Cell lineage in marine nematode Enoplus brevis

Contact area-dependent cell communication and the morphological invariance of ascidian embryogenesis

Marine invertebrate ascidians display embryonic reproducibility: Their early embryonic cell lineages are considered invariant and are conserved between distantly related species, despite rapid genomic divergence. Here, we address the drivers of this reproducibility. We used light-sheet imaging and automated cell segmentation and tracking procedures to systematically quantify the behavior of individual cells every 2 minutes during Phallusia mammillata embryogenesis. Interindividual reproducibility was observed down to the area of individual cell contacts. We found tight links between the reproducibility of embryonic geometries and asymmetric cell divisions, controlled by differential sister cell inductions. We combined modeling and experimental manipulations to show that the area of contact between signaling and responding cells is a key determinant of cell communication. Our work establishes the geometric control of embryonic inductions as an alternative to classical morphogen gradients and suggests that the range of cell signaling sets the scale at which embryonic reproducibility is observed.

Are there gap junctions without connexins or pannexins?

BACKGROUND

Gap junctions (GJ) are one of the most common forms of intercellular communication. GJs are assembled from proteins that form channels connecting the cytoplasm of adjacent cells. They are considered to be the main or the only type of intercellular channels and the universal feature of all multicellular animals. Two unrelated protein families are currently considered to be involved in this function, namely, connexins and pannexins (pannexins/innexins). Pannexins were hypothesized to be the universal GJ proteins of multicellular animals, distinct from connexins that are characteristic of chordates only. Here we have revised this supposition by applying growing high throughput sequencing data from diverse metazoan species.

RESULTS

Pannexins were found in Chordates, Ctenophores, Cnidarians, and in the most major groups of bilateral protostomes. Yet some metazoans appear to have neither connexins nor pannexins in their genomes. We detected no connexins or pannexins/innexins homologues in representatives of all five classes of echinoderms and their closest relatives hemichordates with available genomic sequences. Despite this, our intracellular recordings demonstrate direct electrical coupling between blastomeres at the 2-cell embryo of the echinoderm (starfish Asterias rubens). In these experiments, carboxyfluorescein fluorescent dye did not diffuse between electrically coupled cells. This excludes the possibility that the observed electrical coupling is mediated by incomplete cytoplasm separation during cleavage.

CONCLUSION

Functional GJs are present in representatives of the clade that lack currently recognized GJ protein families. New undiscovered protein families utilized for intercellular channels are predicted. It is possible that the new type(s) of intercellular channels are present in parallel to pannexin and connexin gap junctions in animal groups, other than Echinodermata.

Comparative Transcriptomics of Steinernema and Caenorhabditis Single Embryos Reveals Orthologous Gene Expression Convergence during Late Embryogenesis

Cells express distinct sets of genes in a precise spatio-temporal manner during embryonic development. There is a wealth of information on the deterministic embryonic development of Caenorhabditis elegans, but much less is known about embryonic development in nematodes from other taxa, especially at the molecular level. We are interested in insect pathogenic nematodes from the genus Steinernema as models of parasitism and symbiosis as well as a satellite model for evolution in comparison to C. elegans. To explore gene expression differences across taxa, we sequenced the transcriptomes of single embryos of two Steinernema species and two Caenorhabditis species at 11 stages during embryonic development and found several interesting features. Our findings show that zygotic transcription initiates at different developmental stages in each species, with the Steinernema species initiating transcription earlier than Caenorhabditis. We found that ortholog expression conservation during development is higher at the later embryonic stages than at the earlier ones. The surprisingly higher conservation of orthologous gene expression in later embryonic stages strongly suggests a funnel-shaped model of embryonic developmental gene expression divergence in nematodes. This work provides novel insight into embryonic development across distantly related nematode species and demonstrates that the mechanisms controlling early development are more diverse than previously thought at the transcriptional level.

Early development of the root-knot nematode Meloidogyne incognita

BACKGROUND

Detailed descriptions of the early development of parasitic nematodes are seldom available. The embryonic development of the plant-parasitic nematode Meloidogyne incognita was studied, focusing on the early events.

RESULTS

A fixed pattern of repeated cell cleavages was observed, resulting in the appearance of the six founder cells 3 days after the first cell division. Gastrulation, characterized by the translocation of cells from the ventral side to the center of the embryo, was seen 1 day later. Approximately 10 days after the first cell division a rapidly elongating two-fold stage was reached. The fully developed second stage juvenile hatched approximately 21 days after the first cell division.

CONCLUSIONS

When compared to the development of the free-living nematode Caenorhabditis elegans, the development of M. incognita occurs approximately 35 times more slowly. Furthermore, M. incognita differs from C. elegans in the order of cell divisions, and the early cleavage patterns of the germ line cells. However, cytoplasmic ruffling and nuclear migration prior to the first cell division as well as the localization of microtubules are similar between C. elegans and M. incognita.

Phylum-Level Conservation of Regulatory Information in Nematodes despite Extensive Non-coding Sequence Divergence

Gene regulatory information guides development and shapes the course of evolution. To test conservation of gene regulation within the phylum Nematoda, we compared the functions of putative cis-regulatory sequences of four sets of orthologs (unc-47, unc-25, mec-3 and elt-2) from distantly-related nematode species. These species, Caenorhabditis elegans, its congeneric C. briggsae, and three parasitic species Meloidogyne hapla, Brugia malayi, and Trichinella spiralis, represent four of the five major clades in the phylum Nematoda. Despite the great phylogenetic distances sampled and the extensive sequence divergence of nematode genomes, all but one of the regulatory elements we tested are able to drive at least a subset of the expected gene expression patterns. We show that functionally conserved cis-regulatory elements have no more extended sequence similarity to their C. elegans orthologs than would be expected by chance, but they do harbor motifs that are important for proper expression of the C. elegans genes. These motifs are too short to be distinguished from the background level of sequence similarity, and while identical in sequence they are not conserved in orientation or position. Functional tests reveal that some of these motifs contribute to proper expression. Our results suggest that conserved regulatory circuitry can persist despite considerable turnover within cis elements.

To explore the phylogenetic limits of conservation of cis-regulatory elements, we used transgenesis to test the functions of enhancers of four genes from several species spanning the phylum Nematoda. While we found a striking degree of functional conservation among the examined cis elements, their DNA sequences lacked apparent conservation with the C. elegans orthologs. In fact, sequence similarity between C. elegans and the distantly related nematodes was no greater than would be expected by chance. Short motifs, similar to known regulatory sequences in C. elegans, can be detected in most of the cis elements. When tested, some of these sites appear to mediate regulatory function. However, they seem to have originated through motif turnover, rather than to have been preserved from a common ancestor. Our results suggest that gene regulatory networks are broadly conserved in the phylum Nematoda, but this conservation persists despite substantial reorganization of regulatory elements and could not be detected using naïve comparisons of sequence similarity.

The study of Priapulus caudatus reveals conserved molecular patterning underlying different gut morphogenesis in the Ecdysozoa

Background

The digestive systems of animals can become highly specialized in response to their exploration and occupation of new ecological niches. Although studies on different animals have revealed commonalities in gut formation, the model systems Caenorhabditis elegans and Drosophila melanogaster, which belong to the invertebrate group Ecdysozoa, exhibit remarkable deviations in how their intestines develop. Their morphological and developmental idiosyncrasies have hindered reconstructions of ancestral gut characters for the Ecdysozoa, and limit comparisons with vertebrate models. In this respect, the phylogenetic position, and slow evolving morphological and molecular characters of marine priapulid worms advance them as a key group to decipher evolutionary events that occurred in the lineages leading to C. elegans and D. melanogaster.

Results

In the priapulid Priapulus caudatus, the gut consists of an ectodermal foregut and anus, and a mid region of at least partial endodermal origin. The inner gut develops into a 16-cell primordium devoid of visceral musculature, arranged in three mid tetrads and two posterior duplets. The mouth invaginates ventrally and shifts to a terminal anterior position as the ventral anterior ectoderm differentially proliferates. Contraction of the musculature occurs as the head region retracts into the trunk and resolves the definitive larval body plan. Despite obvious developmental differences with C. elegans and D. melanogaster, the expression in P. caudatus of the gut-related candidate genes NK2.1, foxQ2, FGF8/17/18, GATA456, HNF4, wnt1, and evx demonstrate three distinct evolutionarily conserved molecular profiles that correlate with morphologically identified sub-regions of the gut.

Conclusions

The comparative analysis of priapulid development suggests that a midgut formed by a single endodermal population of vegetal cells, a ventral mouth, and the blastoporal origin of the anus are ancestral features in the Ecdysozoa. Our molecular data on P. caudatus reveal a conserved ecdysozoan gut-patterning program and demonstrates that extreme morphological divergence has not been accompanied by major molecular innovations in transcriptional regulators during digestive system evolution in the Ecdysozoa. Our data help us understand the origins of the ecdysozoan body plan, including those of C. elegans and D. melanogaster, and this is critical for comparisons between these two prominent model systems and their vertebrate counterparts.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0139-z) contains supplementary material, which is available to authorized users.

SeaBase: a multispecies transcriptomic resource and platform for gene network inference

Marine and aquatic animals are extraordinarily useful as models for identifying mechanisms of development and evolution, regeneration, resistance to cancer, longevity and symbiosis, among many other areas of research. This is due to the great diversity of these organisms and their wide-ranging capabilities. Genomics tools are essential for taking advantage of these "free lessons" of nature. However, genomics and transcriptomics are challenging in emerging model systems. Here, we present SeaBase, a tool for helping to meet these needs. Specifically, SeaBase provides a platform for sharing and searching transcriptome data. More importantly, SeaBase will support a growing number of tools for inferring gene network mechanisms. The first dataset available on SeaBase is a developmental transcriptomic profile of the sea anemone Nematostella vectensis (Anthozoa, Cnidaria). Additional datasets are currently being prepared and we are aiming to expand SeaBase to include user-supplied data for any number of marine and aquatic organisms, thereby supporting many potentially new models for gene network studies. SeaBase can be accessed online at: http://seabase.core.cli.mbl.edu.

Plectus - a stepping stone in embryonic cell lineage evolution of nematodes

BACKGROUND

Recent studies have challenged the widespread view that the pattern of embryogenesis found in Caenorhabditis elegans (clade 9) is characteristic of nematodes in general. To understand this still largely unexplored landscape of developmental events, we set out to examine more distantly related nematodes in detail for temporospatial differences in pattern formation and cell specification. Members of the genus Plectus (clade 6) seem to be suitable candidates to show variety, with certain idiosyncratic features during early development and the convenient availability of cultivatable species.

METHODS

The study was conducted using 4-D lineage analysis, 3-D modeling of developing embryos and laser-induced ablation of individual blastomeres.

RESULTS

Detailed cell lineage studies of several Plectus species reveal that pattern formation and cell fate assignment differ markedly from C. elegans. Descendants of the first somatic founder cell S1 (AB) - but not the progeny of other founder cells - demonstrate extremely variable spatial arrangements illustrating that here distinct early cell-cell interactions between invariant partners, as found in C. elegans, cannot take place. Different from C. elegans, in Plectus alternative positional variations among early S1 blastomeres resulting in a 'situs inversus' pattern, nevertheless give rise to adults with normal left-right asymmetries. In addition, laser ablations of early blastomeres uncover inductions between variable cell partners.

CONCLUSIONS

Our results suggest that embryonic cell specification in Plectus is not correlated with cell lineage but with position. With this peculiarity, Plectus appears to occupy an intermediate position between basal nematodes displaying a variable early development and the C. elegans-like invariant pattern. We suggest that indeterminate pattern formation associated with late, position-dependent fate assignment represents a plesiomorphic character among nematodes predominant in certain basal clades but lost in derived clades. Thus, the behavior of S1 cells in Plectus can be considered an evolutionary relict in a transition phase between two different developmental strategies.

The Caenorhabditis elegans epidermis as a model skin. I: development, patterning, and growth

The skin of the nematode Caenorhabditis elegans is composed of a simple epidermal epithelium and overlying cuticle. The skin encloses the animal and plays central roles in body morphology and physiology; its simplicity and accessibility make it a tractable genetic model for several aspects of skin biology. Epidermal precursors are specified by a hierarchy of transcriptional regulators. Epidermal cells form on the dorsal surface of the embryo and differentiate to form the epidermal primordium, which then spreads out in a process of epiboly to enclose internal tissues. Subsequent elongation of the embryo into a vermiform larva is driven by cell shape changes and cell fusions in the epidermis. Most epidermal cells fuse in mid-embryogenesis to form a small number of multinucleate syncytia. During mid-embryogenesis the epidermis also becomes intimately associated with underlying muscles, performing a tendon-like role in transmitting muscle force. Post-embryonic development of the epidermis involves growth by addition of new cells to the syncytia from stem cell-like epidermal seam cells and by an increase in cell size driven by endoreplication of the chromosomes in epidermal nuclei.

Nematode model systems in evolution and development

The free-living nematode Caenorhabditis elegans is one of the most important model organisms in all areas of modern biology. Using the knowledge about C. elegans as a baseline, nematodes are now intensively studied in evolution and development. Evolutionary developmental biology or for short, 'evo-devo' has been developed as a new research discipline during the last two decades to investigate how changes in developmental processes and mechanisms result in the modification of morphological structures and phenotypic novelty. In this article, we review the concepts that make nematode evo-devo a successful approach to evolutionary biology. We introduce selected model systems for nematode evo-devo and provide a detailed discussion of four selected case studies. The most striking finding of nematode evo-devo is the magnitude of developmental variation in the context of a conserved body plan. Detailed investigation of early embryogenesis, gonad formation, vulva development, and sex determination revealed that molecular mechanisms evolve rapidly, often in the context of a conserved body plan. These studies highlight the importance of developmental systems drift and neutrality in evolution.

Cell fate specification in the C. elegans embryo

Cell specification requires that particular subsets of cells adopt unique expression patterns that ultimately define the fates of their descendants. In C. elegans, cell fate specification involves the combinatorial action of multiple signals that produce activation of a small number of "blastomere specification" factors. These initiate expression of gene regulatory networks that drive development forward, leading to activation of "tissue specification" factors. In this review, the C. elegans embryo is considered as a model system for studies of cell specification. The techniques used to study cell fate in this species, and the themes that have emerged, are described.

Embryogenesis of Romanomermis culicivorax: an alternative way to construct a nematode

The current picture of embryonic development in nematodes is essentially shaped by Caenorhabditis elegans and its close relatives. As their pattern of embryogenesis is rather similar, it is often considered to be representative for the taxon Nematoda as a whole. Here we give for the first time a comprehensive description of embryonic development in an ancestrally diverged nematode. Romanomermis culicivorax differs strikingly from C. elegans with respect to cell division pattern, spatial arrangement of blastomeres and tissue formation. Our study reveals a number of unexpected phenomena. These include (i) unique polar interphase microtubule caps forming in early blastomeres destined to undergo asymmetric cleavages, suggesting the presence of a so far undescribed MTOC; (ii) embryonic cell lineages of reduced complexity with predominantly monoclonal sublineages, generating just a single tissue type; (iii) construction of major parts of the body from duplicating building blocks consisting of rings of cells, a pattern showing some resemblance to segmentation; (iv) prominent differences in cell fate assignment which can be best explained with a global shift affecting all somatic founder cells. In summary, our data indicate that during nematode evolution massive alterations in the developmental program took place of how to generate a juvenile.

Urnaloricus gadi nov. gen. et nov. sp. (Loricifera, Urnaloricidae nov. fam.), an aberrant Loricifera with a viviparous pedogenetic life cycle

A new species of Loricifera, Urnaloricus gadi nov. gen. et nov. sp., is described from the Faroe Bank, located Southwest of the Faroe Islands, North Atlantic. The new species does not fit into any known families of Loricifera and therefore it is grouped into a new family Urnaloricidae nov. fam. The new species is characterized by having a very complicated life cycle that involves a large cyst-like mega-larva, two reduced larval instars and the Higgins-larvae eating their maternal stage from within. An adult stage is missing. This form of reproduction is called viviparous pedogenesis and normally is found only in nematodes and insects. In the life cycle of Urnaloricidae nov. fam., there are two types of free-living larval stages: a Higgins-larva and a mega-larva. The latter is found in two different forms, a pre- and a cyst-forming mega-larva. Additionally, there are two reduced life history stages, the reduced larval stage (probably a postlarva) and the ghost-larval stage inside the cyst-forming mega-larva. The external morphology of the two forms of mega-larvae is much reduced, e.g., the introvert has only a few rows of scalids when compared with the Higgins-larva. The pre mega-larva is free-living and can sometimes be covered with coccoliths. Internally, a large ovary with a few oocytes, a digestive system, and an internal armature with retracted scalids are present. The pre mega-larva presumably molts into a cyst-forming mega-larva and thereby the ovary is now seen inside the cyst-forming mega-larva. The cyst-forming mega-larva has the same structures as in the pre mega-larva though here the scalids are protruded and there is a gonopore. Inside the cyst-forming mega-larva the ovary produces more oocytes and begins to fill out the entire lumen. At this stage the cyst-forming mega-larva molts first to the presumed postlarval stage, and then this stage molts to a ghost-larva. Hence, the ovary now matures inside the ghost-larva, which is surrounded by both the cuticle of the reduced postlarval stage and the cuticle of the cyst-forming mega-larva. The oocytes mature into eggs, and then into embryos and finally into Higgins-larvae while reabsorbing all the tissue of their maternal stage, the ghost-larva. During this maturation the cuticle of the cyst-forming mega-larva starts to harden and become cyst-like. The fully developed Higgins-larvae emerge through the gonopore of the cyst-forming mega-larva by penetrating the thin cuticles of the ghost-larva and the postlarva. The embryos have holoblastic radial cleavage and later a fluid-filled blastocoel is formed. The eggshells are extremely elastic; hence, they can become very elongated as the embryos mature into Higgins-larvae.

Cell biology and evolution: molecular modules link it all?

Classical studies comparing developing embryos have suggested the importance of modified cell biological processes in the evolution of new phenotypes. Here, I revisit this connection focusing on embryonic development, in particular nematode embryogenesis. I compare phenotypic differences in nematode embryogenesis in two basic cell biological processes, the cell cycle and the localization of the first division axis. The analysis of these and other processes shows that, at the cell biological level, exhaustive variation is found that does not necessarily translate into morphological differences. Modern molecular analyses have led to a view in which molecular complexes, made up of groups of proteins, or modules, that are working together, are responsible for the proper execution of cell biological programs. I discuss how this modular architecture could facilitate the phenotypic changes observed in cell biological processes. Ultimately, understanding the connection between cellular behavior and phenotypic outcome will further elucidate the mechanisms responsible for phenotypic evolution.

Ascidians and the plasticity of the chordate developmental program

Little is known about the ancient chordates that gave rise to the first vertebrates, but the descendants of other invertebrate chordates extant at the time still flourish in the ocean. These invertebrates include the cephalochordates and tunicates, whose larvae share with vertebrate embryos a common body plan with a central notochord and a dorsal nerve cord. Tunicates are now thought to be the sister group of vertebrates. However, research based on several species of ascidians, a diverse and wide-spread class of tunicates, revealed that the molecular strategies underlying their development appear to diverge greatly from those found in vertebrates. Furthermore, the adult body plan of most tunicates, which arises following an extensive post-larval metamorphosis, shows little resemblance to the body plan of any other chordate. In this review, we compare the developmental strategies of ascidians and vertebrates and argue that the very divergence of these strategies reveals the surprising level of plasticity of the chordate developmental program and is a rich resource to identify core regulatory mechanisms that are evolutionarily conserved in chordates. Further, we propose that the comparative analysis of the architecture of ascidian and vertebrate gene regulatory networks may provide critical insight into the origin of the chordate body plan.

Early embryonic development of the priapulid worm Priapulus caudatus

The early cleavage up to gastrulation is described here for the priapulid worm Priapulus caudatus, contradicting and clarifying earlier partial reports on this topic. The cleavage pattern up to gastrulation is highly symmetrical, total, subequal, radial, and stereotypical. Gastrulation is intermediate between epiboly and invagination, and the mesendoderm may be derived from both cells of the first cleavage, thus differing significantly in its origin from that of many other protostomes. Priapulids occupy an increasingly important position in studies of animal evolution as they appear to be relatively basal within the new clade Ecdysozoa (panarthropods plus cycloneuralians); and have been described as both morphological and genetic living fossils. The insights derived from priapulids combined with new data published recently on kinorhynchs and tardigrades imply a substantial developmental diversity among basal ecdysozoans, and weakens the hypothesis that irregular cleavage is plesiomorphic to the entire clade. Further study is required to reconstruct basal cleavage patterns in both this clade, and indeed, the Bilateria as a whole.

The use of Caenorhabditis elegans in parasitic nematode research

There is increasing interest in the use of the free-living nematode Caenorhabditis elegans as a tool for parasitic nematode research and there are now a number of compelling examples of its successful application. C. elegans has the potential to become a standard tool for molecular helminthology researchers, just as yeast is routinely used by molecular biologists to study vertebrate biology. However, in order to exploit C. elegans in a meaningful manner, we need a detailed understanding of the extent to which different aspects of C. elegans biology have been conserved with particular groups of parasitic nematodes. This review first considers the current state of knowledge regarding the conservation of genome organisation across the nematode phylum and then discusses some recent evolutionary development studies in free-living nematodes. The aim is to provide some important concepts that are relevant to the extrapolation of information from C. elegans to parasitic nematodes and also to the interpretation of experiments that use C. elegans as a surrogate expression system. In general, examples have been specifically chosen because they highlight the importance of careful experimentation and interpretation of data. Consequently, the focus is on the differences that have been found between nematode species rather than the similarities. Finally, there is a detailed discussion of the current status of C. elegans as a heterologous expression system to study parasite gene function and regulation using successful examples from the literature.

Cellular pattern formation, establishment of polarity and segregation of colored cytoplasm in embryos of the nematode Romanomermis culicivorax

We have begun to analyze the early embryogenesis of Romanomermis culicivorax, an insect-parasitic nematode phylogenetically distant to Caenorhabditis elegans. Development of R. culicivorax differs from C. elegans in many aspects including establishment of polarity, formation of embryonic axes and the pattern of asymmetric cleavages. Here, a polarity reversal in the germline takes place already in P(1) rather than P(2), the dorsal-ventral axis appears to be inverted and gut fate is derived from the AB rather than from the EMS blastomere. So far unique for nematodes is the presence of colored cytoplasm and its segregation into one specific founder cell. Normal development observed after experimentally induced abnormal partitioning of pigment indicates that it is not involved in cell specification. Another typical feature is prominent midbodies (MB). We investigated the role of the MB region in the establishment of asymmetry. After its irradiation the potential for unequal cleavage in somatic and germline cells as well as differential distribution of pigment are lost. This indicates a crucial involvement of this region for spindle orientation, positioning, and cytoplasmic segregation. A scenario is sketched suggesting why and how during evolution the observed differences between R. culicivorax and C. elegans may have evolved.

Different roads to form the same gut in nematodes

The morphogenesis of a gut from the endoderm has been well studied among the animal kingdom and is also well described in the nematode Caenorhabditis elegans. But are there other ways to build a nematode intestine? Sulston et al. (1983) described a different intestinal cell lineage in the species Panagrellus redivivus and Turbatrix aceti that includes two programmed cell deaths. However, no details are known about the three-dimensional (3D) configuration and the role of the cell deaths. Here, we describe the intestinal morphogenesis of P. redivivus and five other nematode species by means of four-dimensional microscopy, which gives us a 3D representation of gut formation at the cellular level. The morphological pathway of gut formation is highly conserved among these distantly related species. However, we found the P. redivivus pattern in another related species Halicephalobus gingivalis. In this pattern, the intestinal precursors migrate inward in concert with the mesoderm precursors. Based on the observations, we propose a hypothesis that could explain the differences. The positions of the mesoderm precursors create a possible spatial constraint, by which the establishment of bilateral symmetry in the intestine is delayed. This symmetry is corrected by cell migrations; other cells are eliminated and compensated by supplementary cell divisions. This pattern leads to the same result as in the other nematodes: a bilateral symmetrical intestine with nine rings. This illustrates how conserved body plans can be achieved by different developmental mechanisms.

Nematode Gastrulation: Having a BLASTocoel!

During gastrulation of the nematode worm Caenorhabditis elegans, individual cells ingress into a solid ball of cells. Gastrulation in a basal nematode, in contrast, has now been found to occur by invagination into a blastocoel, revealing an unanticipated embryological affinity between nematodes and all other triploblastic metazoans.

Unusual cleavage and gastrulation in a freshwater nematode: developmental and phylogenetic implications

Early embryogenesis in nematodes as seen in Caenorhabditis elegans and many other species of this phylum features several characteristic events. These include the visible presence of a germline from the very beginning generating different somatic lineages via asymmetric cleavages, the absence of a coeloblastula and a unique type of gastrulation with immigration of just two gut precursor cells. Here it is shown by using Nomarski optics that development of the freshwater nematode Tobrilus diversipapillatus differs from this pattern in two prominent aspects. (1) No asymmetric cleavages and no distinct cell lineages are generated; (2) in contrast to all other nematodes studied so far, a prominent coeloblastula is formed and gastrulation resembles the "classical" pattern found all over the animal kingdom. These developmental peculiarities are considered to be plesiomorphic and thus the order "Triplonchida", to which Tobrilus belongs, may occupy a phylogenetic position at the base of the nematode phylum. The findings reported here allow us to reject a number of conceivable correlations between the type of gastrulation and other developmental parameters.

Early embryogenesis of the pinewood nematode Bursaphelenchus xylophilus

The early embryogenesis and cell lineage of the pinewood nematode Bursaphelenchus xylophilus was followed from a single-cell zygote to a 46-cell embryo under Nomarski optics, and elongation of the microtubules was studied by immunostaining. As a B. xylophilus oocyte matures, it passes through a passage connecting the oviduct with the quadricolumella, the distal part of the uterus, and reaches the quadricolumella where it stays for a few minutes and is fertilized. After fertilization, the germinal vesicle disappears, an eggshell is formed, and the male and female pronuclei appear. The pronuclei move toward each other and fuse at the center of the egg. Around this time, the microtubule-organizing center appears. The presumptive region of sperm entry into the oocyte becomes the future anterior portion of the embryo. This anterior-posterior axis determination is opposite to that of Caenorhabditis elegans, where the sperm entry site becomes the posterior portion of the embryo. The optimal growth temperatures of these two nematodes also differ in that temperatures of about 30 degrees C afford the fastest growth rate and highest hatching frequency in B. xylophilus. Otherwise, the lineage resembles that of C. elegans with respect to timing, positioning and the axis orientation of each cell division.

Embryonic cell lineage of the marine nematode Pellioditis marina

We describe the complete embryonic cell lineage of the marine nematode Pellioditis marina (Rhabditidae) up to somatic muscle contraction, resulting in the formation of 638 cells, of which 67 undergo programmed cell death. In comparison with Caenorhabditis elegans, the overall lineage homology is 95.5%; fate homology, however, is only 76.4%. The majority of the differences in fate homology concern nervous, epidermal, and pharyngeal tissues. Gut and, remarkably, somatic muscle is highly conserved in number and position. Partial lineage data from the slower developing Halicephalobus sp. (Panagrolaimidae) reveal a lineage largely, but not exclusively, built up of monoclonal sublineage blocs with identical fates, unlike the polyclonal fate distribution in C. elegans and P. marina. The fate distribution pattern in a cell lineage could be a compromise between minimizing the number of specification events by monoclonal specification and minimizing the need for migrations by forming the cells close at their final position. The latter could contribute to a faster embryonic development. These results reveal that there is more than one way to build a nematode.

Hox Gene Loss during Dynamic Evolution of the Nematode Cluster

HOX GENES ARE IMPORTANT: their central role in anterior-posterior patterning provides a framework for molecular comparison of animal body plan evolution. The nematode Caenorhabditis elegans stands out as having a greatly reduced Hox gene complement. To address this, orthologs of C. elegans Hox genes were identified in six species from across the Nematoda, and they show that rapid homeodomain sequence evolution is a general feature of nematode Hox genes. Some nematodes express additional Hox genes belonging to orthology groups that are absent from C. elegans but present in other bilaterian animals. Analysis of the genomic environment of a newly identified Brugia malayi Hox6-8 ortholog (Bm-ant-1) revealed that it lay downstream of the Bm-egl-5 Hox gene and that their homeodomain exons are alternately cis spliced to the same 5' exon. This organization may represent an intermediate state in Hox gene loss via redundancy. The Hox clusters of nematodes are the product of a dynamic mix of gene loss and rapid sequence evolution, with the most derived state observed in the model C. elegans.

Evolution of development in closely related species of flies and worms

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

Making worm guts: the gene regulatory network of the Caenorhabditis elegans endoderm

The nematode Caenorhabditis elegans is a triploblastic ecdysozoan, which, although it contains too few cells during embryogenesis to create discernible germ "layers," deploys similar programs for germ layer differentiation used in animals with many more cells. The endoderm arises from a single progenitor, the E cell, and is selected from among three possible fates by a three-state combinatorial regulatory system involving intersecting cell-intrinsic and intercellular signals. The core gene regulatory cascade that drives endoderm development, extending from early maternal regulators to terminal differentiation genes, is characterized by activation of successive tiers of transcription factors, including a sequential cascade of redundant GATA transcription factors. Each tier is punctuated by a cell division, raising the possibility that intercession of one cell cycle round, or DNA replication, is required for activation of the next tier. The existence of each tier in the regulatory hierarchy is justified by the assignment of a unique task and each invariably performs at least two functions: to activate the regulators in the next tier and to perform one other activity distinct from that of the next tier. While the regulatory inputs that initiate endoderm development are highly divergent, they mobilize a gene regulatory network for endoderm development that appears to be common to all triploblastic metazoans. Genome-wide functional genomic approaches, including identification of >800 transcripts that exhibit the same regulatory patterns as a number of endoderm-specific genes, are contributing to elucidation of the complete endoderm gene regulatory network in C. elegans. Dissection of the architecture of the C. elegans endoderm network may provide insights into the evolutionary plasticity and origins of this germ layer.

On the evolution of early development in the Nematoda

The phylum Nematoda serves as an excellent model system for exploring how development evolves, using a comparative approach to developmental genetics. More than 100 laboratories are studying developmental mechanisms in the nematode Caenorhabditis elegans, and many of the methods that have been developed for C. elegans can be applied to other nematodes. This review summarizes what is known so far about steps in early development that have evolved in the nematodes, and proposes potential experiments that could make use of these data to further our understanding of how development evolves. The promise of such a comparative approach to developmental genetics is to fill a wide gap in our understanding of evolution--a gap spanning from mutations in developmental genes through to their phenotypic results, on which natural selection may act.

Microevolutionary studies in nematodes: a beginning

Comparisons between related species often allow the detailed genetic analysis of evolutionary processes. Here we advocate the use of the nematode Caenorhabditis elegans (and several other rhabditid species) as model systems for microevolutionary studies. Compared to Drosophila species, which have been a mainstay of such studies, C. elegans has a self-fertilizing mode of reproduction, a shorter life cycle and a convenient cell-level analysis of phenotypic variation. Data concerning its population genetics and ecology are still scarce, however. We review molecular, behavioral and developmental intraspecific polymorphisms for populations of C. elegans, Oscheius sp. 1 and Pristionchus pacificus. Focusing on vulval development, which has been well characterized in several species, we discuss relationships between patterns of variations: (1) for a given genotype (developmental variants), (2) after mutagenesis (mutability), (3) in different populations of the same species (polymorphisms) and (4) between closely related species. These studies have revealed that evolutionary variations between sister species affect those characters that show phenotypic developmental variants, that are mutable and that are polymorphic within species.

Three sons of fortune: early embryogenesis, evolution and ecology of nematodes

Comparative analysis of nematode development has revealed considerable variations in how the fates of embryonic cells are specified. Such early variations seem enigmatic as they do not influence the resultant structure or performance of the emerging animal. Three different nematode species are used to consider why alternative ways to reach the same goal may have been established during evolution and why early steps of embryogenesis are particularly variable. A scenario is sketched with a shift from late to early cell specification, along with an increase in maternal contribution and developmental tempo and a decrease in regulative potential expressing different developmental strategies. Future studies of larger numbers of species are needed to assess the extent of such variations and to understand more fully the underlying mechanisms, rules and driving forces.

Developmental fates of the first four blastomeres of the chaetognath Paraspadella gotoi: relationship to protostomes

Experimental analysis of the development of chaetognaths is virtually lacking. To elucidate developmental fates, single blastomeres of the 2-cell and 4-cell embryos of Paraspadella gotoi were injected with a lineage-tracing dye (Fluoro-Ruby or DiI). The distribution of the labels was observed in the hatchlings. In a previous study, embryos were injected at the 2-cell stage with Fluoro-Ruby and two sets of complementary labeling patterns (DL and VR, and DR and VL) were found. The same results were obtained when DiI was used as a tracer dye. The 4-cell embryo consists of the animal and vegetal cross-furrow cells in a tetrahedral arrangement and one of the vegetal cross-furrow cells typically contains the germ plasm. When single cells were injected at the 4-cell stage, four labeling patterns were observed (D, V, L and R). These four patterns represent subsets of the four patterns observed in the hatchling injected at the 2-cell stage. The V pattern is probably generated from the blastomere containing the germ plasm. It was found that the positions of the blastomeres at the 4-cell stage corresponded to the future body axes, similar to classic spiralians and modified spiralians such as crustaceans. Furthermore, it was confirmed that second cleavage occurs in a leiotropic fashion, which is seen in the second cleavage of the classic spiralians. Chaetognaths may have some similarities to protostomes in their developmental program.

Body-plan evolution in the Bilateria: early antero-posterior patterning and the deuterostome-protostome dichotomy

Recent molecular analyses reveal common themes in early antero-posterior patterning in the four major groups of invertebrate deuterostomes and vertebrates in spite of large differences in the mode of gastrulation. Comparisons with Drosophila and Cnidarians suggest a scheme for evolution of the Bilaterian body plan and emphasize the pressing need for similar studies in a wider variety of organisms, especially more basal protostomes.

Evolution of nematode development

Multiple evolutionary variations occur in the cellular and genetic programming of nematode development. Many changes involve alterations of inductive interactions. Surprisingly, inductive processes vary during evolution, irrespective of changes in the final cell lineages and morphological structures. Genetic studies in some nematodes also shed light on the underlying mechanisms of evolutionary change.

Regulative development in a nematode embryo: a hierarchy of cell fate transformations

Cell specification during embryogenesis of the model system Caenorhabditis elegans involves a combination of inductive and autonomous mechanisms. We have begun to study the development of other nematodes to investigate how well cell-specification mechanisms are preserved among closely related species. Here we report that the embryo of the soil nematode Acrobeloides nanus expresses a so far undescribed regulative potential. When, for instance, the first somatic founder cell AB is eliminated it is replaced by its posterior neighbor EMS, which in turn is replaced by the C cell. This allows-different from C. elegans-the development of partial embryos up to hatching and sometimes to fertile adults. Thus, early somatic blastomeres in A. nanus are multipotent, each being capable of giving rise to more than one somatic founder cell. Lost germ-line cells, however, are not replaced. A model is presented, according to which in A. nanus cellular identities are assigned by specific reciprocal inhibitory cell-cell interactions absent in C. elegans. Differences and similarities in cell specification between the two species are discussed and related to different developmental strategies.

Patterning the C. elegans embryo: moving beyond the cell lineage

The Caenorhabditis elegans embryo undergoes a series of stereotyped cell cleavages that generates the organs and tissues necessary for an animal to survive. Here we review two models of embryonic patterning, one that is lineage-based, and one that focuses on domains of organ and tissue precursors. Our evolving view of C. elegans embryogenesis suggests that this animal develops by mechanisms that are qualitatively similar to those used by other animals.

Conservation of the spiralian developmental program: cell lineage of the nemertean, Cerebratulus lacteus

Lineage tracers were injected into individual blastomeres in embryos of the indirect-developing nemertean Cerebratulus lacteus through the formation of the fourth quartet of micromeres. Subsequent development was followed to the formation of feeding pilidium larvae to establish their ultimate fates. Results showed that these blastomeres have unique fates, and their clones give rise to highly predictable regions of the larval body. As in other spiralians, four discrete cell quadrants can be identified. For the most part, their identities are homologous to the typical spiralian A, B, C, and D cell quadrants. In some respects their fates differ from the typical spiralian fate map; however, these can be understood in terms of simple modifications of the early cleavage program. Unlike most spiralians, the first quartet micromeres in the eight-celled embryo are larger than the corresponding vegetal macromeres, and generate most of the larval ectoderm. All four of these micromeres contribute to the apical organ and generate four bilaterally situated domains of ectoderm, where the progeny of the 1a and 1d micromeres lie to the left of the median plane while those of 1b and 1c lie to the right. Unlike the progeny of the first quartet, those of the second quartet are situated in left (2a), ventral (2b), right (2c), and dorsal (2d) positions. The third quartet micromeres generate clones situated in a bilaterally symmetrical fashion similar to those of the first quartet. The alternating axial relationships exhibited by successive micromere quartets are a characteristic of spiralian development. Unlike other spiralian larvae possessing a ciliary band, the pilidium larval ciliary band is formed by all blastomeres of the first and second micromere quartets, as well as 3c and 3d. Ectomesoderm is derived from two blastomeres (3a and 3b), which give rise to the extensive array of the larval muscle cells. C. lacteus also possesses a true mesentoblast (4d) which gives rise to a pair of mesodermal bandlets, and scattered mesenchymal cells. The dual origin of the mesoderm, as both ectomesoderm and endomesoderm, appears to be a condition present in all spiralians. The gut is formed by all the fourth quartet micromeres as well as the vegetal macromeres (4A, 4B, 4C, 4D). Despite differences in the determination of axial properties and some modifications in quadrant fates, nemerteans appear to be constructed on the typical spiralian developmental platform.