Annelid functional genomics reveal the origins of bilaterian life cycles

Indirect development with an intermediate larva exists in all major animal lineages1, which makes larvae central to most scenarios of animal evolution2-11. Yet how larvae evolved remains disputed. Here we show that temporal shifts (that is, heterochronies) in trunk formation underpin the diversification of larvae and bilaterian life cycles. We performed chromosome-scale genome sequencing in the annelid Owenia fusiformis with transcriptomic and epigenomic profiling during the life cycles of this and two other annelids. We found that trunk development is deferred to pre-metamorphic stages in the feeding larva of O. fusiformis but starts after gastrulation in the non-feeding larva with gradual metamorphosis of Capitella teleta and the direct developing embryo of Dimorphilus gyrociliatus. Accordingly, the embryos of O. fusiformis develop first into an enlarged anterior domain that forms larval tissues and the adult head12. Notably, this also occurs in the so-called 'head larvae' of other bilaterians13-17, with which the O. fusiformis larva shows extensive transcriptomic similarities. Together, our findings suggest that the temporal decoupling of head and trunk formation, as maximally observed in head larvae, facilitated larval evolution in Bilateria. This diverges from prevailing scenarios that propose either co-option9,10 or innovation11 of gene regulatory programmes to explain larva and adult origins.

A scalable culturing system for the marine annelid Platynereis dumerilii

Platynereis dumerilii is a marine segmented worm (annelid) with externally fertilized embryos and it can be cultured for the full life cycle in the laboratory. The accessibility of embryos and larvae combined with the breadth of the established molecular and functional techniques has made P. dumerilii an attractive model for studying development, cell lineages, cell type evolution, reproduction, regeneration, the nervous system, and behavior. Traditionally, these worms have been kept in rooms dedicated for their culture. This allows for the regulation of temperature and light cycles, which is critical to synchronizing sexual maturation. However, regulating the conditions of a whole room has limitations, especially if experiments require being able to change culturing conditions. Here we present scalable and flexible culture methods that provide ability to control the environmental conditions, and have a multi-purpose culture space. We provide a closed setup shelving design with proper light conditions necessary for P. dumerilii to mature. We also implemented a standardized method of feeding P. dumerilii cultures with powdered spirulina which relieves the ambiguity associated with using frozen spinach, and helps standardize nutrition conditions across experiments and across different labs. By using these methods, we were able to raise mature P. dumerilii, capable of spawning and producing viable embryos for experimentation and replenishing culture populations. These methods will allow for the further accessibility of P. dumerilii as a model system, and they can be adapted for other aquatic organisms.

Effects of dechlorination conditions on the developmental toxicity of a chlorinated saline primary sewage effluent: Excessive dechlorination is better than not enough

Chlorine-disinfected sewage effluents are typically dechlorinated by using NaHSO₃, Na₂SO₃, or Na₂S₂O₃, as chlorine residual could be harmful to aquatic organisms upon discharge of sewage effluents into receiving marine water. In this study, we systematically investigated the effects of dechlorination-related factors on the developmental toxicity of a chlorinated saline primary sewage effluent, via direct exposure of the embryos of a marine polychaete to the effluent. The results showed that dechlorination ratio (i.e., the ratio of the dosed amount to the requisite stoichiometric amount of a dechlorination agent) and mixing condition were critical factors affecting the toxicity of the effluent. The toxicity of the effluent under insufficient dechlorination conditions was mainly caused by residual chlorine, especially monochloramine. Although the three dechlorination agents generally performed similarly, dechlorination with Na₂S₂O₃ required a more vigorous mixing condition than that with NaHSO₃ or Na₂SO₃, as the relatively high density of Na₂S₂O₃ might affect the mixing efficiency. Under insufficient mixing conditions, a prolonged dechlorination time was beneficial to achieving complete dechlorination and thus lowered the toxicity of the effluent. Moreover, because disinfection byproducts (DBPs) may have chronic effects on aquatic organisms, the developmental toxicity of the DBP mixtures in the chlorinated effluent in different dechlorination scenarios was also evaluated. The results indicated that increasing the dechlorination ratio reduced the developmental toxicity of the DBP mixture in the chlorinated saline sewage effluent, which might be ascribed to the decrease of the levels of overall brominated and iodinated DBPs; the dechlorination agent (NaHSO₃ or Na₂S₂O₃) might act as a nucleophile in the nucleophilic substitution and cause the substitution of bromine or iodine atoms in brominated and iodinated DBPs. The results from this study might aid in the design and operation of dechlorination facilities in sewage treatment plants.

From spiral cleavage to bilateral symmetry: the developmental cell lineage of the annelid brain

BACKGROUND

During early development, patterns of cell division-embryonic cleavage-accompany the gradual restriction of blastomeres to specific cell fates. In Spiralia, which include annelids, mollusks, and flatworms, "spiral cleavage" produces a highly stereotypic, spiral-like arrangement of blastomeres and swimming trochophore-type larvae with rotational (spiral) symmetry. However, starting at larval stages, spiralian larvae acquire elements of bilateral symmetry, before they metamorphose into fully bilateral juveniles. How this spiral-to-bilateral transition occurs is not known and is especially puzzling for the early differentiating brain and head sensory organs, which emerge directly from the spiral cleavage pattern. Here we present the developmental cell lineage of the Platynereis larval episphere.

RESULTS

Live-imaging recordings from the zygote to the mid-trochophore stage (~ 30 hpf) of the larval episphere of the marine annelid Platynereis dumerilii reveal highly stereotypical development and an invariant cell lineage of early differentiating cell types. The larval brain and head sensory organs develop from 11 pairs of bilateral founders, each giving rise to identical clones on the right and left body sides. Relating the origin of each bilateral founder pair back to the spiral cleavage pattern, we uncover highly divergent origins: while some founder pairs originate from corresponding cells in the spiralian lineage on each body side, others originate from non-corresponding cells, and yet others derive from a single cell within one quadrant. Integrating lineage and gene expression data for several embryonic and larval stages, we find that the conserved head patterning genes otx and six3 are expressed in bilateral founders representing divergent lineage histories and giving rise to early differentiating cholinergic neurons and head sensory organs, respectively.

CONCLUSIONS

We present the complete developmental cell lineage of the Platynereis larval episphere, and thus the first comprehensive account of the spiral-to-bilateral transition in a developing spiralian. The bilateral symmetry of the head emerges from pairs of bilateral founders, similar to the trunk; however, the head founders are more numerous and show striking left-right asymmetries in lineage behavior that we relate to differential gene expression.

PdumBase: a transcriptome database and research tool for Platynereis dumerilii and early development of other metazoans

BACKGROUND

The marine polychaete annelid Platynereis dumerilii has recently emerged as a prominent organism for the study of development, evolution, stem cells, regeneration, marine ecology, chronobiology and neurobiology within metazoans. Its phylogenetic position within the spiralian/ lophotrochozoan clade, the comparatively high conservation of ancestral features in the Platynereis genome, and experimental access to any stage within its life cycle, make Platynereis an important model for elucidating the complex regulatory and functional molecular mechanisms governing early development, later organogenesis, and various features of its larval and adult life. High resolution RNA-seq gene expression data obtained from specific developmental stages can be used to dissect early developmental mechanisms. However, the potential for discovery of these mechanisms relies on tools to search, retrieve, and compare genome-wide information within Platynereis, and across other metazoan taxa.

RESULTS

To facilitate exploration and discovery by the broader scientific community, we have developed a web-based, searchable online research tool, PdumBase, featuring the first comprehensive transcriptome database for Platynereis dumerilii during early stages of development (2 h ~ 14 h). Our database also includes additional stages over the P. dumerilii life cycle and provides access to the expression data of 17,213 genes (31,806 transcripts) along with annotation information sourced from Swiss-Prot, Gene Ontology, KEGG pathways, Pfam domains, TmHMM, SingleP, and EggNOG orthology. Expression data for each gene includes the stage, the normalized FPKM, the raw read counts, and information that can be leveraged for statistical analyses of differential gene expression and the construction of genome-wide co-expression networks. In addition, PdumBase offers early stage transcriptome expression data from five further species as a valuable resource for investigators interested in comparing early development in different organisms. To understand conservation of Platynereis gene models and to validate gene annotation, most Platynereis gene models include a comprehensive phylogenetic analysis across 18 species representing diverse metazoan taxa.

CONCLUSIONS

PdumBase represents the first online resource for the early developmental transcriptome of Platynereis dumerilii. It serves as a research platform for discovery and exploration of gene expression during early stages, throughout the Platynereis life cycle, and enables comparison to other model organisms. PdumBase is freely available at http://pdumbase.gdcb.iastate.edu .

Anteroposterior axis patterning by early canonical Wnt signaling during hemichordate development

The Wnt family of secreted proteins has been proposed to play a conserved role in early specification of the bilaterian anteroposterior (A/P) axis. This hypothesis is based predominantly on data from vertebrate embryogenesis as well as planarian regeneration and homeostasis, indicating that canonical Wnt (cWnt) signaling endows cells with positional information along the A/P axis. Outside of these phyla, there is strong support for a conserved role of cWnt signaling in the repression of anterior fates, but little comparative support for a conserved role in promotion of posterior fates. We further test the hypothesis by investigating the role of cWnt signaling during early patterning along the A/P axis of the hemichordate Saccoglossus kowalevskii. We have cloned and investigated the expression of the complete Wnt ligand and Frizzled receptor complement of S. kowalevskii during early development along with many secreted Wnt modifiers. Eleven of the 13 Wnt ligands are ectodermally expressed in overlapping domains, predominantly in the posterior, and Wnt antagonists are localized predominantly to the anterior ectoderm in a pattern reminiscent of their distribution in vertebrate embryos. Overexpression and knockdown experiments, in combination with embryological manipulations, establish the importance of cWnt signaling for repression of anterior fates and activation of mid-axial ectodermal fates during the early development of S. kowalevskii. However, surprisingly, terminal posterior fates, defined by posterior Hox genes, are unresponsive to manipulation of cWnt levels during the early establishment of the A/P axis at late blastula and early gastrula. We establish experimental support for a conserved role of Wnt signaling in the early specification of the A/P axis during deuterostome body plan diversification, and further build support for an ancestral role of this pathway in early evolution of the bilaterian A/P axis. We find strong support for a role of cWnt in suppression of anterior fates and promotion of mid-axial fates, but we find no evidence that cWnt signaling plays a role in the early specification of the most posterior axial fates in S. kowalevskii. This posterior autonomy may be a conserved feature of early deuterostome axis specification.

Three-step effluent chlorination increases disinfection efficiency and reduces DBP formation and toxicity

Chlorination is extensively applied for disinfecting sewage effluents, but it unintentionally generates disinfection byproducts (DBPs). Using seawater for toilet flushing introduces a high level of bromide into domestic sewage. Chlorination of sewage effluent rich in bromide causes the formation of brominated DBPs. The objectives of achieving a disinfection goal, reducing disinfectant consumption and operational costs, as well as diminishing adverse effects to aquatic organisms in receiving water body remain a challenge in sewage treatment. In this study, we have demonstrated that, with the same total chlorine dosage, a three-step chlorination (dosing chlorine by splitting it into three equal portions with a 5-min time interval for each portion) was significantly more efficient in disinfecting a primary saline sewage effluent than a one-step chlorination (dosing chlorine at one time). Compared to one-step chlorination, three-step chlorination enhanced the disinfection efficiency by up to 0.73-log reduction of Escherichia coli. The overall DBP formation resulting from one-step and three-step chlorination was quantified by total organic halogen measurement. Compared to one-step chlorination, the DBP formation in three-step chlorination was decreased by up to 23.4%. The comparative toxicity of one-step and three-step chlorination was evaluated in terms of the development of embryo-larva of a marine polychaete Platynereis dumerilii. The results revealed that the primary sewage effluent with three-step chlorination was less toxic than that with one-step chlorination, indicating that three-step chlorination could reduce the potential adverse effects of disinfected sewage effluents to aquatic organisms in the receiving marine water.

Identification, toxicity and control of iodinated disinfection byproducts in cooking with simulated chlor(am)inated tap water and iodized table salt

Chlorine/chloramine residuals are maintained in drinking water distribution systems to prevent microbial contamination and microorganism regrowth. During household cooking processes (e.g., soup making), the residual chlorine/chloramines in tap water may react with the iodide in iodized table salt to form hypoiodous acid, which could react with remaining natural organic matter in tap water and organic matter in food to generate iodinated disinfection byproducts (I-DBPs). However, I-DBPs formed during cooking with chloraminated/chlorinated tap water are almost completely new to researchers. In this work, by adopting precursor ion scan of m/z 127 using ultra performance liquid chromatography/electrospray ionization-triple quadrupole mass spectrometry, many new polar I-DBPs formed during cooking with chloraminated/chlorinated tap water were detected and proposed with structures, of which 3-iodo-4-hydroxybenzaldehyde, 3-iodo-4-hydroxybenzoic acid, 3-iodo-4-hydroxy-5-methylbenzoic acid, diiodoacetic acid, 3,5-diiodo-4-hydroxybenzaldehyde, 3,5-diiodo-4-hydroxybenzoic acid, 2,6-diiodo-4-nitrophenol, 2,4-diiodo-6-nitrophenol, and 2,4,6-triiodophenol were confirmed with standard compounds. With the aid of ultra fast liquid chromatography/ion trap-time of flight-mass spectrometry, molecular formula identification of five new I-DBPs (C8H5O4I, C7H4NO4I, C8H5O5I, C7H4NO5I, and C8H6O3I2) was achieved. A developmental toxicity with a recently developed sensitive bioassay was conducted for the newly identified I-DBPs, suggesting that phenolic I-DBPs (except for iodinated carboxyphenols) were about 50-200 times more developmentally toxic than aliphatic I-DBPs. The major I-DBPs in a baseline simulated cooking water sample were determined to be from 0.72 to 7.63 μg/L. Polar I-DBPs formed under various disinfection and cooking conditions were compared, and suggestions for controlling their formation were provided.

A metameric origin for the annelid pygidium?

BACKGROUND

Segmented body organizations are widely represented in the animal kingdom. Whether the last common bilaterian ancestor was already segmented is intensely debated. Annelids display broad morphological diversity but many species are among the most homonomous metameric animals. The front end (prostomium) and tail piece (pygidium) of annelids are classically described as non-segmental. However, the pygidium structure and development remain poorly studied.

RESULTS

Using different methods of microscopy, immunolabelling and a number of molecular markers, we describe the neural and mesodermal structures of the pygidium of Platynereis dumerilii. We establish that the pygidium possesses a complicated nervous system with a nerve ring and a pair of sensory ganglia, a complex intrinsic musculature, a large terminal circular blood sinus and an unusual unpaired torus-shaped coelomic cavity. We also describe some earlier steps of pygidial development and pygidial structure of mature animals after epitokous transformation.

CONCLUSIONS

We describe a much more complex organization of the pygidium of P. dumerilii than previously suggested. Many of the characteristics are strikingly similar to those found in the trunk segments, opening the debate on whether the pygidium and trunk segments derive from the same ancestral metameric unit. We analyze these scenarios in the context of two classical theories on the origin of segmentation: the cyclomeric/archicoelomate concept and the colonial theory. Both theories provide possible explanations for the partial or complete homology of trunk segments and pygidium.

Acute hypoxic exposure affects gamete quality and subsequent fertilization success and embryonic development in a serpulid polychaete

Hypoxia likely compromises the reproductive success of those marine organisms carrying out external fertilization because their gametes and embryos are inevitably exposed to the external environment. Hydroides elegans, a dominant serpulid polychaete in Hong Kong waters, can spawn throughout the year but the number of recruits drops during summer when hypoxia commonly occurs. This study attempted to explain such observation by investigating the gamete quality, including sperm motility, egg size, complexity and viability, after 1-h hypoxic exposure (1 mg O2 l(-1)). In addition, how gamete quality affects fertilization success and embryonic development was examined. After 1-h hypoxic exposure, sperm motility was significantly reduced, compromising fertilization success. Although the eggs remained viable, more malformed embryos and retarded embryonic development were observed. We interpreted that the harmful effect of hypoxia on embryonic development was attributed to the teratogenicity and induced oxidative stress, ultimately causing the reduction in recruitment during summer.

Maternal inheritance of twist and analysis of MAPK activation in embryos of the polychaete annelid Platynereis dumerilii

In this study, we aimed to identify molecular mechanisms involved in the specification of the 4d (mesentoblast) lineage in Platynereis dumerilii. We employ RT-PCR and in situ hybridization against the Platynereis dumerilii twist homolog (Pdu-twist) to reveal mesodermal specification within this lineage. We show that Pdu-twist mRNA is already maternally distributed. After fertilization, ooplasmatic segregation leads to relocation of Pdu-twist transcripts into the somatoblast (2d) lineage and 4d, indicating that the maternal component of Pdu-twist might be an important prerequisite for further mesoderm specification but does not represent a defining characteristic of the mesentoblast. However, after the primordial germ cells have separated from the 4d lineage, zygotic transcription of Pdu-twist is exclusively observed in the myogenic progenitors, suggesting that mesodermal specification occurs after the 4d stage. Previous studies on spiral cleaving embryos revealed a spatio-temporal correlation between the 4d lineage and the activity of an embryonic organizer that is capable to induce the developmental fates of certain micromeres. This has raised the question if specification of the 4d lineage could be connected to the organizer activity. Therefore, we aimed to reveal the existence of such a proposed conserved organizer in Platynereis employing antibody staining against dpERK. In contrast to former observations in other spiralian embryos, activation of MAPK signaling during 2d and 4d formation cannot be detected which questions the existence of a conserved connection between organizer function and specification of the 4d lineage. However, our experiments unveil robust MAPK activation in the prospective nephroblasts as well as in the macromeres and some micromeres at the blastopore in gastrulating embryos. Inhibition of MAPK activation leads to larvae with a shortened body axis, defects in trunk muscle spreading and improper nervous system condensation, indicating a critical function for MAPK signaling for the reorganization of embryonic tissues during the gastrulation process.

Effect of parental hypoxic exposure on embryonic development of the offspring of two serpulid polychaetes: Implication for transgenerational epigenetic effect

Sperm production and motility, fecundity, and egg size, complexity and viability of serpulid polychaetes Hydroides elegans and Hydroides diramphus after 2-week treatment to hypoxia (2 mg O2 l(-1)) was compared with those under normoxia (6 mg O2 l(-1)). Despite reduced fecundity, the effect of parental hypoxic exposure on gamete quality was not discernible for both species. However, regardless of their subsequent dissolved oxygen environment, eggs spawned by H. elegans after hypoxic exposure were found to have lower fertilization success, slower embryonic development and a significantly higher yield of malformed embryos than those with a parental normoxic treatment. In contrast, neither fertilization success nor rate of embryonic development was affected for H. diramphus. The results implied that hypoxia was a potential stress reducing the recruitment of H. elegans through non-adaptive epigenetic effect, whereas H. diramphus was a more tolerant species to survive hypoxic events.

[Spawning and early ontogenesis of the littoral polychaete Namanereis littoralis (Grube, 1876) (Nereididae, Namanereidinae)]

For the first time, under laboratory conditions, development of the polychaete Namanereis littoralis (Grube, 1876) is investigated. Under conditions of the Sea of Japan, its reproduction occurs in July and is confined to the season of monsoon rains. Fertilization is external. Spawning manifests no epitocous transformations. Fecundity is low, ovicells are rich in yolk, and development is nonpelagic, lecithotrophic, embryonized, characterized by a high rate--5-8 days--and occurs in mucous clutches up to hatching of benthic juveniles. Temperature and salinity optima of development are 22-27 degrees C and 16-21 per thousand, respectively, characterizing the species as subtropical brackish-water by its origin. Archaic and specialized traits are noted in the early ontogenesis of N. littoralis.

[The golden age of comparative morphology: laser scanning microscopy and neurogenesis of trochophore animals]

Immunochemical labeling of neuronal elements and laser confocal microscopy have considerably expanded the capacity of comparative morphology and allowed us to monitor the neurogenesis of various trochophore animals at the level of individual identified neurons and their projections. It has been demonstrated that many generally accepted concepts of the larval nervous system and the phylogenetic theories constructed on this basis are incorrect. Comparative analysis has demonstrated that the orthogonal brain is absent at all developmental stages in the representative Lophotrochozoa members. Fundamental differences in the structure and development of the nervous system have been found in the trochophores belonging to different taxonomic groups within Lophotrochozoa; these differences demonstrate that the trochophore larva in these groups are not homologous, while their similarity is most likely a result of convergence. Our results challenge the concept of trochophore as the ancestral form common for all trochophore animals. It is necessary to exclude from phylogenetic discussions the orthogon as a basic plan for the structure of the nervous system and the trochophore as an ancestral form for all Lophtrochozoa.

Neurogenesis suggests independent evolution of opercula in serpulid polychaetes

BACKGROUND

The internal phylogenetic relationships of Annelida, one of the key lophotrochozoan lineages, are still heavily debated. Recent molecular analyses suggest that morphologically distinct groups, such as the polychaetes, are paraphyletic assemblages, thus questioning the homology of a number of polychaete morphological characters. Serpulid polychaetes are typically recognized by having fused anterior ends bearing a tentacular crown and an operculum. The latter is commonly viewed as a modified tentacle (= radiole) and is often used as an important diagnostic character in serpulid systematics.

RESULTS

By reconstructing the developmental neuroanatomy of the serpulid polychaete Spirorbis cf. spirorbis (Spirorbinae), we found striking differences in the overall neural architecture, the innervation pattern, and the ontogenetic establishment of the nervous supply of the operculum and the radioles in this species. Accordingly, the spirorbin operculum might not be homologous to the radioles or to the opercula of other serpulid taxa such as Serpula and Pomatoceros and is thus probably not a part of the tentacular crown.

CONCLUSION

We demonstrate that common morphological traits such as the prostomial appendages may have evolved independently in respective serpulid sublineages and therefore require reassessment before being used in phylogenetic analyses. Our findings corroborate recent molecular studies that argue for a revision of serpulid systematics. In addition, our data on Spirorbis neurogenesis provide a novel set of characters that highlight the developmental plasticity of the segmented annelid nervous system.

Brefeldin A and monensin inhibit the D quadrant organizer in the polychaete annelids Arctonoe vittata and Serpula columbiana

The D quadrant organizer is a developmental signaling center that is localized to the vegetal D quadrant in different spiral-cleaving lophotrochozoan embryos and may be homologous to axial organizing regions in other metazoans. Patterning by this organizing center creates a secondary developmental axis and is required for the transition from spiral to bilateral cleavage and later establishment of the adult body plan. Organizer specification in equal-cleaving embryos is thought to involve inductive interactions between opposing animal and vegetal blastomeres. To date, experimental demonstration of this interaction has been limited to molluscs and nemerteans. Here, we examine three families of equal-cleaving polychaete annelids for evidence of animal-vegetal contact. We find that contact is present in the polynoid, Arctonoe vittata, but is absent in the serpulid, Serpula columbiana, and in the oweniid, Oweniia fusiformis. To interfere with cell signaling during the period predicted for organizer specification and patterning in A. vittata and S. columbiana, we use two general inhibitors of protein processing and secretion: Brefeldin A (BFA) and monensin. In A. vittata, we detail subsequent embryonic and larval adult development and show that treatment with either chemical results in radialization of the embryo and subsequent body plan. Radialized larvae differentiate many larval and adult structures despite the loss of bilateral symmetry but do so in either a radially symmetric or four-fold radially symmetric fashion. Our results suggest that the D quadrant organizer is functionally conserved in equal-cleaving polychaetes, but that details of its specification, induction, and patterning have diverged relative to other spiral-cleaving phyla.

beta-Catenin asymmetries after all animal/vegetal- oriented cell divisions in Platynereis dumerilii embryos mediate binary cell-fate specification

In response to Wnt signaling during animal development, beta-catenin accumulates in nuclei to mediate the transcriptional activation of target genes. Here, we show that a highly conserved beta-catenin in the annelid Platynereis dumerilii exhibits a reiterative, nearly universal embryonic pattern of nuclear accumulation remarkably similar to that observed in the nematode Caenorhabditis elegans. Platynereis exhibits beta-catenin sister-cell asymmetries after all cell divisions that occur along the animal/vegetal axis beginning early in embryogenesis, but not after two transverse divisions that establish bilateral symmetry in the trunk. Moreover, ectopic activation of nuclear beta-catenin accumulation in Platynereis causes animal-pole sister cells, which normally have low nuclear beta-catenin levels, to adopt the fate of their vegetal-pole sisters, which normally have high nuclear beta-catenin levels. The presence of reiterative and functionally important beta-catenin asymmetries in two distantly related animal phyla suggests an ancient metazoan origin of a beta-catenin-mediated binary cell-fate specification module.

A single origin of the central nervous system?

As Denes et al. (2007) reveal in this issue, the expression profile and roles of genes that pattern the nervous system in embryos of chordates and annelids are surprisingly similar. This extraordinary conservation suggests that the patterning mechanism has been inherited largely unchanged from the bilaterian common ancestor and that the central nervous system, although dorsal in fish and ventral in worms, is an ancient characteristic of animals.

Molecular architecture of annelid nerve cord supports common origin of nervous system centralization in bilateria

To elucidate the evolutionary origin of nervous system centralization, we investigated the molecular architecture of the trunk nervous system in the annelid Platynereis dumerilii. Annelids belong to Bilateria, an evolutionary lineage of bilateral animals that also includes vertebrates and insects. Comparing nervous system development in annelids to that of other bilaterians could provide valuable information about the common ancestor of all Bilateria. We find that the Platynereis neuroectoderm is subdivided into longitudinal progenitor domains by partially overlapping expression regions of nk and pax genes. These domains match corresponding domains in the vertebrate neural tube and give rise to conserved neural cell types. As in vertebrates, neural patterning genes are sensitive to Bmp signaling. Our data indicate that this mediolateral architecture was present in the last common bilaterian ancestor and thus support a common origin of nervous system centralization in Bilateria.

Histone H2A.Z expression in two indirectly developing marine invertebrates correlates with undifferentiated and multipotent cells

The embryos of indirect developers generate an intermediate larval stage that nourishes the proliferation of undifferentiated multipotent cell precursors in charge of postembryonic adult formation. Multipotency affects the regulation of many genes and seems to be mediated in part by chromatin modification. Chromatin transcriptional properties are regulated by histone modification and by incorporation of peculiar histone variants. The histone variant H2A.Z is associated with transcriptionally competent chromatin and silent genes primed for activation or permanent repression. However, despite the extensive mechanistic characterizations in unicellular eukaryotes, the essential role of the highly conserved H2A.Z variant during animal embryogenesis remains obscure. We show that the expression of H2A.Z in the larvae of two distant indirectly developing marine invertebrates, a polychaete and a sea urchin, remains high in all their embryonic and postembryonic developmentally competent cell precursors, and declines during their differentiation. In particular, the expression in undifferentiated multipotent adult precursors during feeding larval stages in both organisms provides unique insight about its general association with developmental potential. Our experiments confirm previous reports indicating that the expression of H2A.Z is proliferation (DNA synthesis) independent, in contrast with the DNA synthesis dependence of "mainstream" histones. We suggest that similar H2A.Z transcriptional functions previously identified in unicellular organisms also help to maintain an open chromatin state competent for transcriptional-regulatory transactions during metazoan development.

Characterization of twist and snail gene expression during mesoderm and nervous system development in the polychaete annelid Capitella sp. I

To investigate the evolutionary history of mesoderm in the bilaterian lineage, we are studying mesoderm development in the polychaete annelid, Capitella sp. I, a representative lophotrochozoan. In this study, we focus on the Twist and Snail families as candidate mesodermal patterning genes and report the isolation and in situ expression patterns of two twist homologs (CapI-twt1 and CapI-twt2) and two snail homologs (CapI-sna1 and CapI-sna2) in Capitella sp. I. CapI-twt1 is expressed in a subset of mesoderm derivatives during larval development, while CapI-twt2 shows more general mesoderm expression at the same stages. Neither twist gene is detected before the completion of gastrulation. The two snail genes have very distinct expression patterns. At cleavage and early gastrula stages, CapI-sna1 is broadly expressed in precursors of all three germ layers and becomes restricted to cells around the closing blastopore during late gastrulation; CapI-sna2 expression is not detected at these stages. After gastrulation, both snail genes are expressed in the developing central nervous system (CNS) at stages when neural precursor cells are internalized, and CapI-sna1 is also expressed laterally within the segmental mesoderm. Based on the expression patterns in this study, we suggest a putative function for Capitella sp. I twist genes in mesoderm differentiation and for snail genes in regulating CNS development and general cell migration during gastrulation.

HeOtx expression in an indirectly developing polychaete correlates with gastrulation by invagination

The expression of an Otx homolog in the indirectly developing polychaete Hydroides elegans was characterized during embryo, trochophore, and feeding-larva stages. In the animal hemisphere, HeOtx is first expressed in 1q(12) blastomeres and their immediate descendants. Such discrete embryonic animal hemisphere Otx expression perhaps relates to cell-type specification functions of the larva. During feeding stages, transcripts are detected in adult cerebral ganglia precursors and putative adult eye precursors, where it may have adult brain regionalization functions. HeOtx is not expressed in primary trochoblast precursors, but it is expressed in cells adjacent to the ciliary band. HeOtx is also expressed in a group of cells in the dorsal midline of the early trochophore larva in putative posterior sensory organ precursors. The vegetal hemisphere expression starts in oral and lateral sides of the blastopore and later expands to central blastomeres that lead the gastrulation movements. During late gastrulation stages, the expression declines in foregut precursors, but it is maintained in midgut precursors, suggesting its involvement in tripartite gut subdivision functions. HeOtx broader and earlier endoderm expression correlates with gastrulation by invagination associated with the formation of the feeding trochophore, in contrast with a later and orally restricted Otx expression found in a polychaete that gastrulates by epiboly and forms a non-feeding trochophore. The endoderm expression and functional roles in other bilaterians suggest an ancestral role of Otx related to gastrulation by invagination.

Vasa unveils a common origin of germ cells and of somatic stem cells from the posterior growth zone in the polychaete Platynereis dumerilii

To elucidate the evolution of germ cell specification in Metazoa, recent comparative studies focus on ancestral animal groups. Here, we followed the germline throughout the life cycle of the polychaete annelid Platynereis dumerilii, by examining mRNA and protein expression of vasa and other germline-specific factors in combination with lineage tracing experiments. In the fertilised egg, maternal Vasa protein localises to the yolk-free cytoplasm at the animal pole. It then asymmetrically segregates first into the micromeres, then into the founder cells of the mesodermal posterior growth zone (MPGZ). Vasa transcripts initially show ubiquitous distribution, but then become progressively restricted to the MPGZ. The cells of the MPGZ are highly proliferative, as evidenced by BrdU pulse labelling experiments. Besides vasa, they express nanos along with the stem cell-specific genes piwi, and PL10. At 4 days of development, four primordial germ cells are singled out from within the MPGZ, and migrate into the anterior segments to colonise a newly discovered "primary gonad". Our data suggest a common origin of germ cells and of somatic stem cells, similar to the situation found in planarians and cnidarians, which may constitute the ancestral mode of germ cell specification in Metazoa.

Polychaete trunk neuroectoderm converges and extends by mediolateral cell intercalation

During frog and fish development, convergent extension movements transform the spherical gastrula into an elongated neurula. Such transformation of a ball- into a worm-shaped embryo is an ancestral and fundamental feature of bilaterian development, yet this is modified or absent in the protostome model organisms Caenorhabditis or Drosophila. In the polychaete annelid Platynereis dumerilii, early embryonic and larval stages resemble a sphere that subsequently elongates into worm shape. Cellular and molecular mechanisms of polychaete body elongation are yet unknown. Our in vivo time-lapse analysis of Platynereis axis elongation reveals that the polychaete neuroectoderm converges and extends by mediolateral cell intercalation. This occurs on both sides of the neural midline, the line of fusion of the slit-like blastopore. Convergent extension moves apart mouth and anus that are both derived from the blastopore. Tissue elongation is actin-dependent but microtubule-independent. Dependence on JNK activity and spatially restricted expression of strabismus indicates involvement of the noncanonical Wnt pathway. We detect a morphogenetic boundary between the converging and extending trunk neuroectoderm and the anterior otx-expressing head neuroectoderm that does not elongate. Our comparative analysis uncovers striking similarities but also differences between convergent extension in the polychaete and in the frog (the classical vertebrate model for convergent extension). Based on these findings, we propose that convergent extension movements of the trunk neuroectoderm represent an ancestral feature of bilaterian development that triggered the separation of mouth and anus along the elongating trunk.

Sinistral equal-size spiral cleavage of the indirectly developing polychaeteHydroides elegans

Two major variants of the stereotypic spiral cleavage correlate with distinct developmental modes in polychaetes. Indirect development through a feeding trochophore larva correlates with development from four equal-sized blastomeres, whereas direct development correlates with unequal cleavage characterized by a large dorsal blastomere precursor maternally predetermined. The equal-size spiral cleavage of the indirectly developing serpulid Hydroides elegans has been reconstructed from serial sections of nuclei-stained embryos. The order of cell divisions has been determined from the 2-cell stage to the 80-cell stage, when gastrulation cell movements start to overlap with late spiral-cleavage divisions. In contrast to related species, the third cleavage in Hydroides elegans is invariably sinistral. The four quadrants remain indistinct until the 60-cell stage, when the small 2d22 and large 2d21 cells are generated. The developmental significance of the invariant spiral cleavage relates to the spatial distribution of gene functions that it partitions and their relation to blastomere fate commitments. The conservation and divergence of the cleavage pattern among spiralians is well suited to study the developmental control of the cell-cleavage machinery and its evolution.

Hox gene expression in larval development of the polychaetes Nereis virens and Platynereis dumerilii (Annelida, Lophotrochozoa)

The bilaterian animals are divided into three great branches: the Deuterostomia, Ecdysozoa, and Lophotrochozoa. The evolution of developmental mechanisms is less studied in the Lophotrochozoa than in the other two clades. We have studied the expression of Hox genes during larval development of two lophotrochozoans, the polychaete annelids Nereis virens and Platynereis dumerilii. As reported previously, the Hox cluster of N. virens consists of at least 11 genes (de Rosa R, Grenier JK, Andreeva T, Cook CE, Adoutte A, Akam M, Carroll SB, Balavoine G, Nature, 399:772-776, 1999; Andreeva TF, Cook C, Korchagina NM, Akam M, Dondua AK, Ontogenez 32:225-233, 2001); we have also cloned nine Hox genes of P. dumerilii. Hox genes are mainly expressed in the descendants of the 2d blastomere, which form the integument of segments, ventral neural ganglia, pre-pygidial growth zone, and the pygidial lobe. Patterns of expression are similar for orthologous genes of both nereids. In Nereis, Hox2, and Hox3 are activated before the blastopore closure, while Hox1 and Hox4 are activated just after this. Hox5 and Post2 are first active during the metatrochophore stage, and Hox7, Lox4, and Lox2 at the late nectochaete stage only. During larval stages, Hox genes are expressed in staggered domains in the developing segments and pygidial lobe. The pattern of expression of Hox cluster genes suggests their involvement in the vectorial regionalization of the larval body along the antero-posterior axis. Hox gene expression in nereids conforms to the canonical patterns postulated for the two other evolutionary branches of the Bilateria, the Ecdysozoa and the Deuterostomia, thus supporting the evolutionary conservatism of the function of Hox genes in development.

Embryonic expression of HeFoxA1 and HeFoxA2 in an indirectly developing polychaete

Two forkhead family transcription factors, HeFoxA1 and HeFoxA2, were isolated from the serpulid annelid Hydroides elegans and their transcript distribution were characterized during embryogenesis. HeFoxA1 is first detected in second quartet blastomeres soon after their formation, and later in all vegetal half blastomeres, which comprise ectoderm, endoderm, and mesoderm precursors. HeFoxA1 expression declines first in subtrochal ectoderm and presumptive midgut precursors, as well as apparently in D quadrant blastomeres in advance of any known signaling events. Later, during gastrulation, HeFoxA1 declines in hindgut precursors, and by the end of gastrulation the expression remains active only in foregut precursors. HeFoxA1 is apparently expressed in ectomesoderm cells involved in forming the larva-specific protonephridium (the so-called head kidney). The other ortholog, HeFoxA2, is expressed in a subset of the cells in which HeFoxA1 is expressed during early stages, but later it is largely restricted to the endoderm-ectoderm boundary of the proctodaeum. In addition, HeFoxA2 has a unique expression in two hindgut cells and abutting ectoderm cells located by the imminent anal opening. The combined expression of HeFoxA1 and HeFoxA2 correlates with mesoderm and endoderm expression of their orthologs in other bilaterians.

Capitella sp. I homeobrain-like, the first lophotrochozoan member of a novel paired-like homeobox gene family

The paired-like class of homeobox genes contains numerous distinct families, many of which have been implicated in a variety of developmental functions. We report the isolation and expression of a gene with high similarity to Drosophila melanogaster homeobrain from the polychaete annelid Capitella sp. I. The homeobrain-like (hbnl) gene is a paired-like gene that contains a conserved homeodomain, octapeptide region, alanine stretches, and an OAR domain. Gene orthology analyses of the homeodomain from CapI-hbnl places this gene in a new family of paired-like homeodomain genes that includes D. melanogaster homeobrain (hbn) and representatives from all major bilaterian clades as well as a cnidarian gene. CapI-hbnl expression is largely restricted to subsets of cells in the brain and eyes during larval development in Capitella sp. I. The earliest expression of CapI-hbnl is in small discrete cell clusters in the cerebral ganglia. This expression persists through late larval developmental stages whereas expression is absent in postmetamorphic juveniles. Outside the brain, expression is present on the ventral side of the larva in two small cell clusters, at the brain/pharyngeal border and in the anterior-most segment. CapI-hbnl shares features of brain expression with hbn, although in contrast to hbn, which is expressed along the length of the ventral nerve cord, CapI-hbnl has a restricted anterior expression pattern. CapI-hbnl represents an important neural marker for characterization of the annelid nervous system.

[Development of the prototroch in embryogenesis of Nereis virens (Polychaeta)]

Prototroch formation was studied in the polychaete Nereis virens using light, scanning electron, and confocal laser microscopy. Cell lineage of trochoblasts was followed and chronology of their appearance was determined. The prototroch ciliary ring is formed by twelve descendants of micromere lm(2). The remaining four primary trochoblasts have no cilia and, together with descendants of accessory trochoblasts, become anterior supporting cells of the prototroch. Posterior supporting cells are formed by secondary trochoblasts, which are derived from the second micromere quartet 2m. The results obtained make it possible to analyze one of the ancient programs of animal development.

A discoidal lipoprotein from the coelomic fluid of the polychaete Nereis virens

A discoidal lipoprotein was isolated from the coelomic fluid of the polychaete, Nereis virens, by density gradient centrifugation. The lipoprotein was present in both sexes and moved as a uniform band in an agarose gel. The average diameter of the lipoprotein particles determined by electron microscopy was 42 nm with a thickness of 10 nm. SDS electrophoresis showed two apoprotein subunits with molecular masses of 247 and 85 kDa, respectively. In lectin blots, both apoproteins were reactive with Concanavalin A indicating the presence of N-glycans. The small subunit was also reactive with peanut lectin, indicating additional O-glycosylation. The total lipid content was 48% and consisted mainly of phospholipids and some diglycerides as judged by thin layer chromatography. The estimated native molecular mass of N. virens lipoprotein ( approximately 675 kDa) lies in the range of vertebrate high-density lipoprotein and insect lipophorins. The size of the apoproteins is similar to those found in insects, while the composition of the lipid fraction is more similar to that of crustacean lipoproteins.

caudal and even-skipped in the annelid Platynereis dumerilii and the ancestry of posterior growth

In order to address the question of the conservation of posterior growth mechanisms in bilaterians, we have studied the expression patterns of the orthologues of the genes caudal, even-skipped, and brachyury in the annelid Platynereis dumerilii. Annelids belong to the still poorly studied third large branch of the bilaterians, the lophotrochozoans, and have anatomic and developmental characteristics, such as a segmented body plan, indirect development through a microscopic ciliated larva, and building of the trunk through posterior addition, which are all hypothesized by some authors (including us) to be present already in Urbilateria, the last common ancestor of bilaterians. All three genes are shown to be likely involved in the building of the anteroposterior axis around the slit-like amphistomous blastopore as well as in the patterning of the terminal anus-bearing piece of the body (the pygidium). In addition, caudal and even-skipped are likely involved in the posterior addition of segments. Together with the emerging results on the conservation of segmentation genes, these results reinforce the hypothesis that Urbilateria had a segmented trunk developing through posterior addition.

Growth patterns during segmentation in the two polychaete annelids, Capitella sp. I and Hydroides elegans: comparisons at distinct life history stages

Many animals generate new body segments sequentially from a posterior growth zone, and this is generally thought to be the case for the annelids. Most annelids, including polychaetes, have an indirect life cycle and generate their earliest segments during larval life. We have characterized the nature of the growth zone in two polychaetes, Hydroides elegans and Capitella sp. I, during both larval and juvenile stages of segment formation by examining cell division patterns with 5-bromo-2'-deoxyuridine incorporation. Cell division patterns show commonalities between the two species, even though they have distinct body plans and life history characteristics. In both polychaetes, larval segments arise from a field of dividing cells located in lateral regions of the body, rather than from a localized posterior growth zone. Circumferential expansion of the forming segmental tissue is particularly pronounced in Capitella sp. I. Post-metamorphic segments, in contrast, originate from a classical posterior growth zone, with the exception of four posterior thoracic segments of H. elegans, which appear to arise from an area in the middle of the body, indicating plasticity of segment-generating mechanisms present in different annelid life histories. The distinct nature of larval versus juvenile growth zones in H. elegans and Capitella sp. I raises the question of the mechanistic relationship between these two growth zones. The results of this study increase our understanding of the cellular origins of segments in annelids, and serve as a basis for interpretation of molecular expression patterns associated with segment formation in polychaetes.

Influence of environmental conditions on early development of the hydrothermal vent polychaete Alvinella pompejana

Dispersal and colonisation processes at deep-sea vents are still not fully understood, essentially because early life stages of vent species remain unknown. The polychaete worm Alvinella pompejana forms colonies on chimney walls at East Pacific Rise vent sites where the temperature can frequently exceed 20°C. In vitro studies in pressure vessels showed that the early embryos tolerate temperatures in a lower range(10–14°C), suggesting that they would have to escape the colony to develop. Pressure vessels offer the advantage that each parameter can be independently controlled, but they do not simulate the more complex and dynamic conditions naturally encountered at vent sites. Accordingly, in addition to incubations in pressure vessels, we incubated embryos directly at a vent site, in different habitats along a gradient of hydrothermal influence. Embryos incubated on an adult A. pompejana colony where temperature and H2S concentrations were relatively high showed a very low survival rate and did not develop, whereas embryos incubated in a Riftia pachyptila clump environment with a lower hydrothermal signature, or at the base of the chimney where the influence of the hydrothermal activity was very weak, survived well and developed. Although the average temperature recorded in the A. pompejana colony was within the range tolerated by embryos (13°C), frequent peaks above 20°C were recorded. Estimated sulphide concentration at this site reached 200 μmol l–1. Punctuated exposure to both high temperature and elevated sulphide levels probably explain the low survival of embryos within the A. pompejanacolony. The in situ experiments further support the idea that embryos require conditions with moderate hydrothermal influence not generally found within an adult colony. However, as much more benign physicochemical conditions can be found within a few tens of cm of adult colonies, embryos do not necessarily have to leave their vent of origin to develop. Further analyses are needed to pinpoint the specific factors that affect the survival and development of embryos at vents.

Clonal domains in postlarvalPlatynereis dumerilii (Annelida: Polychaeta)

Following an enzymatic procedure for softening the egg envelope, blastomeres in the embryo of the polychaete Platynereis dumerilii were injected with TRITC-dextran. Injection was successful in the following blastomeres: AB, CD, A, B, C, D, 1a-1d, 1A-1D, 4d, and 4d(1). The distribution of fluorescent label was recorded by confocal laser scanning microscopy of young, three-segmented worms after 3 or 4 days of development, in some cases also in 1-day-old trochophore larvae. Results were documented by single optical sections, by stacking a limited number or a complete set of optical sections, and by computer-generated surface views of both the labeled tissue domains and the body contours from complete image stacks of whole worms. With respect to their descent from the embryonic cell pattern, five major compartments can be distinguished which together compose the body of the young worm: 1) The epispheric, epidermal, and neural region of the head, composed of four domains arranged as quasi-radial sectors derived from micromeres 1a, 1b (left and right ventral), and 1c and 1d (right and left dorsal). 2) A posttrochal epidermal region of the head originating from micromeres 2a(1)-2c(1) and constituting the ventral and lateral posttrochal epidermis of the head. 3) A stomodeal-ectomesodermal region of the head, including the stomodeum (micromeres 2a(2) and 2c(2)), its mesodermal envelope, and head mesoderm (micromeres 3a-3d). 4) A solid cone composed of the four terminal macromeres 4A-4D, forming the core of the trunk as the endoderm anlage. 5) An epidermal and mesodermal coating of the trunk originating from the dorsal micromeres 2d and 4d. The region of the so-called (first, anterior) peristomial cirri at the posterior flanks of the head is also composed of 2d- and 4d-derived trunk tissue, thus corroborating the postulated descent of this region and its appendages from a cephalized anteriormost trunk segment and its parapodia. The cell-lineage domains of the first and third micromere tiers are arranged left or right of the sagittal plane, while two micromeres of the second quartet are in a lateral and, initially, two in a median position (2b ventral and 2d dorsal). The offspring of micromere 2d expand from a dorsal position toward the ventral midline and those of cell 4d from a posterior-dorsal site toward the anterior, initially forming two lateral bands. In the epispheric part of the head, part of the neurectodermal tissue derived from micromeres 1a and 1b interweaves in a medio-sagittal bar, and part of the first micromere offspring of all four quadrants (1a-1d) combine in forming a central brain neuropil. Each of the latter sends neurites through both of the circumesophageal connectives. Paired muscle tracts extend through the head toward the base of the antennae and are probably derived from micromeres 3a and 3b. A mesodermal envelope of the stomodeum is probably built by the 3c and 3d micromeres. The formation of symmetry and the nature of the body axes in the embryo and adult of Platynereis dumerilii are discussed. J. Morphol.

Trochophora larvae: cell-lineages, ciliary bands, and body regions. 1. Annelida and Mollusca

The trochophora concept and the literature on cleavage patterns and differentiation of ectodermal structures in annelids ("polychaetes") and molluscs are reviewed. The early development shows some variation within both phyla, and the cephalopods have a highly modified development. Nevertheless, there are conspicuous similarities between the early development of the two phyla, related to the highly conserved spiral cleavage pattern. Apical and cerebral ganglia have almost identical origin in the two phyla, and the cell-lineage of the prototroch is identical, except for minor variations between species. The cell-lineage of the metatrochs is almost unknown, but the telotroch of annelids and the "telotroch" of the gastropod Patella originate from the 2d-cell, as does the gastrotroch in the few species which have been studied. The segmented annelid body, i.e. the region behind the peristome, develops through addition of new ectoderm from a ring of 2d-cells just in front of the telotroch. This whole region is thus derived from 2d-cells. Conversely, the mollusc body is covered by descendants of cells from both the C and D quadrants and a growth zone is not apparent. This supports the notion that the molluscs are not segmented like the annelids, and that the repeated structures seen in polyplacophorans and monoplacophorans do not represent a segmentation homologous to that of the annelids.

Regulation of cleavage by protein kinase C inChaetopterus

We report that protein kinase C (PKC) plays a regulatory role in early cleavage in Chaetopterus eggs. Using Western blotting, we assayed the expression patterns of conventional PKCs (cPKC), novel PKCs (nPKC), and atypical PKCs (aPKC). During early development after fertilization, PKC protein levels varied independently by isoform. PKC protein expression during differentiation, without cleavage and after parthenogenetic activation, was very similar to that during normal development indicating that PKC gene expression does not require cellularization. Since PKC has been shown to regulate meiosis in this organism, we also assayed the membrane association of these isoforms as an indicator of their activation during meiosis and early cleavage. PKC-gamma transiently associated with membranes and therefore became activated before meiotic division and cleavage, whereas PKC-alpha and -beta transiently dissociated from membranes and therefore became inactivated at these times. Inhibition of these PKC isoforms by bisindolylmaleimide I had no effect on cleavage or early development to the trochophore larva, indicating that PKC-gamma activation is not essential for cleavage or early development. However, their persistent activation by thymeleatoxin blocked cleavage. The results indicate that the dissociation of PKC-alpha and/or -beta from the membrane fraction, and therefore their inactivation, is essential for normal cleavage. Elevated PKC activity is essential for nuclear envelope breakdown and spindle formation at meiosis I. By contrast, down-regulation of this activity is essential for cleavage after fertilization.

The polychaetePlatynereis dumerilii (Annelida): a laboratory animal with spiralian cleavage, lifelong segment proliferation and a mixed benthic/pelagic life cycle

Platynereis dumerilii, a marine polychaetous annelid with indirect development, can be continuously bred in the laboratory. Here, we describe its spectacular reproduction and development and address a number of open research problems. Oogenesis is easily studied because the oocytes grow while floating in the coelom. Unlike the embryos of other model spiralians, the Platynereis embryo is transparent giving insight into the dynamic structures and processes inside the cells that accompany the prevailing anisotropic cleavages. Functional studies on cell specification and differential gene expression in embryos, larvae, and later stages are underway. Lifelong proliferation of uniform trunk segments qualifies Platynereis as a model for the study of gene expression and of the functional circuitry of this process. Platynereis can also become a stepping stone in the comparison of segmentation between annelids and arthropods because it comes closer to the putative ancestral morphology and style of development than other model annelids.

The MAPK cascade in equally cleaving spiralian embryos

Spiralian development is shared by several protostome phyla and characterized by regularities in early cleavage, fate map, and larva. Experimental evidence from multiple spiralian species implicates cells in the D quadrant lineage as the organizer of future axial development of the embryo. However, the mechanisms by which the D quadrant is specified differ between species with equal and unequal spiral cleavage. Equally cleaving mollusc embryos establish the D quadrant via cell-cell interactions between the micromeres and macromeres at the 24- to 36-cell stage. In unequally cleaving embryos, the D quadrant is established at the 4-cell stage via asymmetries in the first 2 cell divisions. We have begun to explore the molecular mechanisms of D quadrant patterning in spiralians. Previously, we showed that, in the unequally cleaving embryo of the mollusc Ilyanassa obsoleta, the MAPK pathway is activated and functionally required in 3D and also in the micromeres known to require a signal from 3D. Here, we examine the role of MAPK signaling in 4 spiralians with equal cleavage. In 3 equally cleaving molluscs, the chiton Chaetopleura, the limpet Tectura, and the snail Lymnaea, the MAPK pathway is activated in the 3D cell but not in the overlying micromeres. In the equally cleaving embryo of the polychaete annelid Hydroides, MAPK activation was not detected in the 3D macromere but was observed in one of its daughter cells, 4d. In addition, inhibiting Tectura MAPK activation disrupts differentiation of 3D and cells induced by it, supporting a functional role for MAPK in axis specification in equally cleaving spiralians. Thus, MAPK signaling may have a conserved role in the D quadrant organizer cell 3D in molluscs. However, there have been at least 2 evolutionary changes in the activation of the MAPK pathway during spiralian evolution. MAPK function in the Ilyanassa micromeres is a recent cooption and, since the divergence of annelids and molluscs, there has been a shift in onset of MAPK activation between 3D and 4d. We propose that this latter shift correlates with a change in the timing of specification of the secondary embryonic axis.

Specificity of Cellular Expression of C. variopedatus Polychaete Innexin in the Developing Embryo: Evolutionary Aspects of Innexins? Heterogeneous Gene Structures

Innexins are a family of membrane proteins involved in the formation of gap junctions in invertebrates. They have been found to participate in several aspects of cell differentiation and in embryonic patterning through the formation of specific intercellular communication channels. We present here data showing that the recently identified innexin of the marine worm Chaetopterus variopedatus is expressed only in particular cells of the early stage, demonstrating cell specificity of innexin expression also in polychaete annelids. Phylogenetic analysis of all known innexins results in a phylogenetic tree clearly distinguishing insect, nematode, and other invertebrate innexins. Comparative analysis of proteins and known related genes shows that the apparent similarity of protein composition, overall structural organization, and specificity of cellular expression, typical of innexins of all studied organisms, correspond to highly heterogeneous gene structures even for genes that are in close contiguity on the same chromosome. A possible evolutionary motive producing this situation is discussed.

Transcription and translation inhibitors permit metamorphosis up to radiole formation in the serpulid polychaete Hydroides elegans haswell

Settlement and metamorphosis in most well-studied marine invertebrates are rapid processes, triggered by external cues. How this initial environmentally mediated response is transduced into morphogenetic events that culminate in the formation of a functional juvenile is still not well understood for any marine invertebrate. The response of larvae of the serpulid polychaete Hydroides elegans to inhibitors of mRNA and protein synthesis was examined to determine if metamorphosis requires these molecular processes. Competent larvae of H. elegans were induced to metamorphose by exposing them to a bacterial film or a 3-h pulse of 10 mM CsCl in the presence of the gene-transcription inhibitor DRB (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) or the translation inhibitor emetine. When induced to metamorphose in the presence of either inhibitor, larvae of H. elegans progressed through metamorphosis to the point at which branchial radioles start to develop. DRB and emetine inhibited the incorporation of radiolabeled uridine into RNA and radiolabeled methionine into peptides, respectively, indicating that they were effective in blocking the appropriate syntheses. Taken together, these results indicate that the induction of metamorphosis in H. elegans does not require de novo transcription or translation, and that the form of the juvenile worm is achieved in two phases. During the first phase, larvae respond to the inducer by attaching to the substratum, secreting a primary tube, resorbing the prototroch cilia, undergoing caudal elongation, and differentiating the collar; once the collar is formed, they begin secreting the secondary, calcified tube. During the second phase, the small worm develops branchial radioles and begins to grow, requiring new mRNA and protein syntheses.

Inhibition of embryonic development and fertilization in broadcast spawning marine invertebrates by water soluble diatom extracts and the diatom toxin 2-trans,4-trans decadienal

Water soluble diatom extracts and the diatom aldehyde 2-trans,4-trans decadienal were assayed on the gametes and embryos of the broadcast spawning polychaetes Arenicola marina and Nereis virens and the echinoderms Asterias rubens and Psammechinus miliaris. Both crude cellular extracts and purified aldehyde were found to inhibit fertilization, embryogenesis and hatching success in a dose dependent manner. Intact diatom cells had no discernable effect on fertilization or development. Extracts of Skeletonema costatum were generally more effective than Nitzschia commutata in inhibiting development and fertilization. There was considerable interspecific variation in terms of toxin sensitivity. The polychaetes were more sensitive to the effects than the echinoderms. Within the polychaetes A. marina was the more tolerant in terms of developmental competence but N. virens had a higher fertilization rate. Echinoid embryos were more tolerant than asteroid embryos. This is the first study to present data on the inhibition of fertilization success by diatom extracts and aldehydes. Our observations are discussed in relation to temporal patterns in spawning and possible adaptive mechanisms to avoid diatom toxicity.

Numerical chromosomal aberrations in the early life-history stages of a marine tubeworm, Pomatoceros lamarckii (Polychaeta: Serpulidae)

The marine environment provides a sink for a host of toxic chemicals, directly or inadvertently, released as a result of human activity. Some of these chemicals have the potential to act as aneugens, substances that cause numerical chromosomal aberrations (NCAs). NCAs are one of the most important classes of genetic abnormality and are implicated in a variety of deleterious effects, including premature ageing, birth defects and cancer. Clearly, any increase in the incidence of these agents in the marine environment poses a risk to the indigenous biota and its predators, including man. In this paper, we describe our recent success with applying the fluorescence in situ hybridisation technique (FISH) to detect NCAs in the interphase cell nuclei of Pomatoceros lamarckii, a common rocky shore invertebrate. Given the lack of requirement for any detailed cytogenetic knowledge, the method holds considerable promise for laboratory and field studies in general, and should lend itself to automated screening protocols, where large numbers of cells can be screened rapidly, for example, using a flow cytometer. When exposed either under acute or chronic (viz. adult) exposure conditions, colchicine and di-butylphthalate (DBP) (a widely-used plasticiser), two recognised aneugens, induced significant increases in the levels of NCAs, in the dose range 1 x 10(-6)-5 x 10(-6) M, in both four to eight cell embryo stages and 24 h-old larvae. In keeping with the severely debilitating effects of this class of agent, an inverse correlation was observed between the induced levels of NCAs and larval fitness based on the results of a standard 48-h larval bioassay.

Genotoxic, cytotoxic and ontogenetic effects of tri-n-butyltin on the marine worm, Platynereis dumerilii (Polychaeta: Nereidae)

The genotoxic, cytotoxic and ontogenetic (embryo-larval) or developmental effects of tri-n-butyltin (TBT), were investigated in Platynereis dumerilii. Following the determination of maximum tolerated dose with regard to ontogenetic effects and mortality, early life stages of P. dumerilii were exposed to a range of TBT concentrations. Subsequently, the embryo-larvae were analysed for evidence of genotoxicity and cytotoxicity. Genotoxicity was assessed using cytogenetic endpoints that included the frequency of sister chromatid exchanges and chromosomal aberrations from metaphase spreads. Cytotoxicity was evaluated by determining the proliferative rate index of the growing embryo-larval cells using 5-bromodeoxyuridine labelling of the chromosomes or fluorescence plus Giemsa staining technique. TBT-exposed embryo-larvae of P. dumerilii exhibited sensitivity similar to that of other invertebrates, indicating that P. dumerilii is a suitable ecotoxicity test species. The results also suggested dose-dependent effects for genotoxic and cytotoxic end points in relation to TBT exposure. The present study highlights the need to elucidate the relative importance of direct genotoxic and indirect effects through production of genotoxic hormonal derivatives.

Cloning and molecular characterization of the first innexin of the phylum annelida-expression of the gene during development

A novel member of the innexin family (cv-inx) has been isolated from the annelid polychaete worm Chaetopterus variopedatus using a PCR approach on genomic DNA and sequence analysis on genomic DNA clones. The gene is present in a HindIII-HindIII segment of 2250 bp containing an uninterrupted open reading frame of 1196 bp encoding a protein of 399 amino acids. The predicted protein shows the typical structural features of innexins and consensus sites for phosphorylation. Analyses on genomic DNA demonstrate that cv-inx is a single copy gene with no introns in the coding region, exactly corresponding to the cDNA sequence. The gene expression is regulated during development as shown by Northern blots analyses of the RNA and by immunoreaction with antibodies against the protein at several embryonic stages. The finding of an innexin in the phylum Annelida, outside of the Ecdysozoa clade, and its peculiar gene structure suggest the necessity to reconsider the current hypothesis on the origin and evolution of gap junctional proteins.

Deep-sea ecology. Developmental arrest in vent worm embryos

Temperature is a key factor in controlling the distribution of marine organisms and is particularly important at hydrothermal vents, where steep thermal gradients are present over a scale of centimetres. The thermophilic worm Alvinella pompejana, which is found at the vents of the East Pacific Rise (2,500-m depth), has an unusually broad thermotolerance (20-80 degrees C) as an adult, but we show here that the temperature range required by the developing embryo is very different from that tolerated by adults. Our results indicate that early embryos may disperse through cold abyssal water in a state of developmental arrest, completing their development only when they encounter water that is warm enough for their growth and survival.

Phylogenetic analyses of mode of larval development

Phylogenies based on morphological or molecular characters have been used to provide an evolutionary context for analysis of larval evolution. Studies of gastropods, bivalves, tunicates, sea stars, sea urchins, and polychaetes have revealed massive parallel evolution of similar larval forms. Some of these studies were designed to test, and have rejected, the species selection hypothesis for evolutionary trends in the frequency of derived larvae or life history traits. However, the lack of well supported models of larval character evolution leave some doubt about the quality of inferences of larval evolution from phylogenies of living taxa. Better models based on maximum likelihood methods and known prior probabilities of larval character state changes will improve our understanding of the history of larval evolution.

Genotoxic, cytotoxic and developmental effects of tributyltin oxide (TBTO): an integrated approach to the evaluation of the relative sensitivities of two marine species

Adopting an integrated approach the potential genotoxic, cytotoxic and developmental effects of tributyltin oxide (TBTO), a known endocrine-disrupting agent for neogastropods, have been evaluated in two ecologically relevant invertebrates: Mytilus edulis (blue mussel) and Platynereis dumerilli (rag worm). Following determination of the maximum tolerated dose (MTD) in terms of developmental and survival effects, the embryo-larval stages of these organisms were exposed to a range of concentrations of TBTO, and analysed for cytotoxic (proliferation rate index) and genotoxic (sister chromatid exchanges and chromosomal aberrations) effects. The study suggested that: (1) TBTO is both toxic and genotoxic to embryo-larval stages of both species; (2) at comparable concentrations, for developmental and genotoxic effects, P. dumerilii (non-target species) is more sensitive compared to M. edulis (target species); and (3) genotoxic effects are more closely tied with the development and survival of the organisms. The study emphasises the need of the evaluation of genotoxic potential of other endocrine-disrupting agents in different taxonomic groups.