A perfect vulva every time: gradients and signaling cascades in C. elegans

Fusome topology and inheritance during insect gametogenesis

From insects to mammals, oocytes and sperm develop within germline cysts comprising cells connected by intercellular bridges (ICBs). In numerous insects, formation of the cyst is accompanied by growth of the fusome-a membranous organelle that permeates the cyst. Fusome composition and function are best understood in Drosophila melanogaster: during oogenesis, the fusome dictates cyst topology and size and facilitates oocyte selection, while during spermatogenesis, the fusome synchronizes the cyst's response to DNA damage. Despite its distinct and sex-specific roles during insect gametogenesis, elucidating fusome growth and inheritance in females and its structure and connectivity in males has remained challenging. Here, we take advantage of advances in three-dimensional (3D) confocal microscopy and computational image processing tools to reconstruct the topology, growth, and distribution of the fusome in both sexes. In females, our experimental findings inform a theoretical model for fusome assembly and inheritance and suggest that oocyte selection proceeds through an 'equivalency with a bias' mechanism. In males, we find that cell divisions can deviate from the maximally branched pattern observed in females, leading to greater topological variability. Our work consolidates existing disjointed experimental observations and contributes a readily generalizable computational approach for quantitative studies of gametogenesis within and across species.

Quantitative models for building and growing fated small cell networks

Small cell clusters exhibit numerous phenomena typically associated with complex systems, such as division of labour and programmed cell death. A conserved class of such clusters occurs during oogenesis in the form of germline cysts that give rise to oocytes. Germline cysts form through cell divisions with incomplete cytokinesis, leaving cells intimately connected through intercellular bridges that facilitate cyst generation, cell fate determination and collective growth dynamics. Using the well-characterized Drosophila melanogaster female germline cyst as a foundation, we present mathematical models rooted in the dynamics of cell cycle proteins and their interactions to explain the generation of germline cell lineage trees (CLTs) and highlight the diversity of observed CLT sizes and topologies across species. We analyse competing models of symmetry breaking in CLTs to rationalize the observed dynamics and robustness of oocyte fate specification, and highlight remaining gaps in knowledge. We also explore how CLT topology affects cell cycle dynamics and synchronization and highlight mechanisms of intercellular coupling that underlie the observed collective growth patterns during oogenesis. Throughout, we point to similarities across organisms that warrant further investigation and comment on the extent to which experimental and theoretical findings made in model systems extend to other species.

Insulated Switches: Dual-Function Protein RalGEFRGL-1 Promotes Developmental Fidelity

The C. elegans vulva is an excellent model for the study of developmental biology and cell–cell signaling. The developmental induction of vulval precursor cells (VPCs) to assume the 3°-3°-2°-1°-2°-3° patterning of cell fates occurs with 99.8% accuracy. During C. elegans vulval development, an EGF signal from the anchor cell initiates the activation of Ras^LET-60 > Raf^LIN-45 > MEK^MEK-2 > ERK^MPK-1 signaling cascade to induce the 1° cell. The presumptive 1° cell signals its two neighboring cells via Notch^LIN-12 to develop 2° cells. In addition, Ras^LET-60 switches effectors to RalGEF^RGL-1 > Ral^RAL-1 to promote 2° fate. Shin et al. (2019) showed that RalGEF^RGL-1 is a dual-function protein in VPCs fate patterning. RalGEF^RGL-1 functions as a scaffold for PDK^PDK-1 > Akt^AKT-1/2 modulatory signaling to promote 1° fate in addition to propagating the Ras^LET-60 modulatory signal through Ral^RAL-1 to promote 2° fate. The deletion of RalGEF^RGL-1 increases the frequency of VPC patterning errors 15-fold compared to the wild-type control. We speculate that RalGEF^RGL-1 represents an “insulated switch”, whereby the promotion of one signaling activity curtails the promotion of the opposing activity. This property might increase the impact of the switch on fidelity more than two separately encoded proteins could. Understanding how developmental fidelity is controlled will help us to better understand the origins of cancer and birth defects, which occur in part due to the misspecification of cell fates.

The Signaling Network Controlling C. elegans Vulval Cell Fate Patterning

EGF, emitted by the Anchor Cell, patterns six equipotent C. elegans vulval precursor cells to assume a precise array of three cell fates with high fidelity. A group of core and modulatory signaling cascades forms a signaling network that demonstrates plasticity during the transition from naïve to terminally differentiated cells. In this review, we summarize the history of classical developmental manipulations and molecular genetics experiments that led to our understanding of the signals governing this process, and discuss principles of signal transduction and developmental biology that have emerged from these studies.

Ral Signals through a MAP4 Kinase-p38 MAP Kinase Cascade in C. elegans Cell Fate Patterning

C. elegans vulval precursor cell (VPC) fates are patterned by an epidermal growth factor (EGF) gradient. High-dose EGF induces 1° VPC fate, and lower dose EGF contributes to 2° fate in support of LIN-12/Notch. We previously showed that the EGF 2°-promoting signal is mediated by LET-60/Ras switching effectors, from the canonical Raf-MEK-ERK mitogen-activated protein (MAP) kinase cascade that promotes 1° fate to the non-canonical RalGEF-Ral that promotes 2° fate. Of oncogenic Ras effectors, RalGEF-Ral is by far the least well understood. We use genetic analysis to identify an effector cascade downstream of C. elegans RAL-1/Ral, starting with an established Ral binding partner, Exo84 of the exocyst complex. Additionally, RAL-1 signals through GCK-2, a citron-N-terminal-homology-domain-containing MAP4 kinase, and PMK-1/p38 MAP kinase cascade to promote 2° fate. Our study delineates a Ral-dependent developmental signaling cascade in vivo, thus providing the mechanism by which lower EGF dose is transduced.

Gene-free methodology for cell fate dynamics during development

Models of cell function that assign a variable to each gene frequently lead to systems of equations with many parameters whose behavior is obscure. Geometric models reduce dynamics to intuitive pictorial elements that provide compact representations for sparse in vivo data and transparent descriptions of developmental transitions. To illustrate, a geometric model fit to vulval development in Caenorhabditis elegans, implies a phase diagram where cell-fate choices are displayed in a plane defined by EGF and Notch signaling levels. This diagram defines allowable and forbidden cell-fate transitions as EGF or Notch levels change, and explains surprising observations previously attributed to context-dependent action of these signals. The diagram also reveals the existence of special points at which minor changes in signal levels lead to strong epistatic interactions between EGF and Notch. Our model correctly predicts experiments near these points and suggests specific timed perturbations in signals that can lead to additional unexpected outcomes.

At first, embryos are made up of identical cells. Then, as the embryo develops, these cells specialize into different types, such as heart and brain cells. Chemical signals sent and received by the cells are key to forming the right type of cell at the right time and place. The cellular machinery that produces and interprets these signals is exceedingly complex and difficult to understand.

In the 1950s, Conrad Waddington presented an alternative way of thinking about how an unspecialized cell progresses to one of many different fates. He suggested visualizing the developing cell as a ball rolling along a hilly landscape. As the ball travels, obstacles in its way guide it along particular paths. Eventually the ball comes to rest in a valley, with each valley in the landscape representing a different cell fate. Although this “landscape model” is an appealing metaphor for how signaling events guide cell specialization, it was not clear whether it could be put to productive use.

The egg-laying organ in the worm species Caenorhabditis elegans is called the vulva, and is often studied by researchers who want to learn more about how organs develop. The vulva develops from a small number of identical cells that adopt one of three possible cell fates. Two chemical signals, called epidermal growth factor (EGF) and Notch, control this specialization process.

Corson and Siggia have now constructed a simple landscape model that can reproduce the normal arrangement of cell types in the vulva. When adjusted to describe the effect of genetic mutations that affect either EGF or Notch, the model could predict the outcome of mutations that affect both signals at once. The twists and turns of cell paths in the landscape could also account for several non-intuitive cell fate outcomes that had been assumed to result from subtle regulation of EGF and Notch signals.

Landscape models should be easy to apply to other developing tissues and organs. By providing an intuitive picture of how signals shape cellular decisions, the models could help researchers to learn how to control cell and tissue development. This could lead to new treatments to repair or replace failing organs, making regenerative medicine a reality.

Impaired removal of H3K4 methylation affects cell fate determination and gene transcription

Methylation of histone 3 lysine 4 (H3K4) is largely associated with promoters and enhancers of actively transcribed genes and is finely regulated during development by the action of histone methyltransferases and demethylases. H3K4me3 demethylases of the KDM5 family have been previously implicated in development, but how the regulation of H3K4me3 level controls developmental processes is not fully established. Here, we show that the H3K4 demethylase RBR-2, the unique member of the KDM5 family in C. elegans, acts cell-autonomously and in a catalytic-dependent manner to control vulva precursor cells fate acquisition, by promoting the LIN-12/Notch pathway. Using genome-wide approaches, we show that RBR-2 reduces the H3K4me3 level at transcription start sites (TSSs) and in regions upstream of the TSSs, and acts both as a transcription repressor and activator. Analysis of the lin-11 genetic locus, a direct RBR-2 target gene required for vulva precursor cell fate acquisition, shows that RBR-2 controls the epigenetic signature of the lin-11 vulva-specific enhancer and lin-11 expression, providing in vivo evidence that RBR-2 can positively regulate transcription and cell fate acquisition by controlling enhancer activity.

Visualizing and manipulating temporal signaling dynamics with fluorescence-based tools

The use of genome-wide proteomic and RNA interference approaches has moved our understanding of signal transduction from linear pathways to highly integrated networks centered on core nodes. However, probing the dynamics of flow of information through such networks remains technically challenging. In particular, how the temporal dynamics of an individual pathway can elicit distinct outcomes in a single cell type and how multiple pathways may interact sequentially or synchronously to influence cell fate remain open questions in many contexts. The development of fluorescence-based reporters and optogenetic regulators of pathway activity enables the analysis of signaling in living cells and organisms with unprecedented spatiotemporal resolution and holds the promise of addressing these key questions. We present a brief overview of the evidence for the importance of temporal dynamics in cellular regulation, introduce these fluorescence-based tools, and highlight specific studies that leveraged these tools to probe the dynamics of information flow through signaling networks. In particular, we highlight two studies in Caenorhabditis elegans sensory neurons and cultured mammalian cells that demonstrate the importance of signal dynamics in determining cellular responses.

Plasticity and errors of a robust developmental system in different environments

Many developmental processes generate invariant phenotypes in a wide range of ecological conditions. Such robustness to environmental variation is a fundamental biological property, yet its extent, limits, and adaptive significance have rarely been assessed empirically. Here we tested how environmental variation affects vulval formation in Caenorhabditis nematodes. In different environments, a correct vulval pattern develops with high precision, but rare deviant patterns reveal the system's limits and how its mechanisms respond to environmental challenges. Key features of the apparent robustness are functional redundancy among vulval precursor cells and tolerance to quantitative variation in Ras, Notch, and Wnt pathway activities. The observed environmental responses and precision of vulval patterning vary within and between Caenorhabditis species. These results highlight the complex response of developmental systems to the environment and illustrate how a robust and invariant phenotype may result through cellular and molecular processes that are highly plastic--across environments and evolution.

Robustness and evolution: concepts, insights and challenges from a developmental model system

Robustness, the persistence of an organismal trait under perturbations, is a ubiquitous property of complex living systems. We here discuss key concepts related to robustness with examples from vulva development in the nematode Caenorhabditis elegans. We emphasize the need to be clear about the perturbations a trait is (or is not) robust to. We discuss two prominent mechanistic causes of robustness, namely redundancy and distributed robustness. We also discuss possible evolutionary causes of robustness, one of which does not involve natural selection. To better understand robustness is of paramount importance for understanding organismal evolution. Part of the reason is that highly robust systems can accumulate cryptic variation that can serve as a source of new adaptations and evolutionary innovations. We point to some key challenges in improving our understanding of robustness.

EGF signal propagation during C. elegans vulval development mediated by ROM-1 rhomboid

During Caenorhabditis elegans vulval development, the anchor cell (AC) in the somatic gonad secretes an epidermal growth factor (EGF) to activate the EGF receptor (EGFR) signaling pathway in the adjacent vulval precursor cells (VPCs). The inductive AC signal specifies the vulval fates of the three proximal VPCs P5.p, P6.p, and P7.p. The C. elegans Rhomboid homolog ROM-1 increases the range of EGF, allowing the inductive signal to reach the distal VPCs P3.p, P4.p and P8.p, which are further away from the AC. Surprisingly, ROM-1 functions in the signal-receiving VPCs rather than the signal-sending AC. This observation led to the discovery of an AC-independent activity of EGF in the VPCs that promotes vulval cell fate specification and depends on ROM-1. Of the two previously reported EGF splice variants, the longer one requires ROM-1 for its activity, while the shorter form acts independently of ROM-1. We present a model in which ROM-1 relays the inductive AC signal from the proximal to the distal VPCs by allowing the secretion of the LIN-3L splice variant. These results indicate that, in spite of their structural diversity, Rhomboid proteins play a conserved role in activating EGFR signaling in C. elegans, Drosophila, and possibly also in mammals.

The two steps of vulval induction in Oscheius tipulae CEW1 recruit common regulators including a MEK kinase

The cell interactions that specify the spatial pattern of vulval precursor cell (VPC) fates differ between the nematodes Oscheius tipulae CEW1 and Caenorhabditis elegans. In the former, the centered pattern of fates is obtained by two successive inductions from the gonadal anchor cell, whereas in the latter, a single inductive step by the anchor cell (EGF-Ras-MAP kinase pathway) can act as a morphogen and is reinforced by lateral signaling between the vulval precursors (Notch pathway). We performed a genetic screen for vulva mutants in O. tipulae CEW1. Here we present the mutants that specifically affect the vulval induction mechanisms. Phenotypic and epistatic analyses of these mutants show that both vulval induction steps share common components, one of which appears to be MEK kinase(s). Moreover, the inductive pathway (including MEK kinase) influences the competence of the vulval precursor cells and more strikingly their division pattern as well, irrespective of their vulval fate. Finally, a comparison of vulval mutant phenotypes obtained in C. elegans and O. tipulae CEW1 highlights the evolution of vulval induction mechanisms between the two species.

Cki-1 links cell division and cell fate acquisition in the C. elegans somatic gonad

The formation of a complex multicellular organism requires the precise specification of many diverse cell types at the correct time and position throughout development. This may be achieved by coordinating cell fate specification processes with progression through the cell cycle. Here, we show that the extra distal tip cells (DTCs) associated with the loss of cki-1, a Caenorhabditis elegans homologue of the cyclin-dependent kinase inhibitor p27, do not arise from duplications of pre-existing DTCs, but that they are formed from another cell type within the somatic gonad. Results from our laser microsurgery experiments suggest that the extra DTCs are caused by aberrant somatic gonadal precursor cell divisions in the absence of cki-1, resulting in abnormal daughter cell fates. cki-1(RNAi) animals also possess extra anchor cells and ectopic gonad arms with variable sheath cell numbers and positioning. In addition, cki-1(RNAi) animals display an endomitotic oocyte (Emo) phenotype. Our results uncover a novel role of this CKI in cell fate acquisition, either by directly influencing specification, or through a more conventional role in appropriately linking cell cycle phase with this process.

Evolution of development in closely related species of flies and worms

One of the main challenges in evolutionary biology is to identify the molecular changes that underlie phenotypic differences that are of evolutionary significance. Comparative studies of early development have shown that changes in the spatio-temporal use of regulatory genes, as well as changes in the specificity of regulatory proteins, are correlated with important differences in morphology between phylogenetically distant species. However, it is not known how such changes take place in natural populations, and whether they result from a single, or many small, additive events. Extending this approach to the study of development of closely related species promises to enrich this debate.

An EGFR/Ebi/Sno pathway promotes delta expression by inactivating Su(H)/SMRTER repression during inductive notch signaling

The Notch and Epidermal Growth Factor Receptor (EGFR) pathways both regulate proliferation and differentiation, and the cellular response to each is often influenced by the other. Here, we describe a mechanism that links them in a sequential fashion, in the developing compound eye of Drosophila. EGFR activation induces photoreceptor (R cell) differentiation and promotes their expression of Delta. This Notch ligand then induces neighboring cells to become nonneuronal cone cells. ebi and strawberry notch (sno) regulate EGFR-dependent Delta transcription by antagonizing a repressor function of Suppressor of Hairless (Su(H)). Sno binds to Su(H), and Ebi, an F-box/WD40 protein, forms a complex with Su(H) and the corepressor SMRTER. EGFR-activated transcriptional derepression requires ebi and sno, is proteasome-dependent, and correlates with the translocation of SMRTER to the cytoplasm.

Microevolutionary analysis of the nematode genus Pristionchus suggests a recent evolution of redundant developmental mechanisms during vulva formation

To identify the mechanisms by which molecular variation is introduced into developmental systems, microevolutionary approaches to evolutionary developmental biology have to be taken. Here, we describe the molecular and developmental characterization of laboratory strains of the nematode genus Pristionchus, which lays a foundation for a microevolutionary analysis of vulva development. We describe 13 laboratory strains of the Pristionchus genus that are derived from natural isolates from around the world. Mating experiments and ITS sequence analysis indicated that these 13 strains represent four different species: the gonochoristic species P. Iheritieriand three hermaphroditic species, P. pacificus, P. maupasi, and an as yet undescribed species Pristionchus sp., respectively. P. pacificus is represented by five different strains isolated from California, Washington, Hawaii, Ontario, and Poland. Developmental differences during vulva formation are observed between strains from different species but also between strains of P. pacificus, like the strains from California and Poland. In particular, redundant developmental mechanisms present during vulva formation in P. pacificus var. California are absent in other strains. Amplified restriction fragment length polymorphism (AFLP) analyses of the P. pacificus strains revealed that the American strains are highly polymorphic. In contrast, the developmentally distinct strain from Poland is identical to the Californian strain, suggesting that the developmental differences rely on a small number of changes in developmental control genes rather than the accumulation of changes at multiple loci.

Mechanisms of plant embryo development

1. Evolution in plants has favored both a simpler body plan with fewer cell types and the epigenetic flexibility to regenerate, via growth, dedifferentiation, and redifferentiation, to recover from environmental insults. It has become increasingly apparent that a plant cell uses external signals to differentiate and to maintain or to change the differentiated state. A cell-cell signaling and positional information strategy seems to be the predominant mechanism employed in plant development. 2. An axis can be initiated by physical/chemical forces such as light and ion current, requiring no new gene action. Random chemical fluctuations and physicochemical forces could explain the initiation of differences among cells of equal developmental potential. Amplification of chemical polarizing events may lead to biochemical differences, new gene expression, and finally shoot/root axis establishment. 3. Radial and axial patterning may be governed by a mechanism involving polar auxin transport. 4. Because the meristems and the three fundamental tissues formed during embryogenesis are renewed and extended throughout the life of the plant, with some exceptions, most genes expressed in the embryo are also expressed during postgermination development. 5. Embryogenic competence is acquired during reproductive development. While the zygote is determined for embryogenesis, the developing embryo and often the seedling remain embryogenic. Embryogenic potential declines during vegetative development. The embryogenic strength of a tissue is correlated with its developmental distance from the zygote.

Negative regulation of receptor tyrosine kinase signals

In Metazoans a number of cellular functions are controlled by receptor tyrosine kinases (RTKs) during development and in postnatal life. The execution of these programs requires that signals of adequate strength are delivered for the appropriate time within precise spatial boundaries. Several RTK inhibitors have been identified in invertebrate and mammalian organisms. Because they are involved in tuning and termination of receptor signals, negative regulators of RTK activity fulfill a fundamental function in the control of receptor signaling.

A Caenorhabditis elegans MAP kinase kinase, MEK-1, is involved in stress responses

The c-Jun N-terminal kinase (JNK), a member of the mitogen-activated protein kinase (MAPK) family, was shown to be involved in the response to various stresses in cultured cells. However, there is little in vivo evidence indicating a role for a JNK pathway in the stress response of an organism. We identified the Caenorhabditis elegans mek-1 gene, which encodes a 347 amino acid protein highly homologous to mammalian MKK7, an activator of JNK. Mek-1 reporter fusion proteins are expressed in pharyngeal muscle, uterus, a portion of intestine, and neurons. A mek-1 deletion mutant is hypersensitive to copper and cadmium ions and to starvation. A wild-type mek-1 transgene rescued the hypersensitivity to the metal ions. Double mutants of mek-1 with an eat-5, eat-11 or eat-18 mutation, which are characterized by a limited feeding defect, showed distinct growth defects under normal conditions. Expression of an activated form of MEK-1 in the whole animal or specifically in the pharynx inhibited pharyngeal pumping. These results suggest a role for mek-1 in stress responses, with a focus in the pharynx and/or intestine.

Novel cell-cell interactions during vulva development in Pristionchus pacificus

Vulva development differs between Caenorhabditis elegans and Pristionchus pacificus in several ways. Seven of 12 ventral epidermal cells in P. pacificus die of apoptosis, whereas homologous cells in C. elegans fuse with the hypodermal syncytium. Vulva induction is a one-step process in C. elegans, but requires a continuous interaction between the gonad and the epidermis in P. pacificus. Here we describe several novel cell-cell interactions in P. pacificus, focusing on the vulva precursor cell P8.p and the mesoblast M. P8.p in P. pacificus, unlike its homologous cell in C. elegans, is incompetent to respond to gonadal signaling in the absence of other vulva precursor cells, but can respond to lateral signaling from a neighboring vulval precursor. P8.p provides an inhibitory signal that determines the developmental competence of P(5,7).p. This lateral inhibition acts via the mesoblast M and is regulated by the homeotic gene Ppa-mab-5. In Ppa-mab-5 mutants, M is misspecified and provides inductive signaling to the vulval precursor cells, including P8.p. Taken together, vulva development in P. pacificus displays novel cell-cell interactions involving the mesoblast M and P8.p. In particular, P8.p represents a new ventral epidermal cell type, which is characterized by novel interactions and a specific response to gonadal signaling.

Evolution of vulva development in the Cephalobina (Nematoda)

Ventral cord and vulva development are analyzed in a large sample of nematode species of the suborder Cephalobina. We find a specific range of evolutionary variations at distinct developmental steps. (1) Unlike Caenorhabditis elegans and relatives, the vulva is formed from the four precursor cells P(5-8).p or, exceptionally, from P(6, 7).p only. (2) The vulval competence group is restricted to these four cells or is larger. (3) The fates of more anterior and posterior Pn.p cells vary between closely related species (mostly cell death versus epidermal fate). (4) The mechanism of vulval cell fate patterning varies within a single genus, even between strains of the same species. (5) We describe the first example of a vulval cell lineage that is asymmetric between the anterior and the posterior sides of the vulva. For a selection of the investigated taxa, phylogenetic trees were constructed in order to map vulval characters and infer evolutionary polarities. We can conclude that in this group, death of the Pn.p cells probably constitutes a derived character state compared to a syncytial fate. Rhabditophanes sp. and Strongyloides ratti are placed as sister taxa, probably sharing an exclusive common ancestor in which the number of precursor cells forming the vulva was reduced from four to two.

Airway patterning: A paradigm for restricted signalling

Intercellular signalling is limited by the range of cell responsiveness, often mediated by repressors. A recently identified repressor, Sprouty, inhibits MAP kinase signalling in flies, mice and humans and has a conserved function in patterning the airways of these divergent species.

Cell cycle-dependent sequencing of cell fate decisions in Caenorhabditis elegans vulva precursor cells

In Caenorhabditis elegans, the fates of the six multipotent vulva precursor cells (VPCs) are specified by extracellular signals. One VPC expresses the primary (1 degrees) fate in response to a Ras-mediated inductive signal from the gonad. The two VPCs flanking the 1 degrees cell each express secondary (2 degrees) fates in response to lin-12-mediated lateral signaling. The remaining three VPCs each adopt the non-vulval tertiary (3 degrees) fate. Here I describe experiments examining how the selection of these vulval fates is affected by cell cycle arrest and cell cycle-restricted lin-12 activity. The results suggest that lin-12 participates in two developmental decisions separable by cell cycle phase: lin-12 must act prior to the end of VPC S phase to influence a 1 degrees versus 2 degrees cell fate choice, but must act after VPC S phase to influence a 3 degrees versus 2 degrees cell fate choice. Coupling developmental decisions to cell cycle transitions may provide a mechanism for prioritizing or ordering choices of cell fates for multipotential cells.

EGL-17(FGF) expression coordinates the attraction of the migrating sex myoblasts with vulval induction in C. elegans

During the development of the egg-laying system in Caenorhabditis elegans hermaphrodites, central gonadal cells organize the alignment of the vulva with the sex myoblasts, the progenitors of the egg-laying muscles. A fibroblast growth factor [EGL-17(FGF)] and an FGF receptor [EGL-15(FGFR)] are involved in the gonadal signals that guide the migrations of the sex myoblasts. Here we show that EGL-17(FGF) can act as an instructive guidance cue to direct the sex myoblasts to their final destinations. We find that egl-17 reporter constructs are expressed in the primary vulval cell and that EGL-17(FGF) expression in this cell correlates with the precise positioning of the sex myoblasts. We postulate that EGL-17(FGF) helps to coordinate the development of a functional egg-laying system, linking vulval induction with proper sex myoblast migration.

Purification of an extracellular signaling molecule involved in production of aerial mycelium by Streptomyces coelicolor

We have extensively purified a factor from conditioned medium that restores aerial mycelium formation to a mutant of Streptomyces coelicolor that is defective in morphological differentiation. Response to this factor is shown to depend on the presence of the BldK oligopeptide import system. We suggest that this substance acts at the first step in a putative cascade of developmental regulatory signals.

Evolution of cell lineage

Organisms such as nematodes and mollusks, the development of which involves fixed cell lineages, allow studies of the genetic programming of precision and of evolution of its flexibility. Stochastic specification of cell fate may evolve to a fixed cell lineage and specification by cell interactions to autonomous specification. Comparative developmental studies also shed light on the evolution of asymmetry.

Drosophila Jun kinase regulates expression of decapentaplegic via the ETS-domain protein Aop and the AP-1 transcription factor DJun during dorsal closure

During Drosophila embryogenesis, ectodermal cells of the lateral epithelium stretch in a coordinated fashion to internalize the amnioserosa cells and close the embryo dorsally. This process, dorsal closure, requires two signaling pathways: the Drosophila Jun-amino-terminal kinase (DJNK) pathway and the Dpp pathway. We have identified mutations in DJun and show that DJNK controls dorsal closure by activating DJun and inactivating the ETS repressor Aop/Yan by phosphorylation. DJun and Aop regulate dpp expression in the most dorsal row of cells. Secreted Dpp then instructs more ventrally located cells to stretch. Our results provide a causal link between the DJNK and Dpp pathways during dorsal closure. Interestingly, in vertebrates, transforming growth factor-beta and c-Jun regulate collagenase gene expression during wound healing, a process that also involves the closing of an epithelial sheath.

Genetic enhancers of sem-5 define components of the gonad-independent guidance mechanism controlling sex myoblast migration in Caenorhabditis elegans hermaphrodites

The migrations of the sex myoblasts in Caenorhabditis elegans hermaphrodites involve two guidance mechanism: a gonad-dependent attraction that confers precise positioning of the sex myoblasts and a gonad-independent mechanism that is sufficient for coarse positioning in the absence of the gonad (Thomas et al., 1990). Here we show that mutations in unc-53, unc-71, and unc-73 disrupt sex myoblast positioning in the absence of the gonad, while they do not affect positioning in the presence of the gonad. Thus, mutations in these genes appear to compromise the gonad-independent mechanism without affecting motility or the gonad-dependent attraction. Mutations in sem-5 confer dramatic sex myoblast positioning defects in double mutant combinations with unc-53, unc-71, or unc-73 mutations, even in the presence of the gonad. This suggests that sem-5 is required for the gonad-dependent attractive mechanism. Mutations in let-60 ras and let-341 also confer sex myoblast migration defects in an unc-53 background, implicating these genes in gonad-dependent positioning as well.

The ins and outs of notch signaling

The Notch gene encodes a cell surface protein that regulates cell fate choices in vertebrates and invertebrates. Given the wide variety of cell types influenced by Notch, it would seem that the signal relayed through Notch activation is not an instructive one per se. Rather, Notch signaling is thought to influence the cell's ability to respond to instructive signals responsible for specific cell fates. Expression and functional studies of Notch support this idea; however, the possibility of additional functions for Notch cannot be excluded. Much of what we know about the Notch signaling pathway comes from studies with Drosophila Notch and the Caenorhabditis elegans Notch-related genes lin-12 and glp-1. With the isolation of multiple vertebrate Notch genes we are beginning to understand and define Notch signaling in vertebrates as well. A number of excellent reviews have been published summarizing the current status of Notch/LIN-12/GLP-1 signaling in Drosophila and C. elegans, as well as recent findings with the vertebrate counterparts. Here I review the structure of the various Notch proteins and their putative ligands, and discuss possible interactions between Notch, its ligands, and other cellular components that affect Notch signal transduction. A role for Notch signaling during normal development and in disease processes is discussed in an accompanying review by T. Gridley (1997, Mol. Cell. Neurosci. 9: 103-108).

Flk-1, a receptor for vascular endothelial growth factor (VEGF), is expressed by retinal progenitor cells

Throughout development of the vertebrate retina, progenitor cells are multipotential, producing a variety of distinctive cell types. Little is known of the molecular mechanisms directing the determination of cell fate. We have examined retinal progenitor cells for expression of receptor tyrosine kinases in an attempt to define receptors that could allow a progenitor to respond to its environment. We found that the receptor tyrosine kinase Flk-1, previously shown to be expressed in endothelial cells, is also expressed in neural progenitor cells of the mouse retina. Flk-1 RNA expression in the retinal progenitors commences with the onset of neuronal differentiation and persists throughout retinal neurogenesis. Flk-1 RNA and protein levels in the retina vary temporally during development, as shown by in situ hybridization and Western blot analysis. Patterns of beta-galactosidase expression in mice containing the lacZ gene in place of the Flk-1 gene are consistent with Flk-1 being expressed in retinal progenitors. In addition, we show that the ligand of Flk-1, vascular endothelial growth factor (VEGF), is expressed in the developing retina by differentiated cells and that a chimeric ligand of VEGF fused to alkaline phosphatase binds to proliferating retinal progenitors. Furthermore, the neural retina-derived Flk-1 protein kinase is activated by VEGF in vitro. Thus, the Flk-1 receptor protein kinase is expressed on the surface of neural progenitors in mouse retina and may play a critical role in neurogenesis as well as in vasculogenesis.

The Drosophila Jun-N-terminal kinase is required for cell morphogenesis but not for DJun-dependent cell fate specification in the eye

We cloned and characterized the Drosophila homolog of mammalian Jun-N-terminal kinases (DJNK). We show that DJNK is encoded by basket (bsk). Like hemipterous (hep), which encodes the Drosophila JNK kinase, bsk is required in the embryo for dorsal closure, a process involving coordinate cell shape changes of ectodermal cells. Dorsal closure can also be blocked by dominant negative Drosophila cdc42, which has been shown to act upstream of JNKK in vertebrates. Therefore it appears that the JNK pathway is conserved and that it is involved in controlling cell morphogenesis in Drosophila. Although DJNK efficiently phosphorylates DJun in vitro, bsk function is not required for the specification of cell fate in the developing eye, a process that requires MAP kinase and DJun function.

The Notch pathway: democracy and aristocracy in the selection of cell fate

Major advances in the past two years have increased our understanding of the molecular components of the Notch signal-transduction pathway in both invertebrates and vertebrates. Recent studies have begun to address the interaction of other signaling pathways with the Notch pathway. Of particular interest is the integration of signals from the Wingless pathway and from asymmetrically segregating determinants such as numb. Molecular models for Notch-mediated cell-fate selection within groups of developmentally equivalent cells are presented.

Short-range signaling by candidate morphogens of the TGF beta family and evidence for a relay mechanism of induction

The specification and patterning of cell fates by a morphogen gradient is a unifying theme of developmental biology, yet little evidence exists for the presence of gradients in vivo or to show how such putative gradients form. Vg1 and activin are candidate morphogens involved in Xenopus mesoderm induction. This study suggests that these TGF beta family members act on adjacent cells but do not travel through the intact extracellular space to induce distant cells directly. Moreover, we present evidence for the presence of secondary inducing signals that could be involved in relaying signals to distant cells. These results suggest that if a localized cellular source of an inducer acts to pattern mesodermal cells at a distance in Xenopus embryos, it does so by a relay mechanism.

Heterochronic genes control cell cycle progress and developmental competence of C. elegans vulva precursor cells

Heterochronic genes control the timing of vulval development in the C. elegans hermaphrodite. lin-14 or lin-28 loss-of-function mutations cause the vulval precursor cells (VPCs) to enter S phase and to divide one larval stage earlier than in the wild type. A precocious vulva is formed by essentially normal cell lineage patterns, governed by the same intercellular signals as in the wild type. Mutations that prevent the normal developmental down-regulation of lin-14, activity delay or block VPC division and prevent vulval differentiation. A genetic pathway that includes lin-4, lin-14, and lin-28 controls when VPCs complete G1 and also controls when VPCs acquire the competence to respond to the intercellular patterning signals and express vulval fates.

Argos transcription is induced by the Drosophila EGF receptor pathway to form an inhibitory feedback loop

Argos is a secreted molecule with an atypical EGF motif. It was recently shown to function as an inhibitor of the signaling triggered by the Drosophila EGF receptor (DER). In this work, we determine the contribution of Argos to the establishment of cell fates in the embryonic ventral ectoderm. Graded activation of DER is essential for patterning the ventral ectoderm. argos mutant embryos show expansion of ventral cell fates suggesting hyperactivation of the DER pathway. In the embryonic ventral ectoderm, argos is expressed in the ventralmost row of cells. We show that argos expression in the ventral ectoderm is induced by the DER pathway: argos is not expressed in DER mutant embryos, while it is ectopically expressed in the entire ventral ectoderm following ubiquitous activation of the DER pathway. argos expression appears to be triggered directly by the DER pathway, since induction can also be observed in cell culture, following activation of DER by its ligand, Spitz. Argos therefore functions in a sequential manner, to restrict the duration and level of DER signaling. This type of inhibitory feedback loop may represent a general paradigm for signaling pathways inducing diverse cell fates within a population of non-committed cells.

The Caenorhabditis elegans gene lin-1 encodes an ETS-domain protein and defines a branch of the vulval induction pathway

The Caenorhabditis elegans gene lin-1 appears to act after the Ras-Raf-MEK-MAPK signaling cascade that mediates vulval induction. We show that lin-1 is a negative regulator of vulval cell fates and encodes an ETS-domain putative transcription factor containing potential MAPK phosphorylation sites. In lin-1 null mutants, the vulval precursor cells (VPCs) still respond to signaling from the gonadal anchor cell, indicating that lin-1 defines a branch of the inductive signaling pathway. We also provide evidence that the inductive and lateral signaling pathways are integrated to control the 1 degree and 2 degrees vulval cell fates after the point at which lin-1 acts in the inductive pathway and that VPCs can assess the relative rather than absolute levels of inductive and lateral signaling in determining whether to express the 1 degree or 2 degrees vulval cell fates.