Echinoderm Evolution Research Articles

Neural development of the brittlestar Amphiura filiformis

Comparative features of the development of the larval nervous system of ophiuroids have the potential for resolving aspects of echinoderm evolution. In Amphiura filiformis serotonergic neural progenitors appear in the animal plate of late gastrulae. The serotonergic progenitors increase in number and become displaced to the aboral ectoderm side of the developing ciliary band. The ciliary band neurons appear as irregularly spaced neural progenitors on the oral side of the ciliary band lateral to the mouth. These cells extend neurites along the axis of the ciliary band, which meet at the center of the ventral transverse ciliary band. The larval nervous system begins as a U-shaped tract of axons that surrounds the oral field and tracts of axons and neurons in the ciliary bands of the larval arms are added. In addition, the larval nervous system has an extensive pre-oral neuropil, rings of nerves surrounding the anus and pyloric sphincters, and a plexus of axons that surround the esophagus. The nervous system of the juvenile develops beneath the oral ectoderm. The components of the adult nervous system: five segments of radial nerve, commissures that form the nerve ring, and podial nerves all appear as the juvenile develops. The larval nervous system begins to fragment and degenerate as the juvenile grows. The complete description of neural development of an ophiuroid reveals that the four classes so far investigated are consistent with phylogenies based on adult features and comparisons of neural organization help rationalize conflicting hypotheses of the evolution of larval forms in echinoderms.

Ciona intestinalis and Oxycomanthus japonicus, Representatives of Marine Invertebrates

The study of marine invertebrates is useful in various biological research fields. However, genetic analyses of these animals are limited, mainly due to difficulties in culturing them, and the genetic resources of marine invertebrates have not been organized. Recently, advances have been made in the study of two deuterostomes, an ascidian Ciona intestinalis and a feather star Oxycomanthus japonicus. The draft genome sequence of Ciona intestinalis has been determined, and its compact genome, which has less redundancy of genes compared with vertebrates, provides us with a useful experimental system for analyzing the functions of genes during development. The life cycle of Ciona intestinalis is approximately 2-3 months, and the genetic techniques including a perfect inland culture system, germline transformation with a transposon Minos, enhancer detection and insertional mutagenesis, have been established. The feather star Oxycomanthus japonicus conserves the characteristics of the basic echinoderm body plan with a segmented mesoderm, which is a fascinating characteristic for understanding the evolution of echinoderms. Oxycomanthus japonicus shows strong regeneration ability and is a suitable subject for analysis of the mechanisms of regeneration. In consideration of these features, the National BioResource Project (NBRP) has started to support the supply of wild-types, transgenic lines and inbred lines of Ciona intestinalis and Oxycomanthus japonicus.

Lessons from a gene regulatory network: echinoderm skeletogenesis provides insights into evolution, plasticity and morphogenesis

Significant new insights have emerged from the analysis of a gene regulatory network (GRN) that underlies the development of the endoskeleton of the sea urchin embryo. Comparative studies have revealed ways in which this GRN has been modified (and conserved) during echinoderm evolution, and point to mechanisms associated with the evolution of a new cell lineage. The skeletogenic GRN has also recently been used to study the long-standing problem of developmental plasticity. Other recent findings have linked this transcriptional GRN to morphoregulatory proteins that control skeletal anatomy. These new studies highlight powerful new ways in which GRNs can be used to dissect development and the evolution of morphogenesis.

The pre‐radial history of echinoderms

AbstractGene sequence data now identify a robust phylogeny of deuterostomes and provide a framework within which the evolution of echinoderms can be interpreted. The topology of the molecular tree makes a number of important predictions about the morphological characters of the earliest echinoderm at its split from hemichordates: it possessed gill slits (but not a notochord), had a bilaterally symmetrical body plan in the adult and, less certainly, underwent torsion during development. Carpoids, a highly contentious group of extinct deuterostomes with a plated calcite skeleton that have variously been interpreted as stem‐ and crown‐group chordates, stem‐group echinoderms or stem‐ and crown‐group echinoderms, display many of these basal characters and provide critical evidence for how the latest common ancestor of hemichordates and echinoderms was transformed into a pentaradiate crown‐group echinoderm.Cinctans have a large atrial opening in addition to mouth and anus, and are interpreted as pharyngeal basket feeders. The paired grooves associated with the mouth indicate the presence of a hydrovascular system, but not necessarily one built along the echinoderm plan (that is, derived from just the left hydrocoel). Stylophorans have a bilateral body plan that is externally masked by torsion and possess gills, either unpaired and external, or paired and internal, opening into an atrial cavity. Their bilateral appendage is a locomotory organ, not an ambulacrum, and there is no evidence that stylophorans ever possessed a well‐developed hydrovascular system homologous to the water vascular system of echinoderms and the tentacles of pterobranch hemichordates. Solutes are the most crownward, having a true echinodermal ambulacral system with a single hydropore and no pharyngeal gill openings. Copyright © 2005 John Wiley & Sons, Ltd.

Developmental gene regulatory network architecture across 500 million years of echinoderm evolution.

Evolutionary change in morphological features must depend on architectural reorganization of developmental gene regulatory networks (GRNs), just as true conservation of morphological features must imply retention of ancestral developmental GRN features. Key elements of the provisional GRN for embryonic endomesoderm development in the sea urchin are here compared with those operating in embryos of a distantly related echinoderm, a starfish. These animals diverged from their common ancestor 520-480 million years ago. Their endomesodermal fate maps are similar, except that sea urchins generate a skeletogenic cell lineage that produces a prominent skeleton lacking entirely in starfish larvae. A relevant set of regulatory genes was isolated from the starfish Asterina miniata, their expression patterns determined, and effects on the other genes of perturbing the expression of each were demonstrated. A three-gene feedback loop that is a fundamental feature of the sea urchin GRN for endoderm specification is found in almost identical form in the starfish: a detailed element of GRN architecture has been retained since the Cambrian Period in both echinoderm lineages. The significance of this retention is highlighted by the observation of numerous specific differences in the GRN connections as well. A regulatory gene used to drive skeletogenesis in the sea urchin is used entirely differently in the starfish, where it responds to endomesodermal inputs that do not affect it in the sea urchin embryo. Evolutionary changes in the GRNs since divergence are limited sharply to certain cis-regulatory elements, whereas others have persisted unaltered.

Evolution of echinoderms may not have required modification of the ancestral deuterostome HOX gene cluster: first report of PG4 and PG5 Hox orthologues in echinoderms.

Is the extreme derivation of the echinoderm body plan reflected in a derived echinoderm Hox genotype? Building on previous work, we exploited the sequence conservation of the homeobox to isolate putative orthologues of several Hox genes from two asteroid echinoderms. The 5-peptide motif (LPNTK) diagnostic of PG4 Hox genes was identified immediately downstream of one of the partial homeodomains from Patiriella exigua. This constitutes the first unequivocal report of a PG4 Hox gene orthologue from an echinoderm. Subsequent screenings identified genes of both PG4 and PG4/5 in Asterias rubens. Although in echinoids only a single gene (PG4/5) occupies these two contiguous cluster positions, we conclude that the ancestral echinoderm must have had the complete deuterostome suite of medial Hox genes, including orthologues of both PG4 and PG4/5 (=PG5). The reported absence of PG4 in the HOX cluster of echinoids is therefore a derived state, and the ancestral echinoderm probably had a HOX cluster not dissimilar to that of other deuterostomes. Modification of the ancestral deuterostome Hox genotype may not have been required for evolution of the highly derived echinoderm body plan.

Life history evolution and comparative developmental biology of echinoderms.

Evolutionary biologists studying life history variation have used echinoderms in experimental, laboratory, and field studies of life history evolution. This focus on echinoderms grew originally from the tradition of comparative embryology, in which echinoderms were central. The tools for obtaining and manipulating echinoderm gametes and larvae were taken directly from comparative embryological research. In addition, the comparative embryologists employed a diverse array of echinoderms, not a few model species, and this diversity has led to a broad understanding of the development, function, and evolution of echinoderm larvae. As a result, this branch of life history evolution has deep roots in comparative developmental biology of echinoderms. Here two main aspects of this relationship are reviewed. The first is a broad range of studies of fertilization biology, dispersal, population genetics, functional morphology, and asexual reproduction in which developmental biologists might take a keen interest because of the historical origins of this research in echinoderm comparative embryology. The second is a similarly broad variety of topics in life history research in which evolutionary biologists require techniques or data from developmental biology in order to make progress on understanding patterns of life history variation among echinoderm species and higher taxa. Both sets of topics provide opportunities for interaction and collaboration.

Developmental regulatory genes and echinoderm evolution.

Modified interactions among developmental regulatory genes and changes in their expression domains are likely to be an important part of the developmental basis for evolutionary changes in morphology. Although developmental regulatory genes are now being studied in an increasing number of taxa, there has been little attempt to analyze the resulting data within an explicit phylogenetic context. Here we present comparative analyses of expression data from regulatory genes in the phylum Echinodermata, considering the implications for understanding both echinoderm evolution as well as the evolution of regulatory genes in general. Reconstructing the independent evolutionary histories of regulatory genes, their expression domains, their developmental roles, and the structures in which they are expressed reveals a number of distinct evolutionary patterns. A few of these patterns correspond to interpretations common in the literature, whereas others have received little prior mention. Together, the analyses indicate that the evolution of echinoderms involved: (1) the appearance of many apomorphic developmental roles and expression domains, some of which have plesiomorphic bilateral symmetry and others of which have apomorphic radial symmetry or left-right asymmetry; (2) the loss of some developmental roles and expression domains thought to be plesiomorphic for Bilateria; and (3) the retention of some developmental roles thought to be plesiomorphic for Bilateria, although with modification in expression domains. Some of the modifications within the Echinodermata concern adult structures; others, transient larval structures. Some changes apparently appeared early in echinoderm evolution (> 450 Ma), whereas others probably happened more recently (< 50 Ma). Cases of likely convergence in expression domains suggest caution when using developmental regulatory genes to make inferences about homology among morphological structures of distantly related taxa.

The Origin and evolution of larval forms

Introduction: B.K. Hall and M.H. Wake, Larval Development, Evolution and Ecology. Larval Types and Larval Evolution: C. Hickman, Larvae in Invertebrate Development and Evolution. J. Hanken, Larvae in Amphibian Development and Evolution. J.F. Webb, Fish Larvae in Development and Evolution. Mechanisms of Larval Development and Evolution: M.W. Hart and G.A. Wray, Heterochrony. C. Rose, Hormonal Control of Larval Development and Evolution--Amphibians. H.F. Nijhout, Hormonal Control of Larval Development and Evolution--Insects. R.A. Raff, Cell Lineages in Larval Development and Evolution of Echinoderms. L. Nagy and M. Grbic, Cell Lineages in Larval Development and Evolution of Insects. Larval Functional Morphology, Physiology and Ecology: L. Sanderson and S. Kupferberg, Development and Evolution of Aquatic Larval Feeding Mechanisms. E. Greene, Phenotypic Variation in Larval Development and Evolution: Polymorphism, Polyphenism and Developmental Reaction Norms. Epilogue: M.H. Wake and B.K. Hall, Epilogue: Prospects for Research on the Origin and Evolution of Larval Forms. Subject Index.

Erratum: Radical alterations in the roles of homeobox genes during echinoderm evolution

Nature 389, 718–721 (1997) The labelling of Fig. 1 of this Letter was incomplete in several places. The corrected figure is reproduced below.

Radical alterations in the roles of homeobox genes during echinoderm evolution.

Echinoderms possess one of the most highly derived body architectures of all metazoan phyla, with radial symmetry, a calcitic endoskeleton, and a water vascular system. How these dramatic morphological changes evolved has been the subject of extensive speculation and debate, but remains unresolved. Because echinoderms are closely related to chordates and postdate the protostome/deuterostome divergence, they must have evolved from bilaterally symmetrical ancestors. Here we report the expression domains in echinoderms of three important developmental regulatory genes (distal-less, engrailed and orthodenticle), all of which encode transcription factors that contain a homeodomain. Our findings show that the reorganization of body architecture involved extensive changes in the deployment and roles of homeobox genes. These changes include modifications in the symmetry of expression domains and the evolution of several new developmental roles, as well as the loss of roles conserved between arthropods and chordates. Some of these modifications seem to have evolved very early in the history of echinoderms, whereas others probably evolved during the subsequent diversification of adult and larval morphology. These results demonstrate the evolutionary lability of regulatory genes that are widely viewed as conservative.

Echinoderm skeletal crystallography and paleobiological applications

The crystallographic orientations of echinoderm skeletal elements can supplement standard morphological comparisons in the exploration of echinoderm evolution. At a coarse scale, many echinoderms share a crystallographic pattern in whichcaxes radiate away from the axis of pentaradial symmetry. Within this common pattern, however,caxes of different taxa can differ dramatically in their degree of variability, angles of inclination, and relationships to the external morphology of skeletal elements. Crystallographic data reflect a variety of taxon-specific influences and therefore reveal different information in different taxa. In echinoids, orientations ofcaxes in coronal plates correlate well with high-level taxonomic groupings, whilecaxes of apical plates record modes of larval development. In blastoids,caxes of radial plates have a structural interpretation, with thecaxis oriented parallel to the orientation of the surface of the radial plate during its initial growth stages. In crinoids,caxes do not correlate with taxonomic group, plate morphology, or developmental sequence, but instead correlate with relative positions of skeletal elements on the calyx. Although their full potential has yet to be explored, the varied crystallographic patterns in echinoderms have been used to clarify skeletal structure, characterize developmental anomalies, and infer homologies of skeletal plates both within specimens and between groups. A axes are less constrained in their orientations thancaxes and offer less promise of revealing novel paleobiological information.

Phylogenetic relationships among extant classes of echinoderms, as inferred from sequences of 18S rDNA, coincide with relationships deduced from the fossil record.

In spite of the rich fossil record and multiple descriptions of morphological and embryological characteristics, the origin and subsequent evolution of echinoderms remain highly controversial issues. Using sequence data derived from 18S rDNA, we have investigated the phylogenetic relationships among five extant classes of echinoderms--namely, crinoids, asteroids, ophiuroids, echinoids, and holothurians. Almost complete sequences of 18S rDNA were determined for one species in each class, and phylogenetic trees were constructed both by the neighbor-joining method and by the maximum-likelihood method, with a hemichordate as an outgroup. The trees constructed by these methods support the hypothesis that the phylum Echinodermata can be subdivided into two subphyla, Pelmatozoa and Eleutherozoa. The class Holothuroidea, which has been the subject of debate with respect to whether the members are primitive or advanced echinoderms, did not occupy a primitive position but had an affinity for the class Echinoidea. Since both trees gave different branching topologies for the order of emergence of asteroids and ophiuroids, it seems likely that these two groups emerged within a very short period of time. A rough estimate of the timing of the divergence of the five classes from the present molecular analysis coincided with that deduced from the fossil record.

Gene arrangement in sea star mitochondrial DNA demonstrates a major inversion event during echinoderm evolution

The mitochondrial (mt) DNA from the sea star Pisaster ochraceus has been isolated, restriction-mapped, and cloned into plasmid vectors. Both ribosomal RNA genes, the genes for 12 of the 13 mitochondrial proteins, and 11 of the tRNA genes have been localized by DNA sequence analyses. The sequence arrangement of the genes is markedly different from that seen in sea urchin mitochondrial DNA. A segment of the DNA molecule extending from tRNA pro, including the tRNA cluster, ND1, ND2, and 16S genes, is inverted in relation to the sea urchin genome. The resulting gene order in the sea star is 12S, 16S, ND2, ND1, tRNA cluster, COI. As a result of the inversion, the transcriptional polarity of ND1, ND2, and 16S genes are opposite to that of the 12S and COI genes. The arrangement and transcriptional polarity of the other genes mapped here is the same as seen in urchin.

Organization and unusual expression of histone genes in the sea starPisaster ochraceus

The organization and maternal expression of histone genes differ greatly between the two echinoderm classes represented by the sea stars and sea urchins. We have isolated and characterized a 5.3-kb genomic DNA fragment encoding core histone genes in the sea starPisaster ochraceus. The majority of histone genes are organized as tandem repeats of the 5.3-kb fragment, which is present in as many as 700 copies per haploid genome. The identity, precise location, and transcriptional polarity of individual core histone genes within the repeat were determined by DNA sequence analyses. The gene order in the sea star (H2B, H2A, H4, H3) is different from that in sea urchins (H2B, H3, H2A, H4). What is remarkable is the low level of maternal histone mRNA inP. ochraceus eggs relative to that in sea urchins. This observation supports a recent suggestion that major changes in mode of gene expression, rather than in gene organization or copy number, can be correlated with major events in echinoderm evolution.

A comparative study of invertebrate glucose 6-phosphate dehydrogenases.

The enzymes having G6PD activity were isolated from 10 species belonging to 8 invertebrate phyla and their properties were examined in order to compare with those of vertebrate G6PD and H6PD. As a result it has been demonstrated that invertebrate G6PD has properties quite similar to vertebrate G6PD in the following respects; (1) The primary substrates are G6P and NADP; (2) Km's for G6P and NADP are in the majority of cases of the order of 10-5M and 10-6M, respectively; (3) Mg ions have no or only a slight stimulative effect; (4) The molecular weight of the monomer is about 55×103 and the enzyme exists as a dimer and/or a tetramer.No H6PD was found in the species studied.From these and other data it may be speculated that animal G6PD would not have undergone a marked change in the substrate specificity and in the molecular weight of the monomer during evolution and that echinoderms might be the only one among invertebrates that possesses H6PD. This suggests that if G6PD and H6PD might have a common ancestral molecule, the divergence might have occurred at the time of echinoderm evolution.

Hexose 6-phosphate dehydrogenase in starfishes

1. 1. Tissue extracts from 18 species of starfishes contain hexose 6-phosphate dehydrogenase (H6PD) which closely resembled vertebrate H6PD in the following respects: dependence on NADP and NAD, high activities on galactose 6-phosphate and 2-deoxyglucose 6-phosphate, formation of enzymatically active smaller molecules by tryptic digestion, resistance to SH inhibitors and inhibition by Mg and Ca ions. 2. 2. K m 's of partially purified H6PD of Asterias amurensis for glucose 6-phosphate, galactose 6-phosphate, 2-deoxyglucose 6-phosphate and glucose are 8 × 10 −6, 7.1 × 10 −5, 9.4 × 10 −4 and 1.4 M, respectively, being comparable with those of vertebrate H6PD. 3. 3. The present finding on starfish H6PD suggests that the progenitor of vertebrate H6PD might arise near the beginning of or before echinoderm evolution.

Sea Star Platasterias: Ossicle Morphology and Taxonomic Position

The modern sea star Platasterias latiradiata, on the basis of ossicle morphology, is removed from the fossil subclass Somasteroidea and assigned to the genus Luidia of the living subclass Asteroidea. A somasteroid assignment of this species should not be used in inferences concerning evolution and morphology of primitive echinoderms.

Unusual strength properties of echinoderm calcite related to structure

Echinoderm skeletons are composed of high-magnesium calcite having a fenestrate structure. Each skeletal ossicle appears to be a single crystal of calcium carbonate rather than a polycrystalline aggregate as, for example, are mollusc shells. Measurements of crushing strength for spines of three species of sea urchin demonstrate that the strength to weight ratio of these spines is equivalent to or greater than that of mollusc skeletons and most calcareous rocks. Echinoderms utilize calcium carbonate with unusually high efficiency, fulfilling the strength and volumetric requirements of a suitable skeleton with a minimum amount of material. Comparative dimensional analysis of structural and mechanical properties of these skeletal materials may provide further insight into the evolution of echinoderms.

Influence of X-Rays on Limb Regeneration in Urodele Amphibians

Influence of X-Rays on Limb Regeneration in Urodele Amphibians