Ontogeny of the collar cord: Neurulation in the hemichordate Saccoglossus kowalevskii

The chordate body plan is characterized by a central notochord, a pharynx perforated by gill pores, and a dorsal central nervous system. Despite progress in recent years, the evolutionary origin of each of theses characters remains controversial. In the case of the nervous system, two contradictory hypotheses exist. In the first, the chordate nervous system is derived directly from a diffuse nerve net; whereas, the second proposes that a centralized nervous system is found in hemichordates and, therefore, predates chordate evolution. Here, we document the ontogeny of the collar cord of the enteropneust Saccoglossus kowalevskii using transmission electron microscopy and 3D‐reconstruction based on completely serially sectioned stages. We demonstrate that the collar cord develops from a middorsal neural plate that is closed in a posterior to anterior direction. Transversely oriented ependymal cells possessing myofilaments mediate this morphogenetic process and surround the remnants of the neural canal in juveniles. A mid‐dorsal glandular complex is present in the collar. The collar cord in juveniles is clearly separated into a dorsal saddle‐like region of somata and a ventral neuropil. We characterize two cell types in the somata region, giant neurons and ependymal cells. Giant neurons connect via a peculiar cell junction that seems to function in intercellular communication. Synaptic junctions containing different vesicle types are present in the neuropil. These findings support the hypotheses that the collar cord constitutes a centralized element of the nervous system and that the morphogenetic process in the ontogeny of the collar cord is homologous to neurulation in chordates. Moreover, we suggest that these similarities are indicative of a close phylogenetic relationship between enteropneusts and chordates. J. Morphol., 2010. ©2010 Wiley‐Liss, Inc.     

The postanal tail of the enteropneust Saccoglossus kowalevskii is a ciliary creeping organ without distinct similarities to the chordate tail

Stach, T. and Kaul, S. 2011. The postanal tail of the enteropneust Saccoglossus kowalevskii is a ciliary creeping organ without distinct similarities to the chordate tail. —Acta Zoologica (Stockholm) 92 : 150–160. The postanal tail of chordates is one of the key characters in chordate evolution and it has been suggested to be homologous to the postanal tail of harrimaniid enteropneusts. We present electron microscopic data of the ontogeny of the postanal tail in the enteropneust Saccoglossus kowalevskii. The postanal tail develops as a ventral posterior allometric outgrowth with a ventral extension of the telotroch. Transmission electron microscopy of serial sections reveals the epidermal organization of the postanal tail with the exception of short, bilaterally symmetric extensions of the paired metacoels. The epidermis cells are connected by apical junctions, rest basally on the extracellular matrix surrounding the mesoderm, and possess a basiepidermal nerve net. The ventral cells in the postanal tail are multiciliated and used for creeping. Dorsal cells are monociliated with numerous microvilli. Two types of glandular cells are present among the epidermis cells. The mesoderm cells contain myofilaments. We were unable to detect anatomical structures similar to the ones present in the postanal locomotory tail of chordates, such as notochord, neural tube, or endodermal strand. Thus, results of our anatomical study do not support homology of the postanal chordate tail and the postanal tail of harrimaniid enteropneusts.     

Hemichordates and the origin of chordates

Hemichordates, the phylum of bilateral animals most closely related to chordates, could reveal the evolutionary origins of chordate traits such as the nerve cord, notochord, gill slits and tail. The anteroposterior maps of gene expression domains for 38 genes of chordate neural patterning are highly similar for hemichordates and chordates, even though hemichordates have a diffuse nerve-net. About 40% of the domains are not present in protostome maps. We propose that this map, the gill slits and the tail date to the deuterostome ancestor. The map of dorsoventral expression domains, centered on a Bmp-Chordin axis, differs between the two groups; hemichordates resemble protostomes more than they do chordates. The dorsoventral axis might have undergone extensive modification in the chordate line, including centralization of the nervous system, segregation of epidermis, derivation of the notochord, and an inversion of organization.     

Ascidians and the plasticity of the chordate developmental program

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

Early development of coelomic structures in an echinoderm larva and a similarity with coelomic structures in a chordate embryo

Early coelomic development in the abbreviated development of the sea urchin Holopneustes purpurescens is described and then used in a comparison with coelomic development in chordate embryos to support homology between a single arm of the five-armed radial body plan of an echinoderm and the single bilateral axis of a chordate. The homology depends on a positional similarity between the origin of the hydrocoele in echinoderm development and the origin of the notochord in chordate development, and a positional similarity between the respective origins of the coelomic mesoderm and chordate mesoderm in echinoderm and chordate development. The hydrocoele is homologous with the notochord and the secondary podia are homologous with the somites. The homology between a single echinoderm arm and the chordate axis becomes clear when the aboral to oral growth from the archenteron in the echinoderm larva is turned anteriorly, more in line with the anterior–posterior axis of the early zygote. A dorsoventral axis inversion in chordates is not required in the proposed homology.     

The evolution of anural larvae in molgulid ascidians

Ascidians are urochordates, marine invertebrates with non-feeding motile chordate tadpole larvae, except in the family Molgulidae. Urodele, or tailed, Molgulids have typical ascidian chordate tadpole larvae possessing tails with muscle cells, a notochord, and a dorsal hollow nerve cord. In contrast, anural (or tail-less) Molgulids lack a tail and defining chordate features. Molecular phylogenies generated with 18S and 28S ribosomal sequences indicate that Molgulid species fall into at least four distinct clades, three of which have multiple anural members. This refined and expanded phylogeny allows careful examination of the factors that may have influenced the evolution of tail-less ascidians.     

Interaction of Wnt and activin in dorsal mesoderm induction in Xenopus.

Both the activin and Wnt families of peptide growth factors are capable of inducing dorsal mesoderm in Xenopus embryos. Presumptive ventral ectoderm cells isolated from embryos injected with Xwnt8 mRNA were cultured in the presence of activin A to study the possible interactions between these two classes of signaling proteins. We find that overexpression of Xwnt8 RNA alters the response of ventral ectoderm to activin such that ventral explants differentiate dorsoanterior structures including notochord and eyes. This response is similar to the response of dorsal ectoderm to activin alone. When embryos are irradiated with uv light to inhibit dorsal axis formation, ectodermal explants differentiate notochord when they are induced by a combination of both signaling factors, but not when cells receive only one inducing signal (activin or Xwnt8). This result is further supported by the observation that goosecoid (gsc) mRNA, an early marker for dorsal mesoderm, is expressed in these explants only when they are injected with Xwnt8 mRNA followed by exposure to activin. Early morphogenetic movements of the induced cells and activation of muscle-specific actin and Brachyury (Xbra) genes also reveal a cooperation of activin A and Xwnt8 in mesoderm induction.     

Expression of Brachyury-like T-box transcription factor, Xbra3 in Xenopus embryo

The Xenopus Brachyury-like Xbra3 gene is a novel T-box gene that is closely associated with Xenopus Brachyury. The expression pattern of Xbra3 during development is similar to that of Xbra. During gastrulation Xbra3 is expressed in the marginal zone, with a gradient of increasing expression from ventral to dorsal. In the early neurula stage Xbra3 is expressed in the notochord and posterior mesoderm, but by the tailbud stage its expression is restricted to the forming tailbud and the posterior portion of the notochord.     

Essential role of Bmp signaling and its positive feedback loop in the early cell fate evolution of chordates

In chordates, early separation of cell fate domains occurs prior to the final specification of ectoderm to neural and non-neural as well as mesoderm to dorsal and ventral during development. Maintaining such division with the establishment of an exact border between the domains is required for the formation of highly differentiated structures such as neural tube and notochord. We hypothesized that the key condition for efficient cell fate separation in a chordate embryo is the presence of a positive feedback loop for Bmp signaling within the gene regulatory network (GRN), underlying early axial patterning. Here, we therefore investigated the role of Bmp signaling in axial cell fate determination in amphioxus, the basal chordate possessing a centralized nervous system. Pharmacological inhibition of Bmp signaling induces dorsalization of amphioxus embryos and expansion of neural plate markers, which is consistent with an ancestral role of Bmp signaling in chordate axial patterning and neural plate formation. Furthermore, we provided evidence for the presence of the positive feedback loop within the Bmp signaling network of amphioxus. Using mRNA microinjections we found that, in contrast to vertebrate Vent genes, which promote the expression of Bmp4, amphioxus Vent1 is likely not responsible for activation of cephalochordate ortholog Bmp2/4. Cis-regulatory analysis of amphioxus Bmp2/4, Admp and Chordin promoters in medaka embryos revealed remarkable conservation of the gene regulatory information between vertebrates and basal chordates. Our data suggest that emergence of a positive feedback loop within the Bmp signaling network may represent a key molecular event in the evolutionary history of the chordate cell fate determination.     

The Annelid Ancestry of the Chordates and the Origin of the Chordate Central Nervous System and the Notochord

It is maintained that the segmentation of the chordates is not a re-emergent condition and that their ancestors must not be sought among oligomerous coelomates but among the kolygomerous forms. It is proposed that the chordates developed from tube-dwelling Polychaeta resembling the present members of the Sabellidae , by reverting to a free-living existance and a reversal of the dorsoventrality. The chordate neural tube is regarded as a new structure developed from the closed-over faecal groove and the surrounding gland cells of the glandular mass of the ventral gland-shields of the polychaete ancestor.
The disappearance of the annelid cerebral ganglia and their gradual replacement by a new co-ordinating centre in the form of a dilation of the anterior end of the neural tube is postulated. The author accepts D elsman's (1922) suggestion that the paired segmental ganglia of the annelid ancestor persist as the spinal ganglia of the vertebrates.
The chordate mouth is regarded as the same as that of its annelid ancestor.
A strip of dense connective tissue wedged in between the ventral nerve chords and in which are embedded the fibres of the longitudinal muscle overlying the nerve chords of the presumed polychaete ancestor is regarded as the precursor of the notochord, while the longitudinal musculature of the chordate trunk developed from the adjacent longitudinal muscle bands.
A cartilage-like internal skeleton supports the branchial crown of the tubicolous polychaetes examined.     

Anteroposterior patterning in hemichordates and the origins of the chordate nervous system

The chordate central nervous system has been hypothesized to originate from either a dorsal centralized, or a ventral centralized, or a noncentralized nervous system of a deuterostome ancestor. In an effort to resolve these issues, we examined the hemichordate Saccoglossus kowalevskii and studied the expression of orthologs of genes that are involved in patterning the chordate central nervous system. All 22 orthologs studied are expressed in the ectoderm in an anteroposterior arrangement nearly identical to that found in chordates. Domain topography is conserved between hemichordates and chordates despite the fact that hemichordates have a diffuse nerve net, whereas chordates have a centralized system. We propose that the deuterostome ancestor may have had a diffuse nervous system, which was later centralized during the evolution of the chordate lineage.     

Regeneration of the acorn worm pygochord with the implication for its convergent evolution with the notochord

The origin of the notochord is a central issue in chordate evolution. This study examined the development of the acorn worm pygochord, a putative homologue of the notochord. Because the pygochord differentiates only after metamorphosis, the developmental was followed process by inducing regeneration after artificial amputation in Ptychodera flava. It was found that although the regeneration of the posterior part of the body did not proceed via formation of an obvious regeneration bud, pygochord regeneration was observed within a few weeks, possibly via trans-differentiation of endoderm cells. The expression of the fibrillary collagen gene (Fcol) and elav in the pygochord during regeneration was detected. This indicates that pygochord cells are not part of gut epithelial cells, but that they differentiated as a distinct cell type. Our gene expression analyses do not provide supporting evidence for the homology between the pygochord and notochord, but rather favored the convergent evolution between them.     

Retinoids and nonvertebrate chordate development

Retinoic acid (RA) is required for the differentiation and morphogenesis of chordate-specific features, such as the antero-posterior regionalization of the dorsal hollow nerve cord and neural crest cells. RA receptors (RARs) have been reported exclusively in chordates, suggesting that the acquisition of the RAR gene was important for chordate evolution. A scenario is presented here for the establishment of an RAR-mediated developmental regulatory system during the course of chordate evolution. In the common chordate ancestor, RAR came to control the spatial expression pattern of Hox genes in the ectoderm and endoderm along the antero-posterior axis. In these germ layers, RA was required for the differentiation of epidermal sensory neurons and the morphogenesis of pharyngeal gill slits, respectively. As the diffuse epidermal nerve net in the chordate ancestor became centralized to form the dorsal nerve cord, the epidermal Hox expression pattern was carried into the central nervous system. Because the Hox code here came to specify neuronal identity along the antero-posterior axis, RA became inextricably linked to the antero-posterior patterning of the chordate central nervous system.