Building divergent body plans with similar genetic pathways

Deuterostome animals exhibit widely divergent body plans. Echinoderms have either radial or bilateral symmetry, hemichordates include bilateral enteropneust worms and colonial pterobranchs, and chordates possess a defined dorsal-ventral axis imposed on their anterior-posterior axis. Tunicates are chordates only as larvae, following metamorphosis the adults acquire a body plan unique for the deuterostomes. This paper examines larval and adult body plans in the deuterostomes and discusses two distinct ways of evolving divergent body plans. First, echinoderms and hemichordates have similar feeding larvae, but build a new adult body within or around their larvae. In hemichordates and many direct-developing echinoderms, the adult is built onto the larva, with the larval axes becoming the adult axes and the larval mouth becoming the adult mouth. In contrast, indirect-developing echinoderms undergo radical metamorphosis where adult axes are not the same as larval axes. A second way of evolving a divergent body plan is to become colonial, as seen in hemichordates and tunicates. Early embryonic development and gastrulation are similar in all deuterostomes, but, in chordates, the anterior-posterior axis is established at right angles to the animal-vegetal axis, in contrast to hemichordates and indirect-developing echinoderms. Hox gene sequences and anterior-posterior expression patterns illuminate deuterostome phylogenetic relationships and the evolution of unique adult body plans within monophyletic groups. Many genes that are considered vertebrate 'mesodermal' genes, such as nodal and brachyury T, are likely to ancestrally have been involved in the formation of the mouth and anus, and later were evolutionarily co-opted into mesoderm during vertebrate development.     

Hox genes in a pentameral animal

There is renewed interest in how the different body plans of extant phyla are related. This question has traditionally been addressed by comparisons between vertebrates and Drosophila. Fortunately, there is now increasing emphasis on animals representing other phyla. Pentamerally symmetric echinoderms are a bilaterian metazoan phylum whose members exhibit secondarily derived radial symmetry. Precisely how their radially symmetric body plan originated from a bilaterally symmetric ancestor is unknown, however, two recent papers address this subject. Peterson et al. propose a hypothesis on evolution of the anteroposterior axis in echinoderms, and Arenas-Mena et al. examine expression of five posterior Hox genes during development of the adult sea urchin.     

Origins of radial symmetry identified in an echinoderm during adult development and the inferred axes of ancestral bilateral symmetry

How the radial body plan of echinoderms is related to the bilateral body plan of their deuterostome relatives, the hemichordates and the chordates, has been a long-standing problem. Now, using direct development in a sea urchin, I show that the first radially arranged structures, the five primary podia, form from a dorsal and a ventral hydrocoele at the oral end of the archenteron. There is a bilateral plane of symmetry through the podia, the mouth, the archenteron and the blastopore. This adult bilateral plane is thus homologous with the bilateral plane of bilateral metazoans and a relationship between the radial and bilateral body plans is identified. I conclude that echinoderms retain and use the bilateral patterning genes of the common deuterostome ancestor. Homologies with the early echinoderms of the Cambrian era and between the dorsal hydrocoele, the chordate notochord and the proboscis coelom of hemichordates become evident.     

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.     

A hexamer origin of the echinoderms' five rays

Of the major deuterostome groups, the echinoderms with their multiple forms and complex development are arguably the most mysterious. Although larval echinoderms are bilaterally symmetric, the adult body seems to abandon the larval body plan and to develop independently a new structure with different symmetries. The prevalent pentamer structure, the asymmetry of Lovén's rule and the variable location of the periproct and madrepore present enormous difficulties in homologizing structures across the major clades, despite the excellent fossil record. This irregularity in body forms seems to place echinoderms outside the other deuterostomes. Here I propose that the predominant five-ray structure is derived from a hexamer structure that is grounded directly in the structure of the bilaterally symmetric larva. This hypothesis implies that the adult echinoderm body can be derived directly from the larval bilateral symmetry and thus firmly ranks even the adult echinoderms among the bilaterians. In order to test the hypothesis rigorously, a model is developed in which one ray is missing between rays IV-V (Lovén's schema) or rays C-D (Carpenter's schema). The model is used to make predictions, which are tested and verified for the process of metamorphosis and for the morphology of recent and fossil forms. The theory provides fundamental insight into the M-plane and the Ubisch', Lovén's, and Carpenter's planes and generalizes them for all echinoderms. The theory also makes robust predictions about the evolution of the pentamer structure and its developmental basis.     

Apluteal development of the sea urchin Holopneustes purpurescens Agassiz (Echinodermata: Echinoidea: Euechinoidea)

The external features of a shortened, apluteal development (lacking a pluteus larva) are described. Some features are unusual for echinoids. The large egg is distinctively marked by dark and pale coloured yolk. The sperm entry point is marked by a dark yolk spot and the first cleavage plane in most embryos is through the meridian on which the sperm entry point lies. Dark yolk in the animal hemisphere segregates largely to one blastomere in the two-cell embryo and pale yolk segregates to the other as a result of yolk movements during the first cell cycle. Progeny of the pale-yolk blastomere form adult oral structures and progeny of the dark-yolk blastomere form adult aboral structures. There is no feeding planktonic pluteus larva. The gastrula develops into a demersal vestibula larva with bilateral symmetry. The plane of symmetry is coincident with the Carpenter axis that defines a plane of symmetry through the madreporite in adult echinoderms. The coincidence shows that the anterior ambulacrum is vegetal with respect to egg polarity and the interradius originating at the madreporite is animal. The bilateral symmetry of the vestibula offers insight into the origin of radial symmetry in echinoderms and the body plan of an echinoderm ancestor.     

The A/P axis in echinoderm ontogeny and evolution: evidence from fossils and molecules

SUMMARY Even though echinoderms are members of the Bilateria, the location of their anterior/posterior axis has remained enigmatic. Here we propose a novel solution to the problem employing three lines of evidence: the expression of a posterior class Hox gene in the coeloms of the nascent adult body plan within the larva; the anatomy of certain early fossil echinoderms; and finally the relation between endoskeletal plate morphology and the associated coelomic tissues. All three lines of evidence converge on the same answer, namely that the location of the adult mouth is anterior, and the anterior/posterior axis runs from the mouth through the adult coelomic compartments. This axis then orients the animal such that there is but a single plane of symmetry dividing the animal into left and right halves. We tentatively hypothesize that this plane of symmetry is positioned along the dorsal/ventral axis. These axis identifications lead to the conclusion that the five ambulacra are not primary body axes, but instead are outgrowths from the central anterior/posterior axis. These identifications also shed insight into several other evolutionary mysteries of various echinoderm clades such as the independent evolution of bilateral symmetry in irregular echinoids, but do not elucidate the underlying mechanisms of the adult coelomic architecture.     

Gravity influences the position of the dorsoventral axis in medaka fish embryos (Oryzias latipes)

To determine whether gravity influences the plane of bilateral symmetry in medaka embryos, zygotes were placed with their animal-vegetal axis orientated vertically and with their vegetal pole elevated. Then, at regular intervals during the first cell cycle, the zygotes were tilted 90° for about 10 min and subsequently returned to their original orientation. In embryos tilted during the first half of the first cell cycle, the embryonic shield formed on the side that had been lowermost when the zygote was tilted. In embryos that were tilted twice, first in one direction and then in the opposite direction, the embryonic shield formed on the side that was lowermost the first time. When zygotes were centrifuged at 5 g , the embryonic shield formed on the outwardly radial (centrifugal) side of the embryo. The orientation of the array of parallel microtubules in the vegetal pole region was also influenced by tilting or centrifuging zygotes. No correlation was found between the positions of the polar body and the micropyle and the plane of bilateral symmetry. It was concluded that gravity influences both the plane of bilateral symmetry and the orientation of microtubules in the vegetal pole region of medaka embryos.     

Evolutionary convergence in<Emphasis Type="Italic"> Otx</Emphasis> expression in the pentameral adult rudiment in direct-developing sea urchins

Convergence is a significant evolutionary phenomenon. Arrival at similar morphologies from different starting points indicates a strong role for natural selection in shaping morphological phenotypes. There is no evidence yet of convergence in the developmental mechanisms that underlie the evolution of convergent developmental phenotypes. Here we report the expression domains in sea urchins of two important developmental regulatory genes ( Orthodenticle and Runt), and show evidence of molecular convergence in the evolution of direct-developing sea urchins. Indirect development is ancestral in sea urchins. Evolutionary loss of the feeding pluteus stage and precocious formation of the radially symmetric juvenile has evolved independently in numerous sea urchin lineages, thus direct development is an evolutionary convergence. Indirect-developing species do not express Otx during the formation of their five primordial tube feet, the ancestral condition. However, each direct-developing urchin examined does express Otx in the tube feet. Otx expression in the radial arms of direct-developing sea urchins is thus convergent, and may indicate a specific need for Otx use in direct development, a constraint that would make direct development less able to evolve than if there were multiple molecular means for it to evolve. In contrast, Runt is expressed in tube feet in both direct- and indirect-developing species. Because echinoderms are closely related to chordates and postdate the protostome/deuterostome divergence, they must have evolved from bilaterally symmetrical ancestors. Arthropods and chordates use Otx in patterning their anterior axis, and Runt has multiple roles including embryonic patterning in arthropods, and blood and bone cell differentiation in vertebrates. Runt has apparently been co-opted in echinoderms for patterning of pentamery, and Otx in pentameral patterning among direct-developing echinoids. The surprisingly dynamic nature of Otx evolution reinvigorates debate on the role of natural selection vs shared ancestry in the evolution of novel features.     

Flexibility in starfish behavior by multi-layered mechanism of self-organization

Understanding animal behavior as a product of natural selection sometimes result in an underestimation of the animal's adaptability: lower animals with poor mental capabilities are usually considered to simply exhibit innate behavioral patterns. Self-organized behavior may exhibit both stability of certain behavioral patterns and flexibility in adopting those patterns. Thus, the self-organization processes of starfish arm and tube feet movements are investigated, by observing obstacle avoidance behavior and tube feet of moving starfish. As starfish have no central nervous systems, their behaviors are the result of certain self-organization processes. Starfish have hierarchically constructed motor organs consisting of arms and tube feet. The collective behavior of the tube feet does not function only as simple fluctuations in the arms' coordination. As a result, starfish seem to exhibit more versatile behavioral changes than expected from the original model of a self-organized behavior.     

Development of symmetry and asymmetry in the early evolution of the echinoderms

Development of pentameral symmetry in echinoderms occurred by coiling and closing up of bilateral asymmetrical trimeral metamery, characteristic of ancestral echinoderms. It originated in the ambulacral system and further developed in other systems under the influence of the original metamery and primary bilateral asymmetry. Stages of the development of pentameral symmetry on the basis of bilateral symmetry are recognized in the morphology of the ambulacral system of the earliest echinoderms.     

Diverse radial symmetry among the Cambrian Fortunian fossil embryos from northern Sichuan and southern Shaanxi provinces,South China

The Cambrian Fortunian fossil embryos exhibit embryonic development of ancient animals and hence have important bearings on evolutionary developmental biology. They have radial symmetry, and may be early representatives of cnidarians. Here we report new material of three-dimensionally phosphatized fossil embryos from the Fortunian Kuanchuanpu Formation and coeval strata in northern Sichuan and southern Shaanxi provinces, South China. The new material includes previously reported fossil embryos assignable to Pseudooides prima with biradial symmetry or pseudo-hexaradial symmetry, Quadrapyrgites quadratacris with tetraradial symmetry, and Olivooides multisulcatus with pentaradial symmetry. Additionally, we recovered two new types of fossil embryos, i.e., Embryo I with hexaradial symmetry and Embryo II with octaradial symmetry, and they are tentatively suggested to represent new cnidarians. In contrast to the diverse radial symmetry of the Fortunian cnidarians, modern cnidarians exhibit stable tetraradial symmetry in medusozoans, biradial symmetry in anthozoans, and bilateral symmetry in siphonophores (Hydrozoa). The current study supports the view that the tetraradial symmetry of modern medusozoans is a surviving remnant of their Fortunian relatives.     

Arrays in rays: terminal addition in echinoderms and its correlation with gene expression

The echinoderms are deuterostomes that superimpose radial symmetry upon bilateral larval morphology. Consequently, they are not the first animals that come to mind when the concepts of segmentation and terminal addition are being discussed. However, it has long been recognized that echinoderms have serial elements along their radii formed in accordance with the ocular plate rule (OPR). The OPR is a special case of terminal growth, forming elements of the ambulacra that define the rays in echinoderms. New elements are added at the terminus of the ray, which may or may not be marked by a calcified element called the terminal plate (the "ocular" of sea urchins). The OPR operates in every echinoderm, from the occasionally bizarre fossils of the Cambrian to the most familiar extant taxa. Using the OPR and other criteria of recognition, echinoderm body wall can be divided into two main regions: extraxial components are associated with the somatocoels, axial components (formed in accordance with the OPR) with the hydrocoel. We compare patterns of development in axial regions of echinoderms with those found in the anterior-posterior axes of the earliest echinoderms as well as other invertebrates. Although axial and extraxial skeletons appear to be composed of the same biomineral matrix, the genes involved in patterning these two skeletal components are likely distinct. During development of the axial skeleton, for instance, the genes engrailed and orthodenticle are expressed in spatial and temporal patterns consistent with the OPR. Other genes such as distal-less seem to demarcate early ontogenetic boundaries between the axial rudiment and the extraxial larval body. There is a complex and pervasive reorganization of gene expression domains to produce the highly divergent morphologies seen in the Echinodermata. We integrate morphological and genetic information, particularly with respect to the origins of radial symmetry in the rudiment, and the concomitant development of the rays.     

Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution

Echinoderms are unique in being pentaradiate, having diverged from the ancestral bilaterian body plan more radically than any other animal phylum. This transformation arises during ontogeny, as echinoderm larvae are initially bilateral, then pass through an asymmetric phase, before giving rise to the pentaradiate adult. Many fossil echinoderms are radial and a few are asymmetric, but until now none have been described that show the original bilaterian stage in echinoderm evolution. Here we report new fossils from the early middle Cambrian of southern Europe that are the first echinoderms with a fully bilaterian body plan as adults. Morphologically they are intermediate between two of the most basal classes, the Ctenocystoidea and Cincta. This provides a root for all echinoderms and confirms that the earliest members were deposit feeders not suspension feeders.     

Nuclear beta-catenin and the development of bilateral symmetry in normal and LiCl-exposed chick embryos.

Studies in Xenopus laevis and zebrafish suggest a key role for beta-catenin in the specification of the axis of bilateral symmetry. In these organisms, nuclear beta-catenin demarcates the dorsalizing centers. We have asked whether beta-catenin plays a comparable role in the chick embryo and how it is adapted to the particular developmental constraints of chick development. The first nuclear localization of beta-catenin is observed in late intrauterine stages of development in the periphery of the blastoderm, the developing area opaca and marginal zone. Obviously, this early, radially symmetric domain does not predict the future organizing center of the embryo. During further development, cells containing nuclear beta-catenin spread under the epiblast and form the secondary hypoblast. The onset of hypoblast formation thus demarcates the first bilateral symmetry in nuclear beta-catenin distribution. Lithium chloride exposure also causes ectopic nuclear localization of beta-catenin in cells of the epiblast in the area pellucida. Embryos treated before primitive streak formation become completely radialized, as shown by the expression of molecular markers, CMIX and GSC. Lithium treatments performed during early or medium streak stages cause excessive development of the anterior primitive streak, node and notochord, and lead to a degeneration of prospective ventral and posterior structures, as shown by the expression of the molecular markers GSC, CNOT1, BMP2 and Ch-Tbx6L. In summary, we found that in spite of remarkable spatiotemporal differences, beta-catenin acts in the chick in a manner similar to that in fish and amphibia.     

Ventralization of an indirect developing hemichordate by NiCl₂ suggests a conserved mechanism of dorso-ventral (D/V) patterning in Ambulacraria (hemichordates and echinoderms)

One of the earliest steps in embryonic development is the establishment of the future body axes. Morphological and molecular data place the Ambulacraria (echinoderms and hemichordates) within the Deuterostomia and as the sister taxon to chordates. Extensive work over the last decades in echinoid (sea urchins) echinoderms has led to the characterization of gene regulatory networks underlying germ layer specification and axis formation during embryogenesis. However, with the exception of recent studies from a direct developing hemichordate (Saccoglossus kowalevskii), very little is known about the molecular mechanism underlying early hemichordate development. Unlike echinoids, indirect developing hemichordates retain the larval body axes and major larval tissues after metamorphosis into the adult worm. In order to gain insight into dorso-ventral (D/V) patterning, we used nickel chloride (NiCl?), a potent ventralizing agent on echinoderm embryos, on the indirect developing enteropneust hemichordate, Ptychodera flava. Our present study shows that NiCl? disrupts the D/V axis and induces formation of a circumferential mouth when treated before the onset of gastrulation. Molecular analysis, using newly isolated tissue-specific markers, shows that the ventral ectoderm is expanded at expense of dorsal ectoderm in treated embryos, but has little effect on germ layer or anterior-posterior markers. The resulting ventralized phenotype, the effective dose, and the NiCl? sensitive response period of Ptychodera flava, is very similar to the effects of nickel on embryonic development described in larval echinoderms. These strong similarities allow one to speculate that a NiCl? sensitive pathway involved in dorso-ventral patterning may be shared between echinoderms, hemichordates and a putative ambulacrarian ancestor. Furthermore, nickel treatments ventralize the direct developing hemichordate, S. kowalevskii indicating that a common pathway patterns both larval and adult body plans of the ambulacrarian ancestor and provides insight in to the origin of the chordate body plan.     

Cambrian stalked echinoderms show unexpected plasticity of arm construction

Feeding arms carrying coelomic extensions of the theca are thought to be unique to crinoids among stemmed echinoderms. However, a new two-armed echinoderm from the earliest Middle Cambrian of Spain displays a highly unexpected morphology. X-ray microtomographic analysis of its arms shows they are polyplated in their proximal part with a dorsal series of uniserial elements enclosing a large coelomic lumen. Distally, the arm transforms into the more standard biserial structure of a blastozoan brachiole. Phylogenetic analysis demonstrates that this taxon lies basal to rhombiferans as sister-group to pleurocystitid and glyptocystitid blastozoans, drawing those clades deep into the Cambrian. We demonstrate that Cambrian echinoderms show surprising variability in the way their appendages are constructed, and that the appendages of at least some blastozoans arose as direct outgrowths of the body in much the same way as the arms of crinoids.     

Brefeldin A or monensin inhibits the 3D organizer in gastropod,polyplacophoran, and scaphopod molluscs

In molluscs, the 3D vegetal blastomere acts as a developmental signaling center, or organizer, and is required to establish bilateral symmetry in the embryo. 3D is similar to organizing centers in other metazoans, but detailed comparisons are difficult, in part because its organizing function is poorly understood. To elucidate 3D function in a standardized fashion, we used monensin and brefeldin A (BFA) to rapidly and reversibly interfere with protein processing and secretion, thereby inhibiting the signaling interactions that underlie its specification and patterning. In the gastropods, Patella vulgata and Lymnaea stagnalis, the polyplacophoran, Mopalia muscosa, and the scaphopod, Antalis entalis, treatments initiated before the organizer-dependent onset of bilateral cleavage resulted in radialization of subsequent development. In radialized P. vulgata, L. stagnalis, and M. muscosa, organizer specification was blocked, and embryos failed to make the transition to bilateral cleavage. In all four species, the subsequent body plan was radially symmetric and was similarly organized about a novel aboral–oral axis. Our results demonstrate that brefeldin A (BFA) and monensin can be used to inhibit 3D’s organizing function in a comparative fashion and that, at least in M. muscosa, the organizer-dependent developmental architecture of the embryo predicts subsequent patterns of morphogenetic movements in gastrulation and, ultimately, the layout of the adult body plan.     

Bilateral homologues in echinoderms and a predictive model of the bilateral echinoderm ancestor

The problem of the unknown relationship between the radial body form of echinoderms and the bilateral body form of their relatives and ancestor is approached here by applying a hypothetical ancestral gastrulation pattern to the echinoderm body plan. The result is a prediction of the homologue in echinoderms of the bilateral plane of bilateral forms. The predicted plane is identified in the adult rudiment of the echinoid Holopneustes purpurescens as the plane through the oral anlagen, the future anus and future madreporite. It is also, therefore, the plane through the classically described Carpenter axis. Based on this identification, other bilateral homologues are proposed and a model of the bilateral echinoderm ancestor is constructed. The bilateral homologues are potentially testable by the techniques of molecular embryology.