Ancient deuterostome origins of vertebrate brain signalling centres

ERK signaling expands mammalian cortical radial glial cells and extends the neurogenic period

The molecular basis for cortical expansion during evolution remains largely unknown. Here, we report that fibroblast growth factor (FGF)-extracellular signal-regulated kinase (ERK) signaling promotes the self-renewal and expansion of cortical radial glial (RG) cells. Furthermore, FGF-ERK signaling induces bone morphogenic protein 7 (Bmp7) expression in cortical RG cells, which increases the length of the neurogenic period. We demonstrate that ERK signaling and Sonic Hedgehog (SHH) signaling mutually inhibit each other in cortical RG cells. We provide evidence that ERK signaling is elevated in cortical RG cells during development and evolution. We propose that the expansion of the mammalian cortex, notably in human, is driven by the ERK-BMP7-GLI3R signaling pathway in cortical RG cells, which participates in a positive feedback loop through antagonizing SHH signaling. We also propose that the relatively short cortical neurogenic period in mice is partly due to mouse cortical RG cells receiving higher SHH signaling that antagonizes ERK signaling.

Sea lamprey enlightens the origin of the coupling of retinoic acid signaling to vertebrate hindbrain segmentation

Retinoic acid (RA) is involved in antero-posterior patterning of the chordate body axis and, in jawed vertebrates, has been shown to play a major role at multiple levels of the gene regulatory network (GRN) regulating hindbrain segmentation. Knowing when and how RA became coupled to the core hindbrain GRN is important for understanding how ancient signaling pathways and patterning genes can evolve and generate diversity. Hence, we investigated the link between RA signaling and hindbrain segmentation in the sea lamprey Petromyzon marinus, an important jawless vertebrate model providing clues to decipher ancestral vertebrate features. Combining genomics, gene expression, and functional analyses of major components involved in RA synthesis (Aldh1as) and degradation (Cyp26s), we demonstrate that RA signaling is coupled to hindbrain segmentation in lamprey. Thus, the link between RA signaling and hindbrain segmentation is a pan vertebrate feature of the hindbrain and likely evolved at the base of vertebrates.

The Phylotypic Brain of Vertebrates, from Neural Tube Closure to Brain Diversification

BACKGROUND

The phylotypic or intermediate stages are thought to be the most evolutionary conserved stages throughout embryonic development. The contrast with divergent early and later stages derived from the concept of the evo-devo hourglass model. Nonetheless, this developmental constraint has been studied as a whole embryo process, not at organ level. In this review, we explore brain development to assess the existence of an equivalent brain developmental hourglass. In the specific case of vertebrates, we propose to split the brain developmental stages into: (1) Early: Neurulation, when the neural tube arises after gastrulation. (2) Intermediate: Brain patterning and segmentation, when the neuromere identities are established. (3) Late: Neurogenesis and maturation, the stages when the neurons acquire their functionality. Moreover, we extend this analysis to other chordates brain development to unravel the evolutionary origin of this evo-devo constraint.

SUMMARY

Based on the existing literature, we hypothesise that a major conservation of the phylotypic brain might be due to the pleiotropy of the inductive regulatory networks, which are predominantly expressed at this stage. In turn, earlier stages such as neurulation are rather mechanical processes, whose regulatory networks seem to adapt to environment or maternal geometries. The later stages are also controlled by inductive regulatory networks, but their effector genes are mostly tissue-specific and functional, allowing diverse developmental programs to generate current brain diversity. Nonetheless, all stages of the hourglass are highly interconnected: divergent neurulation must have a vertebrate shared end product to reproduce the vertebrate phylotypic brain, and the boundaries and transcription factor code established during the highly conserved patterning will set the bauplan for the specialised and diversified adult brain.

KEY MESSAGES

The vertebrate brain is conserved at phylotypic stages, but the highly conserved mechanisms that occur during these brain mid-development stages (Inducing Regulatory Networks) are also present during other stages. Oppositely, other processes as cell interactions and functional neuronal genes are more diverse and majoritarian in early and late stages of development, respectively. These phenomena create an hourglass of transcriptomic diversity during embryonic development and evolution, with a really conserved bottleneck that set the bauplan for the adult brain around the phylotypic stage.

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.

Three foxg1 paralogues in lampreys and gnathostomes-brothers or cousins?

Foxg1 is a key regulator of the early development of the vertebrate forebrain and sensory organs. In this study, we describe for the first time three foxg1 paralogues in lamprey, representative of one of two basally diverged lineages of vertebrates-the agnathans. We also first describe three foxg1 genes in sterlet-representative of one of the evolutionarily ancient clades of gnathostomes. According to the analysis of local genomic synteny, three foxg1 genes of agnathans and gnathostomes have a common origin as a result of two rounds of genomic duplications in the early evolution of vertebrates. At the same time, it is difficult to reliably establish pairwise orthology between foxg1 genes of agnathans and gnathostomes based on the analysis of phylogeny and local genomic synteny, as well as our studies of the spatiotemporal expression of foxg1 genes in the river lamprey Lampetra fluviatilis and the sterlet Acipenser ruthenus. Thus, the appearance of three foxg1 paralogues in agnathans and gnathostomes could have occurred either as a result of two rounds of duplication of the vertebrate common ancestor genome (2R hypothesis) or as a result of the first common round followed by subsequent independent polyploidizations in two evolutionary lineages (1R hypothesis).

Integrating Complex Life Cycles in Comparative Developmental Biology

The goal of comparative developmental biology is identifying mechanistic differences in embryonic development between different taxa and how these evolutionary changes have led to morphological and organizational differences in adult body plans. Much of this work has focused on direct-developing species in which the adult forms straight from the embryo and embryonic modifications have direct effects on the adult. However, most animal lineages are defined by indirect development, in which the embryo gives rise to a larval body plan and the adult forms by transformation of the larva. Historically, much of our understanding of complex life cycles is viewed through the lenses of ecology and zoology. In this review, we discuss the importance of establishing developmental rather than morphological or ecological criteria for defining developmental mode and explicitly considering the evolutionary implications of incorporating complex life cycles into broad developmental comparisons of embryos across metazoans.

Molecular evidence of anteroposterior patterning in adult echinoderms

The origin of the pentaradial body plan of echinoderms from a bilateral ancestor is one of the most enduring zoological puzzles1,2. Because echinoderms are defined by morphological novelty, even the most basic axial comparisons with their bilaterian relatives are problematic. To revisit this classical question, we used conserved anteroposterior axial molecular markers to determine whether the highly derived adult body plan of echinoderms masks underlying patterning similarities with other deuterostomes. We investigated the expression of a suite of conserved transcription factors with well-established roles in the establishment of anteroposterior polarity in deuterostomes3-5 and other bilaterians6-8 using RNA tomography and in situ hybridization in the sea star Patiria miniata. The relative spatial expression of these markers in P. miniata ambulacral ectoderm shows similarity with other deuterostomes, with the midline of each ray representing the most anterior territory and the most lateral parts exhibiting a more posterior identity. Strikingly, there is no ectodermal territory in the sea star that expresses the characteristic bilaterian trunk genetic patterning programme. This finding suggests that from the perspective of ectoderm patterning, echinoderms are mostly head-like animals and provides a developmental rationale for the re-evaluation of the events that led to the evolution of the derived adult body plan of echinoderms.

Gene expression mapping of the neuroectoderm across phyla - conservation and divergence of early brain anlagen between insects and vertebrates

Gene expression has been employed for homologizing body regions across bilateria. The molecular comparison of vertebrate and fly brains has led to a number of disputed homology hypotheses. Data from the fly Drosophila melanogaster have recently been complemented by extensive data from the red flour beetle Tribolium castaneum with its more insect-typical development. In this review, we revisit the molecular mapping of the neuroectoderm of insects and vertebrates to reconsider homology hypotheses. We claim that the protocerebrum is non-segmental and homologous to the vertebrate fore- and midbrain. The boundary between antennal and ocular regions correspond to the vertebrate mid-hindbrain boundary while the deutocerebrum represents the anterior-most ganglion with serial homology to the trunk. The insect head placode is shares common embryonic origin with the vertebrate adenohypophyseal placode. Intriguingly, vertebrate eyes develop from a different region compared to the insect compound eyes calling organ homology into question. Finally, we suggest a molecular re-definition of the classic concepts of archi- and prosocerebrum.

Molecular characterization of nervous system organization in the hemichordate acorn worm Saccoglossus kowalevskii

Hemichordates are an important group for investigating the evolution of bilaterian nervous systems. As the closest chordate outgroup with a bilaterally symmetric adult body plan, hemichordates are particularly informative for exploring the origins of chordates. Despite the importance of hemichordate neuroanatomy for testing hypotheses on deuterostome and chordate evolution, adult hemichordate nervous systems have not been comprehensively described using molecular techniques, and classic histological descriptions disagree on basic aspects of nervous system organization. A molecular description of hemichordate nervous system organization is important for both anatomical comparisons across phyla and for attempts to understand how conserved gene regulatory programs for ectodermal patterning relate to morphological evolution in deep time. Here, we describe the basic organization of the adult hemichordate Saccoglossus kowalevskii nervous system using immunofluorescence, in situ hybridization, and transgenic reporters to visualize neurons, neuropil, and key neuronal cell types. Consistent with previous descriptions, we found the S. kowalevskii nervous system consists of a pervasive nerve plexus concentrated in the anterior, along with nerve cords on both the dorsal and ventral side. Neuronal cell types exhibited clear anteroposterior and dorsoventral regionalization in multiple areas of the body. We observed spatially demarcated expression patterns for many genes involved in synthesis or transport of neurotransmitters and neuropeptides but did not observe clear distinctions between putatively centralized and decentralized portions of the nervous system. The plexus shows regionalized structure and is consistent with the proboscis base as a major site for information processing rather than the dorsal nerve cord. In the trunk, there is a clear division of cell types between the dorsal and ventral cords, suggesting differences in function. The absence of neural processes crossing the basement membrane into muscle and extensive axonal varicosities suggest that volume transmission may play an important role in neural function. These data now facilitate more informed neural comparisons between hemichordates and other groups, contributing to broader debates on the origins and evolution of bilaterian nervous systems.

Hemichordates' diffuse "skin brain" shows unexpected complexity

Hemichordates are close relatives of chordates. Their nervous system patterning is chordate-like, but their neural architecture remains unexplored. A new study in PLOS Biology reveals an unexpected neuroanatomical complexity in these animals, also informing chordate origins.

Sea lamprey enlightens the origin of the coupling of retinoic acid signaling to vertebrate hindbrain segmentation

Retinoic acid (RA) is involved in antero-posterior patterning of the chordate body axis and, in jawed vertebrates, has been shown to play a major role at multiple levels of the gene regulatory network (GRN) regulating hindbrain segmentation. Knowing when and how RA became coupled to the core hindbrain GRN is important for understanding how ancient signaling pathways and patterning genes can evolve and generate diversity. Hence, we investigated the link between RA signaling and hindbrain segmentation in the sea lamprey Petromyzon marinus, an important jawless vertebrate model providing clues to decipher ancestral vertebrate features. Combining genomics, gene expression, and functional analyses of major components involved in RA synthesis (Aldh1as) and degradation (Cyp26s), we demonstrate that RA signaling is coupled to hindbrain segmentation in lamprey. Thus, the link between RA signaling and hindbrain segmentation is a pan vertebrate feature of the hindbrain and likely evolved at the base of vertebrates.

Phylogenetic tracing of midbrain-specific regulatory sequences suggests single origin of eubilaterian brains

Conserved cis-regulatory elements (CREs) control Engrailed-, Pax2-, and dachshund-related gene expression networks directing the formation and function of corresponding midbrain circuits in arthropods and vertebrates. Polarized outgroup analyses of 31 sequenced metazoan genomes representing all animal clades reveal the emergence of Pax2- and dachshund-related CRE-like sequences in anthozoan Cnidaria. The full complement, including Engrailed-related CRE-like sequences, is only detectable in spiralians, ecdysozoans, and chordates that have a brain; they exhibit comparable genomic locations and extensive nucleotide identities that reveal the presence of a conserved core domain, all of which are absent in non-neural genes and, together, distinguish them from randomly assembled sequences. Their presence concurs with a genetic boundary separating the rostral from caudal nervous systems, demonstrated for the metameric brains of annelids, arthropods, and chordates and the asegmental cycloneuralian and urochordate brain. These findings suggest that gene regulatory networks for midbrain circuit formation evolved within the lineage that led to the common ancestor of protostomes and deuterostomes.

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.

Coevolutionary signals in metabotropic glutamate receptors capture residue contacts and long-range functional interactions

Upon ligand binding to a G protein-coupled receptor, extracellular signals are transmitted into a cell through sets of residue interactions that translate ligand binding into structural rearrangements. These interactions needed for functions impose evolutionary constraints so that, on occasion, mutations in one position may be compensated by other mutations at functionally coupled positions. To quantify the impact of amino acid substitutions in the context of major evolutionary divergence in the G protein-coupled receptor subfamily of metabotropic glutamate receptors (mGluRs), we combined two phylogenetic-based algorithms, Evolutionary Trace and covariation Evolutionary Trace, to infer potential structure-function couplings and roles in mGluRs. We found a subset of evolutionarily important residues at known functional sites and evidence of coupling among distinct structural clusters in mGluR. In addition, experimental mutagenesis and functional assays confirmed that some highly covariant residues are coupled, revealing their synergy. Collectively, these findings inform a critical step toward understanding the molecular and structural basis of amino acid variation patterns within mGluRs and provide insight for drug development, protein engineering, and analysis of naturally occurring variants.

Humans, fish, spiders and bees inherited working memory and attention from their last common ancestor

All brain processes that generate behaviour, apart from reflexes, operate with information that is in an "activated" state. This activated information, which is known as working memory (WM), is generated by the effect of attentional processes on incoming information or information previously stored in short-term or long-term memory (STM or LTM). Information in WM tends to remain the focus of attention; and WM, attention and STM together enable information to be available to mental processes and the behaviours that follow on from them. WM and attention underpin all flexible mental processes, such as solving problems, making choices, preparing for opportunities or threats that could be nearby, or simply finding the way home. Neither WM nor attention are necessarily conscious, and both may have evolved long before consciousness. WM and attention, with similar properties, are possessed by humans, archerfish, and other vertebrates; jumping spiders, honey bees, and other arthropods; and members of other clades, whose last common ancestor (LCA) is believed to have lived more than 600 million years ago. It has been reported that very similar genes control the development of vertebrate and arthropod brains, and were likely inherited from their LCA. Genes that control brain development are conserved because brains generate adaptive behaviour. However, the neural processes that generate behaviour operate with the activated information in WM, so WM and attention must have existed prior to the evolution of brains. It is proposed that WM and attention are widespread amongst animal species because they are phylogenetically conserved mechanisms that are essential to all mental processing, and were inherited from the LCA of vertebrates, arthropods, and some other animal clades.

Homologous gene regulatory networks control development of apical organs and brains in Bilateria

Apical organs are relatively simple larval nervous systems. The extent to which apical organs are evolutionarily related to the more complex nervous systems of other animals remains unclear. To identify common developmental mechanisms, we analyzed the gene regulatory network (GRN) controlling the development of the apical organ in sea urchins. We characterized the developmental expression of 30 transcription factors and identified key regulatory functions for FoxQ2, Hbn, Delta/Notch signaling, and SoxC in the patterning of the apical organ and the specification of neurons. Almost the entire set of apical transcription factors is expressed in the nervous system of worms, flies, zebrafish, frogs, and mice. Furthermore, a regulatory module controlling the axial patterning of the vertebrate brain is expressed in the ectoderm of sea urchin embryos. We conclude that GRNs controlling the formation of bilaterian nervous systems share a common origin and that the apical GRN likely resembles an ancestral regulatory program.

Walking is like slithering: A unifying, data-driven view of locomotion

Legged movement is ubiquitous in nature and of increasing interest for robotics. Most legged animals routinely encounter foot slipping, yet detailed modeling of multiple contacts with slipping exceeds current simulation capacity. Here we present a principle that unifies multilegged walking (including that involving slipping) with slithering and Stokesian (low Reynolds number) swimming. We generated data-driven principally kinematic models of locomotion for walking in low-slip animals (Argentine ant, 4.7% slip ratio of slipping to total motion) and for high-slip robotic systems (BigANT hexapod, slip ratio 12 to 22%; Multipod robots ranging from 6 to 12 legs, slip ratio 40 to 100%). We found that principally kinematic models could explain much of the variability in body velocity and turning rate using body shape and could predict walking behaviors outside the training data. Most remarkably, walking was principally kinematic irrespective of leg number, foot slipping, and turning rate. We find that grounded walking, with or without slipping, is governed by principally kinematic equations of motion, functionally similar to frictional swimming and slithering. Geometric mechanics thus leads to a unified model for swimming, slithering, and walking. Such commonality may shed light on the evolutionary origins of animal locomotion control and offer new approaches for robotic locomotion and motion planning.

Studying development, regeneration, stem cells, and more in the acoel Hofstenia miamia

Acoel worms represent an enigmatic lineage of animals (Acoelomorpha) that has danced around the tree of animal life. Morphology-based classification placed them as flatworms (Phylum Platyhelminthes), with much of their biology being interpreted as a variation on what is observed in better-studied members of that phylum. However, molecular phylogenies suggest that acoels belong to a clade (Xenacoelomorpha) that could be a sister group to other animals with bilateral symmetry (Bilateria) or could belong within deuterostomes, closely related to a group that includes sea stars (Ambulacraria). This change in phylogenetic position has led to renewed interest in the biology of acoels, which can now offer insights into the evolution of many bilaterian traits. The acoel Hofstenia miamia has emerged as a powerful new research organism that enables mechanistic studies of xenacoelomorph biology, especially of developmental and regenerative processes. This article explains the motivation for developing Hofstenia as a new model system, describes Hofstenia biology, highlights the tools and resources that make Hofstenia a good research organism, and considers the questions that Hofstenia is well-positioned to answer. Finally, looking to the future, this article serves as an invitation to new and established scientists to join the growing community of researchers studying this exciting model system.

Saccoglossus kowalevskii: Evo-devo insights from the mud

Hemichordates have long been recognized as a critical group for addressing hypotheses of chordate origins. Historically this was due to anatomical traits that resembled those of chordates, most strikingly the dorsolateral gill slits. As molecular data and phylogenetic analyses were found to support a close phylogenetic relationship between hemichordates and chordates within the deuterostomes, interest was revived in hemichordates. In particular, Saccoglossus kowalevskii has been developed as a molecular model to represent hemichordate developmental biology. Herein, we highlight the considerations when choosing a particular species to study and the challenges we encountered when developing S. kowalevskii. We discuss our findings and how method and tool development enabled them, and how we envision expanding our repertoire of molecular tools in the future. Establishing a new model organism comes with many obstacles-from identifying a reliable season to collect animals, to developing modern molecular techniques. The Saccoglossus research community has benefited greatly from the collaborations and teamwork established over the years. As a result, Saccoglossus is well positioned to contribute to a new century of evolutionary developmental (evo-devo) research.

An evolutionary perspective on chordate brain organization and function: insights from amphioxus, and the problem of sentience

The similarities between amphioxus and vertebrate brains, in their regional subdivision, cell types and circuitry, make the former a useful benchmark for understanding the evolutionary innovations that shaped the latter. Locomotory control systems were already well developed in basal chordates, with the ventral neuropile of the dien-mesencephalon serving to set levels of activity and initiate locomotory actions. A chief deficit in amphioxus is the absence of complex vertebrate-type sense organs. Hence, much of vertebrate story is one of progressive improvement both to these and to sensory experience more broadly. This has two aspects: (i) anatomical and neurocircuitry innovations in the organs of special sense and the brain centres that process and store their output, and (ii) the emergence of primary consciousness, i.e. sentience. With respect to the latter, a bottom up, evolutionary perspective has a different focus from a top down human-centric one. At issue: the obstacles to the emergence of sentience in the first instance, the sequence of addition of new contents to evolving consciousness, and the homology relationship between them. A further question, and a subject for future investigation, is how subjective experience is optimized for each sensory modality. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

Transcriptional Regulation and Implications for Controlling Hox Gene Expression

Hox genes play key roles in axial patterning and regulating the regional identity of cells and tissues in a wide variety of animals from invertebrates to vertebrates. Nested domains of Hox expression generate a combinatorial code that provides a molecular framework for specifying the properties of tissues along the A–P axis. Hence, it is important to understand the regulatory mechanisms that coordinately control the precise patterns of the transcription of clustered Hox genes required for their roles in development. New insights are emerging about the dynamics and molecular mechanisms governing transcriptional regulation, and there is interest in understanding how these may play a role in contributing to the regulation of the expression of the clustered Hox genes. In this review, we summarize some of the recent findings, ideas and emerging mechanisms underlying the regulation of transcription in general and consider how they may be relevant to understanding the transcriptional regulation of Hox genes.

Three topological features of regulatory networks control life-essential and specialized subsystems

Gene regulatory networks (GRNs) play key roles in development, phenotype plasticity, and evolution. Although graph theory has been used to explore GRNs, associations amongst topological features, transcription factors (TFs), and systems essentiality are poorly understood. Here we sought the relationship amongst the main GRN topological features that influence the control of essential and specific subsystems. We found that the K_nn, page rank, and degree are the most relevant GRN features: the ones are conserved along the evolution and are also relevant in pluripotent cells. Interestingly, life-essential subsystems are governed mainly by TFs with intermediary K_nn and high page rank or degree, whereas specialized subsystems are mainly regulated by TFs with low K_nn. Hence, we suggest that the high probability of TFs be toured by a random signal, and the high probability of the signal propagation to target genes ensures the life-essential subsystems' robustness. Gene/genome duplication is the main evolutionary process to rise K_nn as the most relevant feature. Herein, we shed light on unexplored topological GRN features to assess how they are related to subsystems and how the duplications shaped the regulatory systems along the evolution. The classification model generated can be found here: https://github.com/ivanrwolf/NoC/ .

T-Cell Factors as Transcriptional Inhibitors: Activities and Regulations in Vertebrate Head Development

Since its first discovery in the late 90s, Wnt canonical signaling has been demonstrated to affect a large variety of neural developmental processes, including, but not limited to, embryonic axis formation, neural proliferation, fate determination, and maintenance of neural stem cells. For decades, studies have focused on the mechanisms controlling the activity of β-catenin, the sole mediator of Wnt transcriptional response. More recently, the spotlight of research is directed towards the last cascade component, the T-cell factor (TCF)/Lymphoid-Enhancer binding Factor (LEF), and more specifically, the TCF/LEF-mediated switch from transcriptional activation to repression, which in both embryonic blastomeres and mouse embryonic stem cells pushes the balance from pluri/multipotency towards differentiation. It has been long known that Groucho/Transducin-Like Enhancer of split (Gro/TLE) is the main co-repressor partner of TCF/LEF. More recently, other TCF/LEF-interacting partners have been identified, including the pro-neural BarH-Like 2 (BARHL2), which belongs to the evolutionary highly conserved family of homeodomain-containing transcription factors. This review describes the activities and regulatory modes of TCF/LEF as transcriptional repressors, with a specific focus on the functions of Barhl2 in vertebrate brain development. Specific attention is given to the transcriptional events leading to formation of the Organizer, as well as the roles and regulations of Wnt/β-catenin pathway in growth of the caudal forebrain. We present TCF/LEF activities in both embryonic and neural stem cells and discuss how alterations of this pathway could lead to tumors.

Forebrain Architecture and Development in Cyclostomes, with Reference to the Early Morphology and Evolution of the Vertebrate Head

The vertebrate head and brain are characterized by highly complex morphological patterns. The forebrain, the most anterior division of the brain, is subdivided into the diencephalon, hypothalamus, and telencephalon from the neuromeric subdivision into prosomeres. Importantly, the telencephalon contains the cerebral cortex, which plays a key role in higher order cognitive functions in humans. To elucidate the evolution of the forebrain regionalization, comparative analyses of the brain development between extant jawed and jawless vertebrates are crucial. Cyclostomes - lampreys and hagfishes - are the only extant jawless vertebrates, and diverged from jawed vertebrates (gnathostomes) over 500 million years ago. Previous developmental studies on the cyclostome brain were conducted mainly in lampreys because hagfish embryos were rarely available. Although still scarce, the recent availability of hagfish embryos has propelled comparative studies of brain development and gene expression. By integrating findings with those of cyclostomes and fossil jawless vertebrates, we can depict the morphology, developmental mechanism, and even the evolutionary path of the brain of the last common ancestor of vertebrates. In this review, we summarize the development of the forebrain in cyclostomes and suggest what evolutionary changes each cyclostome lineage underwent during brain evolution. In addition, together with recent advances in the head morphology in fossil vertebrates revealed by CT scanning technology, we discuss how the evolution of craniofacial morphology and the changes of the developmental mechanism of the forebrain towards crown gnathostomes are causally related.

Genetic Mechanism for the Cyclostome Cerebellar Neurons Reveals Early Evolution of the Vertebrate Cerebellum

The vertebrate cerebellum arises at the dorsal part of rhombomere 1, induced by signals from the isthmic organizer. Two major cerebellar neuronal subtypes, granule cells (excitatory) and Purkinje cells (inhibitory), are generated from the anterior rhombic lip and the ventricular zone, respectively. This regionalization and the way it develops are shared in all extant jawed vertebrates (gnathostomes). However, very little is known about early evolution of the cerebellum. The lamprey, an extant jawless vertebrate lineage or cyclostome, possesses an undifferentiated, plate-like cerebellum, whereas the hagfish, another cyclostome lineage, is thought to lack a cerebellum proper. In this study, we found that hagfish Atoh1 and Wnt1 genes are co-expressed in the rhombic lip, and Ptf1a is expressed ventrally to them, confirming the existence of r1's rhombic lip and the ventricular zone in cyclostomes. In later stages, lamprey Atoh1 is downregulated in the posterior r1, in which the NeuroD increases, similar to the differentiation process of cerebellar granule cells in gnathostomes. Also, a continuous Atoh1-positive domain in the rostral r1 is reminiscent of the primordium of valvula cerebelli of ray-finned fishes. Lastly, we detected a GAD-positive domain adjacent to the Ptf1a-positive ventricular zone in lampreys, suggesting that the Ptf1a-positive cells differentiate into some GABAergic inhibitory neurons such as Purkinje and other inhibitory neurons like in gnathostomes. Altogether, we conclude that the ancestral genetic programs for the formation of a distinct cerebellum were established in the last common ancestor of vertebrates.

Segmentation and patterning of the vertebrate hindbrain

During early development, the hindbrain is sub-divided into rhombomeres that underlie the organisation of neurons and adjacent craniofacial tissues. A gene regulatory network of signals and transcription factors establish and pattern segments with a distinct anteroposterior identity. Initially, the borders of segmental gene expression are imprecise, but then become sharply defined, and specialised boundary cells form. In this Review, we summarise key aspects of the conserved regulatory cascade that underlies the formation of hindbrain segments. We describe how the pattern is sharpened and stabilised through the dynamic regulation of cell identity, acting in parallel with cell segregation. Finally, we discuss evidence that boundary cells have roles in local patterning, and act as a site of neurogenesis within the hindbrain.

The Enigmatic Reissner's Fiber and the Origin of Chordates

Reissner’s fiber (RF) is a secreted filament that floats in the neural canal of chordates. Since its discovery in 1860, there has been no agreement on its primary function, and its strong conservation across chordate species has remained a mystery for comparative neuroanatomists. Several findings, including the chemical composition and the phylogenetic history of RF, clinical observations associating RF with the development of the neural canal, and more recent studies suggesting that RF is needed to develop a straight vertebral column, may shed light on the functions of this structure across chordates. In this article, we will briefly review the evidence mentioned above to suggest a role of RF in the origin of fundamental innovations of the chordate body plan, especially the elongation of the neural tube and maintenance of the body axis. We will also mention the relevance of RF for medical conditions like hydrocephalus, scoliosis of the vertebral spine and possibly regeneration of the spinal cord.

The dorsoanterior brain of adult amphioxus shares similarities in expression profile and neuronal composition with the vertebrate telencephalon

BACKGROUND

The evolutionary origin of the telencephalon, the most anterior part of the vertebrate brain, remains obscure. Since no obvious counterpart to the telencephalon has yet been identified in invertebrate chordates, it is difficult to trace telencephalic origins. One way to identify homologous brain parts between distantly related animal groups is to focus on the combinatorial expression of conserved regionalisation genes that specify brain regions.

RESULTS

Here, we report the combined expression of conserved transcription factors known to specify the telencephalon in the vertebrates in the chordate amphioxus. Focusing on adult specimens, we detect specific co-expression of these factors in the dorsal part of the anterior brain vesicle, which we refer to as Pars anterodorsalis (PAD). As in vertebrates, expression of the transcription factors FoxG1, Emx and Lhx2/9 overlaps that of Pax4/6 dorsally and of Nkx2.1 ventrally, where we also detect expression of the Hedgehog ligand. This specific pattern of co-expression is not observed prior to metamorphosis. Similar to the vertebrate telencephalon, the amphioxus PAD is characterised by the presence of GABAergic neurons and dorsal accumulations of glutamatergic as well as dopaminergic neurons. We also observe sustained proliferation of neuronal progenitors at the ventricular zone of the amphioxus brain vesicle, as observed in the vertebrate brain.

CONCLUSIONS

Our findings suggest that the PAD in the adult amphioxus brain vesicle and the vertebrate telencephalon evolved from the same brain precursor region in ancestral chordates, which would imply homology of these structures. Our comparative data also indicate that this ancestral brain already contained GABA-, glutamatergic and dopaminergic neurons, as is characteristic for the olfactory bulb of the vertebrate telencephalon. We further speculate that the telencephalon might have evolved in vertebrates via a heterochronic shift in developmental timing.

Molecular insights into deuterostome evolution from hemichordate developmental biology

Hemichordates, along with echinoderms and chordates, belong to the lineage of bilaterians called the deuterostomes. Their phylogenetic position as an outgroup to chordates provides an opportunity to investigate the evolutionary origins of the chordate body plan and reconstruct ancestral deuterostome characters. The body plans of the hemichordates and chordates are organizationally divergent making anatomical comparisons very challenging. The developmental underpinnings of animal body plans are often more conservative than the body plans they regulate, and offer a novel data set for making comparisons between morphologically divergent body architectures. Here I review the hemichordate developmental data generated over the past 20 years that further test hypotheses of proposed morphological affinities between the two taxa, but also compare the conserved anteroposterior, dorsoventral axial patterning programs and germ layer specification programs. These data provide an opportunity to determine which developmental programs are ancestral deuterostome or bilaterian innovations, and which ones occurred in stem chordates or vertebrates representing developmental novelties of the chordate body plan.

Transcription Factors of the bHLH Family Delineate Vertebrate Landmarks in the Nervous System of a Simple Chordate

Tunicates are marine invertebrates whose tadpole-like larvae feature a highly simplified version of the chordate body plan. Similar to their distant vertebrate relatives, tunicate larvae develop a regionalized central nervous system and form distinct neural structures, which include a rostral sensory vesicle, a motor ganglion, and a caudal nerve cord. The sensory vesicle contains a photoreceptive complex and a statocyst, and based on the comparable expression patterns of evolutionarily conserved marker genes, it is believed to include proto-hypothalamic and proto-retinal territories. The evolutionarily conserved molecular fingerprints of these landmarks of the vertebrate brain consist of genes encoding for different transcription factors, and of the gene batteries that they control, and include several members of the bHLH family. Here we review the complement of bHLH genes present in the streamlined genome of the tunicate Ciona robusta and their current classification, and summarize recent studies on proneural bHLH transcription factors and their expression territories. We discuss the possible roles of bHLH genes in establishing the molecular compartmentalization of the enticing nervous system of this unassuming chordate.

GABAergic motor neurons bias locomotor decision-making in C. elegans

Proper threat-reward decision-making is critical to animal survival. Emerging evidence indicates that the motor system may participate in decision-making but the neural circuit and molecular bases for these functions are little known. We found in C. elegans that GABAergic motor neurons (D-MNs) bias toward the reward behavior in threat-reward decision-making by retrogradely inhibiting a pair of premotor command interneurons, AVA, that control cholinergic motor neurons in the avoidance neural circuit. This function of D-MNs is mediated by a specific ionotropic GABA receptor (UNC-49) in AVA, and depends on electrical coupling between the two AVA interneurons. Our results suggest that AVA are hub neurons where sensory inputs from threat and reward sensory modalities and motor information from D-MNs are integrated. This study demonstrates at single-neuron resolution how motor neurons may help shape threat-reward choice behaviors through interacting with other neurons.

Neural anatomy of echinoid early juveniles and comparison of nervous system organization in echinoderms

The echinoderms are a phylum of marine deuterostomes characterized by the pentaradial (five fold) symmetry of their adult bodies. Due to this unusual body plan, adult echinoderms have long been excluded from comparative analyses aimed at understanding the origin and evolution of deuterostome nervous systems. Here, we investigated the neural anatomy of early juveniles of representatives of three of the five echinoderm classes: the echinoid Paracentrotus lividus, the asteroid Patiria miniata, and the holothuroid Parastichopus parvimensis. Using whole mount immunohistochemistry and confocal microscopy, we found that the nervous system of echinoid early juveniles is composed of three main structures: a basiepidermal nerve plexus, five radial nerve cords connected by a circumoral nerve ring, and peripheral nerves innervating the appendages. Our whole mount preparations further allowed us to obtain thorough descriptions of these structures and of several innervation patterns, in particular at the level of the appendages. Detailed comparisons of the echinoid juvenile nervous system with those of asteroid and holothuroid juveniles moreover supported a general conservation of the main neural structures in all three species, including at the level of the appendages. Our results support the previously proposed hypotheses for the existence of two neural units in echinoderms: one consisting of the basiepidermal nerve plexus to process sensory stimuli locally and one composed of the radial nerve cords and the peripheral nerves constituting a centralized control system. This study provides the basis for more in-depth comparisons of the echinoderm adult nervous system with those of other animals, in particular hemichordates and chordates, to address the long-standing controversies about deuterostome nervous system evolution.

Ancestral regulatory mechanisms specify conserved midbrain circuitry in arthropods and vertebrates

Comparative developmental genetics indicate insect and mammalian forebrains form and function in comparable ways. However, these data are open to opposing interpretations that advocate either a single origin of the brain and its adaptive modification during animal evolution; or multiple, independent origins of the many different brains present in extant Bilateria. Here, we describe conserved regulatory elements that mediate the spatiotemporal expression of developmental control genes directing the formation and function of midbrain circuits in flies, mice, and humans. These circuits develop from corresponding midbrain-hindbrain boundary regions and regulate comparable behaviors for balance and motor control. Our findings suggest that conserved regulatory mechanisms specify cephalic circuits for sensory integration and coordinated behavior common to all animals that possess a brain.

Corresponding attributes of neural development and function suggest arthropod and vertebrate brains may have an evolutionarily conserved organization. However, the underlying mechanisms have remained elusive. Here, we identify a gene regulatory and character identity network defining the deutocerebral–tritocerebral boundary (DTB) in Drosophila. This network comprises genes homologous to those directing midbrain-hindbrain boundary (MHB) formation in vertebrates and their closest chordate relatives. Genetic tracing reveals that the embryonic DTB gives rise to adult midbrain circuits that in flies control auditory and vestibular information processing and motor coordination, as do MHB-derived circuits in vertebrates. DTB-specific gene expression and function are directed by cis-regulatory elements of developmental control genes that include homologs of mammalian Zinc finger of the cerebellum and Purkinje cell protein 4. Drosophila DTB-specific cis-regulatory elements correspond to regulatory sequences of human ENGRAILED-2, PAX-2, and DACHSHUND-1 that direct MHB-specific expression in the embryonic mouse brain. We show that cis-regulatory elements and the gene networks they regulate direct the formation and function of midbrain circuits for balance and motor coordination in insects and mammals. Regulatory mechanisms mediating the genetic specification of cephalic neural circuits in arthropods correspond to those in chordates, thereby implying their origin before the divergence of deuterostomes and ecdysozoans.

Gene expression analysis of three homeobox genes throughout early and late development of a feather star Anneissia japonica

Crinoids are considered as the most basal extant echinoderms. They retain aboral nervous system with a nerve center, which has been degraded in the eleutherozoan echinoderms. To investigate the evolution of patterning of the nervous systems in crinoids, we examined temporal and spatial expression patterns of three neural patterning-related homeobox genes, six3, pax6, and otx, throughout the development of a feather star Anneissia japonica. These genes were involved in the patterning of endomesodermal tissues instead of the ectodermal neural tissues in the early planktonic stages. In the stages after larval attachment, the expression of these genes was mainly observed in the podia and the oral nervous systems instead of the aboral nerve center. Our results indicate the involvement of these three genes in the formation of oral nervous system in the common ancestor of the echinoderms and suggest that the aboral nerve center is not evolutionally related to the brain of other bilaterians.

Comparative and Evolutionary Physiology of Vasopressin/ Oxytocin-Type Neuropeptide Signaling in Invertebrates

The identification of structurally related hypothalamic hormones that regulate blood pressure and diuresis (vasopressin, VP; CYFQNCPRG-NH2) or lactation and uterine contraction (oxytocin, OT; CYIQNCPLG-NH2) was a major advance in neuroendocrinology, recognized in the award of the Nobel Prize for Chemistry in 1955. Furthermore, the discovery of central actions of VP and OT as regulators of reproductive and social behavior in humans and other mammals has broadened interest in these neuropeptides beyond physiology into psychology. VP/OT-type neuropeptides and their G-protein coupled receptors originated in a common ancestor of the Bilateria (Urbilateria), with invertebrates typically having a single VP/OT-type neuropeptide and cognate receptor. Gene/genome duplications followed by gene loss gave rise to variety in the number of VP/OT-type neuropeptides and receptors in different vertebrate lineages. Recent advances in comparative transcriptomics/genomics have enabled discovery of VP/OT-type neuropeptides in an ever-growing diversity of invertebrate taxa, providing new opportunities to gain insights into the evolution of VP/OT-type neuropeptide function in the Bilateria. Here we review the comparative physiology of VP/OT-type neuropeptides in invertebrates, with roles in regulation of reproduction, feeding, and water/salt homeostasis emerging as common themes. For example, we highlight recent reports of roles in regulation of oocyte maturation in the sea-squirt Ciona intestinalis, extraoral feeding behavior in the starfish Asterias rubens and energy status and dessication resistance in ants. Thus, VP/OT-type neuropeptides are pleiotropic regulators of physiological processes, with evolutionarily conserved roles that can be traced back to Urbilateria. To gain a deeper understanding of the evolution of VP/OT-type neuropeptide function it may be necessary to not only determine the actions of the peptides but also to characterize the transcriptomic/proteomic/metabolomic profiles of cells expressing VP/OT-type precursors and/or VP/OT-type receptors within the framework of anatomically and functionally identified neuronal networks. Furthermore, investigation of VP/OT-type neuropeptide function in a wider range of invertebrate species is now needed if we are to determine how and when this ancient signaling system was recruited to regulate diverse physiological and behavioral processes in different branches of animal phylogeny and in contrasting environmental contexts.

The genetic program to specify ectodermal cells in ascidian embryos

The ascidian belongs to the sister group of vertebrates and shares many features with them. The gene regulatory network (GRN) controlling gene expression in ascidian embryonic development leading to the tadpole larva has revealed evolutionarily conserved gene circuits between ascidians and vertebrates. These conserved mechanisms are indeed useful to infer the original developmental programs of the ancestral chordates. Simultaneously, these studies have revealed which gene circuits are missing in the ascidian GRN; these gene circuits may have been acquired in the vertebrate lineage. In particular, the GRN responsible for gene expression in ectodermal cells of ascidian embryos has revealed the genetic programs that regulate the regionalization of the brain, formation of palps derived from placode-like cells, and differentiation of sensory neurons derived from neural crest-like cells. We here discuss how these studies have given insights into the evolution of these traits.

Transcription and Beyond: Delineating FOXG1 Function in Cortical Development and Disorders

Forkhead Box G1 (FOXG1) is a member of the Forkhead family of genes with non-redundant roles in brain development, where alteration of this gene's expression significantly affects the formation and function of the mammalian cerebral cortex. FOXG1 haploinsufficiency in humans is associated with prominent differences in brain size and impaired intellectual development noticeable in early childhood, while homozygous mutations are typically fatal. As such, FOXG1 has been implicated in a wide spectrum of congenital brain disorders, including the congenital variant of Rett syndrome, infantile spasms, microcephaly, autism spectrum disorder (ASD) and schizophrenia. Recent technological advances have yielded greater insight into phenotypic variations observed in FOXG1 syndrome, molecular mechanisms underlying pathogenesis of the disease, and multifaceted roles of FOXG1 expression. In this review, we explore the emerging mechanisms of FOXG1 in a range of transcriptional to posttranscriptional events in order to evolve our current view of how a single transcription factor governs the assembly of an elaborate cortical circuit responsible for higher cognitive functions and neurological disorders.

A biphasic role of non-canonical Wnt16 signaling during early anterior-posterior patterning and morphogenesis of the sea urchin embryo

A Wnt signaling network governs early anterior-posterior (AP) specification and patterning of the deuterostome sea urchin embryo. We have previously shown that non-canonical Fzl1/2/7 signaling antagonizes the progressive posterior-to-anterior downregulation of the anterior neuroectoderm (ANE) gene regulatory network (GRN) by canonical Wnt/β-catenin and non-canonical Wnt1/Wnt8-Fzl5/8-JNK signaling. This study focuses on the non-canonical function of the Wnt16 ligand during early AP specification and patterning. Maternally supplied wnt16 is expressed ubiquitously during cleavage and zygotic wnt16 expression is concentrated in the endoderm/mesoderm beginning at mid-blastula stage. Wnt16 antagonizes the ANE restriction mechanism and this activity depends on a functional Fzl1/2/7 receptor. Our results also show that zygotic wnt16 expression depends on both Fzl5/8 and Wnt/β-catenin signaling. Furthermore, Wnt16 is necessary for the activation and/or maintenance of key regulatory endoderm/mesoderm genes and is essential for gastrulation. Together, our data show that Wnt16 has two functions during early AP specification and patterning: (1) an initial role activating the Fzl1/2/7 pathway that antagonizes the ANE restriction mechanism; and (2) a subsequent function in activating key endoderm GRN factors and the morphogenetic movements of gastrulation.

Foxg specifies sensory neurons in the anterior neural plate border of the ascidian embryo

Foxg constitutes a regulatory loop with Fgf8 and plays an important role in the development of anterior placodes and the telencephalon in vertebrate embryos. Ascidians, which belong to Tunicata, the sister group of vertebrates, develop a primitive placode-like structure at the anterior boundary of the neural plate, but lack a clear counterpart of the telencephalon. In this animal, Foxg is expressed in larval palps, which are adhesive organs with sensory neurons. Here, we show that Foxg begins to be expressed in two separate rows of cells within the neural plate boundary region under the control of the MAPK pathway to pattern this region. However, Foxg is not expressed in the brain, and we find no evidence that knockdown of Foxg affects brain formation. Our data suggest that recruitment of Fgf to the downstream of Foxg might have been a critical evolutionary event for the telencephalon in the vertebrate lineage.

Vertebrate telencephalon formation requires Foxg-Fgf8 cross-regulation, but while ascidians express Foxg in the neural plate, they lack a telencephalon. Here the authors show that Foxg loss does not affect ascidian brain formation, indicating that telencephalon evolution required recruitment of Fgf downstream of Foxg.

A developmental perspective on the evolution of the nervous system

The evolution of nervous systems in animals has always fascinated biologists, and thus multiple evolutionary scenarios have been proposed to explain the appearance of neurons and complex neuronal centers. However, the absence of a robust phylogenetic framework for animal interrelationships, the lack of a mechanistic understanding of development, and a recapitulative view of animal ontogeny have traditionally limited these scenarios. Only recently, the integration of advanced molecular and morphological studies in a broad range of animals has allowed to trace the evolution of developmental and neuronal characters on a better-resolved animal phylogeny. This has falsified most traditional scenarios for nervous system evolution, paving the way for the emergence of new testable hypotheses. Here we summarize recent progress in studies of nervous system development in major animal lineages and formulate some of the arising questions. In particular, we focus on how lineage analyses of nervous system development and a comparative study of the expression of neural-related genes has influenced our understanding of the evolution of an elaborated central nervous system in Bilateria. We argue that a phylogeny-guided study of neural development combining thorough descriptive and functional analyses is key to establish more robust scenarios for the origin and evolution of animal nervous systems.

The expression of FoxG1 in the early development of the European river lamprey Lampetra fluviatilis demonstrates significant heterochrony with that in other vertebrates

FoxG1, a member of the Fox/Forkhead family of winged helix transcription factors, plays key roles in the induction and spatial compartmentalization of the telencephalon in vertebrates. Loss- and gain-of-function experiments have established FoxG1 as a maintenance factor for neural progenitors and a crucial player in the specification of the ventral telencephalon (subpallium). For the first time in evolution, the telencephalon appeared in the ancestors of vertebrates, including cyclostomes. However, although FoxG1 homologues are present in cyclostomes (i.e., in lampreys and hagfishes), no systematic study of the spatial-temporal expression of FoxG1 during the embryonic development of these animals has been carried out. Given these findings, we have now studied FoxG1 spatial-temporal expression patterns in the early development of the European river lamprey Lampetra fluviatilis. We show that in contrast to other vertebrates, in which the expression of FoxG1 begins during neurulation, the expression of this gene in L. fluviatilis starts after neurulation, first at stage 21 (early head protrusion) in the area of the otic placodes and then, beginning from stage 22, in the telencephalon. Such heterochrony of FoxG1 expression in the lamprey may reflect the fact that in this basally divergent representative of vertebrates, telencephalon specification occurs relatively late. This heterochrony could be related to the evolutionary history of the telencephalon, with a recent appearance in vertebrates as an extension to more ancient anterior brain regions. Another peculiarity of FoxG1 expression in lamprey, compared to other vertebrates, is that it is not expressed in the lamprey optic structures.

Evolution of the Chordate Telencephalon

The dramatic evolutionary expansion of the neocortex, together with a proliferation of specialized cortical areas, is believed to underlie the emergence of human cognitive abilities. In a broader phylogenetic context, however, neocortex evolution in mammals, including humans, is remarkably conservative, characterized largely by size variations on a shared six-layered neuronal architecture. By contrast, the telencephalon in non-mammalian vertebrates, including reptiles, amphibians, bony and cartilaginous fishes, and cyclostomes, features a great variety of very different tissue structures. Our understanding of the evolutionary relationships of these telencephalic structures, especially those of basally branching vertebrates and invertebrate chordates, remains fragmentary and is impeded by conceptual obstacles. To make sense of highly divergent anatomies requires a hierarchical view of biological organization, one that permits the recognition of homologies at multiple levels beyond neuroanatomical structure. Here we review the origin and diversification of the telencephalon with a focus on key evolutionary innovations shaping the neocortex at multiple levels of organization.

Hox genes: Downstream "effectors" of retinoic acid signaling in vertebrate embryogenesis

One of the major regulatory challenges of animal development is to precisely coordinate in space and time the formation, specification, and patterning of cells that underlie elaboration of the basic body plan. How does the vertebrate plan for the nervous and hematopoietic systems, heart, limbs, digestive, and reproductive organs derive from seemingly similar population of cells? These systems are initially established and patterned along the anteroposterior axis (AP) by opposing signaling gradients that lead to the activation of gene regulatory networks involved in axial specification, including the Hox genes. The retinoid signaling pathway is one of the key signaling gradients coupled to the establishment of axial patterning. The nested domains of Hox gene expression, which provide a combinatorial code for axial patterning, arise in part through a differential response to retinoic acid (RA) diffusing from anabolic centers established within the embryo during development. Hence, Hox genes are important direct effectors of retinoid signaling in embryogenesis. This review focuses on describing current knowledge on the complex mechanisms and regulatory processes, which govern the response of Hox genes to RA in several tissue contexts including the nervous system during vertebrate development.

Evolution of cis-regulatory modules for the head organizer gene goosecoid in chordates: comparisons between Branchiostoma and Xenopus

BACKGROUND

In cephalochordates (amphioxus), the notochord runs along the dorsal to the anterior tip of the body. In contrast, the vertebrate head is formed anterior to the notochord, as a result of head organizer formation in anterior mesoderm during early development. A key gene for the vertebrate head organizer, goosecoid (gsc), is broadly expressed in the dorsal mesoderm of amphioxus gastrula. Amphioxus gsc expression subsequently becomes restricted to the posterior notochord from the early neurula. This has prompted the hypothesis that a change in expression patterns of gsc led to development of the vertebrate head during chordate evolution. However, molecular mechanisms of head organizer evolution involving gsc have never been elucidated.

RESULTS

To address this question, we compared cis-regulatory modules of vertebrate organizer genes between amphioxus, Branchiostoma japonicum, and frogs, Xenopus laevis and Xenopus tropicalis. Here we show conservation and diversification of gene regulatory mechanisms through cis-regulatory modules for gsc, lim1/lhx1, and chordin in Branchiostoma and Xenopus. Reporter analysis using Xenopus embryos demonstrates that activation of gsc by Nodal/FoxH1 signal through the 5' upstream region, that of lim1 by Nodal/FoxH1 signal through the first intron, and that of chordin by Lim1 through the second intron, are conserved between amphioxus and Xenopus. However, activation of gsc by Lim1 and Otx through the 5' upstream region in Xenopus are not conserved in amphioxus. Furthermore, the 5' region of amphioxus gsc recapitulated the amphioxus-like posterior mesoderm expression of the reporter gene in transgenic Xenopus embryos.

CONCLUSIONS

On the basis of this study, we propose a model, in which the gsc gene acquired the cis-regulatory module bound with Lim1 and Otx at its 5' upstream region to be activated persistently in anterior mesoderm, in the vertebrate lineage. Because Gsc globally represses trunk (notochord) genes in the vertebrate head organizer, this cooption of gsc in vertebrates appears to have resulted in inhibition of trunk genes and acquisition of the head organizer and its derivative prechordal plate.

Ambulacrarians and the Ancestry of Deuterostome Nervous Systems

The evolutionary origin and history of metazoan nervous systems has been at the heart of numerous scientific debates for well over a century. This has been a particularly difficult issue to resolve within the deuterostomes, chiefly due to the distinct neural architectures observed within this group of animals. Indeed, deuterosomes feature central nervous systems, apical organs, nerve cords, and basiepidermal nerve nets. Comparative analyses investigating the anatomy and molecular composition of deuterostome nervous systems have nonetheless succeeded in identifying a number of shared and derived features. These analyses have led to the elaboration of diverse theories about the origin and evolutionary history of deuterostome nervous systems. Here, we provide an overview of these distinct theories. Further, we argue that deciphering the adult nervous systems of representatives of all deuterostome phyla, including echinoderms, which have long been neglected in this type of surveys, will ultimately provide answers to the questions concerning the ancestry and evolution of deuterostome nervous systems.

Tissue-Specific Actions of Pax6 on Proliferation and Differentiation Balance in Developing Forebrain Are Foxg1 Dependent

Differences in the growth and maturation of diverse forebrain tissues depend on region-specific transcriptional regulation. Individual transcription factors act simultaneously in multiple regions that develop very differently, raising questions about the extent to which their actions vary regionally. We found that the transcription factor Pax6 affects the transcriptomes and the balance between proliferation and differentiation in opposite directions in the diencephalon versus cerebral cortex. We tested several possible mechanisms to explain Pax6's tissue-specific actions and found that the presence of the transcription factor Foxg1 in the cortex but not in the diencephalon was most influential. We found that Foxg1 is responsible for many of the differences in cell cycle gene expression between the diencephalon and cortex and, in cortex lacking Foxg1, Pax6's action on the balance of proliferation versus differentiation becomes diencephalon like. Our findings reveal a mechanism for generating regional forebrain diversity in which one transcription factor completely reverses the actions of another.