Inversion of the chordate body axis: are there alternatives?

Evolutionary origin of the neural tube in basal deuterostomes

The central nervous system (CNS) of chordates, including humans, develops as a hollow tube with ciliated walls containing cerebrospinal fluid. However, most of the animals inhabiting our planet do not use this design and rather build their centralized brains from non-epithelialized condensations of neurons called ganglia, with no traces of epithelialized tubes or liquid-containing cavities. The evolutionary origin of tube-type CNSs stays enigmatic, especially as non-epithelialized ganglionic-type nervous systems dominate the animal kingdom. Here, I discuss recent findings relevant to understanding the potential homologies and scenarios of the origin, histology and anatomy of the chordate neural tube. The nerve cords of other deuterostomes might relate to the chordate neural tube at histological, developmental and cellular levels, including the presence of radial glia, layered stratification, retained epithelial features, morphogenesis via folding and formation of a lumen filled with liquid. Recent findings inspire a new view of hypothetical evolutionary scenarios explaining the tubular epithelialized structure of the CNS. One such idea suggests that early neural tubes were key for improved directional olfaction, which was facilitated by the liquid-containing internal cavity. The later separation of the olfactory portion of the tube led to the formation of the independent olfactory and posterior tubular CNS systems in vertebrates. According to an alternative hypothesis, the thick basiepithelial nerve cords could provide deuterostome ancestors with additional biomechanical support, which later improved by turning the basiepithelial cord into a tube filled with liquid - a hydraulic skeleton.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Systems biology derived source-sink mechanism of BMP gradient formation

A morphogen gradient of Bone Morphogenetic Protein (BMP) signaling patterns the dorsoventral embryonic axis of vertebrates and invertebrates. The prevailing view in vertebrates for BMP gradient formation is through a counter-gradient of BMP antagonists, often along with ligand shuttling to generate peak signaling levels. To delineate the mechanism in zebrafish, we precisely quantified the BMP activity gradient in wild-type and mutant embryos and combined these data with a mathematical model-based computational screen to test hypotheses for gradient formation. Our analysis ruled out a BMP shuttling mechanism and a bmp transcriptionally-informed gradient mechanism. Surprisingly, rather than supporting a counter-gradient mechanism, our analyses support a fourth model, a source-sink mechanism, which relies on a restricted BMP antagonist distribution acting as a sink that drives BMP flux dorsally and gradient formation. We measured Bmp2 diffusion and found that it supports the source-sink model, suggesting a new mechanism to shape BMP gradients during development.

Before an animal is born, a protein called BMP plays a key role in establishing the difference between the front and the back of the animal. Cells nearer the front of the embryo contain higher amounts of the BMP protein, whilst cells nearer the back have progressively lower levels of BMP. This gradient of BMP ‘concentration’ affects the identity of the cells, with the level of BMP in each cell dictating what parts of the body are made where.

The prevailing view among scientists is that the BMP gradient is created by an opposing gradient of another protein called Chordin, which is found at high levels at the back of the embryo and lower levels near the front. Chordin inhibits BMP and the interaction between the two proteins establishes the gradients that create order across the embryo.

Zinski et al. used computer models to investigate how the BMP gradient is created. Several possibilities were considered, including the effect of Chordin. Comparing the models to precise experimental measurements of BMP activity in zebrafish embryos suggested that a different mechanism known as a source-sink model, rather than the opposing Chordin gradient, may be responsible for the pattern of BMP found in the embryo. In this model, the BMP is produced at the front of the embryo and moves towards the back end by diffusion. At the back of the embryo, BMP is mopped up by Chordin, resulting in a constant gradient of BMP along the embryo.

Many other processes that control how animals grow and develop rely on the formation of similar protein gradients, so these findings may also apply to other aspects of animal development. Understanding how animals grow and develop may help researchers to develop strategies to regrow tissues and organs in human patients.

Neuronal patterning of the tubular collar cord is highly conserved among enteropneusts but dissimilar to the chordate neural tube

A tubular nervous system is present in the deuterostome groups Chordata (cephalochordates, tunicates, vertebrates) and in the non-chordate Enteropneusta. However, the worm-shaped enteropneusts possess a less complex nervous system featuring only a short hollow neural tube, whereby homology to its chordate counterpart remains elusive. Since the majority of data on enteropneusts stem from the harrimaniid Saccoglossus kowalevskii, putative interspecific variations remain undetected resulting in an unreliable ground pattern that impedes homology assessments. In order to complement the missing data from another enteropneust family, we investigated expression of key neuronal patterning genes in the ptychoderid Balanoglossus misakiensis. The collar cord of B. misakiensis shows anterior Six3/6 and posterior Otx + Engrailed expression, in a region corresponding to the chordate brain. Neuronal Nk2.1/Nk2.2 expression is absent. Interestingly, we found median Dlx and lateral Pax6 expression domains, i.e., a condition that is reversed compared to chordates. Comparative analyses reveal that adult nervous system patterning is highly conserved among the enteropneust families Harrimaniidae, Spengelidae and Ptychoderidae. BmiDlx and BmiPax6 have no corresponding expression domains in the chordate brain, which may be indicative of independent acquisition of a tubular nervous system in Enteropneusta and Chordata.

The Fox/Forkhead transcription factor family of the hemichordate Saccoglossus kowalevskii

Background

The Fox gene family is a large family of transcription factors that arose early in organismal evolution dating back to at least the common ancestor of metazoans and fungi. They are key components of many gene regulatory networks essential for embryonic development. Although much is known about the role of Fox genes during vertebrate development, comprehensive comparative studies outside vertebrates are sparse. We have characterized the Fox transcription factor gene family from the genome of the enteropneust hemichordate Saccoglossus kowalevskii, including phylogenetic analysis, genomic organization, and expression analysis during early development. Hemichordates are a sister group to echinoderms, closely related to chordates and are a key group for tracing the evolution of gene regulatory mechanisms likely to have been important in the diversification of the deuterostome phyla.

Results

Of the 22 Fox gene families that were likely present in the last common ancestor of all deuterostomes, S. kowalevskii has a single ortholog of each group except FoxH, which we were unable to detect, and FoxQ2, which has three paralogs. A phylogenetic analysis of the FoxQ2 family identified an ancestral duplication in the FoxQ2 lineage at the base of the bilaterians. The expression analyses of all 23 Fox genes of S. kowalevskii provide insights into the evolution of components of the regulatory networks for the development of pharyngeal gill slits (foxC, foxL1, and foxI), mesoderm patterning (foxD, foxF, foxG), hindgut development (foxD, foxI), cilia formation (foxJ1), and patterning of the embryonic apical territory (foxQ2).

Conclusions

Comparisons of our results with data from echinoderms, chordates, and other bilaterians help to develop hypotheses about the developmental roles of Fox genes that likely characterized ancestral deuterostomes and bilaterians, and more recent clade-specific innovations.

Evolution of bilaterian central nervous systems: a single origin?

The question of whether the ancestral bilaterian had a central nervous system (CNS) or a diffuse ectodermal nervous system has been hotly debated. Considerable evidence supports the theory that a CNS evolved just once. However, an alternative view proposes that the chordate CNS evolved from the ectodermal nerve net of a hemichordate-like ancestral deuterostome, implying independent evolution of the CNS in chordates and protostomes. To specify morphological divisions along the anterior/posterior axis, this ancestor used gene networks homologous to those patterning three organizing centers in the vertebrate brain: the anterior neural ridge, the zona limitans intrathalamica and the isthmic organizer, and subsequent evolution of the vertebrate brain involved elaboration of these ancestral signaling centers; however, all or part of these signaling centers were lost from the CNS of invertebrate chordates. The present review analyzes the evidence for and against these theories. The bulk of the evidence indicates that a CNS evolved just once - in the ancestral bilaterian. Importantly, in both protostomes and deuterostomes, the CNS represents a portion of a generally neurogenic ectoderm that is internalized and receives and integrates inputs from sensory cells in the remainder of the ectoderm. The expression patterns of genes involved in medio/lateral (dorso/ventral) patterning of the CNS are similar in protostomes and chordates; however, these genes are not similarly expressed in the ectoderm outside the CNS. Thus, their expression is a better criterion for CNS homologs than the expression of anterior/posterior patterning genes, many of which (for example, Hox genes) are similarly expressed both in the CNS and in the remainder of the ectoderm in many bilaterians. The evidence leaves hemichordates in an ambiguous position - either CNS centralization was lost to some extent at the base of the hemichordates, or even earlier, at the base of the hemichordates + echinoderms, or one of the two hemichordate nerve cords is homologous to the CNS of protostomes and chordates. In any event, the presence of part of the genetic machinery for the anterior neural ridge, the zona limitans intrathalamica and the isthmic organizer in invertebrate chordates together with similar morphology indicates that these organizers were present, at least in part, at the base of the chordates and were probably elaborated upon in the vertebrate lineage.

The evolution of nervous system patterning: insights from sea urchin development

Recent studies of the sea urchin embryo have elucidated the mechanisms that localize and pattern its nervous system. These studies have revealed the presence of two overlapping regions of neurogenic potential at the beginning of embryogenesis, each of which becomes progressively restricted by separate, yet linked, signals, including Wnt and subsequently Nodal and BMP. These signals act to specify and localize the embryonic neural fields - the anterior neuroectoderm and the more posterior ciliary band neuroectoderm - during development. Here, we review these conserved nervous system patterning signals and consider how the relationships between them might have changed during deuterostome evolution.

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 (NiCl₂), a potent ventralizing agent on echinoderm embryos, on the indirect developing enteropneust hemichordate, Ptychodera flava. Our present study shows that NiCl₂ 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 NiCl₂ 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 NiCl₂ 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.

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.

Conserved and novel roles for the Gsh2 transcription factor in primary neurogenesis

The Gsx genes encode members of the ParaHox family of homeodomain transcription factors, which are expressed in the developing central nervous system in members of all major groups of bilaterians. The Gsx genes in Xenopus show similar patterns of expression to their mammalian homologues during late development. However, they are also expressed from early neurula stages in an intermediate region of the open neural plate where primary interneurons form. The Gsx homologue in the protostome Drosophila is expressed in a corresponding intermediate region of the embryonic neuroectoderm, and is essential for the correct specification of the neuroblasts that arise from it, suggesting that Gsx genes may have played a role in intermediate neural specification in the last common bilaterian ancestor. Here, we show that manipulation of Gsx function disrupts the differentiation of primary interneurons. We demonstrate that, despite their similar expression patterns, the uni-directional system of interactions between homeodomain transcription factors from the Msx, Nkx and Gsx families in the Drosophila neuroectoderm is not conserved between their homologues in the Xenopus open neural plate. Finally, we report the identification of Dbx1 as a direct target of Gsh2-mediated transcriptional repression, and show that a series of cross-repressive interactions, reminiscent of those that exist in the amniote neural tube, act between Gsx, Dbx and Nkx transcription factors to pattern the medial aspect of the central nervous system at open neural plate stages in Xenopus.

A conserved role for the nodal signaling pathway in the establishment of dorso-ventral and left–right axes in deuterostomes

Nodal factors play crucial roles during embryogenesis of chordates. They have been implicated in a number of developmental processes, including mesoderm and endoderm formation and patterning of the embryo along the anterior-posterior and left-right axes. We have analyzed the function of the Nodal signaling pathway during the embryogenesis of the sea urchin, a non-chordate organism. We found that Nodal signaling plays a central role in axis specification in the sea urchin, but surprisingly, its first main role appears to be in ectoderm patterning and not in specification of the endoderm and mesoderm germ layers as in vertebrates. Starting at the early blastula stage, sea urchin nodal is expressed in the presumptive oral ectoderm where it controls the formation of the oral-aboral axis. A second conserved role for nodal signaling during vertebrate evolution is its involvement in the establishment of left-right asymmetries. Sea urchin larvae exhibit profound left-right asymmetry with the formation of the adult rudiment occurring only on the left side. We found that a nodal/lefty/pitx2 gene cassette regulates left-right asymmetry in the sea urchin but that intriguingly, the expression of these genes is reversed compared to vertebrates. We have shown that Nodal signals emitted from the right ectoderm of the larva regulate the asymmetrical morphogenesis of the coelomic pouches by inhibiting rudiment formation on the right side of the larva. This result shows that the mechanisms responsible for patterning the left-right axis are conserved in echinoderms and that this role for nodal is conserved among the deuterostomes. We will discuss the implications regarding the reference axes of the sea urchin and the ancestral function of the nodal gene in the last section of this review.

Phylogenetic correspondence of the body axes in bilaterians is revealed by the right-sided expression of Pitx genes in echinoderm larvae

Chordates and echinoderms are two of the three major deuterostome phyla and show conspicuous left-right (LR) asymmetry. The establishment of LR asymmetry has been explored in vertebrates, but is largely unknown in echinoderms. Here, we report the expression pattern of genes that are orthologous to the chordate left-side specific gene Pitx, cloned from the sea urchin Hemicentrotus pulcherrimus (HpPitx) and the starfish Asterina pectinifera (ApPitx). HpPitx transcripts were first detected bilaterally in one cell of the ventrolateral primary mesenchyme-cell aggregate of early prism larvae. New expression was detected asymmetrically in the right counterpart of a bilateral pair of mesodermal coelomic pouches and in the right ectoderm. In starfish bipinnaria larvae, the ApPitx signal was detected in the right coelomic pouch and in the right half of the ectoderm along the ciliary bands. These results suggest that the function of Pitx in establishing LR asymmetry was introduced in the last common ancestor of echinoderms and chordates. However, the right-side specific expression in echinoderm larvae is inverted compared to chordate embryos. This indicates that the LR axis is inversely represented between echinoderms and chordates, which supports the scenario that dorsoventral axis inversion was introduced into the chordate lineage by turning upside down.

Sog/Chordin is required for ventral-to-dorsal Dpp/BMP transport and head formation in a short germ insect

Bone morphogenetic protein (BMP) signaling plays a major role in dorsoventral patterning in vertebrates and in Drosophila. Remarkably, in Tribolium, a beetle with an ancestral type of insect development, early BMP/dpp exhibits differential expression along the anteroposterior axis. However, the BMP/Dpp inhibitor Sog/chordin is expressed ventrally and establishes a dorsal domain of BMP/Dpp activity by transporting BMPs toward the dorsal side, like in Drosophila. Loss of Tribolium Sog not only abolishes dorsoventral polarity in the ectoderm, but also leads to the complete absence of the CNS. This phenotype suggests that sog is the main BMP antagonist in Tribolium, in contrast to vertebrates and Drosophila, which possess redundant antagonists. Surprisingly, Sog also is required for head formation in Tribolium, as are the BMP antagonists in vertebrates. Thus, in Tribolium, the system of BMP and its antagonists is less complex than in Drosophila or vertebrates and combines features from both, suggesting that it might represent an ancestral state.

Molecular evidence for deep evolutionary roots of bilaterality in animal development

Nearly all metazoans show signs of bilaterality, yet it is believed the bilaterians arose from radially symmetric forms hundreds of millions of years ago. Cnidarians (corals, sea anemones, and "jellyfish") diverged from other animals before the radiation of the Bilateria. They are diploblastic and are often characterized as being radially symmetrical around their longitudinal (oral-aboral) axis. We have studied the deployment of orthologs of a number of family members of developmental regulatory genes that are expressed asymmetrically during bilaterian embryogenesis from the sea anemone, Nematostella vectensis. The secreted TGF-beta genes Nv-dpp, Nv-BMP5-8, six TGF-beta antagonists (NvChordin, NvNoggin1, NvNoggin2, NvGremlin, NvFollistatin, and NvFollistatin-like), the homeodomain proteins NvGoosecoid (NvGsc) and NvGbx, and the secreted guidance factor, NvNetrin, were studied. NvDpp, NvChordin, NvNoggin1, NvGsc, and NvNetrin are expressed asymmetrically along the axis perpendicular to the oral-aboral axis, the directive axis. Furthermore, NvGbx, and NvChordin are expressed in restricted domains on the left and right sides of the body, suggesting that the directive axis is homologous with the bilaterian dorsal-ventral axis. The asymmetric expression of NvNoggin1 and NvGsc appear to be maintained by the canonical Wnt signaling pathway. The asymmetric expression of NvNoggin1, NvNetrin, and Hox orthologs NvAnthox7, NvAnthox8, NvAnthox1a, and NvAnthox6, in conjunction with the observation that NvNoggin1 is able to induce a secondary axis in Xenopus embryos argues that N. vectensis could possess antecedents of the organization of the bilaterian central nervous system.

Comparative Developmental Genetics and the Evolution of Arthropod Body Plans

The arthropods display a wide range of morphological diversity, varying tagmosis, as well as other aspects of the body plan, such as appendage and cuticular morphology. Here we review the roles of developmental regulatory genes in the evolution of arthropod morphology, with an emphasis on what is known from morphologically diverse species. Examination of tagmatic evolution reveals that these changes have been accompanied by changes in the expression patterns of Hox genes. In contrast, review of the modifications to wing morphology seen in insects shows that these body plan changes have generally favored alterations in downstream target genes. These and other examples are used to discuss the evolutionary implications of comparative developmental genetic data.

Evolution of the heart from bacteria to man

This review provides an overview of the evolutionary path to the mammalian heart from the beginnings of life (about four billion years ago ) to the present. Essential tools for cellular homeostasis and for extracting and burning energy are still in use and essentially unchanged since the appearance of the eukaryotes. The primitive coelom, characteristic of early multicellular organisms ( approximately 800 million years ago), is lined by endoderm and is a passive receptacle for gas exchange, feeding, and sexual reproduction. The cells around this structure express genes homologous to NKX2.5/tinman, and gradual specialization of this "gastroderm" results in the appearance of mesoderm in the phylum Bilateria, which will produce the first primitive cardiac myocytes. Investment of the coelom by these mesodermal cells forms a "gastrovascular" structure. Further evolution of this structure in the bilaterian branches Ecdysoa (Drosophila) and Deuterostoma (amphioxus) culminate in a peristaltic tubular heart, without valves, without blood vessels or blood, but featuring a single layer of contracting mesoderm. The appearance of Chordata and subsequently the vertebrates is accompanied by a rapid structural diversification of this primitive linear heart: looping, unidirectional circulation, an enclosed vasculature, and the conduction system. A later innovation is the parallel circulation to the lungs, followed by the appearance of septa and the four-chambered heart in reptiles, birds, and mammals. With differentiation of the cardiac chambers, regional specialization of the proteins in the cardiac myocyte can be detected in the teleost fish and amphibians. In mammals, growth constraints are placed on the heart, presumably to accommodate the constraints of the body plan and the thoracic cavity, and adult cardiac myocytes lose the ability to re-enter the cell cycle on demand. Mammalian cardiac myocyte innervation betrays the ancient link between the heart, the gut, and reproduction: the vagus nerve controlling heart rate emanates from centers in the central nervous system regulating feeding and affective behavior.

Genes and homology in nervous system evolution: comparing gene functions, expression patterns, and cell type molecular fingerprints

The evolution of the nervous system is one of the most fascinating, but also most nebulous fields of homology research. We do not know for example whether the last common ancestors of human, squid, and fly already possessed an elaborate brain and eyes, or rather had a simple, diffuse nervous system. Nevertheless, in the past decade molecular data has greatly advanced our understanding of bilaterian nervous system evolution. In this methodological review, I explain the four levels on which molecular genetic studies advance the quest for homologies between animal nervous systems. (I) Bioinformatic homology research elucidates the evolutionary history of gene families relevant for nervous system evolution such as the opsin superfamily. It tells us when and in what order genes and their functions have emerged. Based on this, we can (II) infer the organismal complexity of some remote ancestor from the functional diversity of its reconstructed proteome. (III) Most common in molecular homology research has been the comparison of expression patterns of developmental control genes. This approach matches and aligns embryonic regions along the body axes, between remote bilaterians. It does not tell us much, however, about the complexity of structures that developed from these regions in Urbilateria. (IV) This is overcome by a novel variant of molecular homology research, the comparison of cell types. Here, a similar "molecular fingerprint" of cells is taken as indication of cross-bilaterian homology. This approach makes it possible to reconstruct the cell-type repertoire of the urbilaterian nervous system.

Ciona intestinalis: chordate development made simple

Thanks to their transparent and rapidly developing mosaic embryos, ascidians (or sea squirts) have been a model system for embryological studies for over a century. Recently, ascidians have entered the postgenomic era, with the sequencing of the Ciona intestinalis genome and the accumulation of molecular resources that rival those available for fruit flies and mice. One strength of ascidians as a model system is their close similarity to vertebrates. Literature reporting molecular homologies between vertebrate and ascidian tissues has flourished over the past 15 years, since the first ascidian genes were cloned. However, it should not be forgotten that ascidians diverged from the lineage leading to vertebrates over 500 million years ago. Here, we review the main similarities and differences so far identified, at the molecular level, between ascidian and vertebrate tissues and discuss the evolution of the compact ascidian genome.

Insights into the urbilaterian brain: conserved genetic patterning mechanisms in insect and vertebrate brain development

Recent molecular genetic analyses of Drosophila melanogaster and mouse central nervous system (CNS) development revealed strikingly similar genetic patterning mechanisms in the formation of the insect and vertebrate brain. Thus, in both insects and vertebrates, the correct regionalization and neuronal identity of the anterior brain anlage is controlled by the cephalic gap genes otd/Otx and ems/Emx, whereas members of the Hox genes are involved in patterning of the posterior brain. A third intermediate domain on the anteroposterior axis of the vertebrate and insect brain is characterized by the expression of the Pax2/5/8 orthologues, suggesting that the tripartite ground plans of the protostome and deuterostome brains share a common evolutionary origin. Furthermore, cross-phylum rescue experiments demonstrate that insect and mammalian members of the otd/Otx and ems/Emx gene families can functionally replace each other in embryonic brain patterning. Homologous genes involved in dorsoventral regionalization of the CNS in vertebrates and insects show remarkably similar patterning and orientation with respect to the neurogenic region (ventral in insects and dorsal in vertebrates). This supports the notion that a dorsoventral body axis inversion occurred after the separation of protostome and deuterostome lineages in evolution. Taken together, these findings demonstrate conserved genetic patterning mechanisms in insect and vertebrate brain development and suggest a monophyletic origin of the brain in protostome and deuterostome bilaterians.

Left-right asymmetry in the sea urchin embryo is regulated by nodal signaling on the right side

The asymmetric positioning of internal organs on the left or right side of the body is highly conserved in vertebrates and relies on a Nodal signaling pathway acting on the left side of the embryo. Whether the same pathway also regulates left-right asymmetry in invertebrates and what is the evolutionary origin of the mechanisms controlling left-right determination are not known. Here, we show that nodal regulates left-right asymmetry in the sea urchin but that, intriguingly, its expression is reversed compared to vertebrates. Nodal signals emitted from the right side of the larva prevent the right coelomic pouch from forming the imaginal rudiment. Inhibition of Nodal signaling after gastrulation causes formation of an ectopic rudiment on the right side, leading to twinned urchins after metamorphosis. In contrast, ectopic activation of the pathway prevents formation of the rudiment. Our results show that the mechanisms responsible for left-right determination are conserved within basal deuterostomes.

Co-option of an oral-aboral patterning mechanism to control left-right differentiation: the direct-developing sea urchin Heliocidaris erythrogramma is sinistralized, not ventralized, by NiCl2

Larval dorsoventral (DV) and left-right (LR) axial patterning unfold progressively in sea urchin development, leading to commitment of the major embryonic regions by the gastrula stage. The direct-developing sea urchin Heliocidaris erythrogramma has lost oral-aboral differentiation along the DV axis but has accelerated vestibular ectoderm development on the left side. NiCl(2) radializes indirect-developing sea urchins by shifting cells toward a ventral fate (oral ectoderm). We treated embryos of H. erythrogramma and the indirect-developing H. tuberculata with NiCl(2). H. tuberculata was ventralized exactly like other indirect developers, establishing that basic patterning mechanisms are conserved in this genus. H. erythrogramma was also radialized; timing, dosage response, and some morphological features were similar to those in other sea urchins. Ectodermal explant and recombination experiments demonstrate that the effect of nickel is autonomous to the ectoderm, another feature in common with indirect developers. However, H. erythrogramma is distinctly sinistralized rather than ventralized, its cells shifting toward a left-side fate (vestibular ectoderm). This geometric contrast in the midst of pervasive functional similarity suggests that nickel-sensitive processes in H. erythrogramma axial patterning, homologous to those in indirect developers, have been redeployed, and hence co-opted, from their ancestral role in DV axis determination to a new role in LR axis determination. We discuss DV and LR axial patterning and their evolutionary transformation.

From larval bodies to adult body plans: patterning the development of the presumptive adult ectoderm in the sea urchin larva

Echinoderms are unique among bilaterians for their derived, nonbilateral adult body plan. Their radial symmetry emerges from the bilateral larval body plan by the establishment of a new axis, the adult oral-aboral axis, involving local mesoderm-ectoderm interactions. We examine the mechanisms underlying this transition in the direct-developing sea urchin Heliocidaris erythrogramma. Adult ectoderm arises from vestibular ectoderm in the left vegetal quadrant. Inductive signals from the left coelom are required for adult ectodermal development but not for initial vestibule formation. We surgically removed gastrula archenteron, making whole-ectoderm explants, left-, right-, and animal-half ectoderm explants, and recombinants of these explants with left coelom. Vestibule formation was analyzed morphologically and with radioactive in situ hybridization with HeET-1, an ectodermal marker. Whole ectodermal explants in the absence of coelom developed vestibules on the left side or ventrally but not on the right side, indicating that left-right polarity is ectoderm autonomous by the gastrula stage. However, right-half ectodermal explants robustly formed vestibules that went on to form adult structures when recombined with the left coelom, indicating that the right side retains vestibule-forming potential that is normally suppressed by signals from the left-side ectoderm. Animal-half explants formed vestibules only about half the time, demonstrating that animal-vegetal axis determination occurs earlier. However, when combined with the left coelom, animal-half ectoderm always formed a vestibule, indicating that the left coelom can induce vestibule formation. This suggests that although coelomic signals are not required for vestibule formation, they may play a role in coordinating the coelom-vestibule interaction that establishes the adult oral-aboral axis.

A phylogeny of the hemichordates based on morphological characters

A comprehensive review of literature on all 15 genera constituting the phylum Hemichordata resulted in a morphological matrix of 105 characters. The echinoderms, tunicates, cephalochordates, and vertebrates were included in the analysis, and the cnidarians, polychaetes, and sipunculids were employed as outgroup taxa. The consensus tree supported the traditional view of a monophyletic Hemichordata, Echinodermata, Ambulacraria, and Chordata. The enteropneust families Spengelidae and Ptychoderidae were each monophyletic and sister-taxa, but there was no resolution among the family Harrimaniidae. A detailed sensitivity analysis provided (i) tree lengths of competing evolutionary hypothesis and (ii) a test of monophyly of groups under a variety of evolutionary models. It is argued that the ancestral deuterostome was a benthic vermiform organism with a terminal mouth and anus, well-developed circular and longitudinal muscles, a simple nerve plexus with little sign of regionalization, a pharynx with gill slits and collagenous gill bars, a cluster of vacuolated cells with myofilaments, produced iodotyrosine, and displayed direct development. The pterobranchs have lost many of these features as a consequence of evolving a small body size and living in tubes, but these features exist in present-day enteropneusts, suggesting that they are a plausible model for the proximate ancestor of deuterostomes.

Protochordate body plan and the evolutionary role of larvae: old controversies resolved?

Motile larvae figure prominently in a number of past scenarios for chordate and vertebrate origins, notably in the writings of Garstang, Berrill, and Romer. All three focus on the motile larva of a primitively sessile tunicate ancestor as a vertebrate progenitor; Garstang went further in deriving chordates themselves by neoteny from a yet more ancient larva of the dipleurula type. Yet the molecular evidence currently available shows convincingly that the part of the tunicate larva that persists to the adult expresses only a subset of the genes required to specify a complete bilaterian body axis, and essentially the same appears to be true of dipleurula larvae. Specifically, both are essentially heads without trunks. Hence, both are highly derived and as such are probably poor models for any real ancestor. A more convincing case can be made for a sequence of ancestral forms that throughout their evolution were active, motile organisms expressing a full complement of axial patterning genes. This implies a basal, ancestral form resembling modern enteropneusts, although a pelagic organism at a hemichordate level of complexity is also possible. A reassessment is thus required of the role played by adult and larval tunicates, and of larvae more generally, in chordate evolution. Tunicates need to be interpreted with caution, since the extreme degree of modification in the adult may have been accompanied by reductions to the larva. Dipleurula larvae may retain some ancestral features (e.g., of apical, oral, and anal organization), but are otherwise probably too specialized to be central players in chordate evolution. Garstang nevertheless remains a key figure in the history of evolutionary thought for his innovative ideas on the relation between ontogeny and phylogeny, and the way in which major innovations in morphology and body plan arise.

The dawn of bilaterian animals: the case of acoelomorph flatworms

The origin of the bilaterian metazoans from radial ancestors is one of the biggest puzzles in animal evolution. A way to solve it is to identify the nature and main features of the last common ancestor of the bilaterians (LCB). Recent progress in molecular phylogeny has shown that many platyhelminth flatworms, regarded for a long time as basal bilaterians, now belong to the lophotrochozoan protostomates. In contrast, the LCB is now considered a complex organism bearing several features of modern bilaterians. Here we discuss an alternative view, in which acoelomorph (Acoela + Nemertodermatida) flatworms, which do not belong to the Platyhelminthes, represent the earliest extant bilaterian clade. Sequences from ribosomal and other nuclear genes, Hox cluster genes, and reinterpretation of some morphological features strongly support the basal position of acoelomorphs arguing against a complex LCB. This reconstruction backs the old planuloid-acoeloid hypothesis and may help our understanding of the evolution of body axes, Hox genes and the Cambrian explosion.

Developmental mechanism and evolutionary origin of vertebrate left/right asymmetries

The systematically 'handed', or directionally asymmetrical way in which the major viscera are packed within the vertebrate body is known as situs. Other less obvious vertebrate lateralisations concern cognitive neural function, and include the human phenomena of hand-use preference and language-associated cognitive partitioning. An overview, rather than an exhaustive scholarly review, is given of recent advances in molecular understanding of the mechanism that ensures normal development of 'correct' situs. While the asymmetry itself and its left/right direction are clearly vertebrate-conserved characters, data available from various embryo types are compared in order to assess the likelihood that the developmental mechanism is evolutionarily conserved in its entirety. A conserved post-gastrular 'phylotypic' stage, with left- and right-specific cascades of key, orthologous gene expressions, clearly exists. It now seems probable that earlier steps, in which symmetry-breaking information is reliably transduced to trigger these cascades on the correct sides, are also conserved at depth although it remains unclear exactly how these steps operate. Earlier data indicated that the initiation of symmetry-breaking had been transformed, among the different vertebrate classes, as drastically as has the anatomy of pre-gastrular development itself, but it now seems more likely that this apparent diversity is deceptive. Ideas concerning the functional advantages to the vertebrate lifestyle of a systematically asymmetrical visceral packing arrangement, while untestable, are accepted because they form a plausible adaptationist 'just-so' story. Nevertheless, two contrasting beliefs are possible about the evolutionary origins of situs. Major recent advances in analysis of its developmental mechanism are largely due not to zoologists, comparative anatomists or evolutionary systematists, but to molecular geneticists, and these workers have generally assumed that the asymmetry is an evolutionary novelty imposed on a true bilateral symmetry, at or close to the origin of the vertebrate clade. A major purpose of this review is to advocate an alternative view, on the grounds of comparative anatomy and molecular systematics together with the comparative study of expressions of orthologous genes in different forms. This view is that situs represents a co-optation of a pre-existing, evolutionarily ancient non-bilaterality of the adult form in a vertebrate ancestor. Viewed this way, vertebrate or chordate origins are best understood as the novel imposition of an adaptively bilateral locomotory-skeletal-neural system, around a retained non-symmetrical 'visceral' animal. One component of neuro-anatomical asymmetry, the habenular/parapineal one that originates in the diencephalon, has recently been found (in teleosts) to be initiated from the same 'phylotypic' gene cascade that controls situs development. But the function of this particular diencephalic asymmetry is currently unclear. Other left-right partitionings of brain function, including the much more recently evolved, cerebral cortically located one associated with human language and hand-use, may be controlled entirely separately from situs even though their directionality has a particular relation to it in a majority of individuals. Finally, possible relationships are discussed between the vertebrate directional asymmetries and those that occur sporadically among protostome bilaterian forms. These may have very different evolutionary and molecular bases, such that there may have been constraints, in protostome evolution, upon any exploitation of left and right for complex organismic, and particularly cognitive neural function.

Proliferation and Differentiation Processes in the Heart Muscle Elements in Different Phylogenetic Groups

This article reviews, discusses, and summarizes data about the generative behavior of muscle tissue cells, the mechanisms of its regulation, and the organization of the endocrine function of the heart in the main phylogenetic groups. With respect to the ratio of processes of proliferation and differentiation, cell organization, and growth mechanism, muscle tissues of propulsive organs can be divided into three types, each revealed in one of three main groups of animals, lophotrochozoans, ecdysozoans, and chordates. Ecdysterone is likely to play the key role in the regulation of proliferation and differentiation processes in the heart muscle of crustaceans, and, most probably, also of molluscs. In each of the three main phylogenetic groups the endocrine function of the heart consisting of secretion of natriuretic peptides has a peculiar organization. Vertebrate cardiomyocytes are known to combine contractile and endocrine differentiation. Such functional dualism is absent in heart muscle elements of Lophotrochozoa and Ecdysozoa; in the heart of lopfotrochozoans, secretion of natriuretic peptides is performed by endothelial cells and their derivatives. Homology of the heart muscle in the animal kingdom as well as possible mechanisms of genomic and epigenomic regulation of different types of cardiomyogenesis are discussed.

The evolutionary origins and significance of vertebrate left-right organisation

In the last few years, an understanding has emerged of the developmental mechanism for the consistent internal left-right structure, termed situs, that characterises vertebrate anatomy. This involves largely vertebrate-conserved (i.e. 'phylotypic') gene expression cascades that encode 'leftness' and 'rightness' in appropriate tissues either side of the embryo's midline soon after gastrulation. Recent evidence indicates that the initial, directional symmetry breaking that initiates these cascades utilises mechanisms that are conserved or at least closely related in different vertebrate types. I describe a scenario whereby the capacity for directional modification of an otherwise bilateral body plan can be viewed as an adaptive innovation rather closely connected with vertebrate origins, enabling optimal 'design' for very active lifestyles. But an alternative scenario, while retaining the view that situs and indeed other vertebrate functional lateralisations are deeply adaptive, proposes that they originated in the co-optation of left-right developmental information inherited from a very early stage in metazoan diversification. It is proposed that a remote chordate ancestor lost its original or 'ur-bilaterian' symmetry to pass through an altogether non-symmetrical stage, and that the vertebrate dorsoventral midline plane is not descended from that original one. I review the considerable evidence in favour of this scenario, and discuss its wider implications for directional asymmetries across the Metazoa.

Descent with modification: the unity underlying homology and homoplasy as seen through an analysis of development and evolution

Homology is at the foundation of comparative studies in biology at all levels from genes to phenotypes. Homology is similarity because of common descent and ancestry, homoplasy is similarity arrived at via independent evolution. However, given that there is but one tree of life, all organisms, and therefore all features of organisms, share some degree of relationship and similarity one to another. That sharing may be similarity or even identity of structure and the sharing of a most recent common ancestor--as in the homology of the arms of humans and apes--or it may reflect some (often small) degree of similarity, such as that between the wings of insects and the wings of birds, groups whose shared ancestor lies deep within the evolutionary history of the Metazoa. It may reflect sharing of entire developmental pathways, partial sharing, or divergent pathways. This review compares features classified as homologous with the classes of features normally grouped as homoplastic, the latter being convergence, parallelism, reversals, rudiments, vestiges, and atavisms. On the one hand, developmental mechanisms may be conserved, even when a complete structure does not form (rudiments, vestiges), or when a structure appears only in some individuals (atavisms). On the other hand, different developmental mechanisms can produce similar (homologous) features. Joint examination of nearness of relationship and degree of shared development reveals a continuum within an expanded category of homology, extending from homology --> reversals --> rudiments --> vestiges --> atavisms --> parallelism, with convergence as the only class of homoplasy, an idea that turns out to be surprisingly old. This realignment provides a glimmer of a way to bridge phylogenetic and developmental approaches to homology and homoplasy, a bridge that should provide a key pillar for evolutionary developmental biology (evo-devo). It will not, and in a practical sense cannot, alter how homoplastic features are identified in phylogenetic analyses. But seeing rudiments, reversals, vestiges, atavisms and parallelism as closer to homology than to homoplasy should guide us toward searching for the common elements underlying the formation of the phenotype (what some have called the deep homology of genetic and/or cellular mechanisms), rather than discussing features in terms of shared or independent evolution.

The urbilaterian brain: developmental insights into the evolutionary origin of the brain in insects and vertebrates

Classical phylogenetic, neuroanatomical and neuroembryological studies propose an independent evolutionary origin of the brains of insects and vertebrates. Contrasting with this, data from three sets of molecular and genetic analyses indicate that the developmental program of brains of insects and vertebrates might be highly conserved and suggest a monophyletic origin of the brain of protostomes and deuterostomes. First, recent results of molecular phylogeny imply that none of the currently living animals correspond to evolutionary intermediates between protostomes and deuterostomes, thus making it impossible to infer the morphological organization of an ancestral bilaterian brain from living specimens. Second, recent molecular genetic evidence provides support for the body axis inversion hypothesis, which implies that a dorsoventral inversion of the body axis occurred in protostomes versus deuterostomes, leading to the inverted location of neurogenic regions in these animal groups. Third, recent developmental genetic analyses are uncovering the existence of structurally and functionally homologous genes that have comparable and interchangeable functions in early brain development in insect and vertebrate model systems. Thus, development of the anteriormost part of the embryonic brain in both insects and vertebrates depends upon the otd/Otx and ems/Emx genes; development of the posterior part of the embryonic brain in both insects and vertebrates involves homologous control genes of the Hox cluster. These findings, which demonstrate the conserved expression and function of key patterning genes involved in embryonic brain development in insects and vertebrates support the hypothesis that the brains of protostomes and deuterostomes are of monophyletic, urbilaterian origin.

Testing putative hemichordate homologues of the chordate dorsal nervous system and endostyle: expression of NK2.1 (TTF-1) in the acorn worm Ptychodera flava (Hemichordata, Ptychoderidae)

Recent phylogenetic investigations have confirmed that hemichordates and echinoderms are sister taxa. However, hemichordates share several cardinal characterstics with chordates and are thus an important taxon for testing hypotheses of homology between key chordate characters and their putative hemichordate antecedents. The chordate dorsal nervous system (DNS) and endostyle are intriguing characters because both hemichordate larval and adult structures have been hypothesized as homologues. This study attempts to test these purported homologies through examination of the expression pattem of a Ptychodera flava NK2 gene, PfNK2.1, because this gene is expressed both in the DNS and endostyle/thyroid in a wide range of chordate taxa. We found that PfNK2.1 is expressed in both neuronal and pharyngeal structures, but its expression pattem is broken up into distinct embryonic and juvenile phases. During embryogenesis, PfNK2.1 is expressed in the apical ectoderm, with transcripts later detected in presumable neuronal structures, including the apical organ and ciliated feeding band. In the developing juvenile we detected PfNK2.1 signal throughout the pharynx, including the stomochord, and later in the hindgut. We conclude that the similar utilization of NK2.1 in apical organ development and chordate DNS is probably due to a more general role for NK2.1 in neurogenesis and that hemichordates do not possess a homologue of the chordate DNS. In addition, we conclude that P. flava most likely does not possess a true endostyle; rather during the evolution of the endostyle NK2.1 was recruited from its more general role in pharynx development.

Vetulicolians--are they deuterostomes? chordates?

A recent paper by Shu et al.(1) reinterprets the fossil Vetulicola and related forms, all from the Lower Cambrian, as basal deuterostomes, assigning them their own phylum, Vetulicolia. Their conclusion is based on the presence of structures resembling gill slits and a trunk-like region that shows evidence of segmentation. This report summarizes the fossil evidence for their interpretation and evaluates a possible alternative, that vetulicolians may instead be tunicate-like chordates. Implications for our understanding of the nature of the primitive deuterostome (and chordate) body plan are discussed. .

Defining phyla: evolutionary pathways to metazoan body plans

Phyla are defined by two sets of criteria, one morphological and the other historical. Molecular evidence permits the grouping of animals into clades and suggests that some groups widely recognized as phyla are paraphyletic, while some may be polyphyletic; the phyletic status of crown phyla is tabulated. Four recent evolutionary scenarios for the origins of metazoan phyla and of supraphyletic clades are assessed in the light of a molecular phylogeny: the trochaea hypothesis of Nielsen; the clonal hypothesis of Dewel; the set-aside cell hypothesis of Davidson et al.; and a benthic hypothesis suggested by the fossil record. It is concluded that a benthic radiation of animals could have supplied the ancestral lineages of all but a few phyla, is consistent with molecular evidence, accords well with fossil evidence, and accounts for some of the difficulties in phylogenetic analyses of phyla based on morphological criteria.

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.

Conservation of a DPP/BMP signaling pathway in the nonbilateral cnidarian Acropora millepora

Members of the TGF-beta superfamily of signaling molecules are widespread in metazoans, but the evolutionary origin of particular subclasses of signaling mechanisms is poorly defined. The DPP/BMP class, for example, is implicated in dorsal-ventral patterning, neural patterning, and limb development. Here we report the presence of several components of a DPP/BMP-specific signal transduction cascade in a nonbilateral animal, the coral Acropora millepora. The discovery of these components, a putative type I receptor and two putative receptor-activated Smads, suggests that DPP/BMP signaling predates both dorsal-ventral pattern formation and limb development. We postulate that an ancestral role in neuroepithelial patterning may account for the high level of conservation between DPP/BMP signaling components found in this nonbilateral animal and the more complex triploblastic organisms of the arthropod and chordate phyla.

Otx genes in brain morphogenesis

Most of the gene candidates for the control of developmental programmes that underlie brain morphogenesis in vertebrates are the homologues of Drosophila genes coding for signalling molecules or transcription factors. Among these, the orthodenticle group includes the Drosophila orthodenticle (otd) and the vertebrate Otx1 and Otx2 genes, which are mostly involved in fundamental processes of anterior neural patterning. These genes encode transcription factors that recognise specific target sequences through the DNA binding properties of the homeodomain. In Drosophila, mutations of otd cause the loss of the anteriormost head neuromere where the gene is transcribed, suggesting that it may act as a segmentation "gap" gene. In mouse embryos, the expression patterns of Otx1 and Otx2 have shown a remarkable similarity with the Drosophila counterpart. This suggested that they could be part of a conserved control system operating in the brain and different from that coded by the HOX complexes controlling the hindbrain and spinal cord. To verify this hypothesis a series of mouse models have been generated in which the functions of the murine genes were: (i) fully inactivated, (ii) replaced with each others, (iii) replaced with the Drosophila otd gene. Otx1-/- mutants suffer from epilepsy and are affected by neurological, hormonal, and sense organ defects. Otx2-/- mice are embryonically lethal, they show gastrulation impairments and fail in specifying anterior neural plate. Analysis of the Otx1-/-; Otx2+/- double mutants has shown that a minimal threshold level of the proteins they encode is required for the correct positioning of the midbrain-hindbrain boundary (MHB). In vivo otd/Otx reciprocal gene replacement experiments have provided evidence of a general functional equivalence among otd, Otx1 and Otx2 in fly and mouse. Altogether these data highlight a crucial role for the Otx genes in specification, regionalization and terminal differentiation of rostral central nervous system (CNS) and lead to hypothesize that modification of their regulatory control may have influenced morphogenesis and evolution of the brain.