Control of vulval competence and centering in the nematode Oscheius sp. 1 CEW1

Programmed cell fusion in development and homeostasis

During multicellular organism development, complex structures are sculpted to form organs and tissues, which are maintained throughout adulthood. Many of these processes require cells to fuse with one another, or with themselves. These plasma membrane fusions merge endoplasmic cellular content across external, exoplasmic, space. In the nematode Caenorhabditis elegans, such cell fusions serve as a unique sculpting force, involved in the embryonic morphogenesis of the skin-like multinuclear hypodermal cells, but also in refining delicate structures, such as valve openings and the tip of the tail. During post-embryonic development, plasma membrane fusions continue to shape complex neuron structures and organs such as the vulva, while during adulthood fusion participates in cell and tissue repair. These processes rely on two fusion proteins (fusogens): EFF-1 and AFF-1, which are part of a broader family of structurally related membrane fusion proteins, encompassing sexual reproduction, viral infection, and tissue remodeling. The established capabilities of these exoplasmic fusogens are further expanded by new findings involving EFF-1 and AFF-1 in endocytic vesicle fission and phagosome sealing. Tight regulation by cell-autonomous and non-cell autonomous mechanisms orchestrates these diverse cell fusions at the correct place and time-these processes and their significance are discussed in this review.

Necessity and Contingency in Developmental Genetic Screens: EGF, Wnt, and Semaphorin Pathways in Vulval Induction of the Nematode Oscheius tipulae

Genetic screens in the nematode Caenorhabditis elegans have identified EGF and Notch pathways as key for vulval precursor cell fate patterning. Here, Vargas-Velazquez, Besnard, and Félix report on the molecular identification of...

Genetic screens in the nematode Caenorhabditis elegans identified the EGF/Ras and Notch pathways as central for vulval precursor cell fate patterning. Schematically, the anchor cell secretes EGF, inducing the P6.p cell to a primary (1°) vulval fate; P6.p in turn induces its neighbors to a secondary (2°) fate through Delta-Notch signaling and represses Ras signaling. In the nematode Oscheius tipulae, the anchor cell successively induces 2° then 1° vulval fates. Here, we report on the molecular identification of mutations affecting vulval induction in O. tipulae. A single Induction Vulvaless mutation was found, which we identify as a cis-regulatory deletion in a tissue-specific enhancer of the O. tipulae lin-3 homolog, confirmed by clustered regularly interspaced short palindromic repeats/Cas9 mutation. In contrast to this predictable Vulvaless mutation, mutations resulting in an excess of 2° fates unexpectedly correspond to the plexin/semaphorin pathway. Hyperinduction of P4.p and P8.p in these mutants likely results from mispositioning of these cells due to a lack of contact inhibition. The third signaling pathway found by forward genetics in O. tipulae is the Wnt pathway; a decrease in Wnt pathway activity results in loss of vulval precursor competence and induction, and 1° fate miscentering on P5.p. Our results suggest that the EGF and Wnt pathways have qualitatively similar activities in vulval induction in C. elegans and O. tipulae, albeit with quantitative differences in the effects of mutation. Thus, the derived induction process in C. elegans with an early induction of the 1° fate appeared during evolution, after the recruitment of the EGF pathway for vulval induction.

Liposome-based transfection enhances RNAi and CRISPR-mediated mutagenesis in non-model nematode systems

Nematodes belong to one of the most diverse animal phyla. However, functional genomic studies in nematodes, other than in a few species, have often been limited in their reliability and success. Here we report that by combining liposome-based technology with microinjection, we were able to establish a wide range of genomic techniques in the newly described nematode genus Auanema. The method also allowed heritable changes in dauer larvae of Auanema, despite the immaturity of the gonad at the time of the microinjection. As proof of concept for potential functional studies in other nematode species, we also induced RNAi in the free-living nematode Pristionchus pacificus and targeted the human parasite Strongyloides stercoralis.

From "the Worm" to "the Worms" and Back Again: The Evolutionary Developmental Biology of Nematodes

Since the earliest days of research on nematodes, scientists have noted the developmental and morphological variation that exists within and between species. As various cellular and developmental processes were revealed through intense focus on Caenorhabditis elegans, these comparative studies have expanded. Within the genus Caenorhabditis, they include characterization of intraspecific polymorphisms and comparisons of distinct species, all generally amenable to the same laboratory culture methods and supported by robust genomic and experimental tools. The C. elegans paradigm has also motivated studies with more distantly related nematodes and animals. Combined with improved phylogenies, this work has led to important insights about the evolution of nematode development. First, while many aspects of C. elegans development are representative of Caenorhabditis, and of terrestrial nematodes more generally, others vary in ways both obvious and cryptic. Second, the system has revealed several clear examples of developmental flexibility in achieving a particular trait. This includes developmental system drift, in which the developmental control of homologous traits has diverged in different lineages, and cases of convergent evolution. Overall, the wealth of information and experimental techniques developed in C. elegans is being leveraged to make nematodes a powerful system for evolutionary cellular and developmental biology.

Toward Universal Forward Genetics: Using a Draft Genome Sequence of the Nematode Oscheius tipulae To Identify Mutations Affecting Vulva Development

Mapping-by-sequencing has become a standard method to map and identify phenotype-causing mutations in model species. Here, we show that a fragmented draft assembly is sufficient to perform mapping-by-sequencing in nonmodel species. We generated a draft assembly and annotation of the genome of the free-living nematode Oscheius tipulae, a distant relative of the model Caenorhabditis elegans We used this draft to identify the likely causative mutations at the O. tipulae cov-3 locus, which affect vulval development. The cov-3 locus encodes the O. tipulae ortholog of C. elegans mig-13, and we further show that Cel-mig-13 mutants also have an unsuspected vulval-development phenotype. In a virtuous circle, we were able to use the linkage information collected during mutant mapping to improve the genome assembly. These results showcase the promise of genome-enabled forward genetics in nonmodel species.

Oscheius tipulae in Italy: Evidence of an Alien Isolate in the Integral Natural Reserve of Montecristo Island (Tuscany)

Montecristo Island is an integral natural reserve of the Tuscan Archipelago National Park (Central Italy), characterized by a peculiar assemblage of flora and fauna, with several endemic taxa, and also with a high number of alien species. During a soil survey, we found an alien Oscheius tipulae Lam & Webster, 1971 isolate, phylogenetically close to others from South America. In this article, we examined the possible pathways of introduction of this nematode. Because of the high number of alien plants in this protected area and the low desiccation survival ability of O. tipulae, we hypothesized that the presence of this alien nematode isolate may be related to the soil of introduced plants, although historical association with plant-associated invertebrates is also possible. Further studies with more populations and marker molecules are necessary to investigate the distribution of O. tipulae and the possible impact on this natural reserve.

Non-coding stem-bulge RNAs are required for cell proliferation and embryonic development in C. elegans

Stem bulge RNAs (sbRNAs) are a family of small non-coding stem-loop RNAs present in Caenorhabditis elegans and other nematodes, the function of which is unknown. Here, we report the first functional characterisation of nematode sbRNAs. We demonstrate that sbRNAs from a range of nematode species are able to reconstitute the initiation of chromosomal DNA replication in the presence of replication proteins in vitro, and that conserved nucleotide sequence motifs are essential for this function. By functionally inactivating sbRNAs with antisense morpholino oligonucleotides, we show that sbRNAs are required for S phase progression, early embryonic development and the viability of C. elegans in vivo. Thus, we demonstrate a new and essential role for sbRNAs during the early development of C. elegans. sbRNAs show limited nucleotide sequence similarity to vertebrate Y RNAs, which are also essential for the initiation of DNA replication. Our results therefore establish that the essential function of small non-coding stem-loop RNAs during DNA replication extends beyond vertebrates.

Cell division and targeted cell cycle arrest opens and stabilizes basement membrane gaps

Large gaps in basement membrane (BM) occur during organ remodelling and cancer cell invasion. Whether dividing cells, which temporarily reduce their attachment to BM, influence these breaches is unknown. Here we analyse uterine-vulval attachment during development across 21 species of rhabditid nematodes and find that the BM gap that forms between these organs is always bounded by a non-dividing vulval cell. Through cell cycle manipulation and live cell imaging in Caenorhabditis elegans, we show that actively dividing vulval cells facilitate enlargement of this breach by promoting BM movement. In contrast, targeted cell cycle arrest halts BM movement and limits gap opening. Further, we demonstrate that the BM component laminin accumulates at the BM gap edge and promotes increased integrin levels in non-dividing vulval cells, stabilizing gap position. Together, these studies reveal that cell division can be used as a mechanism to regulate BM breaches, thus controlling the exchange of cells between tissues.

The wing-patterning network in the wingless castes of Myrmicine and Formicine ant species is a mix of evolutionarily labile and non-labile genes

Wing polyphenism in ants is the ability of a single genome to produce winged or wingless castes in a colony in response to environmental cues. Although wing polyphenism is a universal and homologous feature of ants, the gene network underlying wing polyphenism is conserved in the winged castes, but is labile in the wingless castes, that is, the network is interrupted at different points in the wingless castes of different ant species. Because the expression of all genes sampled so far in this network in the wingless castes is evolutionarily labile across species, an important question is whether all "interruption points" in the network are evolutionarily labile or are there interruption points that are evolutionarily non-labile. Here we show that in the wingless castes, the expression of the gene brinker (brk), which mediates growth, patterning, and apoptosis in the Drosophila wing disc, is non-labile; it is absent in vestigial wing discs of four ants species. In contrast, the expression of engrailed (en), a gene upstream of brk is labile; it is present in some species but absent in others. In the winged castes, both brk and en expression are conserved relative to their expression in Drosophila wing discs. The differential lability of genes in the network in wingless castes may be a general feature of networks underlying polyphenic traits. This raises the possibility that some genes, like brk, may be under stabilizing selection while most others, like en, may be evolving via directional selection or neutral drift.

Genetic control of vulval development in Caenorhabditis briggsae

The nematode Caenorhabditis briggsae is an excellent model organism for the comparative analysis of gene function and developmental mechanisms. To study the evolutionary conservation and divergence of genetic pathways mediating vulva formation, we screened for mutations in C. briggsae that cause the egg-laying defective (Egl) phenotype. Here, we report the characterization of 13 genes, including three that are orthologs of Caenorhabditis elegans unc-84 (SUN domain), lin-39 (Dfd/Scr-related homeobox), and lin-11 (LIM homeobox). Based on the morphology and cell fate changes, the mutants were placed into four different categories. Class 1 animals have normal-looking vulva and vulva-uterine connections, indicating defects in other components of the egg-laying system. Class 2 animals frequently lack some or all of the vulval precursor cells (VPCs) due to defects in the migration of P-cell nuclei into the ventral hypodermal region. Class 3 animals show inappropriate fusion of VPCs to the hypodermal syncytium, leading to a reduced number of vulval progeny. Finally, class 4 animals exhibit abnormal vulval invagination and morphology. Interestingly, we did not find mutations that affect VPC induction and fates. Our work is the first study involving the characterization of genes in C. briggsae vulva formation, and it offers a basis for future investigations of these genes in C. elegans.

Several Grassland Soil Nematode Species Are Insensitive to RNA-Mediated Interference

Phenotypic analysis of defects caused by RNA mediated interference (RNAi) in Caenorhabditis elegans has proven to be a powerful tool for determining gene function. In this study we investigated the effectiveness of RNAi in four non-model grassland soil nematodes, Oscheius sp FVV-2., Rhabditis sp, Mesorhabditis sp., and Acrobeloides sp. In contrast to reference experiments performed using C. elegans and Caenorhabditis briggsae, feeding bacteria expressing dsRNA and injecting dsRNA into the gonad did not produce the expected RNAi knockdown phenotypes in any of the grassland nematodes. Quantitative reverse-transcribed PCR (qRT-PCR) assays did not detect a statistically significant reduction in the mRNA levels of endogenous genes targeted by RNAi in Oscheius sp., and Mesorhabditis sp. From these studies we conclude that due to low effectiveness and inconsistent reproducibility, RNAi knockdown phenotypes in non-Caenorhabditis nematodes should be interpreted cautiously.

Evolution of susceptibility to ingested double-stranded RNAs in Caenorhabditis nematodes

Background

The nematode Caenorhabditis elegans is able to take up external double-stranded RNAs (dsRNAs) and mount an RNA interference response, leading to the inactivation of specific gene expression. The uptake of ingested dsRNAs into intestinal cells has been shown to require the SID-2 transmembrane protein in C. elegans. By contrast, C. briggsae was shown to be naturally insensitive to ingested dsRNAs, yet could be rendered sensitive by transgenesis with the C. elegans sid-2 gene. Here we aimed to elucidate the evolution of the susceptibility to external RNAi in the Caenorhabditis genus.

Principal Findings

We study the sensitivity of many new species of Caenorhabditis to ingested dsRNAs matching a conserved actin gene sequence from the nematode Oscheius tipulae. We find ample variation in the Caenorhabditis genus in the ability to mount an RNAi response. We map this sensitivity onto a phylogenetic tree, and show that sensitivity or insensitivity have evolved convergently several times. We uncover several evolutionary losses in sensitivity, which may have occurred through distinct mechanisms. We could render C. remanei and C. briggsae sensitive to ingested dsRNAs by transgenesis of the Cel-sid-2 gene. We thus provide tools for RNA interference studies in these species. We also show that transgenesis by injection is possible in many Caenorhabditis species.

Conclusions

The ability of animals to take up dsRNAs or to respond to them by gene inactivation is under rapid evolution in the Caenorhabditis genus. This study provides a framework and tools to use RNA interference and transgenesis in various Caenorhabditis species for further comparative and evolutionary studies.

Considering RNAi experimental design in parasitic helminths

Almost a decade has passed since the first report of RNA interference (RNAi) in a parasitic helminth. Whilst much progress has been made with RNAi informing gene function studies in disparate nematode and flatworm parasites, substantial and seemingly prohibitive difficulties have been encountered in some species, hindering progress. An appraisal of current practices, trends and ideals of RNAi experimental design in parasitic helminths is both timely and necessary for a number of reasons: firstly, the increasing availability of parasitic helminth genome/transcriptome resources means there is a growing need for gene function tools such as RNAi; secondly, fundamental differences and unique challenges exist for parasite species which do not apply to model organisms; thirdly, the inherent variation in experimental design, and reported difficulties with reproducibility undermine confidence. Ideally, RNAi studies of gene function should adopt standardised experimental design to aid reproducibility, interpretation and comparative analyses. Although the huge variations in parasite biology and experimental endpoints make RNAi experimental design standardization difficult or impractical, we must strive to validate RNAi experimentation in helminth parasites. To aid this process we identify multiple approaches to RNAi experimental validation and highlight those which we deem to be critical for gene function studies in helminth parasites.

RNAi effector diversity in nematodes

While RNA interference (RNAi) has been deployed to facilitate gene function studies in diverse helminths, parasitic nematodes appear variably susceptible. To test if this is due to inter-species differences in RNAi effector complements, we performed a primary sequence similarity survey for orthologs of 77 Caenorhabditis elegans RNAi pathway proteins in 13 nematode species for which genomic or transcriptomic datasets were available, with all outputs subjected to domain-structure verification. Our dataset spanned transcriptomes of Ancylostoma caninum and Oesophagostomum dentatum, and genomes of Trichinella spiralis, Ascaris suum, Brugia malayi, Haemonchus contortus, Meloidogyne hapla, Meloidogyne incognita and Pristionchus pacificus, as well as the Caenorhabditis species C. brenneri, C. briggsae, C. japonica and C. remanei, and revealed that: (i) Most of the C. elegans proteins responsible for uptake and spread of exogenously applied double stranded (ds)RNA are absent from parasitic species, including RNAi-competent plant-nematodes; (ii) The Argonautes (AGOs) responsible for gene expression regulation in C. elegans are broadly conserved, unlike those recruited during the induction of RNAi by exogenous dsRNA; (iii) Secondary Argonautes (SAGOs) are poorly conserved, and the nuclear AGO NRDE-3 was not identified in any parasite; (iv) All five Caenorhabditis spp. possess an expanded RNAi effector repertoire relative to the parasitic nematodes, consistent with the propensity for gene loss in nematode parasites; (v) In spite of the quantitative differences in RNAi effector complements across nematode species, all displayed qualitatively similar coverage of functional protein groups. In summary, we could not identify RNAi effector deficiencies that associate with reduced susceptibility in parasitic nematodes. Indeed, similarities in the RNAi effector complements of RNAi refractory and competent nematode parasites support the broad applicability of this research genetic tool in nematodes.

Many organisms regulate gene expression through an RNA interference (RNAi) pathway, first characterized in the nematode Caenorhabditis elegans. This pathway can be triggered experimentally using double-stranded (ds)RNA to selected gene targets, thereby allowing researchers to ‘silence’ individual genes and so investigate their function. It is hoped that this technology will facilitate gene silencing in important parasitic nematodes that impose a considerable health and economic burden on mankind. Unfortunately, differences in RNAi susceptibility have been observed between species. Here we investigated the possibility that differences in the complement of effector proteins involved in the RNAi pathway are responsible for these differences in susceptibility. Our data revealed that most facets of the RNAi pathway are well represented across parasitic nematodes, although there were fewer pathway proteins in other nematodes compared to C. elegans. In contrast, the proteins responsible for uptake and spread of dsRNA are not well represented in parasitic nematodes. However, the importance of these differences is undermined by our observation that the protein complements in all the parasites were qualitatively similar, regardless of RNAi-susceptibility. Clearly, differences in the RNAi pathway of parasitic nematodes do not explain the variations in susceptibility to experimental RNAi.

Evolution of programmed cell fusion: common mechanisms and distinct functions

Eukaryotic cells have evolved diverged mechanisms to merge cells. Here, we discuss three types of cell fusion: (1) Non-self-fusion, cells with different genetic contents fuse to start a new organism and fusion between enveloped viruses and host cells; (2) Self-fusion, genetically identical cells fuse to form a multinucleated cell; and (3) Auto-fusion, a single cell fuses with itself by bringing specialized cell membrane domains into contact and transforming itself into a ring-shaped cell. This is a new type of selfish fusion discovered in C. elegans. We divide cell fusion into three stages: (1) Specification of the cell-fusion fate; (2) Cell attraction, attachment, and recognition; (3) Execution of plasma membrane fusion, cytoplasmic mixing and cytoskeletal rearrangements. We analyze cell fusion in diverse biological systems in development and disease emphasizing the mechanistic contributions of C. elegans to the understanding of programmed cell fusion, a genetically encoded pathway to merge specific cells.

Ascaris suum: RNAi mediated silencing of enolase gene expression in infective larvae

Ascaris suum is an important parasite of pigs that causes tremendous economic losses globally to agriculture and animal husbandry annually. RNA interference (RNAi) technology has been described as a successful and useful approach for the elucidation of gene function in parasitic nematodes. In the present study, RNAi was used to silence the expression of a gene encoding enolase in A. suum by soaking infective larvae in double-stranded RNA derived from an EST (representing As-enol-1) selected from an A. suum infective larvae-specific cDNA library. The mRNA levels of RNAi-treated larvae were examined by Reverse-Transcription PCR (RT-PCR) analysis. The survival of RNAi-treated larvae was compared with larvae treated with dsRNA-free culture medium. The effect of enolase depletion on the development of A. suum larvae was assessed by infecting BALB/c mice with RNAi-treated larvae. The results showed that enolase gene expression was silenced completely and the survival rate of the RNAi-treated nematodes was reduced by 20.11% (P<0.01) after soaking for 72 h. Although no significant difference was detected in the numbers of larvae recovered from the liver and lungs of infected mice 4 days post infection, RNAi knockdown of the A. suum enolase mRNA led to significant shorter larvae, indicating that loss of enolase expression may cause delays in larval development.

Conserved mechanism of Wnt signaling function in the specification of vulval precursor fates in C. elegans and C. briggsae

The C. elegans hermaphrodite vulva serves as a paradigm for understanding how signaling pathways control organ formation. Previous studies have shown that Wnt signaling plays important roles in vulval development. To understand the function and evolution of Wnt signaling in Caenorhabditis nematodes we focused on C. briggsae, a species that is substantially divergent from C. elegans in terms of the evolutionary time scale yet shares almost identical morphology. We isolated mutants in C. briggsae that display multiple pseudo-vulvae resulting from ectopic VPC induction. We cloned one of these loci and found that it encodes an Axin homolog, Cbr-PRY-1. Our genetic studies revealed that Cbr-pry-1 functions upstream of the canonical Wnt pathway components Cbr-bar-1 (beta-catenin) and Cbr-pop-1(tcf/lef) as well as the Hox target Cbr-lin-39 (Dfd/Scr). We further characterized the pry-1 vulval phenotype in C. briggsae and C. elegans using 8 cell fate markers, cell ablation, and genetic interaction approaches. Our results show that ectopically induced VPCs in pry-1 mutants adopt 2° fates independently of the gonad-derived inductive and LIN-12/Notch-mediated lateral signaling pathways. We also found that Cbr-pry-1 mutants frequently show a failure of P7.p induction. A similar, albeit low penetrant, defect is also observed in C. elegans pry-1 mutants. The genetic analysis of the P7.p induction defect revealed that it was caused by altered regulation of lin-12 and its transcriptional target lip-1 (MAP kinase phosphatase). Thus, our results provide evidence for LIN-12/Notch-dependent and independent roles of Wnt signaling in promoting 2 degrees VPC fates in both nematode species.

RNA interference in nematodes and the chance that favored Sydney Brenner

The efficiency of RNA interference varies between different organisms, even among nematodes. A recent report of successful RNA interference in the nematode Panagrolaimus superbus in BMC Molecular Biology has implications for the comparative study of the functional genomics of nematode species, and prompts reflections on the choice of Caenorhabditis elegans as a model organism.

Systemic RNAi mediated gene silencing in the anhydrobiotic nematode Panagrolaimus superbus

BACKGROUND

Gene silencing by RNA interference (RNAi) is a powerful tool for functional genomics. Although RNAi was first described in Caenorhabditis elegans, several nematode species are unable to mount an RNAi response when exposed to exogenous double stranded RNA (dsRNA). These include the satellite model organisms Pristionchus pacificus and Oscheius tipulae. Available data also suggest that the RNAi pathway targeting exogenous dsRNA may not be fully functional in some animal parasitic nematodes. The genus Panagrolaimus contains bacterial feeding nematodes which occupy a diversity of niches ranging from polar, temperate and semi-arid soils to terrestrial mosses. Thus many Panagrolaimus species are adapted to tolerate freezing and desiccation and are excellent systems to study the molecular basis of environmental stress tolerance. We investigated whether Panagrolaimus is susceptible to RNAi to determine whether this nematode could be used in large scale RNAi studies in functional genomics.

RESULTS

We studied two species: Panagrolaimus sp. PS1159 and Panagrolaimus superbus. Both nematode species displayed embryonic lethal RNAi phenotypes following ingestion of Escherichia coli expressing dsRNA for the C. elegans embryonic lethal genes Ce-lmn-1 and Ce-ran-4. Embryonic lethal RNAi phenotypes were also obtained in both species upon ingestion of dsRNA for the Panagrolaimus genes ef1b and rps-2. Single nematode RT-PCR showed that a significant reduction in mRNA transcript levels occurred for the target ef1b and rps-2 genes in RNAi treated Panagrolaimus sp. 1159 nematodes. Visible RNAi phenotypes were also observed when P. superbus was exposed to dsRNA for structural genes encoding contractile proteins. All RNAi phenotypes were highly penetrant, particularly in P. superbus.

CONCLUSION

This demonstration that Panagrolaimus is amenable to RNAi by feeding will allow the development of high throughput methods of RNAi screening for P. superbus. This greatly enhances the utility of this nematode as a model system for the study of the molecular biology of anhydrobiosis and cryobiosis and as a possible satellite model nematode for comparative and functional genomics. Our data also identify another nematode infraorder which is amenable to RNAi and provide additional information on the diversity of RNAi phenotypes in nematodes.

Oscheius tipulae, a widespread hermaphroditic soil nematode, displays a higher genetic diversity and geographical structure than Caenorhabditis elegans

The nematode Oscheius tipulae belongs to the same family (Rhabditidae) as the model species Caenorhabditis elegans. Both species reproduce through self-fertilizing hermaphrodites and facultative males. Recent studies have shown that the self-fertile C. elegans and C. briggsae displayed a 20-fold lower genetic diversity than the male-female species C. remanei. Several explanations have been put forward to account for this difference, including their mode of reproduction and dynamic population structure. Here, we present the results of extensive worldwide sampling of O. tipulae, which we previously used as a laboratory organism for developmental genetics. We found that O. tipulae is much more widespread and common in soil throughout the world than Caenorhabditis species. We analysed 63 O. tipulae isolates from several continents using amplified fragment length polymorphism (AFLP). We found that O. tipulae harbours a 5-fold higher genetic diversity than C. elegans and C. briggsae. As in C. elegans, a high proportion of this diversity was found locally. Yet, we detected significant geographical differentiation, both at the worldwide scale with a latitudinal structure and between three localities in France. In summary, O. tipulae exhibited significantly higher levels of genetic diversity and large-scale geographical structure than C. elegans, despite their shared mode of reproduction. This species difference in genetic diversity may be explained by a number of other differences, such as population size, distribution, migration and dynamics. Due to its widespread occurrence and relatively high genetic diversity, O. tipulae may be a promising study species for evolutionary studies.

Two hypotheses to explain why RNA interference does not work in animal parasitic nematodes

RNA interference (RNAi) has been used extensively in model organisms such as Caenorhabditis elegans. Methods developed for RNAi in C. elegans have also been used in parasitic nematodes. However, RNAi in parasitic nematodes has been unsuccessful or has had limited success. Studies of genes essential for RNAi in C. elegans and of RNAi in Caenorhabditis spp. other than C. elegans suggest two complementary, and testable, hypotheses for the limited success of RNAi in animal parasitic nematodes. These are: (i) that the external supply of double stranded RNA (dsRNA) to parasitic nematodes is inappropriate to achieve RNAi and (ii) that parasitic nematodes are functionally defective in genes required to initiate RNAi from externally supplied dsRNA.

The genome ofOscheius tipulae: determination of size, complexity, and structure by DNA reassociation using fluorescent dye

This work describes the physicochemical characterization of the genome and telomere structure from the nematode Oscheius tipulae CEW1. Oscheius tipulae is a free-living nematode belonging to the family Rhabditidae and has been used as a model system for comparative genetic studies. A new protocol that combines fluorescent detection of double-stranded DNA and S1 nuclease was used to determine the genome size of O. tipulae as 100.8 Mb (approximately 0.1 pg DNA/haploid nucleus). The genome of this nematode is made up of 83.4% unique copy sequences, 9.4% intermediate repetitive sequences, and 7.2% highly repetitive sequences, suggesting that its structure is similar to those of other nematodes of the genus Caenorhabditis. We also showed that O. tipulae has the same telomere repeats already found in Caenorhabditis elegans at the ends and in internal regions of the chromosomes. Using a cassette-ligation-mediated PCR protocol we were able to obtain 5 different putative subtelomeric sequences of O. tipulae, which show no similarity to C. elegans or C. briggsae subtelomeric regions. DAPI staining of hermaphrodite gonad cells show that, as detected in C. elegans and other rhabditids, O. tipulae have a haploid complement of 6 chromosomes.Key words: Oscheius tipulae, Caenorhabditis elegans, DNA reassociation, telomere, genome size, karyotype.

Hox gene evolution in nematodes: novelty conserved

The conserved homeobox (Hox) gene cluster is neither conserved nor clustered in the nematode Caenorhabditis elegans. Instead, C. elegans has a reduced and dispersed gene complement that is the result the loss of Hox genes in stages throughout its evolutionary history. The roles of Hox genes in patterning the nematode body axis are also divergent, although there are tantalising remnants of ancient regulatory systems. Hox patterning also differs greatly between C. elegans and a second 'model' nematode, Pristionchus pacificus. The pattern of Hox gene evolution may be indicative of the move to deterministic developmental modes in nematodes.

Pristionchus pacificus vulva formation: polarized division, cell migration, cell fusion, and evolution of invagination

Tube formation is a widespread process during organogenesis. Specific cellular behaviors participate in the invagination of epithelial monolayers that form tubes. However, little is known about the evolutionary mechanisms of cell assembly into tubes during development. In Caenorhabditis elegans, the detailed step-to-step process of vulva formation has been studied in wild type and in several mutants. Here we show that cellular processes during vulva development, which involve toroidal cell formation and stacking of rings, are conserved between C. elegans and Pristionchus pacificus, two species of nematodes that diverged approximately 100 million years ago. These cellular behaviors are divided into phases of cell proliferation, short-range migration, and cell fusion that are temporally distinct in C. elegans but not in P. pacificus. Thus, we identify heterochronic changes in the cellular events of vulva development between these two species. We find that alterations in the division axes of two equivalent vulval cells from Left-Right cleavage in C. elegans to Anterior-Posterior division in P. pacificus can cause the formation of an additional eighth ring. Thus, orthogonal changes in cell division axes with alterations in the number and sequence of cell fusion events result in dramatic differences in vulval shape and in the number of rings in the species studied. Our characterization of vulva formation in P. pacificus compared to C. elegans provides an evolutionary-developmental foundation for molecular genetic analyses of organogenesis in different species within the phylum Nematoda.

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

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

The story of cell fusion: big lessons from little worms

The ability of two or more cells to unite to form a new syncytial cell has been utilized in metazoans throughout evolution to form many complex organs, such as muscles, bones and placentae. This requires migration, recognition and adhesion between cells together with fusion of their plasma membranes and rearrangement of their cytoplasmic contents. Until recently, understanding of the mechanisms of cell fusion was restricted to fusion between enveloped viruses and their target cells. The identification of new factors that take part in developmental cell fusion in C. elegans opens the way to understanding how cells fuse and what the functions of this process are. In this review, we describe current knowledge on the mechanisms and putative roles of developmental cell fusion in C. elegans and how cell fusion is regulated, together with other intercellular processes to promote organogenesis.

LIN-39/Hox triggers cell division and represses EFF-1/fusogen-dependent vulval cell fusion

General mechanisms by which Hox genes establish cell fates are known. However, a few Hox effectors mediating cell behaviors have been identified. Here we found the first effector of LIN-39/HoxD4/Dfd in Caenorhabditis elegans. In specific vulval precursor cells (VPCs), LIN-39 represses early and late expression of EFF-1, a membrane protein essential for cell fusion. Repression of eff-1 is also achieved by the activity of CEH-20/Exd/Pbx, a known cofactor of Hox proteins. Unfused VPCs in lin-39(-);eff-1(-) double mutants fail to divide but migrate, executing vulval fates. Thus, lin-39 is essential for inhibition of EFF-1-dependent cell fusion and stimulation of cell proliferation during vulva formation. Supplemental material is available at http://www.genesdev.org.