Autosomal genes of autosomal/X-linked duplicated gene pairs and germ-line proliferation in Caenorhabditis elegans

Exportin 1 modulates life span by regulating nucleolar dynamics via the autophagy protein LGG-1/GABARAP

Altered nucleolar and ribosomal dynamics are key hallmarks of aging, but their regulation remains unclear. Building on the knowledge that the conserved nuclear export receptor Exportin 1 (XPO-1/XPO1) modulates proteostasis and life span, we systematically analyzed the impact of nuclear export on protein metabolism. Using transcriptomic and subcellular proteomic analyses in nematodes, we demonstrate that XPO-1 modulates the nucleocytoplasmic distribution of key proteins involved in nucleolar dynamics and ribosome function, including fibrillarin (FIB-1/FBL) and RPL-11 (RPL11). Silencing xpo-1 led to marked reduction in global translation, which was accompanied by decreased nucleolar size and lower fibrillarin levels. A targeted screen of known proteostatic mediators revealed that the autophagy protein LGG-1/GABARAP modulates nucleolar size by regulating RPL-11 levels, linking specific protein degradation to ribosome metabolism. Together, our study reveals that nucleolar size and life span are regulated by LGG-1/GABARAP via ribosome protein surveillance.

Post-Transcriptional Regulation of Homeostatic, Stressed, and Malignant Stem Cells

Cellular identity is not driven by differences in genomic content but rather by epigenomic, transcriptomic, and proteomic heterogeneity. Although regulation of the epigenome plays a key role in shaping stem cell hierarchies, differential expression of transcripts only partially explains protein abundance. The epitranscriptome, translational control, and protein degradation have emerged as fundamental regulators of proteome complexity that regulate stem cell identity and function. Here, we discuss how post-transcriptional mechanisms enable stem cell homeostasis and responsiveness to developmental cues and environmental stressors by rapidly shaping the content of their proteome and how these processes are disrupted in pre-malignant and malignant states.

Biology of the Caenorhabditis elegans Germline Stem Cell System

Stem cell systems regulate tissue development and maintenance. The germline stem cell system is essential for animal reproduction, controlling both the timing and number of progeny through its influence on gamete production. In this review, we first draw general comparisons to stem cell systems in other organisms, and then present our current understanding of the germline stem cell system in Caenorhabditis elegans In contrast to stereotypic somatic development and cell number stasis of adult somatic cells in C. elegans, the germline stem cell system has a variable division pattern, and the system differs between larval development, early adult peak reproduction and age-related decline. We discuss the cell and developmental biology of the stem cell system and the Notch regulated genetic network that controls the key decision between the stem cell fate and meiotic development, as it occurs under optimal laboratory conditions in adult and larval stages. We then discuss alterations of the stem cell system in response to environmental perturbations and aging. A recurring distinction is between processes that control stem cell fate and those that control cell cycle regulation. C. elegans is a powerful model for understanding germline stem cells and stem cell biology.

Functional Interactions Between rsks-1/S6K, glp-1/Notch, and Regulators of Caenorhabditis elegans Fertility and Germline Stem Cell Maintenance

The proper accumulation and maintenance of stem cells is critical for organ development and homeostasis. The Notch signaling pathway maintains stem cells in diverse organisms and organ systems. In Caenorhabditis elegans, GLP-1/Notch activity prevents germline stem cell (GSC) differentiation. Other signaling mechanisms also influence the maintenance of GSCs, including the highly-conserved TOR substrate ribosomal protein S6 kinase (S6K). Although C. elegans bearing either a null mutation in rsks-1/S6K or a reduction-of-function (rf) mutation in glp-1/Notch produce half the normal number of adult germline progenitors, virtually all these single mutant animals are fertile. However, glp-1(rf) rsks-1(null) double mutant animals are all sterile, and in about half of their gonads, all GSCs differentiate, a distinctive phenotype associated with a significant reduction or loss of GLP-1 signaling. How rsks-1/S6K promotes GSC fate is unknown. Here, we determine that rsks-1/S6K acts germline-autonomously to maintain GSCs, and that it does not act through Cyclin-E or MAP kinase in this role. We found that interfering with translation also enhances glp-1(rf), but that regulation through rsks-1 cannot fully account for this effect. In a genome-scale RNAi screen for genes that act similarly to rsks-1/S6K, we identified 56 RNAi enhancers of glp-1(rf) sterility, many of which were previously not known to interact functionally with Notch. Further investigation revealed at least six candidates that, by genetic criteria, act linearly with rsks-1/S6K. These include genes encoding translation-related proteins, cacn-1/Cactin, an RNA exosome component, and a Hedgehog-related ligand. We found that additional Hedgehog-related ligands may share functional relationships with glp-1/Notch and rsks-1/S6K in maintaining germline progenitors.

CDK-11-Cyclin L is required for gametogenesis and fertility in C. elegans

CDK11, a member of the cyclin-dependent kinase family, has been implicated in a diverse array of functions including transcription, RNA processing, sister chromatid cohesion, spindle assembly, centriole duplication and apoptosis. Despite its involvement in many essential functions, little is known about the requirements for CDK11 and its partner Cyclin L in a developing multicellular organism. Here we investigate the function of CDK11 and Cyclin L during development of the nematode Caenorhabditis elegans. Worms express two CDK11 proteins encoded by distinct loci: CDK-11.1 is essential for normal male and female fertility and is broadly expressed in the nuclei of somatic and germ line cells, while CDK-11.2 is nonessential and is enriched in hermaphrodite germ line nuclei beginning in mid pachytene. Hermaphrodites lacking CDK-11.1 develop normally but possess fewer mature sperm and oocytes and do not fully activate the RAS-ERK pathway that is required for oocyte production in response to environmental cues. Most of the sperm and eggs that are produced in cdk-11.1 null animals appear to complete development normally but fail to engage in sperm-oocyte signaling suggesting that CDK-11.1 is needed at multiple points in gametogenesis. Finally, we find that CDK-11.1 and CDK-11.2 function redundantly during embryonic and postembryonic development and likely do so in association with Cyclin L. Our results thus define multiple requirements for CDK-11-Cyclin L during animal development.

Genomic Identification and Functional Characterization of Essential Genes in Caenorhabditis elegans

Using combined genetic mapping, Illumina sequencing, bioinformatics analyses, and experimental validation, we identified 60 essential genes from 104 lethal mutations in two genomic regions of Caenorhabditis elegans totaling ∼14 Mb on chromosome III(mid) and chromosome V(left). Five of the 60 genes had not previously been shown to have lethal phenotypes by RNA interference depletion. By analyzing the regions around the lethal missense mutations, we identified four putative new protein functional domains. Furthermore, functional characterization of the identified essential genes shows that most are enzymes, including helicases, tRNA synthetases, and kinases in addition to ribosomal proteins. Gene Ontology analysis indicated that essential genes often encode for enzymes that conduct nucleic acid binding activities during fundamental processes, such as intracellular DNA replication, transcription, and translation. Analysis of essential gene shows that they have fewer paralogs, encode proteins that are in protein interaction hubs, and are highly expressed relative to nonessential genes. All these essential gene traits in C. elegans are consistent with those of human disease genes. Most human orthologs (90%) of the essential genes in this study are related to human diseases. Therefore, functional characterization of essential genes underlines their importance as proxies for understanding the biological functions of human disease genes.

HOP-1 Presenilin Deficiency Causes a Late-Onset Notch Signaling Phenotype That Affects Adult Germline Function in Caenorhabditis elegans

Functionally redundant genes present a puzzle as to their evolutionary preservation, and offer an interesting opportunity for molecular specialization. In Caenorhabditis elegans, either one of two presenilin genes (sel-12 or hop-1) facilitate Notch activation, providing the catalytic subunit for the γ secretase proteolytic enzyme complex. For all known Notch signaling events, sel-12 can mediate Notch activation, so the conservation of hop-1 remains a mystery. Here, we uncover a novel "late-onset" germline Notch phenotype in which HOP-1-deficient worms fail to maintain proliferating germline stem cells during adulthood. Either SEL-12 or HOP-1 presenilin can impart sufficient Notch signaling for the establishment and expansion of the germline, but maintenance of an adult stem cell pool relies exclusively on HOP-1-mediated Notch signaling. We also show that HOP-1 is necessary for maximum fecundity and reproductive span. The low-fecundity phenotype of hop-1 mutants can be phenocopied by switching off glp-1/Notch function during the last stage of larval development. We propose that at the end of larval development, dual presenilin usage switches exclusively to HOP-1, perhaps offering opportunities for differential regulation of the germline during adulthood. Additional defects in oocyte size and production rate in hop-1 and glp-1 mutants indicate that the process of oogenesis is compromised when germline Notch signaling is switched off. We calculate that in wild-type adults, as much as 86% of cells derived from the stem cell pool function to support oogenesis. This work suggests that an important role for Notch signaling in the adult germline is to furnish a large and continuous supply of nurse cells to support the efficiency of oogenesis.

Polyadenylation is the key aspect of GLD-2 function in C. elegans

The role of many enzymes extends beyond their dedicated catalytic activity by fulfilling important cellular functions in a catalysis-independent fashion. In this aspect, little is known about 3'-end RNA-modifying enzymes that belong to the class of nucleotidyl transferases. Among these are noncanonical poly(A) polymerases, a group of evolutionarily conserved enzymes that are critical for gene expression regulation, by adding adenosines to the 3'-end of RNA targets. In this study, we investigate whether the functions of the cytoplasmic poly(A) polymerase (cytoPAP) GLD-2 in C. elegans germ cells exclusively depend on its catalytic activity. To this end, we analyzed a specific missense mutation affecting a conserved amino acid in the catalytic region of GLD-2 cytoPAP. Although this mutated protein is expressed to wild-type levels and incorporated into cytoPAP complexes, we found that it cannot elongate mRNA poly(A) tails efficiently or promote GLD-2 target mRNA abundance. Furthermore, germ cell defects in animals expressing this mutant protein strongly resemble those lacking the GLD-2 protein altogether, arguing that only the polyadenylation activity of GLD-2 is essential for gametogenesis. In summary, we propose that all known molecular and biological functions of GLD-2 depend on its enzymatic activity, demonstrating that polyadenylation is the key mechanism of GLD-2 functionality. Our findings highlight the enzymatic importance of noncanonical poly(A) polymerases and emphasize the pivotal role of poly(A) tail-centered cytoplasmic mRNA regulation in germ cell biology.

Chromosome-wide mechanisms to decouple gene expression from gene dose during sex-chromosome evolution

Changes in chromosome number impair fitness by disrupting the balance of gene expression. Here we analyze mechanisms to compensate for changes in gene dose that accompanied the evolution of sex chromosomes from autosomes. Using single-copy transgenes integrated throughout the Caenorhabditis elegans genome, we show that expression of all X-linked transgenes is balanced between XX hermaphrodites and XO males. However, proximity of a dosage compensation complex (DCC) binding site (rex site) is neither necessary to repress X-linked transgenes nor sufficient to repress transgenes on autosomes. Thus, X is broadly permissive for dosage compensation, and the DCC acts via a chromosome-wide mechanism to balance transcription between sexes. In contrast, no analogous X-chromosome-wide mechanism balances transcription between X and autosomes: expression of compensated hermaphrodite X-linked transgenes is half that of autosomal transgenes. Furthermore, our results argue against an X-chromosome dosage compensation model contingent upon rex-directed positioning of X relative to the nuclear periphery.

DOI:http://dx.doi.org/10.7554/eLife.17365.001

DNA inside cells is packaged into structures called chromosomes, each of which contains numerous genes. Many organisms, including humans, have two copies of most chromosomes in their cells. If the process of cell division goes awry, cells can end up with too many or too few copies of their chromosomes, which can cause serious illnesses.

Sex chromosomes pose a conundrum for cells. In humans, females have two copies of the X chromosome, whereas males only have one. This means that males have half the copy number (dose) of genes on the X chromosome. Human cells correct this imbalance by suppressing the activity, or expression, of most of the genes on one of the X chromosomes in females.

“Dosage compensation” also occurs in the roundworm species Caenorhabditis elegans, because male worms have one X chromosome whilst hermaphrodites have two. The dosage compensation mechanism in roundworms differs from that in humans. It involves turning down the expression of both hermaphrodite X chromosomes by half. The process is enacted by a dosage compensation complex that binds to specific sites along both hermaphrodite X chromosomes.

Dosage compensation mechanisms that reduce X chromosome expression in females cause sex chromosomes to have lower gene expression than non-sex chromosomes. Modern sex chromosomes evolved from a pair of non-sex chromosomes, and males lost one copy of all of the genes located on those ancestral chromosomes. This evolutionary history causes both sexes to have lower gene expression from X chromosomes than the other chromosomes, raising the question of whether a mechanism exists to balance out the difference in gene expression between sex chromosomes and non-sex chromosomes.

Wheeler et al. now show that the expression of any foreign gene artificially added to the X chromosomes of C. elegans is equalized between males and hermaphrodites despite the difference in gene dose. The equalization works regardless of where on the X chromosome the new gene is added. The foreign gene does not need to be adjacent to a binding site for the dosage compensation complex. These results indicate that dosage compensation mechanisms regulate gene expression on a chromosome-wide scale.

Wheeler et al. also show that genes added to X chromosomes are expressed at half the level as the same genes added to non-sex chromosomes. These results mean that no chromosome-wide mechanism balances gene expression levels between the X chromosome and the non-sex chromosomes.

It remains unknown how C. elegans, and many other living organisms, evolved to tolerate a lower level of gene expression from the sex chromosomes. Instead of a chromosome-wide mechanism, it is likely that individual genes evolved different ways to alter their expression levels. Working out what these mechanisms are remains a challenge for further research.

DOI:http://dx.doi.org/10.7554/eLife.17365.002

Cell cycle features of C. elegans germline stem/progenitor cells vary temporally and spatially

Many organisms accumulate a pool of germline stem cells during development that is maintained in later life. The dynamics of establishment, expansion and homeostatic maintenance of this pool are subject to both developmental and physiological influences including the availability of a suitable niche microenvironment, nutritional status, and age. Here, we investigated the dynamics of germline proliferation during stages of expansion and homeostasis, using the C. elegans germ line as a model. The vast majority of germ cells in the proliferative zone are in interphase stages of mitosis (G1, S, G2) rather than in the active mitotic (M) phase. We examined mitotic index and DNA content, comparing different life stages, mutants, and physiological conditions. We found that germ cells in larval stages cycle faster than in adult stages, but that this difference could not be attributed to sexual fate of the germ cells. We also found that larval germ cells exhibit a lower average DNA content compared to adult germ cells. We extended our analysis to consider the effects of distance from the niche and further found that the spatial pattern of DNA content differs between larval and adult stages in the wild type and among mutants in pathways that interfere with cell cycle progression, cell fate, or both. Finally, we characterized expansion of the proliferative pool of germ cells during adulthood, using a regeneration paradigm (ARD recovery) in which animals are starved and re-fed. We compared adult stage regeneration and larval stage expansion, and found that the adult germ line is capable of rapid accumulation but does not sustain a larval-level mitotic index nor does it recapitulate the larval pattern of DNA content. The regenerated germ line does not reach the number of proliferative zone nuclei seen in the continuously fed adult. Taken together, our results suggest that cell cycle dynamics are under multiple influences including distance from the niche, age and/or maturation of the germ line, nutrition and, possibly, latitude for physical expansion.

Lipid signalling couples translational surveillance to systemic detoxification in Caenorhabditis elegans

Translation in eukaryotes is followed to detect toxins and virulence factors and coupled to the induction of defence pathways. Caenorhabditis elegans germline-specific mutations in translation components are detected by this system to induce detoxification and immune responses in distinct somatic cells. An RNA interference screen revealed gene inactivations that act at multiple steps in lipid biosynthetic and kinase pathways upstream of MAP kinase to mediate the systemic communication of translation defects to induce detoxification genes. Mammalian bile acids can rescue the defect in detoxification gene induction caused by C. elegans lipid biosynthetic gene inactivations. Extracts prepared from C. elegans with translation deficits but not from the wild type can also rescue detoxification gene induction in lipid-biosynthesis-defective strains. These eukaryotic antibacterial countermeasures are not ignored by bacteria: particular bacterial species suppress normal C. elegans detoxification responses to mutations in translation factors.

Poly(A)-binding proteins are required for diverse biological processes in metazoans

PABPs [poly(A)-binding proteins] bind to the poly(A) tail of eukaryotic mRNAs and are conserved in species ranging from yeast to human. The prototypical cytoplasmic member, PABP1, is a multifunctional RNA-binding protein with roles in global and mRNA-specific translation and stability, consistent with a function as a central regulator of mRNA fate in the cytoplasm. More limited insight into the molecular functions of other family members is available. However, the consequences of disrupting PABP function in whole organisms is less clear, particularly in vertebrates, and even more so in mammals. In the present review, we discuss current and emerging knowledge with respect to the functions of PABP family members in whole animal studies which, although incomplete, already underlines their biological importance and highlights the need for further intensive research in this area.

GLD-4-mediated translational activation regulates the size of the proliferative germ cell pool in the adult C. elegans germ line

To avoid organ dysfunction as a consequence of tissue diminution or tumorous growth, a tight balance between cell proliferation and differentiation is maintained in metazoans. However, cell-intrinsic gene expression mechanisms controlling adult tissue homeostasis remain poorly understood. By focusing on the adult Caenorhabditis elegans reproductive tissue, we show that translational activation of mRNAs is a fundamental mechanism to maintain tissue homeostasis. Our genetic experiments identified the Trf4/5-type cytoplasmic poly(A) polymerase (cytoPAP) GLD-4 and its enzymatic activator GLS-1 to perform a dual role in regulating the size of the proliferative zone. Consistent with a ubiquitous expression of GLD-4 cytoPAP in proliferative germ cells, its genetic activity is required to maintain a robust proliferative adult germ cell pool, presumably by regulating many mRNA targets encoding proliferation-promoting factors. Based on translational reporters and endogenous protein expression analyses, we found that gld-4 activity promotes GLP-1/Notch receptor expression, an essential factor of continued germ cell proliferation. RNA-protein interaction assays documented also a physical association of the GLD-4/GLS-1 cytoPAP complex with glp-1 mRNA, and ribosomal fractionation studies established that GLD-4 cytoPAP activity facilitates translational efficiency of glp-1 mRNA. Moreover, we found that in proliferative cells the differentiation-promoting factor, GLD-2 cytoPAP, is translationally repressed by the stem cell factor and PUF-type RNA-binding protein, FBF. This suggests that cytoPAP-mediated translational activation of proliferation-promoting factors, paired with PUF-mediated translational repression of differentiation factors, forms a translational control circuit that expands the proliferative germ cell pool. Our additional genetic experiments uncovered that the GLD-4/GLS-1 cytoPAP complex promotes also differentiation, forming a redundant translational circuit with GLD-2 cytoPAP and the translational repressor GLD-1 to restrict proliferation. Together with previous findings, our combined data reveals two interconnected translational activation/repression circuitries of broadly conserved RNA regulators that maintain the balance between adult germ cell proliferation and differentiation.

Throughout adulthood, animal tissue homeostasis requires adult stem cell activities. A tight balance between self-renewal and differentiation protects against tissue overgrowth or loss. This balance is strongly influenced by niche-mediated signaling pathways that primarily trigger a transcriptional response in stem cells to promote self-renewal/proliferation. However, the cell-intrinsic mechanisms that modulate signaling pathways to promote proliferation or differentiation are poorly understood. Recently, post-transcriptional mRNA regulation emerged in diverse germline stem cell systems as an important gene expression mechanism, primarily preventing the protein synthesis of factors that promote the switch to differentiation. In the adult C. elegans germ line, this study finds that the evolutionarily conserved cytoplasmic poly(A) polymerase, GLD-4, plays an crucial role in maintaining a healthy balance between proliferation and differentiation forces. This is in part due to translational activation of the mRNA that encodes the germ cell-expressed Notch signaling receptor, an essential regulator of proliferation. Moreover, GLD-4 activity is part of a redundant genetic network downstream of Notch that, together with several other conserved mRNA regulators, promotes differentiation onset. Given the widespread expression of these conserved RNA regulators in metazoans, cell fate balances that are reinforced by translational activation and repression circuitries may therefore be a general mechanism of adult tissue maintenance.

The cytoplasmic poly(A) polymerases GLD-2 and GLD-4 promote general gene expression via distinct mechanisms

Post-transcriptional gene regulation mechanisms decide on cellular mRNA activities. Essential gatekeepers of post-transcriptional mRNA regulation are broadly conserved mRNA-modifying enzymes, such as cytoplasmic poly(A) polymerases (cytoPAPs). Although these non-canonical nucleotidyltransferases efficiently elongate mRNA poly(A) tails in artificial tethering assays, we still know little about their global impact on poly(A) metabolism and their individual molecular roles in promoting protein production in organisms. Here, we use the animal model Caenorhabditis elegans to investigate the global mechanisms of two germline-enriched cytoPAPs, GLD-2 and GLD-4, by combining polysome profiling with RNA sequencing. Our analyses suggest that GLD-2 activity mediates mRNA stability of many translationally repressed mRNAs. This correlates with a general shortening of long poly(A) tails in gld-2-compromised animals, suggesting that most if not all targets are stabilized via robust GLD-2-mediated polyadenylation. By contrast, only mild polyadenylation defects are found in gld-4-compromised animals and few mRNAs change in abundance. Interestingly, we detect a reduced number of polysomes in gld-4 mutants and GLD-4 protein co-sediments with polysomes, which together suggest that GLD-4 might stimulate or maintain translation directly. Our combined data show that distinct cytoPAPs employ different RNA-regulatory mechanisms to promote gene expression, offering new insights into translational activation of mRNAs.

Sex-biased gene expression and evolution of the x chromosome in nematodes

Studies of X chromosome evolution in various organisms have indicated that sex-biased genes are nonrandomly distributed between the X and autosomes. Here, to extend these studies to nematodes, we annotated and analyzed X chromosome gene content in four Caenorhabditis species and in Pristionchus pacificus. Our gene expression analyses comparing young adult male and female mRNA-seq data indicate that, in general, nematode X chromosomes are enriched for genes with high female-biased expression and depleted of genes with high male-biased expression. Genes with low sex-biased expression do not show the same trend of X chromosome enrichment and depletion. Combined with the observation that highly sex-biased genes are primarily expressed in the gonad, differential distribution of sex-biased genes reflects differences in evolutionary pressures linked to tissue-specific regulation of X chromosome transcription. Our data also indicate that X dosage imbalance between males (XO) and females (XX) is influential in shaping both expression and gene content of the X chromosome. Predicted upregulation of the single male X to match autosomal transcription (Ohno's hypothesis) is supported by our observation that overall transcript levels from the X and autosomes are similar for highly expressed genes. However, comparison of differentially located one-to-one orthologs between C. elegans and P. pacificus indicates lower expression of X-linked orthologs, arguing against X upregulation. These contradicting observations may be reconciled if X upregulation is not a global mechanism but instead acts locally on a subset of tissues and X-linked genes that are dosage sensitive.

Regulation of the X chromosomes in Caenorhabditis elegans

Dosage compensation, which regulates the expression of genes residing on the sex chromosomes, has provided valuable insights into chromatin-based mechanisms of gene regulation. The nematode Caenorhabditis elegans has adopted various strategies to down-regulate and even nearly silence the X chromosomes. This article discusses the different chromatin-based strategies used in somatic tissues and in the germline to modulate gene expression from the C. elegans X chromosomes and compares these strategies to those used by other organisms to cope with similar X-chromosome dosage differences.

PAB-1, a Caenorhabditis elegans poly(A)-binding protein, regulates mRNA metabolism in germline by interacting with CGH-1 and CAR-1

Poly(A)-binding proteins are highly conserved among eukaryotes and regulate stability of mRNA and translation. Among C. elegans homologues, pab-1 mutants showed defects in germline mitotic proliferation. Unlike pab-1 mutants, pab-1 RNAi at every larval stage caused arrest of germline development at the following stage, indicating that pab-1 is required for the entire postembryonic germline development. This idea is supported by the observations that the mRNA level of pab-1 increased throughout postembryonic development and its protein expression was germline-enriched. PAB-1 localized to P granules and the cytoplasm in the germline. PAB-1 colocalized with CGH-1 and CAR-1 and affected their localization, suggesting that PAB-1 is a component of processing (P)-bodies that interacts with them. The mRNA and protein levels of representative germline genes, rec-8, GLP-1, rme-2, and msp-152, were decreased after pab-1 RNAi. Although the mRNA level of msp-152 was increased in cgh-1 mutant, it was also significantly reduced by pab-1 RNAi. Our results suggest that PAB-1 positively regulates the mRNA levels of germline genes, which is likely facilitated by the interaction of PAB-1 with other P-body components, CGH-1 and CAR-1.

Germline stem cells and their regulation in the nematode Caenorhabditis elegans

C. elegans germline stem cells exist within a stem cell pool that is maintained by a single-celled mesenchymal niche and Notch signaling. Downstream of Notch signaling, a regulatory network governs stem cells and differentiation. Central to that network is the FBF RNA-binding protein, a member of the widely conserved PUF family that functions by either of two broadly conserved mechanisms to repress its target mRNAs. Without FBF, germline stem cells do not proliferate and they do not maintain their naïve, undifferentiated state. Therefore, FBF is a pivotal regulator of germline self-renewal. Validated FBF targets include several key differentiation regulators as well as a major cell cycle regulator. A genomic analysis identifies many other developmental and cell cycle regulators as likely FBF targets and suggests that FBF is a broad-spectrum regulator of the genome with >1,000 targets. A comparison of the FBF target list with similar lists for human PUF proteins, PUM1 and PUM2, reveals ∼200 shared targets. The FBF hub works within a network controlling self-renewal vs. differentiation. This network consists of classical developmental cell fate regulators and classical cell cycle regulators. Recent results have begun to integrate developmental and cell cycle regulation within the network. The molecular dynamics of the network remain a challenge for the future, but models are proposed. We suggest that molecular controls of C. elegans germline stem cells provide an important model for controls of stem cells more broadly.

Translational control in the Caenorhabditis elegans germ line

Translational control is a prevalent form of gene expression regulation in the Caenorhabditis elegans germ line. Linking the amount of protein synthesis to mRNA quantity and translational accessibility in the cell cytoplasm provides unique advantages over DNA-based controls for developing germ cells. This mode of gene expression is especially exploited in germ cell fate decisions and during oogenesis, when the developing oocytes stockpile hundreds of different mRNAs required for early embryogenesis. Consequently, a dense web of RNA regulators, consisting of diverse RNA-binding proteins and RNA-modifying enzymes, control the translatability of entire mRNA expression programs. These RNA regulatory networks are tightly coupled to germ cell developmental progression and are themselves under translational control. The underlying molecular mechanisms and RNA codes embedded in the mRNA molecules are beginning to be understood. Hence, the C. elegans germ line offers fertile grounds for discovering post-transcriptional mRNA regulatory mechanisms and emerges as great model for a systems level understanding of translational control during development.

C. elegans rrf-1 mutations maintain RNAi efficiency in the soma in addition to the germline

Gene inactivation through RNA interference (RNAi) has proven to be a valuable tool for studying gene function in C. elegans. When combined with tissue-specific gene inactivation methods, RNAi has the potential to shed light on the function of a gene in distinct tissues. In this study we characterized C. elegans rrf-1 mutants to determine their ability to process RNAi in various tissues. These mutants have been widely used in RNAi studies to assess the function of genes specifically in the C. elegans germline. Upon closer analysis, we found that two rrf-1 mutants carrying different loss-of-function alleles were capable of processing RNAi targeting several somatically expressed genes. Specifically, we observed that the intestine was able to process RNAi triggers efficiently, whereas cells in the hypodermis showed partial susceptibility to RNAi in rrf-1 mutants. Other somatic tissues in rrf-1 mutants, such as the muscles and the somatic gonad, appeared resistant to RNAi. In addition to these observations, we found that the rrf-1(pk1417) mutation induced the expression of several transgenic arrays, including the FOXO transcription factor DAF-16. Unexpectedly, rrf-1(pk1417) mutants showed increased endogenous expression of the DAF-16 target gene sod-3; however, the lifespan and thermo-tolerance of rrf-1(pk1417) mutants were similar to those of wild-type animals. In sum, these data show that rrf-1 mutants display several phenotypes not previously appreciated, including broader tissue-specific RNAi-processing capabilities, and our results underscore the need for careful characterization of tissue-specific RNAi tools.

A high-resolution C. elegans essential gene network based on phenotypic profiling of a complex tissue

High-content screening for gene profiling has generally been limited to single cells. Here, we explore an alternative approach-profiling gene function by analyzing effects of gene knockdowns on the architecture of a complex tissue in a multicellular organism. We profile 554 essential C. elegans genes by imaging gonad architecture and scoring 94 phenotypic features. To generate a reference for evaluating methods for network construction, genes were manually partitioned into 102 phenotypic classes, predicting functions for uncharacterized genes across diverse cellular processes. Using this classification as a benchmark, we developed a robust computational method for constructing gene networks from high-content profiles based on a network context-dependent measure that ranks the significance of links between genes. Our analysis reveals that multi-parametric profiling in a complex tissue yields functional maps with a resolution similar to genetic interaction-based profiling in unicellular eukaryotes-pinpointing subunits of macromolecular complexes and components functioning in common cellular processes.

Epigenetic patterns maintained in early Caenorhabditis elegans embryos can be established by gene activity in the parental germ cells

Epigenetic information, such as parental imprints, can be transmitted with genetic information from parent to offspring through the germ line. Recent reports show that histone modifications can be transmitted through sperm as a component of this information transfer. How the information that is transferred is established in the parent and maintained in the offspring is poorly understood. We previously described a form of imprinted X inactivation in Caenorhabditis elegans where dimethylation on histone 3 at lysine 4 (H3K4me2), a mark of active chromatin, is excluded from the paternal X chromosome (Xp) during spermatogenesis and persists through early cell divisions in the embryo. Based on the observation that the Xp (unlike the maternal X or any autosome) is largely transcriptionally inactive in the paternal germ line, we hypothesized that transcriptional activity in the parent germ line may influence epigenetic information inherited by and maintained in the embryo. We report that chromatin modifications and histone variant patterns assembled in the germ line can be retained in mature gametes. Furthermore, despite extensive chromatin remodeling events at fertilization, the modification patterns arriving with the gametes are largely retained in the early embryo. Using transgenes, we observe that expression in the parental germline correlates with differential chromatin assembly that is replicated and maintained in the early embryo. Expression in the adult germ cells also correlates with more robust expression in the somatic lineages of the offspring. These results suggest that differential expression in the parental germ lines may provide a potential mechanism for the establishment of parent-of-origin epigenomic content. This content can be maintained and may heritably affect gene expression in the offspring.

Chromosome-biased binding and gene regulation by the Caenorhabditis elegans DRM complex

DRM is a conserved transcription factor complex that includes E2F/DP and pRB family proteins and plays important roles in development and cancer. Here we describe new aspects of DRM binding and function revealed through genome-wide analyses of the Caenorhabditis elegans DRM subunit LIN-54. We show that LIN-54 DNA-binding activity recruits DRM to promoters enriched for adjacent putative E2F/DP and LIN-54 binding sites, suggesting that these two DNA-binding moieties together direct DRM to its target genes. Chromatin immunoprecipitation and gene expression profiling reveals conserved roles for DRM in regulating genes involved in cell division, development, and reproduction. We find that LIN-54 promotes expression of reproduction genes in the germline, but prevents ectopic activation of germline-specific genes in embryonic soma. Strikingly, C. elegans DRM does not act uniformly throughout the genome: the DRM recruitment motif, DRM binding, and DRM-regulated embryonic genes are all under-represented on the X chromosome. However, germline genes down-regulated in lin-54 mutants are over-represented on the X chromosome. We discuss models for how loss of autosome-bound DRM may enhance germline X chromosome silencing. We propose that autosome-enriched binding of DRM arose in C. elegans as a consequence of germline X chromosome silencing and the evolutionary redistribution of germline-expressed and essential target genes to autosomes. Sex chromosome gene regulation may thus have profound evolutionary effects on genome organization and transcriptional regulatory networks.

Poly(A)-binding proteins are functionally distinct and have essential roles during vertebrate development

Translational control of many mRNAs in developing metazoan embryos is achieved by alterations in their poly(A) tail length. A family of cytoplasmic poly(A)-binding proteins (PABPs) bind the poly(A) tail and can regulate mRNA translation and stability. However, despite the extensive biochemical characterization of one family member (PABP1), surprisingly little is known about their in vivo roles or functional relatedness. Because no information is available in vertebrates, we address their biological roles, establishing that each of the cytoplasmic PABPs conserved in Xenopus laevis [PABP1, embryonic PABP (ePABP), and PABP4] is essential for normal development. Morpholino-mediated knockdown of PABP1 or ePABP causes both anterior and posterior phenotypes and embryonic lethality. In contrast, depletion of PABP4 results mainly in anterior defects and lethality at later stages. Unexpectedly, cross-rescue experiments reveal that neither ePABP nor PABP4 can fully rescue PABP1 depletion, establishing that PABPs have distinct functions. Comparative analysis of the uncharacterized PABP4 with PABP1 and ePABP shows that it shares a mechanistically conserved core role in promoting global translation. Consistent with this analysis, each morphant displays protein synthesis defects, suggesting that their roles in mRNA-specific translational regulation and/or mRNA decay, rather than global translation, underlie the functional differences between PABPs. Domain-swap experiments reveal that the basis of the functional specificity is complex, involving multiple domains of PABPs, and is conferred, at least in part, by protein-protein interactions.

mRNA-specific regulation of translation by poly(A)-binding proteins

The regulation of translation has emerged as a major determinant of gene expression and is critical for both normal cellular function and the development of disease. Numerous studies have highlighted the diverse, and sometimes related, mechanisms which underlie the regulation of global translation rates and the translational control of specific mRNAs. In the present paper, we discuss the emerging roles of the basal translation factor PABP [poly(A)-binding protein] in mRNA-specific translational control in metazoa which suggest that PABP function is more complex than first recognized.

Two mutations in pab-1 encoding poly(A)-binding protein show similar defects in germline stem cell proliferation but different longevity in C. elegans

Four new alleles, bn116, bn117, bn118, and bn119, on LG I were isolated in C. elegans with defects in germline stem cell proliferation. Using genetic mapping and snip-SNP mapping, bn116, bn117, bn118, and bn119 were located 5.0 cM, 1.3 cM, 2.3 cM, and 5.0 cM, respectively, to the right of dpy-5 on LG I. Further, bn116 and bn119 were grouped into the same complementation group by a complementation test. They are loss-of-function recessive alleles that produce homozygous sterile worms whose germ cells do not proliferate during larval development. However, the worms contained normal somatic gonadal structures including distal tip cells and gonadal sheath cells, suggesting that the defect in germline proliferation was not caused by the absence of somatic signaling. Although DAF-16 was localized to the nucleus in all four mutants, the life span was extended only in the three mutants except bn116. These results suggest that the defect in germline stem cell proliferation, the presence of normal somatic gonadal tissues, and DAF-16 nuclear translocation were sufficient for extending the lifespan of the bn117, bn118, and bn119 mutants, but not the bn116 mutant. Intriguingly, bn116 and bn119 were identified as two different mutations on the same gene, pab-1, which encodes a poly(A)-binding protein. Therefore, although the bn116 and bn119 mutations cause similar defects in germ cell proliferation, their effects on life span are different.

Meiotic silencing in Caenorhabditis elegans

In many animals and some fungi, mechanisms have been described that target unpaired chromosomes and chromosomal regions for silencing during meiotic prophase. These phenomena, collectively called "meiotic silencing," target sex chromosomes in the heterogametic sex, for example, the X chromosome in male nematodes and the XY-body in male mice, and also target any other chromosomes that fail to synapse due to mutation or chromosomal rearrangement. Meiotic silencing phenomena are hypothesized to maintain genome integrity and perhaps function in setting up epigenetic control of embryogenesis. This review focuses on meiotic silencing in the nematode, Caenorhabditis elegans, including its mechanism and function(s), and its relationship to other gene silencing processes in the germ line. One hallmark of meiotic silencing in C. elegans is that unpaired/unsynapsed chromosomes and chromosomal regions become enriched for a repressive histone modification, dimethylation of histone H3 on lysine 9 (H3K9me2). Accumulation and proper targeting of H3K9me2 rely on activity of an siRNA pathway, suggesting that histone methyltransferase activity may be targeted/regulated by a small RNA-based transcriptional silencing mechanism.

Soma-germline interactions that influence germline proliferation in Caenorhabditis elegans

Caenorhabditis elegans boasts a short lifecycle and high fecundity, two features that make it an attractive and powerful genetic model organism. Several recent studies indicate that germline proliferation, a prerequisite to optimal fecundity, is tightly controlled over the course of development. Cell proliferation control includes regulation of competence to proliferate, a poorly understood aspect of cell fate specification, as well as cell-cycle control. Furthermore, dynamic regulation of cell proliferation occurs in response to multiple external signals. The C. elegans germ line is proving a valuable model for linking genetic, developmental, systemic, and environmental control of cell proliferation. Here, we consider recent studies that contribute to our understanding of germ cell proliferation in C. elegans. We focus primarily on somatic control of germline proliferation, how it differs at different life stages, and how it can be altered in the context of the life cycle and changes in environmental status.

Evolutionary insights into insecticide resistance gene families of Anopheles gambiae

Insecticide resistance (IR) is one of the major obstacles in insect pests and insect borne disease control strategies, the mechanism of which is known to be genetically controlled. Three major gene families (CYP, GST and COE) have been identified encoding various proteins to metabolize endogenous as well as exogenous compounds that are responsible for IR mechanisms in insects. Understanding evolutionary patterns of genes of such important functions could lead to important understanding, based on which, further studies to control various insect borne infectious diseases could be initiated. We herein utilized the whole genome sequence information of the malaria vector Anopheles gambiae and inferred evolutionary pattern of the three known IR gene families (CYP, GST and COE). The pattern of conservation of IR genes across 38 other taxa was determined to infer evolutionary pattern of these gene families. Chromosomal distribution of IR genes was ascertained and each individual gene of IR gene families was also mapped on the chromosomal arms of An. gambiae. Differential distributional and quantitative aspects of introns in each gene were determined and genetic architecture of genes from all three gene families was compared to draw differential evolution of IR gene families. Further, phylogenetic relationships among genes of each of the three gene families were also inferred. These results in correlation with chromosomal location of each gene have provided valuable information about evolutionary history of IR gene families.

Adaptive evolution of genes duplicated from the Drosophila pseudoobscura neo-X chromosome

Drosophila X chromosomes are disproportionate sources of duplicated genes, and these duplications are usually the result of retrotransposition of X-linked genes to the autosomes. The excess duplication is thought to be driven by natural selection for two reasons: X chromosomes are inactivated during spermatogenesis, and the derived copies of retroposed duplications tend to be testis expressed. Therefore, autosomal derived copies of retroposed genes provide a mechanism for their X-linked paralogs to "escape" X inactivation. Once these duplications have fixed, they may then be selected for male-specific functions. Throughout the evolution of the Drosophila genus, autosomes have fused with X chromosomes along multiple lineages giving rise to neo-X chromosomes. There has also been excess duplication from the two independent neo-X chromosomes that have been examined--one that occurred prior to the common ancestor of the willistoni species group and another that occurred along the lineage leading to Drosophila pseudoobscura. To determine what role natural selection plays in the evolution of genes duplicated from the D. pseudoobscura neo-X chromosome, we analyzed DNA sequence divergence between paralogs, polymorphism within each copy, and the expression profiles of these duplicated genes. We found that the derived copies of all duplicated genes have elevated nonsynonymous polymorphism, suggesting that they are under relaxed selective constraints. The derived copies also tend to have testis- or male-biased expression profiles regardless of their chromosome of origin. Genes duplicated from the neo-X chromosome appear to be under less constraints than those duplicated from other chromosome arms. We also find more evidence for historical adaptive evolution in genes duplicated from the neo-X chromosome, suggesting that they are under a unique selection regime in which elevated nonsynonymous polymorphism provides a large reservoir of functional variants, some of which are fixed by natural selection.

Two conserved regulatory cytoplasmic poly(A) polymerases, GLD-4 and GLD-2, regulate meiotic progression in C. elegans

Translational regulation is heavily employed during developmental processes to control the timely accumulation of proteins independently of gene transcription. In particular, mRNA poly(A) tail metabolism in the cytoplasm is a key determinant for balancing an mRNA's translational output and its decay rate. Noncanonical poly(A) polymerases (PAPs), such as germline development defective-2 (GLD-2), can mediate poly(A) tail extension. Little is known about the regulation and functional complexity of cytoplasmic PAPs. Here we report the discovery of Caenorhabditis elegans GLD-4, a cytoplasmic PAP present in P granules that is orthologous to Trf4/5p from budding yeast. GLD-4 enzymatic activity is enhanced by its interaction with GLS-1, a protein associated with the RNA-binding protein GLD-3. GLD-4 is predominantly expressed in germ cells, and its activity is essential for early meiotic progression of male and female gametes in the absence of GLD-2. For commitment into female meiosis, both PAPs converge on at least one common target mRNA-i.e., gld-1 mRNA-and, as a consequence, counteract the repressive action of two PUF proteins and the putative deadenylase CCR-4. Together our findings suggest that two different cytoplasmic PAPs stabilize and translationally activate several meiotic mRNAs to provide a strong fail-safe mechanism for early meiotic progression.

The DAZL and PABP families: RNA-binding proteins with interrelated roles in translational control in oocytes

Gametogenesis is a highly complex process that requires the exquisite temporal, spatial and amplitudinal regulation of gene expression at multiple levels. Translational regulation is important in a wide variety of cell types but may be even more prevalent in germ cells, where periods of transcriptional quiescence necessitate the use of post-transcriptional mechanisms to effect changes in gene expression. Consistent with this, studies in multiple animal models have revealed an essential role for mRNA translation in the establishment and maintenance of reproductive competence. While studies in humans are less advanced, emerging evidence suggests that translational regulation plays a similarly important role in human germ cells and fertility. This review highlights specific mechanisms of translational regulation that play critical roles in oogenesis by activating subsets of mRNAs. These mRNAs are activated in a strictly determined temporal manner via elements located within their 3′UTR, which serve as binding sites fortrans-acting factors. While we concentrate on oogenesis, these regulatory events also play important roles during spermatogenesis. In particular, we focus on the deleted in azoospermia-like (DAZL) family of proteins, recently implicated in the translational control of specific mRNAs in germ cells; their relationship with the general translation initiation factor poly(A)-binding protein (PABP) and the process of cytoplasmic mRNA polyadenylation.

The Caenorhabditis elegans ekl (enhancer of ksr-1 lethality) genes include putative components of a germline small RNA pathway

A canonical Ras-ERK signaling pathway specifies the fate of the excretory duct cell during Caenorhabditis elegans embryogenesis. The paralogs ksr-1 and ksr-2 encode scaffolding proteins that facilitate signaling through this pathway and that act redundantly to promote the excretory duct fate. In a genomewide RNAi screen for genes that, like ksr-2, are required in combination with ksr-1 for the excretory duct cell fate, we identified 16 "ekl" (enhancer of ksr-1 lethality) genes that are largely maternally required and that have molecular identities suggesting roles in transcriptional or post-transcriptional gene regulation. These include the Argonaute gene csr-1 and a specific subset of other genes implicated in endogenous small RNA processes, orthologs of multiple components of the NuA4/Tip60 histone acetyltransferase and CCR4/NOT deadenylase complexes, and conserved enzymes involved in ubiquitination and deubiquitination. The identification of four small RNA regulators (csr-1, drh-3, ego-1, and ekl-1) that share the Ekl phenotype suggests that these genes define a functional pathway required for the production and/or function of particular germline small RNA(s). These small RNAs and the other ekl genes likely control the expression of one or more regulators of Ras-ERK signaling that function at or near the level of kinase suppressor of Ras (KSR).

Controls of germline stem cells, entry into meiosis, and the sperm/oocyte decision in Caenorhabditis elegans

The Caenorhabditis elegans germ line provides an exceptional model for analysis of the molecular controls governing stem cell maintenance, the cell cycle transition from mitosis to meiosis, and the choice of sexual identity-sperm or oocyte. Germline stem cells are maintained in an undifferentiated state within a well-defined niche formed by a single somatic cell, the distal tip cell (DTC). In both sexes, the DTC employs GLP-1/Notch signaling and FBF/PUF RNA-binding proteins to maintain stem cells and promote mitotic divisions, three additional RNA regulators (GLD-1/quaking, GLD-2/poly(A) polymerase, and GLD-3/Bicaudal-C) control entry into meiosis, and FOG-1/CPEB and FOG-3/Tob proteins govern sperm specification. These key regulators are part of a robust regulatory network that controls germ cell proliferation, stem cell maintenance, and sex determination. Parallels with controls in other organisms include the use of PUF proteins for stem cell maintenance and the prominence of mRNA regulation for the control of germline development.

Meiotic silencing and the epigenetics of sex

The sensing of accurate homologous recognition and pairing between discreet chromosomal regions and/or entire chromosomes entering meiosis is an essential step in ensuring correct alignment for recombination. A component of this is the recognition of heterology, which is required to prevent recombination at ectopic sites and between non-homologous chromosomes. It has been observed that a number of diverged organisms add an additional layer to this process: regions or chromosomes without a homologous counterpart are targeted for silencing during meiotic prophase I. This phenomenon was originally described in filamentous fungi, but has since been observed in nematodes and mammals. In this review we will generally group these phenomena under the title of meiotic silencing, and describe what is known about the process in the organisms in which it is observed. We will additionally propose that the functions of meiotic silencing originate in genome defense, and discuss its potential contributions to genome evolution and speciation.

Parent genes of retrotransposition-generated gene duplicates in Drosophila melanogaster have distinct expression profiles

Genes arising by retrotransposition are always different from their parent genes from the outset. In addition, the cDNA must insert into a region that allows expression or it will become a processed pseudogene. We sought to determine whether this class of gene duplication differs from other gene duplications based on functional criteria. Using amino acid sequences from Drosophila melanogaster, we identified retroduplicated gene pairs at various levels of sequence identity. Analysis of gene ontology annotations showed some enrichment of retroduplications in the cellular physiological processes class. Retroduplications show a higher level of nucleotide substitution than other gene duplications, suggesting a higher rate of divergence. Remarkably, analysis of microarray data for gene expression during embryogenesis showed that parent genes are more highly expressed relative to their retroduplicated copies, tandem duplications, and all genes. Furthermore, an expressed sequence tag library representation shows a broader distribution for parent genes than for all other genes and, as found previously by others, retroduplicated gene transcripts are found most abundantly in testes. Therefore, in examining retroduplicated gene pairs, we have found that parent genes of retroduplications are also a distinctive class in terms of transcript expression levels and distribution.

ASB-1, a germline-specific isoform of mitochondrial ATP synthase b subunit, is required to maintain the rate of germline development in Caenorhabditis elegans

The developmental timing of all types of cells must be synchronized and spatially coordinated to achieve the organized development of a multicellular organism. Previously, we found RNAi of asb-1, encoding a germline-specific isoform of mitochondrial ATP synthase b subunit, caused 100% penetrant sterility in Caenorhabditis elegans. ATP synthase is one of the five complexes of the mitochondrial respiratory chain, and defects in some of the components of the chain are known to slow the growth and extend the lifespan of worms. We found that development of asb-1 mutant germ line was not arrested at any stage, but did slow to half the rate of wild type, whereas the rate of somatic development was the same in asb-1 mutants as that of wild type, indicating that asb-1 is required to maintain the rate of germline development but has no effect on somatic development. Among ATP synthase subunit genes, RNAi of asg-1, encoding a germline-specific isoform of the g subunit, also caused asb-1-like sterility, indicating that some other germline-specific components are also required to maintain the rate of germline development. Both asb-1 and asg-1 are located on autosomes while they possess counterparts, asb-2 and asg-2, respectively, on X chromosome, which are both required for somatic development. Chromosomal locations of the genes may be the basis of the segregation of germline/somatic functions of each gene, as were demonstrated for other autosomal/X-linked duplicated gene pairs.

The GLD-2 poly(A) polymerase activates gld-1 mRNA in the Caenorhabditis elegans germ line

mRNA regulation is crucial for many aspects of metazoan development and physiology, including regulation of stem cells and synaptic plasticity. In the nematode germ line, RNA regulators control stem cell maintenance, the sperm/oocyte decision, and progression through meiosis. Of particular importance to this work are three GLD (germ-line development) regulatory proteins, each of which promotes entry into the meiotic cell cycle: GLD-1 is a STAR/Quaking translational repressor, GLD-2 is a cytoplasmic poly(A) polymerase, and GLD-3 is a homolog of Bicaudal-C. Here we report that the gld-1 mRNA is a direct target of the GLD-2 poly(A) polymerase: polyadenylation of gld-1 mRNA depends on GLD-2, the abundance of GLD-1 protein is dependent on GLD-2, and the gld-1 mRNA coimmunoprecipitates with both GLD-2 and GLD-3 proteins. We suggest that the GLD-2 poly(A) polymerase enhances entry into the meiotic cell cycle at least in part by activating GLD-1 expression. The importance of this conclusion is twofold. First, the activation of gld-1 mRNA by GLD-2 identifies a positive regulatory step that reinforces the decision to enter the meiotic cell cycle. Second, gld-1 mRNA is initially repressed by FBF (for fem-3 binding factor) to maintain stem cells but then becomes activated by the GLD-2 poly(A) polymerase once stem cells begin to make the transition into the meiotic cell cycle. Therefore, a molecular switch regulates gld-1 mRNA activity to accomplish the transition from mitosis to meiosis.

Eukaryotic translation initiation factor 5B activity regulates larval growth rate and germline development inCaenorhabditis elegans

In C. elegans, a population of proliferating germ cells is maintained via GLP-1/Notch signaling; in the absence of GLP-1 signaling, germ cells prematurely enter meiosis and differentiate. We previously identified ego (enhancer of glp-1) genes that promote germline proliferation and interact genetically with the GLP-1 signaling pathway. Here, we report that iffb-1 (initiation factor five B) is an ego gene. iffb-1 encodes the sole C. elegans isoform of eukaryotic translation initiation factor 5B, a protein essential for translation. We have used RNA interference and a deletion mutation to determine the developmental consequences of reduced iffb-1 activity. Our data indicate that maternal iffb-1 gene expression is sufficient for embryogenesis, and zygotic iffb-1 expression is required for development beyond late L1/early L2 stage. Partial reduction in iffb-1 expression delays larval development and can severely disrupt proliferation and differentiation of germ cells. We hypothesize that germline development is particularly sensitive to iffb-1 expression level.