The emergence of the chordate body plan: some puzzles and problems

The Cambrian fossil Pikaia, and the origin of chordate somites

The Middle Cambrian fossil Pikaia has a regular series of vertical bands that, assuming chordate affinities, can be interpreted as septa positioned between serial myotomes. Whether Pikaia has a notochord and nerve cord is less certain, as the dorsal organ, which has no obvious counterpart in living chordates, is the only clearly defined axial structure extending the length of the body. Without a notochord to serve as a reference point, the location of the nerve cord is then conjectural, which begs the question of how a dorsal neural center devoted to somite innervation would first have arisen from a more diffuse ancestral plexus of intraepithelial nerves. This question is examined using hemichordates as a reference point, first for the information they provide on the organization of the ancestral deuterostome nervous system, and second, extending the analysis of E. E. Ruppert, to explain why neural infoldings like the enteropneust collar cord would first have evolved. Both implicate the medial surface of the anterior-most part of the metacoel as the likely site for the evolution of the first somites. The analysis highlights the importance of the somatobranchial condition in chordates, meaning the linkage between the anterior trunk, hox1 expression, and the beginning of the gill series and somites. This feature is arguably a valid criterion by which to assess extinct taxa from the Cambrian that resemble chordates (e.g., vetulicolians and yunnanozoans), but may be unrelated to them. In a more speculative vein, the nature of the dorsal organ is discussed, including the possibility that it is an expanded neural tube combining neural and support functions in one structure.

The origin and evolution of vertebrate neural crest cells

The neural crest is a vertebrate-specific migratory stem cell population that generates a remarkably diverse set of cell types and structures. Because many of the morphological, physiological and behavioural novelties of vertebrates are derived from neural crest cells, it is thought that the origin of this cell population was an important milestone in early vertebrate history. An outstanding question in the field of vertebrate evolutionary-developmental biology (evo-devo) is how this cell type evolved in ancestral vertebrates. In this review, we briefly summarize neural crest developmental genetics in vertebrates, focusing in particular on the gene regulatory interactions instructing their early formation within and migration from the dorsal neural tube. We then discuss how studies searching for homologues of neural crest cells in invertebrate chordates led to the discovery of neural crest-like cells in tunicates and the potential implications this has for tracing the pre-vertebrate origins of the neural crest population. Finally, we synthesize this information to propose a model to explain the origin of neural crest cells. We suggest that at least some of the regulatory components of early stages of neural crest development long pre-date vertebrate origins, perhaps dating back to the last common bilaterian ancestor. These components, originally directing neuroectodermal patterning and cell migration, served as a gene regulatory 'scaffold' upon which neural crest-like cells with limited migration and potency evolved in the last common ancestor of tunicates and vertebrates. Finally, the acquisition of regulatory programmes controlling multipotency and long-range, directed migration led to the transition from neural crest-like cells in invertebrate chordates to multipotent migratory neural crest in the first vertebrates.

Reiterative use of FGF signaling in mesoderm development during embryogenesis and metamorphosis in the hemichordate Ptychodera flava

BACKGROUND

Mesoderm is generally considered to be a germ layer that is unique to Bilateria, and it develops into diverse tissues, including muscle, and in the case of vertebrates, the skeleton and notochord. Studies on various deuterostome animals have demonstrated that fibroblast growth factor (FGF) signaling is required for the formation of many mesodermal structures, such as vertebrate somites, from which muscles are differentiated, and muscles in sea urchin embryos, suggesting an ancient role of FGF signaling in muscle development. However, the formation of trunk muscles in invertebrate chordates is FGF-independent, leading to ambiguity about this ancient role in deuterostomes. To further understand the role of FGF signaling during deuterostome evolution, we investigated the development of mesodermal structures during embryogenesis and metamorphosis in Ptychodera flava, an indirect-developing hemichordate that has larval morphology similar to echinoderms and adult body features that are similar to chordates.

RESULTS

Here we show that genes encoding FGF ligands, FGF receptors and transcription factors that are known to be involved in mesoderm formation and myogenesis are expressed dynamically during embryogenesis and metamorphosis. FGF signaling at the early gastrula stage is required for the specification of the mesodermal cell fate in P. flava. The mesoderm cells are then differentiated stepwise into the hydroporic canal, the pharyngeal muscle and the muscle string; formation of the last two muscular structures are controlled by FGF signaling. Moreover, augmentation of FGF signaling during metamorphosis accelerated the process, facilitating the transformation from cilia-driven swimming larvae into muscle-driven worm-like juveniles.

CONCLUSIONS

Our data show that FGF signaling is required for mesoderm induction and myogenesis in the P. flava embryo, and it is reiteratively used for the morphological transition during metamorphosis. The dependence of muscle development on FGF signaling in both planktonic larvae and sand-burrowing worms supports its ancestral role in deuterostomes.

Evolutionary development of embryonic cerebrospinal fluid composition and regulation: an open research field with implications for brain development and function

Within the consolidated field of evolutionary development, there is emerging research on evolutionary aspects of central nervous system development and its implications for adult brain structure and function, including behaviour. The central nervous system is one of the most intriguing systems in complex metazoans, as it controls all body and mind functions. Its failure is responsible for a number of severe and largely incurable diseases, including neurological and neurodegenerative ones. Moreover, the evolution of the nervous system is thought to be a critical step in the adaptive radiation of vertebrates. Brain formation is initiated early during development. Most embryological, genetic and evolutionary studies have focused on brain neurogenesis and regionalisation, including the formation and function of organising centres, and the comparison of homolog gene expression and function among model organisms from different taxa. The architecture of the vertebrate brain primordium also reveals the existence of connected internal cavities, the cephalic vesicles, which in fetuses and adults become the ventricular system of the brain. During embryonic and fetal development, brain cavities and ventricles are filled with a complex, protein-rich fluid called cerebrospinal fluid (CSF). However, CSF has not been widely analysed from either an embryological or evolutionary perspective. Recently, it has been demonstrated in higher vertebrates that embryonic cerebrospinal fluid has key functions in delivering diffusible signals and nutrients to the developing brain, thus contributing to the proliferation, differentiation and survival of neural progenitor cells, and to the expansion and patterning of the brain. Moreover, it has been shown that the composition and homeostasis of CSF are tightly controlled in a time-dependent manner from the closure of the anterior neuropore, just before the initiation of primary neurogenesis, up to the formation of functional choroid plexuses. In this review, we draw together existing literature about the formation, function and homeostatic regulation of embryonic cerebrospinal fluid, from the closure of the anterior neuropore to the formation of functional fetal choroid plexuses, from an evolutionary perspective. The relevance of these processes to the normal functions and diseases of adult brain will also be discussed.

The deuterostome context of chordate origins

Our understanding of vertebrate origins is powerfully informed by comparative morphology, embryology and genomics of chordates, hemichordates and echinoderms, which together make up the deuterostome clade. Striking body-plan differences among these phyla have historically hindered the identification of ancestral morphological features, but recent progress in molecular genetics and embryology has revealed deep similarities in body-axis formation and organization across deuterostomes, at stages before morphological differences develop. These developmental genetic features, along with robust support of pharyngeal gill slits as a shared deuterostome character, provide the foundation for the emergence of chordates.

Evolution of basal deuterostome nervous systems

Understanding the evolution of deuterostome nervous systems has been complicated by the ambiguous phylogenetic position of the Xenocoelomorpha (Xenoturbellids, acoel flat worms, nemertodermatids), which has been placed either as basal bilaterians, basal deuterostomes or as a sister group to the hemichordate/echinoderm clade (Ambulacraria), which is a sister group of the Chordata. None of these groups has a single longitudinal nerve cord and a brain. A further complication is that echinoderm nerve cords are not likely to be evolutionarily related to the chordate central nervous system. For hemichordates, opinion is divided as to whether either one or none of the two nerve cords is homologous to the chordate nerve cord. In chordates, opposition by two secreted signaling proteins, bone morphogenetic protein (BMP) and Nodal, regulates partitioning of the ectoderm into central and peripheral nervous systems. Similarly, in echinoderm larvae, opposition between BMP and Nodal positions the ciliary band and regulates its extent. The apparent loss of this opposition in hemichordates is, therefore, compatible with the scenario, suggested by Dawydoff over 65 years ago, that a true centralized nervous system was lost in hemichordates.

Echinoderm conundrums: Hox genes, heterochrony, and an excess of mouths

Two issues relating to the translocation of anterior Hox genes in echinoderms to the 5' end of the Hox cluster are discussed: i) that developmental changes associated with fixation to the substratum have led to an acceleration of mesodermal development relative to that of ectoderm, resulting in a mismatch of anteroposterior registry between the two tissues and a larger role for mesoderm in patterning control, and ii) whether this helps explain the ability of some echinoderms to form separate mouths at different locations, one for the larva and one for the juvenile rudiment. Freeing the mesoderm from ectodermal influences may have encouraged morphogenetic innovation, paralleling the situation in tunicates, where an early genomic (or genomic and developmental) change has allowed the body to evolve in novel ways.

Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia

The Middle Cambrian Pikaia gracilens (Walcott) has an iconic position as a Cambrian chordate, but until now no detailed description has been available. Here on the basis of the 114 available specimens we review its anatomy, confirm its place in the chordates and explore with varying degrees of confidence its relationships to both extant and extinct chordates and other deuterostomes. The body of Pikaia is fusiform, laterally compressed and possesses about 100 myomeres. The head is small, bilobed and bears two narrow tentacles. There is no evidence for eyes. Apart from a thin dorsal fin (without finrays) and a series of at least nine bilaterally arranged appendages with possible pharyngeal pores at the anterior end, there are no other external features. In addition to the musculature the internal anatomy includes an alimentary canal, the anterior of which forms a prominent lenticular unit that is almost invariably preserved in positive relief. The cavity is interpreted as pharyngeal, implying that the mouth itself was almost terminal. The posterior extension of the gut is unclear although the anus appears to have been terminal. The most prominent internal structure is a reflectively preserved unit, possibly hollow, termed here the dorsal organ. Although formerly interpreted as a notochord its position and size make this less likely. Its original function remains uncertain, but it could have formed a storage organ. Ventral to the dorsal organ a narrower strand of tissue is interpreted as representing the nerve chord and notochord. In addition to these structures, there is also evidence for a vascular system, including a ventral blood vessel. The position of Pikaia in the chordates is largely based on the presence of sigmoidal myomeres, and the more tentative identification of a notochord. In many other respects, Pikaia differs from the expected nature of primitive chordates, especially as revealed in amphioxus and the Cambrian record (including Cathaymyrus, Haikouichthys, Metaspriggina, Myllokunmingia, and Zhongxiniscus). Whilst the possibility that Pikaia is simply convergent on the chordates cannot be dismissed, we prefer to build a scenario that regards Pikaia as the most stem-ward of the chordates with links to the phylogenetically controversial yunnanozoans. This hypothesis has implications for the evolution of the myomeres, notochord and gills. Finally, the wealth of material of Pikaia indicates that, although by definition there must be some sort of taphonomic imprint, the consistency of preservational details allows a reliable reconstruction of the anatomy and does not significantly erode phylogenetically relevant characters.

Development of the nervous system in hatchlings of Spadella cephaloptera (Chaetognatha), and implications for nervous system evolution in Bilateria

Chaetognaths (arrow worms) play an important role as predators in planktonic food webs. Their phylogenetic position is unresolved, and among the numerous hypotheses, affinities to both protostomes and deuterostomes have been suggested. Many aspects of their life history, including ontogenesis, are poorly understood and, though some aspects of their embryonic and postembryonic development have been described, knowledge of early neural development is still limited. This study sets out to provide new insights into neurogenesis of newly hatched Spadella cephaloptera and their development during the following days, with attention to the two main nervous centers, the brain and the ventral nerve center. These were examined with immunohistological methods and confocal laser-scan microscopic analysis, using antibodies against tubulin, FMRFamide, and synapsin to trace the emergence of neuropils and the establishment of specific peptidergic subsystems. At hatching, the neuronal architecture of the ventral nerve center is already well established, whereas the brain and the associated vestibular ganglia are still rudimentary. The development of the brain proceeds rapidly over the next 6 days to a state that resembles the adult pattern. These data are discussed in relation to the larval life style and behaviors such as feeding. In addition, we compare the larval chaetognath nervous system and that of other bilaterian taxa in order to extract information with phylogenetic value. We conclude that larval neurogenesis in chaetognaths does not suggest an especially close relationship to either deuterostomes or protostomes, but instead displays many apomorphic features.

Walter Garstang: a retrospective

Although, Walter Garstang died over 60 years ago, his work is still cited--sometimes praised, but sometimes belittled. On the negative side, he often appropriated ideas of others without attribution, ignored earlier studies conflicting with his theories, and clung to notions like inheritance of acquired characters, progressive evolution, and saltation after many of his contemporaries were advancing toward the modern synthesis. Moreover, his evolutionary scenarios--especially his derivation of vertebrates from a sessile ascidian--have not been well supported by recent work in developmental genetics and molecular phylogenetics. On the positive side, Garstang firmly established several points of view that remain useful in the age of evolutionary development (evo-devo). He popularized the valid idea that adaptive changes in larvae combined with shifts in developmental timing (heterochrony) could radically change adult morphology and provide an escape from overspecialization. Moreover, his re-statement of the biogenetic law is now widely accepted: namely, that recapitulation results when characters at one stage of development are required for the correct formation of other characters at subsequent stages (his stepping stone model). In other words, ontogeny creates phylogeny because some developmental features are constraints, favoring particular evolutionary outcomes while excluding others. This viewpoint is a useful basis for advancing concepts of homology and for comparing the phylogeny of ontogenies across a series of animals to ascertain the timing and the nature of the underlying ontogenetic changes.

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.