Control of sex-specific apoptosis in C. elegans by the BarH homeodomain protein CEH-30 and the transcriptional repressor UNC-37/Groucho

C. elegans males optimize mate-preference decisions via sex-specific responses to multimodal sensory cues

For sexually reproducing animals, selecting optimal mates is important for maximizing reproductive fitness. In the nematode C. elegans, populations reproduce largely by hermaphrodite self-fertilization, but the cross-fertilization of hermaphrodites by males also occurs. Males' ability to recognize hermaphrodites involves several sensory cues, but an integrated view of the ways males use these cues in their native context to assess characteristics of potential mates has been elusive. Here, we examine the mate-preference behavior of C. elegans males evoked by natively produced cues. We find that males use a combination of volatile sex pheromones (VSPs), ascaroside sex pheromones, surface-associated cues, and other signals to assess multiple features of potential mates. Specific aspects of mate preference are communicated by distinct signals: developmental stage and sex are signaled by ascaroside pheromones and surface cues, whereas the presence of a self-sperm-depleted hermaphrodite is likely signaled by VSPs. Furthermore, males prefer to interact with virgin over mated, and well-fed over food-deprived, hermaphrodites; these preferences are likely adaptive and are also mediated by ascarosides and other cues. Sex-typical mate-preference behavior depends on the sexual state of the nervous system, such that pan-neuronal genetic masculinization in hermaphrodites generates male-typical social behavior. We also identify an unexpected role for the sex-shared ASH sensory neurons in male attraction to ascaroside sex pheromones. Our findings lead to an integrated view in which the distinct physical properties of various mate-preference cues guide a flexible, stepwise behavioral program by which males assess multiple features of potential mates to optimize mate preference.

Let's talk about sex: Mechanisms of neural sexual differentiation in Bilateria

In multicellular organisms, sexed gonads have evolved that facilitate release of sperm versus eggs, and bilaterian animals purposefully combine their gametes via mating behaviors. Distinct neural circuits have evolved that control these physically different mating events for animals producing eggs from ovaries versus sperm from testis. In this review, we will describe the developmental mechanisms that sexually differentiate neural circuits across three major clades of bilaterian animals-Ecdysozoa, Deuterosomia, and Lophotrochozoa. While many of the mechanisms inducing somatic and neuronal sex differentiation across these diverse organisms are clade-specific rather than evolutionarily conserved, we develop a common framework for considering the developmental logic of these events and the types of neuronal differences that produce sex-differentiated behaviors. This article is categorized under: Congenital Diseases > Stem Cells and Development Neurological Diseases > Stem Cells and Development.

Fast and easy method to culture and obtain large populations of male nematodes

Caenorhabditis elegans is a model system widely used in fundamental research. Even though, nematodes are easy to maintain in the laboratory, obtaining large populations of worms require a lot of work and is time consuming. Furthermore, because C. elegans are mainly hermaphrodite it is even more complicated to obtain large amounts of males which make high-throughput experiments using C. elegans males very challenging. In order to overcome these limitations, we developed affordable and rapid methods to: (1) grow large synchronous worm populations (2) easily obtain large amounts of males We developed a culture method on plates to grow big synchronized worm populations with the standard incubators used on all worm labs. We also established an easy filtration method allowing to obtain large male populations in an hour. After filtering, the worm population contains more than 90% of adult males and no adult hermaphrodites since all the contaminants are larva and embryos. The culture and the filtering methods we developed are easy to implement and require a very limited investment in equipment and consumables beside the standard one present in worm labs. In addition, this filtering method could be applied to nematode's species similar in size to C. elegans.

A sex-specific switch in a single glial cell patterns the apical extracellular matrix

Apical extracellular matrix (aECM) constitutes the interface between every tissue and the outside world. It is patterned into diverse tissue-specific structures through unknown mechanisms. Here, we show that a male-specific genetic switch in a single C. elegans glial cell patterns the overlying aECM from a solid sheet to an ∼200 nm pore, thus allowing a male sensory neuron to access the environment. Using cell-specific genetic sex reversal, we find that this switch reflects an inherent sex difference in the glial cell that is independent of the sex identity of the surrounding neurons. Through candidate and unbiased genetic screens, we find that this glial sex difference is controlled by factors shared with neurons (mab-3, lep-2, and lep-5) as well as previously unidentified regulators whose effects may be glia specific (nfya-1, bed-3, and jmjd-3.1). The switch results in male-specific glial expression of a secreted Hedgehog-related protein, GRL-18, that we discover localizes to transient nanoscale rings at sites where aECM pores will form. Using electron microscopy, we find that blocking male-specific gene expression in glia prevents pore formation, whereas forcing male-specific glial gene expression induces an ectopic pore. Thus, a switch in gene expression in a single cell is necessary and sufficient to pattern aECM into a specific structure. Our results highlight that aECM is not a simple homogeneous meshwork, but instead is composed of discrete local features that reflect the identity of the underlying cells.

An antibody free approach to probe the presence of poly-ubiquitin chains on C. elegans sperm derived organelles after fertilization

Upon C. elegans 's oocyte fertilization, the sperm brings mitochondria and membranous organelles (MOs) which are rapidly eliminated by autophagy. Their poly-ubiquitylation is suspected to be a signal for their recognition and degradation but mitochondria poly-ubiquitylation remains debated. Using fluorescent Tandem-repeated Ubiquitin-Binding Entities (TUBEs) we confirmed the presence of K48- and K63-ubiquitin chains on MOs contrasting with the absence of signal on sperm mitochondria. This new and sensitive approach confirmed the poly-ubiquitylation of the MOs while providing additional arguments for the absence of substantial poly-ubiquitylation of sperm-derived mitochondria, suggesting that K63- and K48-poly-ubiquitylation are unlikely acting as a common targeting signal for their degradation.

C. elegans males optimize mate-choice decisions via sex-specific responses to multimodal sensory cues

For sexually reproducing animals, selecting optimal mates is essential for maximizing reproductive fitness. Because the nematode C. elegans reproduces mostly by self-fertilization, little is known about its mate-choice behaviors. While several sensory cues have been implicated in males' ability to recognize hermaphrodites, achieving an integrated understanding of the ways males use these cues to assess relevant characteristics of potential mates has proven challenging. Here, we use a choice-based social-interaction assay to explore the ability of C. elegans males to make and optimize mate choices. We find that males use a combination of volatile sex pheromones (VSPs), ascaroside pheromones, surface-bound chemical cues, and other signals to robustly assess a variety of features of potential mates. Specific aspects of mate choice are communicated by distinct signals: the presence of a sperm-depleted, receptive hermaphrodite is likely signaled by VSPs, while developmental stage and sex are redundantly specified by ascaroside pheromones and surface-associated cues. Ascarosides also signal nutritional information, allowing males to choose well-fed over starved mates, while both ascarosides and surface-associated cues cause males to prefer virgin over previously mated hermaphrodites. The male-specificity of these behavioral responses is determined by both male-specific neurons and the male state of sex-shared circuits, and we reveal an unexpected role for the sex-shared ASH sensory neurons in male attraction to endogenously produced hermaphrodite ascarosides. Together, our findings lead to an integrated view of the signaling and behavioral mechanisms by which males use diverse sensory cues to assess multiple features of potential mates and optimize mate choice.

A sex-specific switch in a single glial cell patterns the apical extracellular matrix

Apical extracellular matrix (aECM) constitutes the interface between every tissue and the outside world. It is patterned into diverse tissue-specific structures through unknown mechanisms. Here, we show that a male-specific genetic switch in a single C. elegans glial cell patterns the aECM into a ∼200 nm pore, allowing a male sensory neuron to access the environment. We find that this glial sex difference is controlled by factors shared with neurons ( mab-3, lep-2, lep-5 ) as well as previously unidentified regulators whose effects may be glia-specific ( nfya-1, bed-3, jmjd-3.1 ). The switch results in male-specific expression of a Hedgehog-related protein, GRL-18, that we discover localizes to transient nanoscale rings at sites of aECM pore formation. Blocking male-specific gene expression in glia prevents pore formation, whereas forcing male-specific expression induces an ectopic pore. Thus, a switch in gene expression in a single cell is necessary and sufficient to pattern aECM into a specific structure.

Sexual dimorphism in Caenorhabditis elegans stress resistance

Physiological responses to the environment, disease, and aging vary by sex in many animals, but mechanisms of dimorphism have only recently begun to receive careful attention. The genetic model nematode Caenorhabditis elegans has well-defined mechanisms of stress response, aging, and sexual differentiation. C. elegans has males, but the vast majority of research only uses hermaphrodites. We found that males of the standard N2 laboratory strain were more resistant to hyperosmolarity, heat, and a natural pro-oxidant than hermaphrodites when in mixed-sex groups. Resistance to heat and pro-oxidant were also male-biased in three genetically and geographically diverse C. elegans strains consistent with a species-wide dimorphism that is not specific to domestication. N2 males were also more resistant to heat and pro-oxidant when keep individually indicating that differences in resistance do not require interactions between worms. We found that males induce canonical stress response genes by similar degrees and in similar tissues as hermaphrodites suggesting the importance of other mechanisms. We find that resistance to heat and pro-oxidant are influenced by the sex differentiation transcription factor TRA-1 suggesting that downstream organ differentiation pathways establish differences in stress resistance. Environmental stress influences survival in natural environments, degenerative disease, and aging. Understanding mechanisms of stress response dimorphism can therefore provide insights into sex-specific population dynamics, disease, and longevity.

T-Cell Factors as Transcriptional Inhibitors: Activities and Regulations in Vertebrate Head Development

Since its first discovery in the late 90s, Wnt canonical signaling has been demonstrated to affect a large variety of neural developmental processes, including, but not limited to, embryonic axis formation, neural proliferation, fate determination, and maintenance of neural stem cells. For decades, studies have focused on the mechanisms controlling the activity of β-catenin, the sole mediator of Wnt transcriptional response. More recently, the spotlight of research is directed towards the last cascade component, the T-cell factor (TCF)/Lymphoid-Enhancer binding Factor (LEF), and more specifically, the TCF/LEF-mediated switch from transcriptional activation to repression, which in both embryonic blastomeres and mouse embryonic stem cells pushes the balance from pluri/multipotency towards differentiation. It has been long known that Groucho/Transducin-Like Enhancer of split (Gro/TLE) is the main co-repressor partner of TCF/LEF. More recently, other TCF/LEF-interacting partners have been identified, including the pro-neural BarH-Like 2 (BARHL2), which belongs to the evolutionary highly conserved family of homeodomain-containing transcription factors. This review describes the activities and regulatory modes of TCF/LEF as transcriptional repressors, with a specific focus on the functions of Barhl2 in vertebrate brain development. Specific attention is given to the transcriptional events leading to formation of the Organizer, as well as the roles and regulations of Wnt/β-catenin pathway in growth of the caudal forebrain. We present TCF/LEF activities in both embryonic and neural stem cells and discuss how alterations of this pathway could lead to tumors.

Distinct neuropeptide-receptor modules regulate a sex-specific behavioral response to a pheromone

Dioecious species are a hallmark of the animal kingdom, with opposing sexes responding differently to identical sensory cues. Here, we study the response of C. elegans to the small-molecule pheromone, ascr#8, which elicits opposing behavioral valences in each sex. We identify a novel neuropeptide-neuropeptide receptor (NP/NPR) module that is active in males, but not in hermaphrodites. Using a novel paradigm of neuropeptide rescue that we established, we leverage bacterial expression of individual peptides to rescue the sex-specific response to ascr#8. Concurrent biochemical studies confirmed individual FLP-3 peptides differentially activate two divergent receptors, NPR-10 and FRPR-16. Interestingly, the two of the peptides that rescued behavior in our feeding paradigm are related through a conserved threonine, suggesting that a specific NP/NPR combination sets a male state, driving the correct behavioral valence of the ascr#8 response. Receptor expression within pre-motor neurons reveals novel coordination of male-specific and core locomotory circuitries.

One template, two outcomes: How does the sex-shared nervous system generate sex-specific behaviors?

Sex-specific behaviors are common in nature and are crucial for reproductive fitness and species survival. A key question in the field of sex/gender neurobiology is whether and to what degree the sex-shared nervous system differs between the sexes in the anatomy, connectivity and molecular identity of its components. An equally intriguing issue is how does the same sex-shared neuronal template diverge to mediate distinct behavioral outputs in females and males. This chapter aims to present the most up-to-date understanding of how this task is achieved in C. elegans. The vast majority of neurons in C. elegans are shared among the two sexes in terms of their lineage history, anatomical position and neuronal identity. Yet a substantial amount of evidence points to the hermaphrodite-male counterparts of some neurons expressing different genes and forming different synaptic connections. This, in turn, enables the same cells and circuits to transmit discrete signals in the two sexes and ultimately execute different functions. We review the various sex-shared behavioral paradigms that have been shown to be sexually dimorphic in recent years, discuss the mechanisms that underlie these examples, refer to the developmental regulation of neuronal dimorphism and suggest evolutionary concepts that emerge from the data.

Mcl1 protein levels and Caspase-7 executioner protease control axial organizer cells survival

BACKGROUND

Organizing centers are groups of specialized cells that secrete morphogens, thereby influencing development of their neighboring territories. Apoptosis is a form of programmed cell death reported to limit the size of organizers. Little is known about the identity of intracellular signals driving organizer cell death. Here we investigated in Xenopus the role of both the anti-apoptotic protein Myeloid-cell-leukemia 1 (Mcl1) and the cysteine proteases Caspase-3 and Caspase-7 in formation of the axial organizing center-the notochord-that derives from the Spemann organizer, and participates in the induction and patterning of the neuroepithelium.

RESULTS

We confirm a role for apoptosis in establishing the axial organizer in early neurula. We show that the expression pattern of mcl1 is coherent with a role for this gene in early notochord development. Using loss of function approaches, we demonstrate that Mcl1 depletion decreases neuroepithelium width and increases notochord cells apoptosis, a process that relies on Caspase-7, and not on Caspase-3, activity. Our data provide evidence that Mcl1 protein levels physiologically control notochord cells' survival and that Caspase-7 is the executioner protease in this developmental process.

CONCLUSIONS

Our study reveals new functions for Mcl1 and Caspase-7 in formation of the axial signalling center.

Brn3/POU-IV-type POU homeobox genes-Paradigmatic regulators of neuronal identity across phylogeny

One approach to understand the construction of complex systems is to investigate whether there are simple design principles that are commonly used in building such a system. In the context of nervous system development, one may ask whether the generation of its highly diverse sets of constituents, that is, distinct neuronal cell types, relies on genetic mechanisms that share specific common features. Specifically, are there common patterns in the function of regulatory genes across different neuron types and are those regulatory mechanisms not only used in different parts of one nervous system, but are they conserved across animal phylogeny? We address these questions here by focusing on one specific, highly conserved and well-studied regulatory factor, the POU homeodomain transcription factor UNC-86. Work over the last 30 years has revealed a common and paradigmatic theme of unc-86 function throughout most of the neuron types in which Caenorhabditis elegans unc-86 is expressed. Apart from its role in preventing lineage reiterations during development, UNC-86 operates in combination with distinct partner proteins to initiate and maintain terminal differentiation programs, by coregulating a vast array of functionally distinct identity determinants of specific neuron types. Mouse orthologs of unc-86, the Brn3 genes, have been shown to fulfill a similar function in initiating and maintaining neuronal identity in specific parts of the mouse brain and similar functions appear to be carried out by the sole Drosophila ortholog, Acj6. The terminal selector function of UNC-86 in many different neuron types provides a paradigm for neuronal identity regulation across phylogeny. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Invertebrate Organogenesis > Worms Nervous System Development > Vertebrates: Regional Development.

Signatures of sex: Sex differences in gene expression in the vertebrate brain

Women and men differ in disease prevalence, symptoms, and progression rates for many psychiatric and neurological disorders. As more preclinical studies include both sexes in experimental design, an increasing number of sex differences in physiology and behavior have been reported. In the brain, sex-typical behaviors are thought to result from sex-specific patterns of neural activity in response to the same sensory stimulus or context. These differential firing patterns likely arise as a consequence of underlying anatomic or molecular sex differences. Accordingly, gene expression in the brains of females and males has been extensively investigated, with the goal of identifying biological pathways that specify or modulate sex differences in brain function. However, there is surprisingly little consensus on sex-biased genes across studies and only a handful of robust candidates have been pursued in the follow-up experiments. Furthermore, it is not known how or when sex-biased gene expression originates, as few studies have been performed in the developing brain. Here we integrate molecular genetic and neural circuit perspectives to provide a conceptual framework of how sex differences in gene expression can arise in the brain. We detail mechanisms of gene regulation by steroid hormones, highlight landmark studies in rodents and humans, identify emerging themes, and offer recommendations for future research. This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Gene Expression and Transcriptional Hierarchies > Sex Determination.

The Hox gene Abdominal-B uses DoublesexF as a cofactor to promote neuroblast apoptosis in the Drosophila central nervous system

Highly conserved DM domain-containing transcription factors (Doublesex/MAB-3/DMRT1) are responsible for generating sexually dimorphic features. In the Drosophila central nervous system, a set of Doublesex (Dsx)-expressing neuroblasts undergo apoptosis in females whereas their male counterparts proliferate and give rise to serotonergic neurons crucial for adult mating behaviour. Our study demonstrates that the female-specific isoform of Dsx collaborates with Hox gene Abdominal-B (Abd-B) to bring about this apoptosis. Biochemical results suggest that proteins AbdB and Dsx interact through their highly conserved homeodomain and DM domain, respectively. This interaction is translated into a cooperative binding of the two proteins on the apoptotic enhancer in the case of females but not in the case of males, resulting in female-specific activation of apoptotic genes. The capacity of AbdB to use the sex-specific isoform of Dsx as a cofactor underlines the possibility that these two classes of protein are capable of cooperating in selection and regulation of target genes in a tissue- and sex-specific manner. We propose that this interaction could be a common theme in generating sexual dimorphism in different tissues across different species.

A Single-Neuron Chemosensory Switch Determines the Valence of a Sexually Dimorphic Sensory Behavior

Biological sex, a fundamental dimension of internal state, can modulate neural circuits to generate behavioral variation. Understanding how and why circuits are tuned by sex can provide important insights into neural and behavioral plasticity. Here we find that sexually dimorphic behavioral responses to C. elegans ascaroside sex pheromones are implemented by the functional modulation of shared chemosensory circuitry. In particular, the sexual state of a single sensory neuron pair, ADF, determines the nature of an animal's behavioral response regardless of the sex of the rest of the body. Genetic feminization of ADF causes males to be repelled by, rather than attracted to, ascarosides, whereas masculinization of ADF has the opposite effect in hermaphrodites. When ADF is ablated, both sexes are weakly repelled by ascarosides. Genetic sex modulates ADF function by tuning chemosensation: although ADF is functional in both sexes, it detects the ascaroside ascr#3 only in males, a consequence of cell-autonomous action of the master sexual regulator tra-1. This occurs in part through the conserved DM-domain gene mab-3, which promotes the male state of ADF. The sexual modulation of ADF has a key role in reproductive fitness, as feminization or ablation of ADF renders males unable to use ascarosides to locate mates. Our results reveal an economical mechanism in which sex-specific behavioral valence arises through the cell-autonomous regulation of a chemosensory switch by genetic sex, allowing a social cue with salience for both sexes to elicit navigational responses commensurate with the differing needs of each.

Sexual Dimorphism and Sex Differences in Caenorhabditis elegans Neuronal Development and Behavior

As fundamental features of nearly all animal species, sexual dimorphisms and sex differences have particular relevance for the development and function of the nervous system. The unique advantages of the nematode Caenorhabditis elegans have allowed the neurobiology of sex to be studied at unprecedented scale, linking ultrastructure, molecular genetics, cell biology, development, neural circuit function, and behavior. Sex differences in the C. elegans nervous system encompass prominent anatomical dimorphisms as well as differences in physiology and connectivity. The influence of sex on behavior is just as diverse, with biological sex programming innate sex-specific behaviors and modifying many other aspects of neural circuit function. The study of these differences has provided important insights into mechanisms of neurogenesis, cell fate specification, and differentiation; synaptogenesis and connectivity; principles of circuit function, plasticity, and behavior; social communication; and many other areas of modern neurobiology.

Programmed Cell Death During Caenorhabditis elegans Development

Programmed cell death is an integral component of Caenorhabditis elegans development. Genetic and reverse genetic studies in C. elegans have led to the identification of many genes and conserved cell death pathways that are important for the specification of which cells should live or die, the activation of the suicide program, and the dismantling and removal of dying cells. Molecular, cell biological, and biochemical studies have revealed the underlying mechanisms that control these three phases of programmed cell death. In particular, the interplay of transcriptional regulatory cascades and networks involving multiple transcriptional regulators is crucial in activating the expression of the key death-inducing gene egl-1 and, in some cases, the ced-3 gene in cells destined to die. A protein interaction cascade involving EGL-1, CED-9, CED-4, and CED-3 results in the activation of the key cell death protease CED-3, which is tightly controlled by multiple positive and negative regulators. The activation of the CED-3 caspase then initiates the cell disassembly process by cleaving and activating or inactivating crucial CED-3 substrates; leading to activation of multiple cell death execution events, including nuclear DNA fragmentation, mitochondrial elimination, phosphatidylserine externalization, inactivation of survival signals, and clearance of apoptotic cells. Further studies of programmed cell death in C. elegans will continue to advance our understanding of how programmed cell death is regulated, activated, and executed in general.

A High-Throughput Small Molecule Screen for C. elegans Linker Cell Death Inhibitors

Programmed cell death is a ubiquitous process in metazoan development. Apoptosis, one cell death form, has been studied extensively. However, mutations inactivating key mammalian apoptosis regulators do not block most developmental cell culling, suggesting that other cell death pathways are likely important. Recent work in the nematode Caenorhabditis elegans identified a non-apoptotic cell death form mediating the demise of the male-specific linker cell. This cell death process (LCD, linker cell-type death) is morphologically conserved, and its molecular effectors also mediate axon degeneration in mammals and Drosophila. To develop reagents to manipulate LCD, we established a simple high-throughput screening protocol for interrogating the effects of small molecules on C. elegans linker cell death in vivo. From 23,797 compounds assayed, 11 reproducibly block linker cell death onset. Of these, five induce animal lethality, and six promote a reversible developmental delay. These results provide proof-of principle validation of our screening protocol, demonstrate that developmental progression is required for linker cell death, and suggest that larger scale screens may identify LCD-specific small-molecule regulators that target the LCD execution machinery.

Function of the C. elegans T-box factor TBX-2 depends on interaction with the UNC-37/Groucho corepressor

T-box transcription factors are important regulators of development in all animals, and altered expression of T-box factors has been identified in an increasing number of diseases and cancers. Despite these important roles, the mechanism of T-box factor activity is not well understood. We have previously shown that the Caenorhabditis elegans Tbx2 subfamily member TBX-2 functions as a transcriptional repressor to specify ABa-derived pharyngeal muscle, and that this function depends on SUMOylation. Here we show that TBX-2 function also depends on interaction with the Groucho-family corepressor UNC-37. TBX-2 interacts with UNC-37 in yeast two-hybrid assays via a highly conserved engrailed homology 1 (eh1) motif located near the TBX-2 C-terminus. Reducing unc-37 phenocopies tbx-2 mutants, resulting in a specific loss of anterior ABa-derived pharyngeal muscles and derepression of the tbx-2 promoter. Moreover, double mutants containing hypomorphic alleles of unc-37 and tbx-2 exhibit enhanced phenotypes, providing strong genetic evidence that unc-37 and tbx-2 share common functions in vivo. To test whether interaction with UNC-37 is necessary for TBX-2 activity, we developed a transgene rescue assay using a tbx-2 containing fosmid and found that mutating the tbx-2 eh1 motif reduced rescue of a tbx-2 null mutant. These results indicate that TBX-2 function in vivo depends on interaction with UNC-37. As many T-box factors contain eh1 motifs, we suggest that interaction with Groucho-family corepressors is a common mechanism contributing to their activity.

Programmed cell death and clearance of cell corpses in Caenorhabditis elegans

Programmed cell death is critical to the development of diverse animal species from C. elegans to humans. In C. elegans, the cell death program has three genetically distinguishable phases. During the cell suicide phase, the core cell death machinery is activated through a protein interaction cascade. This activates the caspase CED-3, which promotes numerous pro-apoptotic activities including DNA degradation and exposure of the phosphatidylserine "eat me" signal on the cell corpse surface. Specification of the cell death fate involves transcriptional activation of the cell death initiator EGL-1 or the caspase CED-3 by coordinated actions of specific transcription factors in distinct cell types. In the cell corpse clearance stage, recognition of cell corpses by phagocytes triggers several signaling pathways to induce phagocytosis of apoptotic cell corpses. Cell corpse-enclosing phagosomes ultimately fuse with lysosomes for digestion of phagosomal contents. This article summarizes our current knowledge about programmed cell death and clearance of cell corpses in C. elegans.

Neural circuits for sexually dimorphic and sexually divergent behaviors in Caenorhabditis elegans

Increasing interest in sex differences in Caenorhabditis elegans neurobiology is resulting from several advances, including the completion of the male tail connectome and the surprising discovery of two 'new' neurons in the male head. In this species, sex-specific circuits in the hermaphrodite and male control reproductive behaviors such as egg-laying and copulation, respectively. Studies of these systems are revealing interesting similarities and contrasts, particularly in the mechanisms by which nutritional status influences reproductive behaviors. Other studies have highlighted the importance of sexual modulation of shared neurons and circuits in optimizing behavioral strategies. Together, these findings indicate that C. elegans uses intertwined, distributed sex differences in circuit structure and function to implement sex-specific as well as sexually divergent, shared behaviors.

Contrasting responses within a single neuron class enable sex-specific attraction in Caenorhabditis elegans

Animals find mates and food, and avoid predators, by navigating to regions within a favorable range of available sensory cues. How are these ranges set and recognized? Here we show that male Caenorhabditis elegans exhibit strong concentration preferences for sex-specific small molecule cues secreted by hermaphrodites, and that these preferences emerge from the collective dynamics of a single male-specific class of neurons, the cephalic sensory neurons (CEMs). Within a single worm, CEM responses are dissimilar, not determined by anatomical classification and can be excitatory or inhibitory. Response kinetics vary by concentration, suggesting a mechanism for establishing preferences. CEM responses are enhanced in the absence of synaptic transmission, and worms with only one intact CEM show nonpreferential attraction to all concentrations of ascaroside for which CEM is the primary sensor, suggesting that synaptic modulation of CEM responses is necessary for establishing preferences. A heterogeneous concentration-dependent sensory representation thus appears to allow a single neural class to set behavioral preferences and recognize ranges of sensory cues.

Engulfment pathways promote programmed cell death by enhancing the unequal segregation of apoptotic potential

Components of the conserved engulfment pathways promote programmed cell death in Caenorhabditis elegans (C. elegans) through an unknown mechanism. Here we report that the phagocytic receptor CED-1 mEGF10 is required for the formation of a dorsal–ventral gradient of CED-3 caspase activity within the mother of a cell programmed to die and an increase in the level of CED-3 protein within its dying daughter. Furthermore, CED-1 becomes enriched on plasma membrane regions of neighbouring cells that appose the dorsal side of the mother, which later forms the dying daughter. Therefore, we propose that components of the engulfment pathways promote programmed cell death by enhancing the polar localization of apoptotic factors in mothers of cells programmed to die and the unequal segregation of apoptotic potential into dying and surviving daughters. Our findings reveal a novel function of the engulfment pathways and provide a better understanding of how apoptosis is initiated during C. elegans development.

Programed cell death occurs in a stereotypic fashion during C. elegans development, and it is thought that engulfment promotes programmed cell death. Here the authors present evidence that a signaling function of the conserved engulfment pathways, not the process of engulfment itself, promotes apoptotic cell death.

Cell Death in C. elegans Development

Cell death is a common and important feature of animal development, and cell death defects underlie many human disease states. The nematode Caenorhabditis elegans has proven fertile ground for uncovering molecular and cellular processes controlling programmed cell death. A core pathway consisting of the conserved proteins EGL-1/BH3-only, CED-9/BCL2, CED-4/APAF1, and CED-3/caspase promotes most cell death in the nematode, and a conserved set of proteins ensures the engulfment and degradation of dying cells. Multiple regulatory pathways control cell death onset in C. elegans, and many reveal similarities with tumor formation pathways in mammals, supporting the idea that cell death plays key roles in malignant progression. Nonetheless, a number of observations suggest that our understanding of developmental cell death in C. elegans is incomplete. The interaction between dying and engulfing cells seems to be more complex than originally appreciated, and it appears that key aspects of cell death initiation are not fully understood. It has also become apparent that the conserved apoptotic pathway is dispensable for the demise of the C. elegans linker cell, leading to the discovery of a previously unexplored gene program promoting cell death. Here, we review studies that formed the foundation of cell death research in C. elegans and describe new observations that expand, and in some cases remodel, this edifice. We raise the possibility that, in some cells, more than one death program may be needed to ensure cell death fidelity.

The Homeobox Genes of Caenorhabditis elegans and Insights into Their Spatio-Temporal Expression Dynamics during Embryogenesis

Homeobox genes play crucial roles for the development of multicellular eukaryotes. We have generated a revised list of all homeobox genes for Caenorhabditis elegans and provide a nomenclature for the previously unnamed ones. We show that, out of 103 homeobox genes, 70 are co-orthologous to human homeobox genes. 14 are highly divergent, lacking an obvious ortholog even in other Caenorhabditis species. One of these homeobox genes encodes 12 homeodomains, while three other highly divergent homeobox genes encode a novel type of double homeodomain, termed HOCHOB. To understand how transcription factors regulate cell fate during development, precise spatio-temporal expression data need to be obtained. Using a new imaging framework that we developed, Endrov, we have generated spatio-temporal expression profiles during embryogenesis of over 60 homeobox genes, as well as a number of other developmental control genes using GFP reporters. We used dynamic feedback during recording to automatically adjust the camera exposure time in order to increase the dynamic range beyond the limitations of the camera. We have applied the new framework to examine homeobox gene expression patterns and provide an analysis of these patterns. The methods we developed to analyze and quantify expression data are not only suitable for C. elegans, but can be applied to other model systems or even to tissue culture systems.

Early development of the neural plate: new roles for apoptosis and for one of its main effectors caspase-3

Despite its tremendous complexity, the vertebrate nervous system emerges from a homogenous layer of neuroepithelial cells, the neural plate. Its formation relies on the time- and space-controlled progression of developmental programs. Apoptosis is a biological process that removes superfluous and potentially dangerous cells and is implemented through the activation of a molecular pathway conserved during evolution. Apoptosis and an unconventional function of one of its main effectors, caspase-3, contribute to the patterning and growth of the neuroepithelium. Little is known about the intrinsic and extrinsic cues controlling activities of the apoptotic machinery during development. The BarH-like (Barhl) proteins are homeodomain-containing transcription factors. The observations in Caenorhabditis elegans, Xenopus, and mice document that Barhl proteins act in cell survival and as cell type-specific regulators of a caspase-3 function that limits neural progenitor proliferation. In this review, we discuss the roles and regulatory modes of the apoptotic machinery in the development of the neural plate. We focus on the Barhl2, the Sonic Hedgehog, and the Wnt pathways and their activities in neural progenitor survival and proliferation.

Developmental alterations of the C. elegans male anal depressor morphology and function require sex-specific cell autonomous and cell non-autonomous interactions

We studied the Caenorhabditis elegans anal depressor development in larval males and hermaphrodites to address how a differentiated cell sex-specifically changes its morphology prior to adulthood. In both sexes, the larval anal depressor muscle is used for defecation behavior. However in the adult males, the muscle's sarcomere is reorganized to facilitate copulation. To address when the changes occur in the anal depressor, we used YFP:actin to monitor, and mutant analysis, laser-ablation and transgenic feminization to perturb the cell's morphological dynamics. In L1 and L2 stage larva, the muscle of both sexes has similar sarcomere morphology, but the hermaphrodite sex-determination system promotes more growth. The male anal depressor begins to change in the L3 stage, first by retracting its muscle arm from the neurons of the defecation circuit. Then the muscle's ventral region develops a slit that demarcates an anterior and posterior domain. This demarcation is not dependent on the anal depressor's intrinsic genetic sex, but is influenced by extrinsic interactions with the developing male sex muscles. However, subsequent changes are dependent on the cell's sex. In the L4 stage, the anterior domain first disassembles the dorsal-ventral sarcomere region and develops filopodia that elongates anteriorly towards the spicule muscles. Later, the posterior domain dissembles the remnants of its sarcomere, but still retains a vestigial attachment to the ventral body wall. Finally, the anterior domain attaches to the sex muscles, and then reassembles an anterior-posteriorly oriented sarcomere. Our work identifies key steps in the dimorphic re-sculpting of the anal depressor that are regulated by genetic sex and by cell-cell signaling.

Patterning of sexually dimorphic neurogenesis in the caenorhabditis elegans ventral cord by Hox and TALE homeodomain transcription factors

BACKGROUND

Reproduction in animals requires development of distinct neurons in each sex. In C. elegans, most ventral cord neurons (VCNs) are present in both sexes, with the exception of six hermaphrodite-specific neurons (VCs) and nine pairs of male-specific neurons (CAs and CPs) that arise from analogous precursor cells. How are the activities of sexual regulators and mediators of neuronal survival, division, and fate coordinated to generate sex-specificity in VCNs?

RESULTS

To address this, we have developed a toolkit of VCN markers that allows us to examine sex-specific neurogenesis, asymmetric fates of daughters of a neuroblast division, and regional specification on the anteroposterior axis. Here, we describe the roles of the Hox transcription factors LIN-39 and MAB-5 in promoting survival, differentiation, and regionalization of VCNs. We also find that the TALE class homeodomain proteins CEH-20 and UNC-62 contribute to specification of neurotransmitter fate in males. Furthermore, we identify that VCN sex is determined during the L1 larval stage.

CONCLUSIONS

These findings, combined with future analyses made possible by the suite of VCN markers described here, will elucidate how Hox-mediated cell fate decisions and sex determination intersect to influence development of neuronal sex differences.

The development of sexual dimorphism: studies of the Caenorhabditis elegans male

Studies of the development of the Caenorhabditis elegans male have been carried out with the aim of understanding the basis of sexual dimorphism. Postembryonic development of the two C. elegans sexes differs extensively. Development along either the hermaphrodite or male pathway is specified initially by the X to autosome ratio. The regulatory events initiated by this ratio include a male-determining paracrine intercellular signal. Expression of this signal leads to different consequences in three regions of the body: the nongonadal soma, the somatic parts of the gonad, and the germ line. In the nongonadal soma, activity of the key Zn-finger transcription factor TRA-1 determines hermaphrodite development; in its absence, the male pathway is followed. Only a few genes directly regulated by TRA-1 are currently known, including members of the evolutionarily conserved, male-determining DM domain Zn-finger transcription factors. In the somatic parts of the gonad and germ line, absence of TRA-1 activity is not sufficient for full expression of the male pathway. Several additional transcription factors involved have been identified. In the germ line, regulatory genes for sperm development that act at the level of RNA in the cytoplasm play a prominent role.

Sexual modulation of neural circuits and behavior in Caenorhabditis elegans

Sex differences in behavior-both sex-specific and shared behaviors-are fundamental to nearly all animal species. One often overlooked mechanism by which these behavioral differences can be generated is through sex-specific modulation of shared circuitry (i.e., circuits present in both sexes). In vertebrates this modulation is likely regulated by hormone-dependent mechanisms as well as by somatic sex itself; invertebrate models have particular promise for understanding the latter of these. Here we review molecular and behavioral evidence of sexual modulation of shared circuitry in the nematode Caenorhabditis elegans. Multiple behaviors in this species, both copulatory and not, are modulated by the genetic sex of shared neurons and circuit. These studies are close to uncovering the molecular mechanisms by which somatic sex modulates neural function in the worm, mechanisms which may be well conserved in more complex organisms. Improving our understanding of the modulation of neural circuit development and function by somatic sex may lend important insight into sex differences in the mammalian nervous system which, in turn, may have important implications for sex biases in disease.

Noncanonical cell death in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has served as a fruitful setting for cell death research for over three decades. A conserved pathway of four genes, egl-1/BH3-only, ced-9/Bcl-2, ced-4/Apaf-1, and ced-3/caspase, coordinates most developmental cell deaths in C. elegans. However, other cell death forms, programmed and pathological, have also been described in this animal. Some of these share morphological and/or molecular similarities with the canonical apoptotic pathway, while others do not. Indeed, recent studies suggest the existence of an entirely novel mode of programmed developmental cell destruction that may also be conserved beyond nematodes. Here, we review evidence for these noncanonical pathways. We propose that different cell death modalities can function as backup mechanisms for apoptosis, or as tailor-made programs that allow specific dying cells to be efficiently cleared from the animal.

TRA-1 ChIP-seq reveals regulators of sexual differentiation and multilevel feedback in nematode sex determination

How sexual regulators translate global sexual fate into appropriate local sexual differentiation events is perhaps the least understood aspect of sexual development. Here we have used ChIP followed by deep sequencing (ChIP-seq) to identify direct targets of the nematode global sexual regulator Transformer 1 (TRA-1), a transcription factor acting at the interface between organism-wide and cell-specific sexual regulation to control all sex-specific somatic differentiation events. We identified 184 TRA-1-binding sites in Caenorhabditis elegans, many with temporal- and/or tissue-specific TRA-1 association. We also identified 78 TRA-1-binding sites in the related nematode Caenorhabditis briggsae, 19 of which are conserved between the two species. Some DNA segments containing TRA-1-binding sites drive male-specific expression patterns, and RNAi depletion of some genes adjacent to TRA-1-binding sites results in defects in male sexual development. TRA-1 binds to sites adjacent to a number of heterochronic regulatory genes, some of which drive male-specific expression, suggesting that TRA-1 imposes sex specificity on developmental timing. We also found evidence for TRA-1 feedback regulation of the global sex-determination pathway: TRA-1 binds its own locus and those of multiple upstream masculinizing genes, and most of these associations are conserved in C. briggsae. Thus, TRA-1 coordinates sexual development by reinforcing the sex-determination decision and directing downstream sexual differentiation events.

Transcriptional regulation of gene expression in C. elegans

Protein coding gene sequences are converted to mRNA by the highly regulated process of transcription. The precise temporal and spatial control of transcription for many genes is an essential part of development in metazoans. Thus, understanding the molecular mechanisms underlying transcriptional control is essential to understanding cell fate determination during embryogenesis, post-embryonic development, many environmental interactions, and disease-related processes. Studies of transcriptional regulation in C. elegans exploit its genomic simplicity and physical characteristics to define regulatory events with single-cell and minute-time-scale resolution. When combined with the genetics of the system, C. elegans offers a unique and powerful vantage point from which to study how chromatin-associated proteins and their modifications interact with transcription factors and their binding sites to yield precise control of gene expression through transcriptional regulation.

The Caenorhabditis elegans homeobox gene ceh-19 is required for MC motorneuron function

Simplicity has made C. elegans pharyngeal development a particularly well-studied subject. Nevertheless, here we add the previously uncharacterized homeobox gene F20D12.6/ceh-19 to the set of transcription factor genes involved. GFP reporter assays revealed that ceh-19 is expressed in three pairs of neurons, the pharyngeal pace-maker neurons MC, the amphid neurons ADF and the phasmid neurons PHA. ceh-19(tm452) mutants are viable and fertile, but grow slightly slower, produce less progeny over a prolonged period, and live longer than the wild type. These phenotypes are likely due to the moderately reduced pharyngeal pumping speed arising from the impairment of MC activity. MC neurons are still born in the ceh-19 mutants but display various morphological defects. ceh-19 expression in MC is completely lost in progeny from animals subject to RNAi for pha-4, which encodes an organ-specifying forkhead transcription factor. CEH-19 is required for the activation in MCs of the excitatory FMRFamide-like neuropeptide-encoding gene flp-2. A regulatory pathway from pha-4 through ceh-19 to flp-2 is thereby defined. The resilience of MC identity in the absence of CEH-19 may reflect the buffering qualities of transcription factor regulatory networks. genesis 51:163–178, 2013. © 2013 Wiley Periodicals, Inc.

SPE-44 implements sperm cell fate

The sperm/oocyte decision in the hermaphrodite germline of Caenorhabditis elegans provides a powerful model for the characterization of stem cell fate specification and differentiation. The germline sex determination program that governs gamete fate has been well studied, but direct mediators of cell-type-specific transcription are largely unknown. We report the identification of spe-44 as a critical regulator of sperm gene expression. Deletion of spe-44 causes sperm-specific defects in cytokinesis, cell cycle progression, and organelle assembly resulting in sterility. Expression of spe-44 correlates precisely with spermatogenesis and is regulated by the germline sex determination pathway. spe-44 is required for the appropriate expression of several hundred sperm-enriched genes. The SPE-44 protein is restricted to the sperm-producing germline, where it localizes to the autosomes (which contain sperm genes) but is excluded from the transcriptionally silent X chromosome (which does not). The orthologous gene in other Caenorhabditis species is similarly expressed in a sex-biased manner, and the protein likewise exhibits autosome-specific localization in developing sperm, strongly suggestive of an evolutionarily conserved role in sperm gene expression. Our analysis represents the first identification of a transcriptional regulator whose primary function is the control of gamete-type-specific transcription in this system.

Stem cells give rise to the variety of specialized cell types within an organism. The decision to adopt a particular cell fate, a process known as specification or determination, requires the coordinated expression of all of the genes needed for that specialized cell to develop and function properly. Understanding the mechanisms that govern these patterns of gene expression is critical to our understanding of stem cell fate specification. We study this process in a nematode species that makes both sperm and eggs from the same stem cell population. We have identified a gene, named spe-44, that is required for the proper expression of sperm genes (but not egg genes). Mutation in spe-44 produces sterile sperm with developmental defects. spe-44 is controlled by factors that govern the sperm/egg decision, and its function in controlling sperm gene expression appears to be conserved in other nematode species.

Evolutionary relationships and diversification of barhl genes within retinal cell lineages

Background

Basic helix-loop-helix and homeodomain transcription factors have been shown to specify all different neuronal cell subtypes composing the vertebrate retina. The appearance of gene paralogs of such retina-specific transcription factors in lower vertebrates, with differently evolved function and/or conserved non-coding elements, might provide an important source for the generation of neuronal diversity within the vertebrate retinal architecture. In line with this hypothesis, we investigated the evolution of the homeobox Barhl family of transcription factors, barhl1 and barhl2, in the teleost and tetrapod lineages. In tetrapod barhl2, but not barhl1, is expressed in the retina and is important for amacrine cell specification. Zebrafish has three barhl paralogs: barhl1.1, barhl1.2 and barhl2, but their precise spatio-temporal retinal expression, as well as their function is yet unknown.

Results

Here we performed a meticulous expression pattern comparison of all known barhl fish paralogs and described a novel barhl paralog in medaka. Our detailed analysis of zebrafish barhl gene expression in wild type and mutant retinas revealed that only barhl1.2 and barhl2 are present in the retina. We also showed that these two paralogs are expressed in distinct neuronal lineages and are differently regulated by Atoh7, a key retinal-specific transcription factor. Finally, we found that the two retained medaka fish barhl paralogs, barhl1 and barhl2, are both expressed in the retina, in a pattern reminiscent of zebrafish barhl1.2 and barhl2 respectively. By performing phylogenetic and synteny analysis, we provide evidence that barhl retinal expression domain is an ancestral feature, probably lost in tetrapods due to functional redundancy.

Conclusions

Functional differences among retained paralogs of key retina-specific transcription factors between teleosts and tetrapods might provide important clues for understanding their potential impact on the generation of retinal neuronal diversity. Intriguingly, within teleosts, retention of zebrafish barhl1.2 and its medaka ortholog barhl1 appears to correlate with the acquisition of distinct signalling mechanisms by the two genes within distinct retinal cell lineages. Our findings provide a starting point for the study of barhl gene evolution in relation to the generation of cell diversity in the vertebrate retina.

Barhl2 limits growth of the diencephalic primordium through Caspase3 inhibition of beta-catenin activation

Little is known about the respective contributions of cell proliferation and cell death to the control of vertebrate forebrain growth. The homeodomain protein barhl2 is expressed in the diencephalons of Xenopus, zebrafish, and mouse embryos, and we previously showed that Barhl2 overexpression in Xenopus neuroepithelial cells induces Caspase3-dependent apoptosis. Here, barhl2 is shown to act as a brake on diencephalic proliferation through an unconventional function of Caspase3. Depletion of Barhl2 or Caspase3 causes an increase in diencephalic cell number, a disruption of the neuroepithelium architecture, and an increase in Wnt activity. Surprisingly, these changes are not caused by decreased apoptosis but instead, are because of an increase in the amount and activation of β-catenin, which stimulates excessive neuroepithelial cell proliferation and induces defects in β-catenin intracellular localization and an up-regulation of axin2 and cyclinD1, two downstream targets of β-catenin/T-cell factor/lymphoïd enhancer factor signaling. Using two different sets of complementation experiments, we showed that, in the developing diencephalon, Caspase3 acts downstream of Barhl2 in limiting neuroepithelial cell proliferation by inhibiting β-catenin activation. Our data argue that Bar homeodomain proteins share a conserved function as cell type-specific regulators of Caspase3 activities.

Cell lineage and cell death: Caenorhabditis elegans and cancer research

Cancer is a complex disease in which cells have circumvented normal restraints on tissue growth and have acquired complex abnormalities in their genomes, posing a considerable challenge to identifying the pathways and mechanisms that drive fundamental aspects of the malignant phenotype. Genetic analyses of the normal development of the nematode Caenorhabditis elegans have revealed evolutionarily conserved mechanisms through which individual cells establish their fates, and how they make and execute the decision to survive or undergo programmed cell death. The pathways identified through these studies have mammalian counterparts that are co-opted by malignant cells. Effective cancer drugs now target some of these pathways, and more are likely to be discovered.

Cell death and sexual differentiation of behavior: worms, flies, and mammals

Sex differences in the nervous system are found throughout the animal kingdom. Here, we discuss three prominent genetic models: nematodes, fruit flies, and mice. In all three, differential cell death is central to sexual differentiation and shared molecular mechanisms have been identified. Our knowledge of the precise function of neural sex differences lags behind. One fruitful approach to the 'function' question is to contrast sexual differentiation in standard laboratory animals with differentiation in species exhibiting unique social and reproductive organizations. Advanced genetic strategies are also addressing this question in worms and flies, and may soon be applicable to vertebrates.

Functional specialization of sensory cilia by an RFX transcription factor isoform

In animals, RFX transcription factors govern ciliogenesis by binding to an X-box motif in the promoters of ciliogenic genes. In Caenorhabditis elegans, the sole RFX transcription factor (TF) daf-19 null mutant lacks all sensory cilia, fails to express many ciliogenic genes, and is defective in many sensory behaviors, including male mating. The daf-19c isoform is expressed in all ciliated sensory neurons and is necessary and sufficient for activating X-box containing ciliogenesis genes. Here, we describe the daf-19(n4132) mutant that is defective in expression of the sensory polycystic kidney disease (PKD) gene battery and male mating behavior, without affecting expression of ciliogenic genes or ciliogenesis. daf-19(n4132) disrupts expression of a new isoform, daf-19m (for function in male mating). daf-19m is expressed in male-specific PKD and core IL2 neurons via internal promoters and remote enhancer elements located in introns of the daf-19 genomic locus. daf-19m genetically programs the sensory functions of a subset of ciliated neurons, independent of daf-19c. In the male-specific HOB neuron, DAF-19(M) acts downstream of the zinc finger TF EGL-46, indicating that a TF cascade controls the PKD gene battery in this cell-type specific context. We conclude that the RFX TF DAF-19 regulates ciliogenesis via X-box containing ciliogenic genes and controls ciliary specialization by regulating non-X-box containing sensory genes. This study reveals a more extensive role for RFX TFs in generating fully functional cilia.

Transcriptional upregulation of both egl-1 BH3-only and ced-3 caspase is required for the death of the male-specific CEM neurons

Most of the 131 cells that die during the development of a C. elegans hermaphrodite do so ~30 min after being generated. Furthermore, in these cells, the pro-caspase proCED-3 is inherited from progenitors and the transcriptional upregulation of the BH3-only gene egl-1 is thought to be sufficient for apoptosis induction. In contrast, the four CEM neurons, which die in hermaphrodites, but not males, die ~150 min after being generated. We found that in the CEMs, the transcriptional activation of both the egl-1 and ced-3 gene is necessary for apoptosis induction. In addition, we show that the Bar homeodomain transcription factor CEH-30 represses egl-1 and ced-3 transcription in the CEMs, thereby permitting their survival. Furthermore, we identified three genes, unc-86, lrs-1 and unc-132, which encode a POU homeodomain transcription factor, a leucyl-tRNA synthetase and a novel protein with limited sequence similarity to the mammalian proto-oncoprotein and kinase PIM-1, respectively, that promote the expression of the ceh-30 gene in the CEMs. Based on these results, we propose that egl-1 and ced-3 transcription are co-regulated in the CEMs to compensate for limiting proCED-3 levels, which most probably are a result of proCED-3 turn over. Similar co-regulatory mechanisms for BH3-only proteins and pro-caspases may function in higher organisms to allow efficient apoptosis induction during development. Finally, we present evidence that the timing of the death of the CEMs is controlled by TRA-1 Gli, the terminal global regulator of somatic sexual fate in C. elegans.

xol-1, the master sex-switch gene in C. elegans, is a transcriptional target of the terminal sex-determining factor TRA-1

In the nematode Caenorhabditis elegans, sex is determined by the ratio of X chromosomes to sets of autosomes: XX animals (2X:2A=1.0) develop as hermaphrodites and XO animals (1X:2A=0.5) develop as males. TRA-1, the worm ortholog of Drosophila Cubitus interruptus and mammalian Gli (Glioma-associated homolog) proteins, is the terminal transcription factor of the C. elegans sex-determination pathway, which specifies hermaphrodite fate by repressing male-specific genes. Here we identify a consensus TRA-1 binding site in the regulatory region of xol-1, the master switch gene controlling sex determination and dosage compensation. xol-1 is normally expressed in males, where it promotes male development and prevents dosage compensation. We show that TRA-1 binds to the consensus site in the xol-1 promoter in vitro and inhibits the expression of xol-1 in XX animals in vivo. Furthermore, inactivation of tra-1 enhances, whereas hyperactivation of tra-1 suppresses, lethality in animals with elevated xol-1 activity. These data imply the existence of a regulatory feedback loop within the C. elegans sex-determination and dosage-compensation cascade that ensures the accurate dose of X-linked genes in cells destined to adopt hermaphrodite fate.

Genetic and molecular insights into the development and evolution of sexual dimorphism

Sexual dimorphism is common throughout the animal kingdom. However, a molecular understanding of how sex-specific traits develop and evolve has been elusive. Recently, substantial progress has been made in elucidating how diverse sex-determination systems are integrated into developmental gene networks. One common theme from these studies is that sex-limited traits and gene expression are produced by the combined action of transcriptional effectors of sex-determination pathways and other transcription factors on target gene cis-regulatory elements. Sex-specific traits evolve by the gain, loss or modification of linkages in the genetic networks regulated by sex-determination transcription factors.

Regulation of sperm gene expression by the GATA factor ELT-1

Cell fate specification is mediated primarily through the expression of cell-type-specific genes. The regulatory pathway that governs the sperm/egg decision in the hermaphrodite germ line of Caenorhabditis elegans has been well characterized, but the transcription factors that drive these developmental programs remain unknown. We report the identification of ELT-1, a GATA transcription factor that specifies hypodermal fate in the embryo, as a regulator of sperm-specific transcription in the germ line. Computational analysis identified a conserved bipartite sequence element that is found almost exclusively in the promoters of a number of sperm genes. ELT-1 was recovered in a yeast one-hybrid screen for factors that bind to that sperm consensus site. In vitro assays defined the sperm consensus sequence as an optimal binding site for ELT-1. We determined that expression of elt-1 is elevated in the sperm-producing germ line, and that ELT-1 is required for sperm function. Deletion of the ELT-1 binding site from a sperm promoter abrogates sperm-specific expression of a reporter transgene. This work demonstrates a role for the ELT-1 transcription factor in sperm, and provides a critical link between the germ line sex determination program and gamete-specific gene expression.

Programmed cell death in the nervous system--a programmed cell fate?

Studies of developmental cell death in the nervous system have revealed two different modes of programmed cell death (PCD). One results from competition for target-derived trophic factors and leads to the stochastic removal of neurons and/or glia. A second, hard-wired form of PCD involves the lineage-specific, stereotypical death of identifiable neurons, glia or undifferentiated cells. Although traditionally associated with invertebrates, this 'programmed PCD' can also occur in vertebrates. Recent studies have shed light on its genetic control and have revealed that activation of the apoptotic machinery can be under the same complex, combinatorial control as the expression of terminal differentiation genes. This review will highlight these findings and will suggest why such complex control evolved.

TRA-1/GLI controls the expression of the Hox gene lin-39 during C. elegans vulval development

The vulva of the Caenorhabditis elegans hermaphrodite develops from a subset of six vulval precursor cells (VPCs) by the combined effect of the Ras, Wingless and Notch signaling cascades, and of three redundant synMuv (synthetic Multivulva) pathways grouped into classes A, B and C. Here we show that signaling via the GLI- (Glioma-associated protein) like transcription factor TRA-1, which is the terminal regulator of the C. elegans sex determination cascade, is a newly discovered pathway specifying vulval cell fates. We found that TRA-1 accumulates in, and regulates the fusion process of, cells (including the VPCs and hypodermal cells) involved in vulval patterning. TRA-1 also influenced the expression of the Hox gene lin-39, a central regulator of vulval development. Furthermore, inactivation of tra-1, which transforms animals with hermaphrodite-specific karyotype into males, promoted vulval induction in synMuv A, but not in synMuv B, mutant background. This implies that TRA-1 interacts with the class B synMuv genes, many of which are involved in chromatin-mediated transcriptional repression of cell proliferation. These results may help to understand how compromised GLI activity in humans leads to cancer. Together, we suggest that the GLI protein family involved in several key developmental processes in both invertebrates and vertebrates regulates somatic cell fates through influencing, at least in part, the expression of specific Hox genes.