Cleavage patterns and the topology of the metazoan tree of life

Comparative Analysis of Bivalve and Sea Urchin Genetics and Development: Investigating the Dichotomy in Bilateria

This comprehensive review presents a comparative analysis of early embryogenesis in Protostomia and Deuterostomia, the first of which exhibit a mosaic pattern of development, where cells are fated deterministically, while Deuterostomia display a regulatory pattern of development, where the fate of cells is indeterminate. Despite these fundamental differences, there are common transcriptional mechanisms that underline their evolutionary linkages, particularly in the field of functional genomics. By elucidating both conserved and unique regulatory strategies, this review provides essential insights into the comparative embryology and developmental dynamics of these groups. The objective of this review is to clarify the shared and distinctive characteristics of transcriptional regulatory mechanisms. This will contribute to the extensive areas of functional genomics, evolutionary biology and developmental biology, and possibly lay the foundation for future research and discussion on this seminal topic.

The invertebrate chordate amphioxus gives clues to vertebrate origins

Amphioxus (cepholochordates) have long been used to infer how the vertebrates evolved from their invertebrate ancestors. However, some of the body part homologies between amphioxus and vertebrates have been controversial. This is not surprising as the amphioxus and vertebrate lineages separated half a billion years ago-plenty of time for independent loss and independent gain of features. The development of new techniques in the late 20th and early 21st centuries including transmission electron microscopy and serial blockface scanning electron microscopy in combination with in situ hybridization and immunocytochemistry to reveal spatio-temporal patterns of gene expression and gene products have greatly strengthened inference of some homologies (like those between regions of the central nervous system), although others (like nephridia) still need further support. These major advances in establishing homologies between amphioxus and vertebrates, together with strong support from comparative genomics, have firmly established amphioxus as a stand-in or model for the ancestral vertebrate.

Second-order division in sectors as a prepattern for sensory organs in vertebrate development

We present in vivo observations of chicken embryo development which show that the early chicken embryo presents a principal structure made out of concentric rings and a secondary structure composed of radial sectors. During development, physical forces deform the main rings into axially directed, antero-posterior tubes, while the sectors roll up to form cylinders that are perpendicular to the antero-posterior axis. As a consequence, the basic structure of the chicken embryo is a series of encased antero-posterior tubes (gut, neural tube, body envelope, amnion, chorion) decorated with smaller orifices (ear duct, eye stalk, nasal duct, gills, mouth) forming at right angles to the main body axis. We argue that the second-order divisions reflect the early pattern of cell cleavage, and that the transformation of radial and orthoradial lines into a body with sensory organs is a generic biophysical mechanism more general than the chicken embryo.

Simulations of sea urchin early development delineate the role of oriented cell division in the morula-to-blastula transition

The sea urchin morula to blastula transition has long been thought to require oriented cell divisions and blastomere adherence to the enveloping hyaline layer. In a computer simulation model, cell divisions constrained by a surface plane division rule are adequate to effect morphological transition. The hyaline membrane acts as an enhancer but is not essential. The model is consistent with the orientation of micromere divisions and the open blastulae of direct developing species. The surface plane division rule precedes overt epithelization of surface cells and acts to organize the developing epithelium. It is a universal feature of early metazoan development and simulations of non-echinoid cleavage patterns support its role throughout Metazoa. The surface plane division rule requires only local cues and cells need not reference global positional information or embryonic axes.

Differential methylation and expression of genes in the hypoxia-inducible factor 1 signaling pathway are associated with paclitaxel-induced peripheral neuropathy in breast cancer survivors and with preclinical models of chemotherapy-induced neuropathic pain

BACKGROUND

Paclitaxel is an important chemotherapeutic agent for the treatment of breast cancer. Paclitaxel-induced peripheral neuropathy (PIPN) is a major dose-limiting toxicity that can persist into survivorship. While not all survivors develop PIPN, for those who do, it has a substantial negative impact on their functional status and quality of life. No interventions are available to treat PIPN. In our previous studies, we identified that the HIF-1 signaling pathway (H1SP) was perturbed between breast cancer survivors with and without PIPN. Preclinical studies suggest that the H1SP is involved in the development of bortezomib-induced and diabetic peripheral neuropathy, and sciatic nerve injury. The purpose of this study was to identify H1SP genes that have both differential methylation and differential gene expression between breast cancer survivors with and without PIPN.

METHODS

A multi-staged integrated analysis was performed. In peripheral blood, methylation was assayed using microarray and gene expression was assayed using RNA-seq. Candidate genes in the H1SP having both differentially methylation and differential expression were identified between survivors who received paclitaxel and did (n = 25) and did not (n = 25) develop PIPN. Then, candidate genes were evaluated for differential methylation and differential expression in public data sets of preclinical models of PIPN and sciatic nerve injury.

RESULTS

Eight candidate genes were identified as both differential methylation and differential expression in survivors. Of the eight homologs identified, one was found to be differential expression in both PIPN and "normal" mice dorsal root ganglia; three were differential methylation in sciatic nerve injury versus sham rats in both pre-frontal cortex and T-cells; and two were differential methylation in sciatic nerve injury versus sham rats in the pre-frontal cortex.

CONCLUSIONS

This study is the first to evaluate for methylation in cancer survivors with chronic PIPN. The findings provide evidence that the expression of H1SP genes associated with chronic PIPN in cancer survivors may be regulated by epigenetic mechanisms and suggests genes for validation as potential therapeutic targets.

Electrical stimulation of developmental forces reveals the mechanism of limb formation in vertebrate embryos

Current knowledge on limbs development lacks a physical description of the forces leading to formation of the limbs precursors or "buds". Earlier stages of development are driven by large scale morphogenetic movements, such as dipolar vortical flows and mechanical buckling, pulled by rings of cells. It is a natural hypothesis that similar phenomena occur during limb formation. However it is difficult to experiment on the developmental forces, in such a complex dynamic system. Here, we report a physical study of hindlimb bud formation in the chicken embryo. We use electrical stimulation to enhance the physical forces present in the tissue, prior to limb bud formation. By triggering the physical forces in a rapid and amplified pattern, we reveal the mechanism of formation of the hindlimbs: the early presumptive embryonic territory is composed of a set of rings encased like Russian dolls. Each ring constricts in an excitable pattern of force, and the limb buds are generated by folding at a pre-existing boundary between two rings, forming the dorsal and ventral ectoderms. The amniotic sac buckles at another boundary. Physiologically, the actuator of the excitable force is the tail bud pushing posteriorly along the median axis. The developmental dynamics suggests how animals may evolve by modification of the magnitude of these forces, within a common broken symmetry. On a practical level, localized electrical stimulation of morphogenetic forces opens the way to in vivo electrical engineering of tissues.

Cleavage modification did not alter blastomere fates during bryozoan evolution

BACKGROUND

Stereotypic cleavage patterns play a crucial role in cell fate determination by precisely positioning early embryonic blastomeres. Although misplaced cell divisions can alter blastomere fates and cause embryonic defects, cleavage patterns have been modified several times during animal evolution. However, it remains unclear how evolutionary changes in cleavage impact the specification of blastomere fates. Here, we analyze the transition from spiral cleavage - a stereotypic pattern remarkably conserved in many protostomes - to a biradial cleavage pattern, which occurred during the evolution of bryozoans.

RESULTS

Using 3D-live imaging time-lapse microscopy (4D-microscopy), we characterize the cell lineage, MAPK signaling, and the expression of 16 developmental genes in the bryozoan Membranipora membranacea. We found that the molecular identity and the fates of early bryozoan blastomeres are similar to the putative homologous blastomeres in spiral-cleaving embryos.

CONCLUSIONS

Our work suggests that bryozoans have retained traits of spiral development, such as the early embryonic fate map, despite the evolution of a novel cleavage geometry. These findings provide additional support that stereotypic cleavage patterns can be modified during evolution without major changes to the molecular identity and fate of embryonic blastomeres.

Underwater flight by the planktonic sea butterfly

In a remarkable example of convergent evolution, we show that the zooplanktonic sea butterfly Limacina helicina 'flies' underwater in the same way that very small insects fly in the air. Both sea butterflies and flying insects stroke their wings in a characteristic figure-of-eight pattern to produce lift, and both generate extra lift by peeling their wings apart at the beginning of the power stroke (the well-known Weis-Fogh 'clap-and-fling' mechanism). It is highly surprising to find a zooplankter 'mimicking' insect flight as almost all zooplankton swim in this intermediate Reynolds number range (Re=10-100) by using their appendages as paddles rather than wings. The sea butterfly is also unique in that it accomplishes its insect-like figure-of-eight wing stroke by extreme rotation of its body (what we call 'hyper-pitching'), a paradigm that has implications for micro aerial vehicle (MAV) design. No other animal, to our knowledge, pitches to this extent under normal locomotion.

Re-evaluating the phylogeny of Sipuncula through transcriptomics

Sipunculans (also known as peanut worms) are an ancient group of exclusively marine worms with a global distribution and a fossil record that dates back to the Early Cambrian. The systematics of sipunculans, now considered a distinct subclade of Annelida, has been studied for decades using morphological and molecular characters, and has reached the limits of Sanger-based approaches. Here, we reevaluate their family-level phylogeny by comparative transcriptomic analysis of eight species representing all known families within Sipuncula. Two data matrices with alternative gene occupancy levels (large matrix with 675 genes and 62% missing data; reduced matrix with 141 genes and 23% missing data) were analysed using concatenation and gene-tree methods, yielding congruent results and resolving each internal node with maximum support. We thus corroborate prior phylogenetic work based on molecular data, resolve outstanding issues with respect to the familial relationships of Aspidosiphonidae, Antillesomatidae and Phascolosomatidae, and highlight the next area of focus for sipunculan systematics.

The molecular symplesiomorphies shared by the stem groups of metazoan evolution: can sites as few as 1% have a significant impact on recognizing the phylogenetic position of myzostomida?

Although it is clear that taxon sampling, alignments, gene sampling, tree reconstruction methods and the total length of the sequences used are critical to the reconstruction of evolutionary history, weakly supported or misleading nodes exist in phylogenetic studies with no obvious flaw in those aspects. The phylogenetic studies focusing on the basal part of bilaterian evolution are such a case. During the past decade, Myzostomida has appeared in the basal part of Bilateria in several phylogenetic studies of Metazoa. However, most researchers have entertained only two competing hypotheses about the position of Myzostomida-an affinity with Annelida and an affinity with Platyhelminthes. In this study, dozens of symplesiomorphies were discovered by means of ancestral state reconstruction in the complete 18S and 28S rDNAs shared by the stem groups of Metazoa. By contrastive analysis on the datasets with or without such symplesiomorphic sites, we discovered that Myzostomida and other basal groups are basal lineages of Bilateria due to the corresponding symplesiomorphies shared with earlier lineages. As such, symplesiomorphies account for approximately 1-2% of the whole dataset have an essential impact on phylogenetic inference, and this study reminds molecular systematists of the importance of carrying out ancestral state reconstruction at each site in sequence-based phylogenetic studies. In addition, reasons should be explored for the low support of the hypothesis that Myzostomida belongs to Annelida in the results of phylogenomic studies. Future phylogenetic studies concerning Myzostomida should include all of the basal lineages of Bilateria to avoid directly neglecting the stand-alone basal position of Myzostomida as a potential hypothesis.

Embryogenesis and larval biology of the cold-water coral Lophelia pertusa

Cold-water coral reefs form spectacular and highly diverse ecosystems in the deep sea but little is known about reproduction, and virtually nothing about the larval biology in these corals. This study is based on data from two locations of the North East Atlantic and documents the first observations of embryogenesis and larval development in Lophelia pertusa, the most common framework-building cold-water scleractinian. Embryos developed in a more or less organized radial cleavage pattern from ∼ 160 µm large neutral or negatively buoyant eggs, to 120-270 µm long ciliated planulae. Embryogenesis was slow with cleavage occurring at intervals of 6-8 hours up to the 64-cell stage. Genetically characterized larvae were sexually derived, with maternal and paternal alleles present. Larvae were active swimmers (0.5 mm s(-1)) initially residing in the upper part of the water column, with bottom probing behavior starting 3-5 weeks after fertilization. Nematocysts had developed by day 30, coinciding with peak bottom-probing behavior, and possibly an indication that larvae are fully competent to settle at this time. Planulae survived for eight weeks under laboratory conditions, and preliminary results indicate that these planulae are planktotrophic. The late onset of competency and larval longevity suggests a high dispersal potential. Understanding larval biology and behavior is of paramount importance for biophysical modeling of larval dispersal, which forms the basis for predictions of connectivity among populations.

Early cleavage in Phoronis muelleri (Phoronida) displays spiral features

The view that early cleavage in Phoronida follows a radial pattern is widely accepted. However, data supporting this characterization are ambiguous. Studies have been repeatedly reporting variation between individual embryos, and the occurrence of embryos exhibiting oblique divisions or nonradial cell arrangements. Such embryos were often considered to represent variation within radial cleavage, or artificial appearances. Cleavage in Phoronis muelleri was previously characterized as "derived radial," but also oblique spindles and cell elongations, and shifted cell arrangements were observed. We studied the early cleavage in P. muelleri applying 4D microscopy, fluorescent staining, and confocal laser scanning microscopy. To deal with the problem of variation we provide statistical evaluations of our data. These show that oblique divisions do not represent variational abnormalities. In fact, they reveal that most cells divide obliquely from the third cleavage onwards. What is more, in almost all cells the axis of the third cleavage is inclined dextrally. The fourth cleavage is even stronger sinistrally pronounced. Subsequently, the pattern of alternating cleavage orientation is largely restricted to animal and vegetal blastomeres. As a result of the obliqueness of divisions, four cells encircle the poles in most embryos. Cross furrows are occasionally present. We found no indications for radial cleavage in P. muelleri. In contrast, the observed cleavage displays several characters consistent with the pattern of spiral cleavage. A close relation of phoronid and spiralian cleavage is also suggested by molecular phylogenies, allying both groups in the Lophotrochozoa. We suggest our findings to represent morphological support for this lophotrochozoan/spiralian affinity of Phoronida.

The rhizome of life: what about metazoa?

The increase in huge number of genomic sequences in recent years has contributed to various genetic events such as horizontal gene transfer (HGT), gene duplication and hybridization of species. Among them HGT has played an important role in the genome evolution and was believed to occur only in Bacterial and Archaeal genomes. As a result, genomes were found to be chimeric and the evolution of life was represented in different forms such as forests, networks and species evolution was described more like a rhizome, rather than a tree. However, in the last few years, HGT has also been evidenced in other group such as metazoa (for example in root-knot nematodes, bdelloid rotifers and mammals). In addition to HGT, other genetic events such as transfer by retrotransposons and hybridization between more closely related lineages are also well established. Therefore, in the light of such genetic events, whether the evolution of metazoa exists in the form of a tree, network or rhizome is highly questionable and needs to be determined. In the current review, we will focus on the role of HGT, retrotransposons and hybridization in the metazoan evolution.

Spiral cleavage and early embryology of a loxosomatid entoproct and the usefulness of spiralian apical cross patterns for phylogenetic inferences

Background

Among the four major bilaterian clades, Deuterostomia, Acoelomorpha, Ecdysozoa, and Lophotrochozoa, the latter shows an astonishing diversity of bodyplans. While the largest lophotrochozoan assemblage, the Spiralia, which at least comprises Annelida, Mollusca, Entoprocta, Platyhelminthes, and Nemertea, show a spiral cleavage pattern, Ectoprocta, Brachiopoda and Phoronida (the Lophophorata) cleave radially. Despite a vast amount of recent molecular phylogenetic analyses, the interrelationships of lophotrochozoan phyla remain largely unresolved. Thereby, Entoprocta play a key role, because they have frequently been assigned to the Ectoprocta, despite their differently cleaving embryos. However, developmental data on entoprocts employing modern methods are virtually non-existent and the data available rely exclusively on sketch drawings, thus calling for thorough re-investigation.

Results

By applying fluorescence staining in combination with confocal microscopy and 3D-imaging techniques, we analyzed early embryonic development of a basal loxosomatid entoproct. We found that cleavage is asynchronous, equal, and spiral. An apical rosette, typical for most spiralian embryos, is formed. We also identified two cross-like cellular arrangements that bear similarities to both, a "molluscan-like" as well as an "annelid-like" cross, respectively.

Conclusions

A broad comparison of cleavage types and apical cross patterns across Lophotrochozoa shows high plasticity of these character sets and we therefore argue that these developmental traits should be treated and interpreted carefully when used for phylogenetic inferences.

A twist in time--the evolution of spiral cleavage in the light of animal phylogeny

Recent progress in reconstructing animal relationships enables us to draw a better picture of the evolution of important characters such as organ systems and developmental processes. By mapping these characters onto the phylogenetic framework, we can detect changes that have occurred in them during evolution. The spiral mode of development is a complex of characters that is present in many lineages, such as nemerteans, annelids, mollusks, and polyclad platyhelminthes. However, some of these lineages show variations of this general program in which sub-characters are modified without changing the overlying pattern. Recent molecular phylogenies suggest that spiral cleavage was lost, or at least has deviated from its original pattern, in more lineages than was previously thought (e.g., in rotifers, gastrotrichs, bryozoans, brachiopods, and phoronids). Here, I summarize recent progress in reconstructing the spiralian tree of life and discuss its significance for our understanding of the spiral-cleavage character complex. I conclude that more detailed knowledge of the development of spiralian taxa is necessary to understand the mechanisms behind these changes, and to understand the evolutionary changes and adaptations of spiralian embryos.

Deciphering the fossil record of early bilaterian embryonic development in light of experimental taphonomy

Experimental analyses of decay in a tunicate deuterostome and three lophotrochozoans indicate that the controls on decay and preservation of embryos, identified previously based on echinoids, are more generally applicable. Four stages of decay are identified regardless of the environment of death and decay. Embryos decay rapidly in oxic and anoxic conditions, although the gross morphology of embryos is maintained for longer under anoxic conditions. Under anoxic reducing conditions, the gross morphology of the embryos is maintained for the longest period of time, compatible with the timescale required for bacterially mediated mineralization of soft tissues. All four stages of decay were encountered under all environmental conditions, matching the spectrum of preservational qualities encountered in all fossil embryo assemblages. The preservation potential of embryos of deuterostomes and lophotrochozoans is at odds with the lack of such embryos in the fossil record. Rather, the fossil record of embryos, as sparse as it is, is dominated by forms interpreted as ecdysozoans, cnidarians, and stem-metazoans. The dearth of deuterostome and lophotrochozoan embryos may be explained by the fact that ecdysozoans, at least, tend to deposit their eggs in the sediment rather than through broadcast spawning. However, fossil embryos remain very rare and the main controlling factor on their fossilization may be the unique conspiracy of environmental conditions at a couple of sites. The preponderance of fossilized embryos of direct developers should not be used in evidence against the existence of indirect development at this time in animal evolutionary history.

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.

Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons

Monoplacophorans are among the rarest members of the phylum Mollusca. Previously only known from fossils since the Cambrian, the first living monoplacophoran was discovered during the famous second Galathea deep-sea expedition. The anatomy of these molluscs shocked the zoological community for presenting serially repeated gills, nephridia, and eight sets of dorsoventral pedal retractor muscles. Seriality of organs in supposedly independent molluscan lineages, i.e., in chitons and the deep-sea living fossil monoplacophorans, was assumed to be a relic of ancestral molluscan segmentation and was commonly accepted to support a direct relationship with annelids. We were able to obtain one specimen of a monoplacophoran Antarctic deep-sea species for molecular study. The first molecular data on monoplacophorans, analyzed together with the largest data set of molluscs ever assembled, clearly illustrate that monoplacophorans and chitons form a clade. This "Serialia" concept may revolutionize molluscan systematics and may have important implications for metazoan evolution as it allows for new interpretations for primitive segmentation in molluscs.

Embryogenesis and larval differentiation in sponges

Having descended from the first multicellular animals on earth, sponges are a key group in which to seek innovations that form the basis of the metazoan body plan, but sponges themselves have a body plan that is extremely difficult to reconcile with that of other animals. Adult sponges lack overt anterior–posterior polarity and sensory organs, and whether they possess true tissues is even debated. Nevertheless, sexual reproduction occurs as in other metazoans, with the development of embryos through a structured series of cellular divisions and organized rearrangements of cellular material, using both mesenchymal and epithelial movements to form a multicellular embryo. In most cases, the embryo undergoes morphogenesis into a spatially organized larva that has several cell layers, anterior–posterior polarity, and sensory capabilities. Here we review original data on the mode of cleavage, timing of cellular differentiation, and the mechanisms involved in the organization of differentiated cells to form the highly structured sponge larva. Our ultimate goal is to develop interpretations of the phylogenetic importance of these data within the Porifera and among basal Metazoa.

The convoluted evolution of snail chirality

The direction that a snail (Mollusca: Gastropoda) coils, whether dextral (right-handed) or sinistral (left-handed), originates in early development but is most easily observed in the shell form of the adult. Here, we review recent progress in understanding snail chirality from genetic, developmental and ecological perspectives. In the few species that have been characterized, chirality is determined by a single genetic locus with delayed inheritance, which means that the genotype is expressed in the mother's offspring. Although research lags behind the studies of asymmetry in the mouse and nematode, attempts to isolate the loci involved in snail chirality have begun, with the final aim of understanding how the axis of left-right asymmetry is established. In nature, most snail taxa (>90%) are dextral, but sinistrality is known from mutant individuals, populations within dextral species, entirely sinistral species, genera and even families. Ordinarily, it is expected that strong frequency-dependent selection should act against the establishment of new chiral types because the chiral minority have difficulty finding a suitable mating partner (their genitalia are on the 'wrong' side). Mixed populations should therefore not persist. Intriguingly, however, a very few land snail species, notably the subgenus Amphidromus sensu stricto, not only appear to mate randomly between different chiral types, but also have a stable, within-population chiral dimorphism, which suggests the involvement of a balancing factor. At the other end of the spectrum, in many species, different chiral types are unable to mate and so could be reproductively isolated from one another. However, while empirical data, models and simulations have indicated that chiral reversal must sometimes occur, it is rarely likely to lead to so-called 'single-gene' speciation. Nevertheless, chiral reversal could still be a contributing factor to speciation (or to divergence after speciation) when reproductive character displacement is involved. Understanding the establishment of chirality, the preponderance of dextral species and the rare instances of stable dimorphism is an important target for future research. Since the genetics of chirality have been studied in only a few pulmonate species, we also urge that more taxa, especially those from the sea, should be investigated.

Evidence from Hox genes that bryozoans are lophotrochozoans

Bryozoans, or moss animals, are small colonial organisms that possess a suspension-feeding apparatus called a lophophore. Traditionally, this "phylum" has been grouped with brachiopods and phoronids because of the feeding structure. Available molecular and morphological data refute this notion of a monophyletic "Lophophorata." Alternative hypotheses place bryozoans either at the base of the Lophotrochozoa or basal to the Lophotrochozoa/Ecdysozoa split. Surprisingly, the only molecular data bearing on this issue are from the 18S nuclear ribosomal gene. Here we report the results of a Hox gene survey using degenerate polymerase chain reaction primers in a gymnolaemate bryozoan, Bugula turrita. Putative orthologs to both the Post2 and the Lox5 genes were found, suggesting that bryozoans are not a basal protostome group but closely allied to other lophotrochozoan taxa. We also found the first definitive evidence of two Deformed/Hox4 class genes in a nonvertebrate animal.

The eutardigrade Thulinia stephaniae has an indeterminate development and the potential to regulate early blastomere ablations

We present a detailed analysis of the cell lineage of the tardigrade Thulinia stephaniae with a 4D-microscopy system (3D time-lapse recording). The recording, of the entire development from embryogenesis until hatching, allowed us to analyze the fate of single descendants from early blastomeres up to germ layer formation and tissue development. The embryo undergoes an irregular indeterminate cleavage pattern without early fate restriction. During gastrulation, mesodermal and endodermal precursors, and a pair of primordial germ cells migrate through a blastopore at the prospective position of the mouth. Our results are not consistent with earlier descriptions of mesoderm formation by enterocoely in tardigrades. The mesoderm in Thulinia stephaniae originates from a variable number of blastomeres, which form mesodermal bands that later produce the serial somites. The nervous system is formed by neural progenitor cells, which delaminate from the neurogenic ectoderm. Early embryogenesis of Thulinia stephaniae is highly regulative, even after laser ablations of blastomeres at the two- and four-cell stages 'normal' juveniles are formed. This has never been observed before for a protostome. Germ cell specification occurs late during development between the sixth and seventh cell generation. Comparing the development of other protostomes with that of the Tardigrada, which occupy a basal position within the Arthropoda, suggests that an indeterminate cleavage and regulatory development is not only part of the ground pattern of the Arthropoda, but probably of the entire Ecdysozoa.

Evolution of development in the sea star genus Patiriella: clade-specific alterations in cleavage

Examination of early development in five species of the Patiriella sea star species complex indicates that the ancestral-type radial holoblastic cleavage (Type I) is characteristic of P. regularis and P. exigua, whereas cleavage in species from the calcar clade followed multiple alternatives (Types II-IV) from holoblastic to meroblastic. Considering that invariant radial cleavage is thought to play a role in embryonic axis formation in echinoderms, we documented the details of blastomere formation in Patiriella sp. and followed development of the embryos. In Type II cleavage, the first and second cleavage planes appeared simultaneously at one pole of the embryo, dividing it directly into four equally sized blastomeres. In Type III cleavage, the first and second cleavage planes appeared simultaneously, followed promptly by the third cleavage plane, dividing the embryo directly into eight equally sized blastomeres. In Type IV cleavage, numerous furrows appeared simultaneously at one end of the embryo, dividing it into 32-40 equally sized blastomeres. Confocal sections revealed that embryos with cleavage Types II-IV were initially syncytial. The timing of karyokinesis in embryos with Types II and III cleavage was similar to that seen in clutch mates with Type I cleavage. Karyokinesis in embryos with Type IV cleavage, however, differed in timing compared with Type I clutch mates. Alteration in cleavage was not associated with polarized distribution of maternally provided nutrients. For each cleavage type, development was normal to the competent larval stage. Although variable blastomere configuration in the calcar clade may be linked to possession of a lecithotrophic development, other Patiriella species with this mode of development have typical cleavage. The presence of variable cleavage in all calcar clade species indicates that phylogenetic history has played a role in the distribution of this embryonic trait in Patiriella. The plasticity in early cleavage in these sea stars indicates that this aspect of early development is not constrained against change and that there are many ways to achieve multicellularity.

Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome

Determining the phylogenetic position of enigmatic phyla such as Chaetognatha is a longstanding challenge for biologists. Chaetognaths (or arrow worms) are small, bilaterally symmetrical metazoans. In the past decades, their relationships within the metazoans have been strongly debated because of embryological and morphological features shared with the two main branches of Bilateria: the deuterostomes and protostomes. Despite recent attempts based on molecular data, the Chaetognatha affinities have not yet been convincingly defined. To answer this fundamental question, we determined the complete mitochondrial DNA genome of Spadella cephaloptera. We report three unique features: it is the smallest metazoan mitochondrial genome known and lacks both atp8 and atp6 and all tRNA genes. Furthermore phylogenetic reconstructions show that Chaetognatha belongs to protostomes. This implies that some embryological characters observed in chaetognaths, such as a gut with a mouth not arising from blastopore (deuterostomy) and a mesoderm derived from archenteron (enterocoely), could be ancestral features (plesiomorphies) of bilaterians.

Phyletic patterns of early development in gastropod molluscs

Cell lineage data for 30 exemplar gastropod taxa representing all major subclades and the outgroup Polyplacophora were examined for phylogenetic signal using cladistic analysis. Most cell lineages show phyletic trends of acceleration or retardation relative to the outgroup and more basal ingroup taxa, and when coded this variation is phylo-genetically informative. PAUP analyses of a cell lineage data set under three sets of character ordering assumptions produced similar tree topologies. The topologies of the strict consensus trees for both ordered and Dollo (near irreversibility of character transformations) character assumptions were similar, whereas the unordered character assumption recovers the least phyletic information. The cell lineage cladograms are also in agreement with the fossil record of the timing and sequence of gastropod subclade origination. A long branch lies between the Patellogastropoda+Vetigastropoda grade and the Neritopsina+Apogastropoda clade. The geological timing of this long branch is correlated with the first large-scale terrestrially derived eutrophication of the near-shore marine habitat, and one possible explanation for this branch may be a developmental shift associated with the evolution of feeding larvae in response to the more productive conditions in the near-shore water column. Although character transformations are highly ordered in this data set, developmental rate characters (like all other morphological and molecular characters) are also subject to homoplasy. Finally, this study further supports the hypothesis that early development of gastropod molluscs has conserved a strong phyletic signal for about half a billion years.

Hox gene survey in the chaetognath Spadella cephaloptera: evolutionary implications

We present the isolation of six Hox genes in the chaetognath Spadella cephaloptera. We identified one member of the paralogy group 3, four median genes and a mosaic gene that shares features of both median and posterior classes ( SceMedPost). Several hypotheses may account for the presence of a mosaic Hox gene in this animal. Here we propose that SceMedPost may represent an ancestral gene, which has not diverged totally into a posterior or a median one. This hypothesis has interesting implications for the reconstruction of the evolutionary history of Hox genes and suggests that Chaetognatha lineage divergence could predate the deuterostome/protostome split. Such a phylogenetic position is considered in the light of their embryological and morphological characters.

Molecules, development and fossils in the study of metazoan evolution; Articulata versus Ecdysozoa revisited

Two conflicting hypotheses of protostome relationships, Articulata and Ecdysozoa, are reviewed by evaluating the evidence in favor and against each one of them. Understanding early embryonic development and segmentation in non-arthropod non-annelid protostomes seems crucial to the debate. New ways of coding metazoan matrices, avoiding ground-patterns and higher taxa, and incorporating fossil evidence seems the best way to avoid circular debates. Molecular data served as the catalyzer for the Ecdysozoa hypothesis, although morphological support had been implicitly suggested. Most molecular analyses published so far have shown some support for Ecdysozoa, whereas none has ever supported Articulata. Here, new analyses of up to four nuclear loci, including 18S rRNA, myosin heavy chain II, histone H3 and elongation factor 1-alpha are conducted to test the molecular support for Ecdysozoa, and, at least under some parameter sets, most data sets show a clade formed by the molting animals. In contrast, support for Articulata is not found under any analytical conditions.

Evaluating hypotheses of deuterostome phylogeny and chordate evolution with new LSU and SSU ribosomal DNA data

We investigated evolutionary relationships among deuterostome subgroups by obtaining nearly complete large-subunit ribosomal RNA (LSU rRNA)-gene sequences for 14 deuterostomes and 3 protostomes and complete small-subunit (SSU) rRNA-gene sequences for five of these animals. With the addition of previously published sequences, we compared 28 taxa using three different data sets (LSU only, SSU only, and combined LSU + SSU) under minimum evolution (with LogDet distances), maximum likelihood, and maximum parsimony optimality criteria. Additionally, we analyzed the combined LSU + SSU sequences with spectral analysis of LogDet distances, a technique that measures the amount of support and conflict within the data for every possible grouping of taxa. Overall, we found that (1) the LSU genes produced a tree very similar to the SSU gene tree, (2) adding LSU to SSU sequences strengthened the bootstrap support for many groups above the SSU-only values (e.g., hemichordates plus echinoderms as Ambulacraria; lancelets as the sister group to vertebrates), (3) LSU sequences did not support SSU-based hypotheses of pterobranchs evolving from enteropneusts and thaliaceans evolving from ascidians, and (4) the combined LSU + SSU data are ambiguous about the monophyly of chordates. No tree-building algorithm united urochordates conclusively with other chordates, although spectral analysis did so, providing our only evidence for chordate monophyly. With spectral analysis, we also evaluated several major hypotheses of deuterostome phylogeny that were constructed from morphological, embryological, and paleontological evidence. Our rRNA-gene analysis refutes most of these hypotheses and thus advocates a rethinking of chordate and vertebrate origins.

Testing the new animal phylogeny: first use of combined large-subunit and small-subunit rRNA gene sequences to classify the protostomes

Although the small-subunit ribosomal RNA (SSU rRNA) gene is widely used in the molecular systematics, few large-subunit (LSU) rRNA gene sequences are known from protostome animals, and the value of the LSU gene for invertebrate systematics has not been explored. The goal of this study is to test whether combined LSU and SSU rRNA gene sequences support the division of protostomes into Ecdysozoa (molting forms) and Lophotrochozoa, as was proposed by Aguinaldo et al. (1997) (Nature 387:489) based on SSU rRNA sequences alone. Nearly complete LSU gene sequences were obtained, and combined LSU + SSU sequences were assembled, for 15 distantly related protostome taxa plus five deuterostome outgroups. When the aligned LSU + SSU sequences were analyzed by tree-building methods (minimum evolution analysis of LogDet-transformed distances, maximum likelihood, and maximum parsimony) and by spectral analysis of LogDet distances, both Ecdysozoa and Lophotrochozoa were indeed strongly supported (e.g., bootstrap values >90%), with higher support than from the SSU sequences alone. Furthermore, with the LogDet-based methods, the LSU + SSU sequences resolved some accepted subgroups within Ecdysozoa and Lophotrochozoa (e.g., the polychaete sequence grouped with the echiuran, and the annelid sequences grouped with the mollusc and lophophorates)-subgroups that SSU-based studies do not reveal. Also, the mollusc sequence grouped with the sequences from lophophorates (brachiopod and phoronid). Like SSU sequences, our LSU + SSU sequences contradict older hypotheses that grouped annelids with arthropods as Articulata, that said flatworms and nematodes were basal bilateralians, and considered lophophorates, nemerteans, and chaetognaths to be deuterostomes. The position of chaetognaths within protostomes remains uncertain: our chaetognath sequence associated with that of an onychophoran, but this was unstable and probably artifactual. Finally, the benefits of combining LSU with SSU sequences for phylogenetic analyses are discussed: LSU adds signal, it can be used at lower taxonomic levels, and its core region is easy to align across distant taxa-but its base frequencies tend to be nonstationary across such taxa. We conclude that molecular systematists should use combined LSU + SSU rRNA genes rather than SSU alone.

Heads or tails? Amphioxus and the evolution of anterior-posterior patterning in deuterostomes

In Xenopus, the canonical Wnt-signaling pathway acting through beta-catenin functions both in establishing the dorso-ventral axis and in patterning the anterior-posterior axis. This pathway also acts in patterning the animal-vegetal axis in sea urchins. However, because sea urchin development is typically indirect, and adult sea urchins have pentamerous symmetry and lack a longitudinal nerve cord, it has not been clear how the roles of the canonical Wnt-signaling pathway in axial patterning in sea urchins and vertebrates are evolutionarily related. The developmental expression patterns of Notch, brachyury, caudal, and eight Wnt genes have now been determined for the invertebrate chordate Amphioxus, which, like sea urchins, has an early embryo that gastrulates by invagination, but like vertebrates, has a later embryo with a dorsal hollow nerve cord that elongates posteriorly from a tail bud. Comparisons of Amphioxus with other deuterostomes suggest that patterning of the ancestral deuterostome embryo along its anterior-posterior axis during the late blastula and subsequent stages involved a posterior signaling center including Wnts, Notch, and transcription factors such as brachyury and caudal. In tunicate embryos, in which cell numbers are reduced and cell fates largely determined during cleavage stages, only vestiges of this signaling center are still apparent; these include localization of Wnt-5 mRNA to the posterior cytoplasm shortly after fertilization and localization of beta-catenin to vegetal nuclei during cleavage stages. Neither in tunicates nor in Amphioxus is there any evidence that the canonical Wnt-signaling pathway functions in establishment of the dorso-ventral axis. Thus, roles for Wnt-signaling in dorso-ventral patterning of embryos may be a vertebrate innovation that arose in connection with the evolution of yolky eggs and gastrulation by extensive involution.

Molecular studies of hemichordate development: a key to understanding the evolution of bilateral animals and chordates

Using the Hawaiian acorn worm, Ptychodera flava, we began molecular studies on the development of hemichordates, a phylum previously unstudied at this level. Here we review results garnered from the examination of a few specific genes selected to help understand the evolution of vertebrate structures. These studies suggest new ideas about the evolution of developmental mechanisms in the deuterostomes. In a seminal observation, we noted an unexpected zone of expression of the Brachyurygene in the early anterior embryonic ectoderm where the mouth will form. Typically, the Brachyury gene is closely linked to development of the notochord and is expressed around the blastopore and in the posterior mesoderm in most animals. This first expression of Brachyury at the blastopore may represent a regulatory program associated with organizing the original animal head and gut opening, as suggested by the expression of Brachyury during hypostome formation in hydra. We believe that the anterior expression of Brachyury in deuterostomes represents the cooption of the program for organizing the original animal gut opening to form the deuterostome mouth. Recent data from the trochophore larva of a polychaete show that an anterior zone of expression of Brachyury is produced in this protostome by splitting of the Brachyury field during the formation of a gut with a mouth and anus by the lateral fusion of the sides of the blastopore. The ability to initiate independently a secondary regulatory program to organize the new mouth leading to an anterior field of Brachyury expression may be a signal event in the evolution of the deuterostomes. We also noted that the P. flava homolog of T-brain/Eomes, a gene closely related by sequence and expression around the blastopore to Brachyury and associated with development of the vertebrate brain, also exhibits early posterior expression around the blastopore and a field of de novo anterior ectoderm expression during later embryogenesis. The tissue in the zone of de novo anterior ectoderm expression of Pf-Tbrain produces the apical organ, a larval neural structure that has been touted as an evolutionary precursor of the chordate dorsal brain. The gene regulatory mechanisms responsible for initiating the anterior zone of de novo expression of T-brain may represent a cooption to specify early neuroectoderm of the regulatory program evolved first to drive anterior Brachyury expression for deuterostome mouth formation. It will be interesting to examine the possibilities that an ability to initiate the de novo anterior expression of the program that includes T-brain may be a key event in the evolution of the developmental mechanisms leading to the chordate dorsal nervous system.

Evaluating hypotheses of basal animal phylogeny using complete sequences of large and small subunit rRNA

We studied the evolutionary relationships among basal metazoan lineages by using complete large subunit (LSU) and small subunit (SSU) ribosomal RNA sequences for 23 taxa. After identifying competing hypotheses, we performed maximum likelihood searches for trees conforming to each hypothesis. Kishino-Hasegawa tests were used to determine whether the data (LSU, SSU, and combined) reject any of the competing hypotheses. We also conducted unconstrained tree searches, compared the resulting topologies, and calculated bootstrap indices. Shimodaira-Hasegawa tests were applied to determine whether the data reject any of the topologies resulting from the constrained and unconstrained tree searches. LSU, SSU, and the combined data strongly contradict two assertions pertaining to sponge phylogeny. Hexactinellid sponges are not likely to be the basal lineage of a monophyletic Porifera or the sister group to all other animals. Instead, Hexactinellida and Demospongia form a well-supported clade of siliceous sponges, Silicea. It remains unclear, on the basis of these data alone, whether the calcarean sponges are more closely related to Silicea or to nonsponge animals. The SSU and combined data reject the hypothesis that Bilateria is more closely related to Ctenophora than it is to Cnidaria, whereas LSU data alone do not refute either hypothesis. LSU and SSU data agree in supporting the monophyly of Bilateria, Cnidaria, Ctenophora, and Metazoa. LSU sequence data reveal phylogenetic structure in a data set with limited taxon sampling. Continued accumulation of LSU sequences should increase our understanding of animal phylogeny.

Reconnecting cell and animal lineages: what do cell lineages tell us about the evolution and development of Spiralia?

Cell lineage studies in the clade Eutrochozoa, and especially the Spiralia, remains a rich and relatively untapped source for understanding broad evolutionary developmental problems; including (1) the utility of cell timing formation for phylogenetic hypotheses; (2) the evolution of cell timing changes and its relation to heterochronic patterns; (3) stereotypy or lack thereof in rates of change of cell growth during evolution and its relation to both evolutionary history and current usage; and (4) how mosaic cleavage timing variation may be expected to differ from other groups. A compilation of available cell timing information was made from previous studies where each division was explicitly followed and the total number of cells followed was greater than 24. From that compilation, we performed a series of heuristic and quantitative analyses, including a phylogenetic analysis using cell timing data as characters and analyses of timing variation across all taxa. Our results show that: (1) cell lineage data reconstructs a phylogenetic hypothesis that has similarities, especially among the Mollusca. to the patterns found in morphological and molecular analyses; (2) the mesentoblast (4d) is a unique cell compared to other cell in that it speeds up and slows down relative to other cells in taxa with both unequal and equal cell sizes; (3) some cells that form in the same quartet at the same point in the cell lineage hierarchy have much lower variations than analogous other cells, arguing for architectural constraint or stabilizing selection acting on those cells; and (4) although variation in cell timing generally increases during development, timing of formation of progeny cells in the first quartet has lower variation than the parent cells, arguing that some regulation-like behavior might be present.

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.

Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences

Insight into the origin and early evolution of the animal phyla requires an understanding of how animal groups are related to one another. Thus, we set out to explore animal phylogeny by analyzing with maximum parsimony 138 morphological characters from 40 metazoan groups, and 304 18S rDNA sequences, both separately and together. Both types of data agree that arthropods are not closely related to annelids: the former group with nematodes and other molting animals (Ecdysozoa), and the latter group with molluscs and other taxa with spiral cleavage. Furthermore, neither brachiopods nor chaetognaths group with deuterostomes; brachiopods are allied with the molluscs and annelids (Lophotrochozoa), whereas chaetognaths are allied with the ecdysozoans. The major discordance between the two types of data concerns the rooting of the bilaterians, and the bilaterian sister-taxon. Morphology suggests that the root is between deuterostomes and protostomes, with ctenophores the bilaterian sister-group, whereas 18S rDNA suggests that the root is within the Lophotrochozoa with acoel flatworms and gnathostomulids as basal bilaterians, and with cnidarians the bilaterian sister-group. We suggest that this basal position of acoels and gnathostomulids is artifactal because for 1,000 replicate phylogenetic analyses with one random sequence as outgroup, the majority root with an acoel flatworm or gnathostomulid as the basal ingroup lineage. When these problematic taxa are eliminated from the matrix, the combined analysis suggests that the root lies between the deuterostomes and protostomes, and Ctenophora is the bilaterian sister-group. We suggest that because chaetognaths and lophophorates, taxa traditionally allied with deuterostomes, occupy basal positions within their respective protostomian clades, deuterostomy most likely represents a suite of characters plesiomorphic for bilaterians.

Micrognathozoa: a new class with complicated jaws like those of Rotifera and Gnathostomulida

A new microscopic aschelminth-like animal, Limnognathia maerski nov. gen. et sp., is described from a cold spring at Disko Island, West Greenland, and assigned to Micrognathozoa nov. class. It has a complex of jaws in its pharynx, and the ultrastructure of the main jaws is similar to that of the jaws of advanced scleroperalian gnathostomulids. However, other jaw elements appear also to have characteristics of the trophi of Rotifera. Jaw-like structures are found in other protostome taxa as well-for instance, in proboscises of kalyptorhynch platyhelminths, in dorvilleid polychaetes and aplacophoran mollusks-but studies of their ultrastructure show that none of these jaws is homologous with jaws found in Gnathostomulida, Rotifera, and Micrognathozoa. The latter three groups have recently been joined into the monophylum Gnathifera Ahlrichs, 1995, an interpretation supported by the presence of jaw elements with cuticular rods with osmiophilic cores in all three groups. Such tubular structures are found in the fulcrum of all Rotifera and in several cuticular sclerites of both Gnathostomulida and Micrognathozoa. The gross morphology of the pharyngeal apparatus is similar in the three groups. It consists of a ventral pharyngeal bulb and a dorsal pharyngeal lumen. The absence of pharyngeal ciliation cannot be used as an autapomorphy in the ground pattern of the Gnathifera because the Micrognathozoa has the plesiomorphic alternative with a ciliated pharyngeal epithelium. The body of Limnognathia maerski nov. gen. et sp. consists of a head, thorax, and abdomen. The dorsal and lateral epidermis have plates formed by an intracellular matrix, as in Rotifera and Acanthocephala; however, the epidermis is not syncytial. The ventral epidermis lacks internal plates, but has a cuticular oral plate without ciliary structures. Two ventral rows of multiciliated cells form a locomotory organ. These ciliated cells resemble the ciliophores present in some interstitial annelids. An adhesive ciliated pad is located ventrally close to a caudal plate. As in many marine interstitial animals-e.g., gnathostomulids, gastrotrichs, and polychaetes-a special form of tactile bristles or sensoria is found on the body. Two pairs of protonephridia with unicellular terminal cells are found in the trunk; this unicellular condition may be the plesiomorphic condition in Bilateria. Only specimens with the female reproductive system have been found, indicating that all adult animals are parthenogenetic females. We suggest that 1) jaws of Gnathostomulida, Rotifera, and the new taxon, Micrognathozoa, are homologous structures; 2) Rotifera (including Acanthocephala) and the new group might be sister groups, while Gnathostomulida could be the sister-group to this assemblage; and 3) the similarities to certain gastrotrichs and interstitial polychaetes are convergent.

Micrognathozoa: a new class with complicated jaws like those of Rotifera and Gnathostomulida

A new microscopic aschelminth-like animal, Limnognathia maerski nov. gen. et sp., is described from a cold spring at Disko Island, West Greenland, and assigned to Micrognathozoa nov. class. It has a complex of jaws in its pharynx, and the ultrastructure of the main jaws is similar to that of the jaws of advanced scleroperalian gnathostomulids. However, other jaw elements appear also to have characteristics of the trophi of Rotifera. Jaw-like structures are found in other protostome taxa as well-for instance, in proboscises of kalyptorhynch platyhelminths, in dorvilleid polychaetes and aplacophoran mollusks-but studies of their ultrastructure show that none of these jaws is homologous with jaws found in Gnathostomulida, Rotifera, and Micrognathozoa. The latter three groups have recently been joined into the monophylum Gnathifera Ahlrichs, 1995, an interpretation supported by the presence of jaw elements with cuticular rods with osmiophilic cores in all three groups. Such tubular structures are found in the fulcrum of all Rotifera and in several cuticular sclerites of both Gnathostomulida and Micrognathozoa. The gross morphology of the pharyngeal apparatus is similar in the three groups. It consists of a ventral pharyngeal bulb and a dorsal pharyngeal lumen. The absence of pharyngeal ciliation cannot be used as an autapomorphy in the ground pattern of the Gnathifera because the Micrognathozoa has the plesiomorphic alternative with a ciliated pharyngeal epithelium. The body of Limnognathia maerski nov. gen. et sp. consists of a head, thorax, and abdomen. The dorsal and lateral epidermis have plates formed by an intracellular matrix, as in Rotifera and Acanthocephala; however, the epidermis is not syncytial. The ventral epidermis lacks internal plates, but has a cuticular oral plate without ciliary structures. Two ventral rows of multiciliated cells form a locomotory organ. These ciliated cells resemble the ciliophores present in some interstitial annelids. An adhesive ciliated pad is located ventrally close to a caudal plate. As in many marine interstitial animals-e.g., gnathostomulids, gastrotrichs, and polychaetes-a special form of tactile bristles or sensoria is found on the body. Two pairs of protonephridia with unicellular terminal cells are found in the trunk; this unicellular condition may be the plesiomorphic condition in Bilateria. Only specimens with the female reproductive system have been found, indicating that all adult animals are parthenogenetic females. We suggest that 1) jaws of Gnathostomulida, Rotifera, and the new taxon, Micrognathozoa, are homologous structures; 2) Rotifera (including Acanthocephala) and the new group might be sister groups, while Gnathostomulida could be the sister-group to this assemblage; and 3) the similarities to certain gastrotrichs and interstitial polychaetes are convergent.

"Alien" wasps and evolution of development

A comparative analysis of early developmental programs in a group of parasitic wasps reveals that closely related species can undergo dramatic evolutionary shifts in their patterns of embryogenesis. Developmental changes detected include alterations in early cleavage divisions, the establishment of embryonic anteroposterior polarity and modifications of the segmentation gene hierarchy described from Drosophila. These changes appear to be adaptations to parasitic development, taking place within the body of the host. Wasps illustrate a surprising plasticity in their early development and embryogenesis. The alterations associated with different parasitic strategies suggest that ecological adaptations may have profound influences on developmental processes in animals.

The epitome of hand waving? Larval feeding and hypotheses of metazoan phylogeny

The homology of larval forms, and particularly their feeding methods, has been a major element in some recent discussions about animal phylogeny. "Downstream feeding" is one of two main larval-feeding modes and is usually equated to an opposed-band system with ciliary bands called the prototroch and metatroch. Feeding in larvae is reviewed here and the homology hypothesis of downstream larval feeding is expanded, encompassing any feeding involving the prototroch. It is often argued that the presence of planktotrophic larvae using downstream feeding is plesiomorphic among spiralian animals, and that there is a bias in transformations, such that feeding larvae tend to be lost rather than gained. These hypotheses are assessed using cladistic parsimony methodology, in relation to Spiralia, Trochozoa, and with particular reference to polychaete annelids. Methods adopted for the possibility of a bias in transformations toward loss of downstream larval feeding include: expanded primary homology arguments, character reconstructions favoring reversals, and polymorphic terminals coded as having downstream larval feeding. Nevertheless, all analyses show that downstream larval feeding appears to have evolved multiple times from a lecithotrophic condition. The results support a conclusion that the prototroch was primarily locomotory, and has become associated with feeding a number of times. Hypotheses of metazoan phylogeny predicated on the assumption that downstream-feeding larvae are plesiomorphic are re-assessed.