, ventral fates can be specified at any position around the blastodermal margins and are excluded from the dorsal position. The diblastodermic system of the janus-mutant embryo allows for the study of the interactions of dorsal and ventral determinants in varying spatial arrangements. We have studied pattern formation in dorsal half-blastoderms that contain the entire shield region but only a reduced ventrolateral marginal zone. As assessed by the presence of the most ventral cell type, blood, ventral specification within a dorsal half-blastoderm is not suppressed.

In vivo gene transfer into the blastoderm of early developmental stage of chicken

An attempt was made to improve gene transfer into chick embryos in order to produce transgenic chickens. The beta-actin-lacZ/MiwZ, a marker gene in transfection reagent, was injected into the blastodisc of either unincubated fertilized eggs (stage X) or eggs induced from the shell gland by treating the hens intravenously with oxytocin or arginine vasotocin (stages IV-VI). All the manipulated embryos were incubated to reach stage XIV, the period at which primordial germ cells (PGCs) migrate from the germinal crescent to the gonadal anlage via the blood stream. MiwZ was detected in the embryos, extraembryonic tissues and blood by the histochemical staining method of beta-galactosidase. The MiwZ DNA was detected in 57% (127/221) of the survival embryos and in 9% (12/127) of the embryonic tissues. The expression was observed mosaically in the epidermis, heart and neural tube. The PGCs in the blood collected from the vitelline artery or dorsal aorta also showed a positive histochemical staining. However, the expression of MiwZ using the soft shelled eggs was more intense in the extraembryonic tissues, although it did not emerge in the embryos. Thus, it is possible to introduce an exogenous gene into the embryonic tissues using incubated fertilized eggs without sacrificing the hens. This technique for successive genetic operations should facilitate the production of transgenic chickens.

The Drosophila short gastrulation gene prevents Dpp from autoactivating and suppressing neurogenesis in the neuroectoderm

The short gastrulation (sog) gene is expressed in broad lateral stripes comprising the neuroectoderm of the Drosophila blastoderm embryo. sog encodes a predicted secreted protein that functions nonautonomously to antagonize the activity of the TGF-beta-like Decapentaplegic (Dpp) signaling pathway in the dorsal region of the embryo. Recently, it has been shown that sog and dpp are functionally equivalent to their respective Xenopus homologs chordin and BMP-4. In this report we provide the first direct evidence that sog plays a local role in the lateral region of the blastoderm embryo to oppose Dpp activity in the neuroectoderm. In the dorsal region, Dpp signaling both suppresses neurogenesis and maintains expression of genes that promote dorsal cell fates (dorsalization). We show that Dpp also can perform both of these functions in the neuroectoderm. In wild-type embryos, the ability of Dpp to induce expression of dorsal markers including itself (autoactivation) in the neuroectoderm is blocked by sog. We propose that Sog protects the neuroectoderm from an invasive positive feedback loop created by Dpp diffusion and autoactivation. We show that the two functions of Dpp signaling, neural suppression and dorsalization, are triggered by distinct thresholds of Dpp activity. Epistasis experiments reveal that all observed sog activity can be accounted for by Sog functioning as a dedicated Dpp antagonist. Finally, we provide evidence that Sog functions as a diffusible morphogen in the blastoderm embryo. These data strongly support the view that the primary phylogenetically conserved function of the Drosophila sog and dpp genes and the homologous Xenopus chordin and BMP-4 genes is to subdivide the primitive embryonic ectoderm into neural versus non-neural domains.

The basis for a heat-induced developmental defect: defining crucial lesions

Because lethal heat shocks perturb a multitude of cellular processes, the primary lesions responsible for death from heat stress remain to be defined. In Drosophila, sublethal heat treatments produce developmental anomalies that frequently mimic the effects of known mutations and are hence referred to as phenocopies. Mutations subject to phenocopy mimicry provide signposts to those biological processes most sensitive to heat and most important for the function and survival of the organism as a whole. We have analyzed a particular developmental defect inducible in early embryos of Drosophila melanogaster. By molecular, phenotypic, and genetic criteria, we have found extensive parallels between this phenocopy and certain dominant mutations in the segmentation gene fushi tarazu (ftz). Our analysis of this phenocopy indicates that the crucial lesion is interference with proper turnover of ftz protein, resulting in ftz overexpression. Our results provide a novel explanation for a heat-induced developmental defect. Perturbations in relative amounts of important regulatory proteins may be a common mechanism by which heat-shock phenocopies arise.

scute (sis-b) function in Drosophila sex determination

The primary sex determination signal, the X chromosome-to-autosome (X/A) ratio, controls the choice of sexual identity in the Drosophila melanogaster embryo by regulating the activity of the early promoter of the Sex-lethal gene, Sxl-Pe. This promoter is activated in females (2X/2A), while it remains off in males (1X/2A). Promoter activation in females is dependent upon X-linked numerator genes. One of these genes, sisterless-b (sis-b), corresponds to the scute (sc) locus of the achaete-scute complex, and it encodes a helix-loop-helix transcription factor. In the studies reported here we have used monoclonal antibodies to study the expression and functioning of the sc(sis-b) protein. Sc is first detected at nuclear cycle 6 to 7, well before Sxl-Pe is first active. At this stage, the protein is in the cytoplasm, not the nucleus. Only after the formation of the syncytial blastoderm, at nuclear cycle 10 to 11, does a substantial fraction of the protein enter the nucleus, and this nuclear import closely coincides with the initial activation of Sxl-Pe. Consistent with the idea that the dose of sc(sis-b) is critical for its function as an X-chromosome counting element, wild-type syncytial blastoderm embryos could be grouped into two classes based on the level of protein. Western blot (immunoblot) analysis demonstrates that this difference in protein level correlates directly with the activity state of the Sxl gene. Finally, we provide the first direct evidence that Sc forms heteromeric complexes in vivo in early embryos with the maternally derived helix-loop-helix protein Daughterless. This in vivo complex is likely to be critical for Sc function in Sxl-Pe activation.

The decapentaplegic core promoter region plays an integral role in the spatial control of transcription

The Drosophila melanogaster decapentaplegic (dpp) gene encodes a transforming growth factor beta-related cell signaling molecule that plays a critical role in dorsal/ventral pattern formation. The dpp expression pattern in the Drosophila embryo is dynamic, consisting of three phases. Phase I, in which dpp is expressed in a broad dorsal domain, depends on elements in the dpp second intron that interact with the Dorsal transcription factor to repress transcription ventrally. In contrast, phases II and III, in which dpp is expressed first in broad longitudinal stripes (phase II) and subsequently in narrow longitudinal stripes (phase III), depend on multiple independent elements in the dpp 5'-flanking region. Several aspects of the normal dpp expression pattern appear to depend on the unique properties of the dpp core promoter. For example, this core promoter (extending from -22 to +6) is able to direct a phase II expression pattern in the absence of additional upstream or downstream regulatory elements. In addition, a ventral-specific enhancer in the dpp 5'-flanking region that binds the Dorsal factor activates the heterologous hsp70 core promoter but not the dpp core promoter. Thus, the dpp core promoter region may contribute to spatially regulated transcription both by interacting directly with spatially restricted activators and by modifying the activity of proteins bound to enhancer elements.

Distribution of PCNA during postblastoderm cell division cycles in the Drosophila melanogaster embryo: effect of a string- mutation

We used immunocytochemical methods and a specific antibody to identify proliferating cell nuclear antigen (PCNA) in Drosophila embryos during the postblastoderm cell division cycles. The strong nuclear staining observed during interphase disappeared at prophase, staining remained nil throughout the remaining mitotic (M) phases and reappeared in nuclei at the next interphase. As the embryos with the homozygous string- mutation sustained the strong staining signal in nuclei throughout embryonic development, disappearance of the staining signal probably depends on string function and is coupled with the onset of mitosis. When cells in embryos were arrested at M phase following treatment with TN-16 or Vinblastine, the staining signal with the anti-PCNA antibody was lost. However, Western blots showed that the level of PCNA protein in M phase-arrested embryos did not decrease. Therefore, the disappearance of staining signals is apparently due to reorganization of PCNA protein in the multiprotein complex in nuclei, rendering it inaccessible to the antibody, rather than to the degradation of PCNA protein. In contrast to findings in developing embryos, cultured Drosophila Kc cells stained strongly with the anti-PCNA antibody, during both interphase and the M phase. In genetic crossing experiments of transgenic flies carrying the lacZ gene under the control of the PCNA gene regulatory region (-607 to +137 with respect to the transcription initiation site) with string- mutant files, the PCNA gene promoter seems to function in a manner independent of cell cycle progression or of functions of the string gene.

Differential aldehyde sensitivity of newly replicated chromatin from Drosophila melanogaster embryos

By using a series of formalin concentrations we have found that high aldehyde levels in the fixation buffer of Miller spreads are correlated with the appearance of nonnucleosomal stretches in newly replicated chromatin of embryos from Drosophila melanogaster. These nucleosome-free gaps are found 0-500 nm behind the replication fork and do not correspond to naked DNA. The analysis of the distribution of nucleosome-free gaps on newly replicated DNA has revealed some structural details about the maturation of nucleosomes and provides direct evidence that parental nucleosomes have an altered structure at the replication fork. Finally, these stretches of nonnucleosomal chromatin are located in a trans disposition inside the active replicon, although there exists a considerable variability.

Evolution of segmentation genes in insects

Systematic genetic analysis of the segmentation process in Drosophila has established a paradigm for the molecular control of the formation of metameric segments. However, it has been suggested that some of the mechanisms involved in this process in Drosophila are uniquely adapted to the syncytial mode of embryogenesis in such higher dipterans. A particularly contentious problem is the role of early segmentation genes in short-germ insects, in which development proceeds by sequential addition of segments in a cellular environment. However, analysis of the expression of presumptive homologs of segmentation genes in holometabolous short-germ insects suggests that they do indeed have a role in segmentation and that the Drosophila paradigm may be more widely applicable than is usually assumed. Most interestingly, these results suggest that the molecular mechanisms of pattern formation in noncellular and in cellular environments may not be as radically different as it is often thought to be.

Sp1/egr-like zinc-finger protein required for endoderm specification and germ-layer formation in Drosophila

Much of our present knowledge of the biological processes involved in pattern formation in Drosophila is derived from segmentation analysis. Comparatively little is known about the genetic requirement and mechanisms underlying the formation and separation of germ layers by morphogenetic movements during gastrulation. Here we show that the Drosophila gene huckebein (hkb), a member of the gap-gene class of segmentation genes, is required for germ-layer formation at blastoderm. Absence of the hkb product, an Sp1/egr-like zinc-finger protein, causes the ectodermal and mesodermal primordia to expand at the expense of endoderm anlagen. Conversely, ectopic expression of hkb inhibits the formation of the major gastrulation fold which gives rise to the mesoderm and prevents normal segmentation in the ectoderm. Thus, hkb is necessary for endoderm development and its activity defines spatial limits within the blastoderm embryo in which the germ layers are established.