Evolutionary conservation of the presumptive neural plate markers AmphiSox1/2/3 and AmphiNeurogenin in the invertebrate chordate amphioxus

Evolutionary origin of the chordate nervous system revealed by amphioxus developmental trajectories

The common ancestor of all vertebrates had a highly sophisticated nervous system, but questions remain about the evolution of vertebrate neural cell types. The amphioxus, a chordate that diverged before the origin of vertebrates, can inform vertebrate evolution. Here we develop and analyse a single-cell RNA-sequencing dataset from seven amphioxus embryo stages to understand chordate cell type evolution and to study vertebrate neural cell type origins. We identified many new amphioxus cell types, including homologues to the vertebrate hypothalamus and neurohypophysis, rooting the evolutionary origin of these structures. On the basis of ancestor-descendant reconstruction of cell trajectories of the amphioxus and other species, we inferred expression dynamics of transcription factor genes throughout embryogenesis and identified three ancient developmental routes forming chordate neurons. We characterized cell specification at the mechanistic level and generated mutant lines to examine the function of five key transcription factors involved in neural specification. Our results show three developmental origins for the vertebrate nervous system: an anterior FoxQ2-dependent mechanism that is deeply conserved in invertebrates, a less-conserved route leading to more posterior neurons in the vertebrate spinal cord and a mechanism for specifying neuromesoderm progenitors that is restricted to chordates. The evolution of neuromesoderm progenitors may have led to a dramatic shift in posterior neural and mesodermal cell fate decisions and the body elongation process in a stem chordate.

SUMO-dependent transcriptional repression by Sox2 inhibits the proliferation of neural stem cells

Sox2 is known for its roles in maintaining the stem cell state of embryonic stem cells and neural stem cells. In particular, it has been shown to slow the proliferation of these cell types. It is also known for its effects as an activating transcription factor. Despite this, analysis of published studies shows that it represses as many genes as it activates. Here, we identify a new set of target genes that Sox2 represses in neural stem cells. These genes are associated with centrosomes, centromeres and other aspects of cell cycle control. In addition, we show that SUMOylation of Sox2 is necessary for the repression of these genes and for its repressive effects on cell proliferation. Together, these data suggest that SUMO-dependent repression of this group of target genes is responsible for the role of Sox2 in regulating the proliferation of neural stem cells.

Gene regulatory networks of epidermal and neural fate choice in a chordate

Neurons are a highly specialized cell type only found in metazoans. They can be scattered throughout the body or grouped together, forming ganglia or nerve cords. During embryogenesis, centralized nervous systems develop from the ectoderm, which also forms the epidermis. How pluripotent ectodermal cells are directed toward neural or epidermal fates, and to which extent this process is shared among different animal lineages, are still open questions. Here, by using micromere explants, we were able to define in silico the putative gene regulatory networks (GRNs) underlying the first steps of the epidermis and the central nervous system formation in the cephalochordate amphioxus. We propose that although the signal triggering neural induction in amphioxus (i.e., Nodal) is different from vertebrates, the main transcription factors implicated in this process are conserved. Moreover, our data reveal that transcription factors of the neural program seem to not only activate neural genes but also to potentially have direct inputs into the epidermal GRN, suggesting that the Nodal signal might also contribute to neural fate commitment by repressing the epidermal program. Our functional data on whole embryos support this result and highlight the complex interactions among the transcription factors activated by the signaling pathways that drive ectodermal cell fate choice in chordates.

The evolutionary origins of the vertebrate olfactory system

Vertebrates develop an olfactory system that detects odorants and pheromones through their interaction with specialized cell surface receptors on olfactory sensory neurons. During development, the olfactory system forms from the olfactory placodes, specialized areas of the anterior ectoderm that share cellular and molecular properties with placodes involved in the development of other cranial senses. The early-diverging chordate lineages amphioxus, tunicates, lampreys and hagfishes give insight into how this system evolved. Here, we review olfactory system development and cell types in these lineages alongside chemosensory receptor gene evolution, integrating these data into a description of how the vertebrate olfactory system evolved. Some olfactory system cell types predate the vertebrates, as do some of the mechanisms specifying placodes, and it is likely these two were already connected in the common ancestor of vertebrates and tunicates. In stem vertebrates, this evolved into an organ system integrating additional tissues and morphogenetic processes defining distinct olfactory and adenohypophyseal components, followed by splitting of the ancestral placode to produce the characteristic paired olfactory organs of most modern vertebrates.

Investigating cellular and molecular mechanisms of neurogenesis in Capitella teleta sheds light on the ancestor of Annelida

BACKGROUND

Diverse architectures of nervous systems (NSs) such as a plexus in cnidarians or a more centralized nervous system (CNS) in insects and vertebrates are present across Metazoa, but it is unclear what selection pressures drove evolution and diversification of NSs. One underlying aspect of this diversity lies in the cellular and molecular mechanisms driving neurogenesis, i.e. generation of neurons from neural precursor cells (NPCs). In cnidarians, vertebrates, and arthropods, homologs of SoxB and bHLH proneural genes control different steps of neurogenesis, suggesting that some neurogenic mechanisms may be conserved. However, data are lacking for spiralian taxa.

RESULTS

To that end, we characterized NPCs and their daughters at different stages of neurogenesis in the spiralian annelid Capitella teleta. We assessed cellular division patterns in the neuroectoderm using static and pulse-chase labeling with thymidine analogs (EdU and BrdU), which enabled identification of NPCs that underwent multiple rounds of division. Actively-dividing brain NPCs were found to be apically-localized, whereas actively-dividing NPCs for the ventral nerve cord (VNC) were found apically, basally, and closer to the ventral midline. We used lineage tracing to characterize the changing boundary of the trunk neuroectoderm. Finally, to start to generate a genetic hierarchy, we performed double-fluorescent in-situ hybridization (FISH) and single-FISH plus EdU labeling for neurogenic gene homologs. In the brain and VNC, Ct-soxB1 and Ct-neurogenin were expressed in a large proportion of apically-localized, EdU+ NPCs. In contrast, Ct-ash1 was expressed in a small subset of apically-localized, EdU+ NPCs and subsurface, EdU- cells, but not in Ct-neuroD+ or Ct-elav1+ cells, which also were subsurface.

CONCLUSIONS

Our data suggest a putative genetic hierarchy with Ct-soxB1 and Ct-neurogenin at the top, followed by Ct-ash1, then Ct-neuroD, and finally Ct-elav1. Comparison of our data with that from Platynereis dumerilii revealed expression of neurogenin homologs in proliferating NPCs in annelids, which appears different than the expression of vertebrate neurogenin homologs in cells that are exiting the cell cycle. Furthermore, differences between neurogenesis in the head versus trunk of C. teleta suggest that these two tissues may be independent developmental modules, possibly with differing evolutionary trajectories.

A Notch-regulated proliferative stem cell zone in the developing spinal cord is an ancestral vertebrate trait

Vertebrates have evolved the most sophisticated nervous systems we know. These differ from the nervous systems of invertebrates in several ways, including the evolution of new cell types, and the emergence and elaboration of patterning mechanisms to organise cells in time and space. Vertebrates also generally have many more cells in their central nervous systems than invertebrates, and an increase in neural cell number may have contributed to the sophisticated anatomy of the brain and spinal cord. Here, we study how increased cell number evolved in the vertebrate central nervous system, investigating the regulation of cell proliferation in the lamprey spinal cord. Markers of proliferation show that a ventricular progenitor zone is found throughout the lamprey spinal cord. We show that inhibition of Notch signalling disrupts the maintenance of this zone. When Notch is blocked, progenitor cells differentiate precociously, the proliferative ventricular zone is lost and differentiation markers become expressed throughout the spinal cord. Comparison with other chordates suggests that the emergence of a persistent Notch-regulated proliferative progenitor zone was a crucial step for the evolution of vertebrate spinal cord complexity.

Molecular regionalization of the developing amphioxus neural tube challenges major partitions of the vertebrate brain

All vertebrate brains develop following a common Bauplan defined by anteroposterior (AP) and dorsoventral (DV) subdivisions, characterized by largely conserved differential expression of gene markers. However, it is still unclear how this Bauplan originated during evolution. We studied the relative expression of 48 genes with key roles in vertebrate neural patterning in a representative amphioxus embryonic stage. Unlike nonchordates, amphioxus develops its central nervous system (CNS) from a neural plate that is homologous to that of vertebrates, allowing direct topological comparisons. The resulting genoarchitectonic model revealed that the amphioxus incipient neural tube is unexpectedly complex, consisting of several AP and DV molecular partitions. Strikingly, comparison with vertebrates indicates that the vertebrate thalamus, pretectum, and midbrain domains jointly correspond to a single amphioxus region, which we termed Di-Mesencephalic primordium (DiMes). This suggests that these domains have a common developmental and evolutionary origin, as supported by functional experiments manipulating secondary organizers in zebrafish and mice.

Gonadotropins in the Russian Sturgeon: Their Role in Steroid Secretion and the Effect of Hormonal Treatment on Their Secretion

In the reproduction process of male and female fish, pituitary derived gonadotropins (GTHs) play a key role. To be able to specifically investigate certain functions of Luteinizing (LH) and Follicle stimulating hormone (FSH) in Russian sturgeon (Acipenser gueldenstaedtii; st), we produced recombinant variants of the hormones using the yeast Pichia pastoris as a protein production system. We accomplished to create in vitro biologically active heterodimeric glycoproteins consisting of two associated α- and β-subunits in sufficient quantities. Three dimensional modelling of both GTHs was conducted in order to study the differences between the two GTHs. Antibodies were produced against the unique β-subunit of each of the GTHs, in order to be used for immunohistochemical analysis and to develop an ELISA for blood and pituitary hormone quantification. This detection technique revealed the specific localization of the LH and FSH cells in the sturgeon pituitary and pointed out that both cell types are present in substantially higher numbers in mature males and females, compared to immature fish. With the newly attained option to prevent cross-contamination when investigating on the effects of GTH administration, we compared the steroidogeneic response (estradiol and 11-Keto testosterone (11-KT) in female and males, respectively) of recombinant stLH, stFSH, and carp pituitary extract in male and female sturgeon gonads at different developmental stages. Finally, we injected commercially available gonadotropin releasing hormones analog (GnRH) to mature females, and found a moderate effect on the development of ovarian follicles. Application of only testosterone (T) resulted in a significant increase in circulating levels of 11-KT whereas the combination of GnRH + T did not affect steroid levels at all. The response pattern for estradiol demonstrated a similar situation. FSH levels showed significant increases when GnRH + T was administered, while no changes were present in LH levels.

The evolution of early neurogenesis

The foundation of the diverse metazoan nervous systems is laid by embryonic patterning mechanisms, involving the generation and movement of neural progenitors and their progeny. Here we divide early neurogenesis into discrete elements, including origin, pattern, proliferation, and movement of neuronal progenitors, which are controlled by conserved gene cassettes. We review these neurogenetic mechanisms in representatives of the different metazoan clades, with the goal to build a conceptual framework in which one can ask specific questions, such as which of these mechanisms potentially formed part of the developmental "toolkit" of the bilaterian ancestor and which evolved later.

Vertebrate cranial placodes as evolutionary innovations--the ancestor's tale

Evolutionary innovations often arise by tinkering with preexisting components building new regulatory networks by the rewiring of old parts. The cranial placodes of vertebrates, ectodermal thickenings that give rise to many of the cranial sense organs (ear, nose, lateral line) and ganglia, originated as such novel structures, when vertebrate ancestors elaborated their head in support of a more active and exploratory life style. This review addresses the question of how cranial placodes evolved by tinkering with ectodermal patterning mechanisms and sensory and neurosecretory cell types that have their own evolutionary history. With phylogenetic relationships among the major branches of metazoans now relatively well established, a comparative approach is used to infer, which structures evolved in which lineages and allows us to trace the origin of placodes and their components back from ancestor to ancestor. Some of the core networks of ectodermal patterning and sensory and neurosecretory differentiation were already established in the common ancestor of cnidarians and bilaterians and were greatly elaborated in the bilaterian ancestor (with BMP- and Wnt-dependent patterning of dorsoventral and anteroposterior ectoderm and multiple neurosecretory and sensory cell types). Rostral and caudal protoplacodal domains, giving rise to some neurosecretory and sensory cells, were then established in the ectoderm of the chordate and tunicate-vertebrate ancestor, respectively. However, proper cranial placodes as clusters of proliferating progenitors producing high-density arrays of neurosecretory and sensory cells only evolved and diversified in the ancestors of vertebrates.

Insulin-like genes in ascidians: findings in Ciona and hypotheses on the evolutionary origins of the pancreas

Insulin plays an extensively characterized role in the control of sugar metabolism, growth and homeostasis in a wide range of organisms. In vertebrate chordates, insulin is mainly produced by the beta cells of the endocrine pancreas, while in non-chordate animals insulin-producing cells are mainly found in the nervous system and/or scattered along the digestive tract. However, recent studies have indicated the notochord, the defining feature of the chordate phylum, as an additional site of expression of insulin-like peptides. Here we show that two of the three insulin-like genes identified in Ciona intestinalis, an invertebrate chordate with a dual life cycle, are first expressed in the developing notochord during embryogenesis and transition to distinct areas of the adult digestive tract after metamorphosis. In addition, we present data suggesting that the transcription factor Ciona Brachyury is involved in the control of notochord expression of at least one of these genes, Ciona insulin-like 2. Finally, we review the information currently available on insulin-producing cells in ascidians and on pancreas-related transcription factors that might control their expression.

The evolutionary history of vertebrate cranial placodes II. Evolution of ectodermal patterning

Cranial placodes are evolutionary innovations of vertebrates. However, they most likely evolved by redeployment, rewiring and diversification of preexisting cell types and patterning mechanisms. In the second part of this review we compare vertebrates with other animal groups to elucidate the evolutionary history of ectodermal patterning. We show that several transcription factors have ancient bilaterian roles in dorsoventral and anteroposterior regionalisation of the ectoderm. Evidence from amphioxus suggests that ancestral chordates then concentrated neurosecretory cells in the anteriormost non-neural ectoderm. This anterior proto-placodal domain subsequently gave rise to the oral siphon primordia in tunicates (with neurosecretory cells being lost) and anterior (adenohypophyseal, olfactory, and lens) placodes of vertebrates. Likewise, tunicate atrial siphon primordia and posterior (otic, lateral line, and epibranchial) placodes of vertebrates probably evolved from a posterior proto-placodal region in the tunicate-vertebrate ancestor. Since both siphon primordia in tunicates give rise to sparse populations of sensory cells, both proto-placodal domains probably also gave rise to some sensory receptors in the tunicate-vertebrate ancestor. However, proper cranial placodes, which give rise to high density arrays of specialised sensory receptors and neurons, evolved from these domains only in the vertebrate lineage. We propose that this may have involved rewiring of the regulatory network upstream and downstream of Six1/2 and Six4/5 transcription factors and their Eya family cofactors. These proteins, which play ancient roles in neuronal differentiation were first recruited to the dorsal non-neural ectoderm in the tunicate-vertebrate ancestor but subsequently probably acquired new target genes in the vertebrate lineage, allowing them to adopt new functions in regulating proliferation and patterning of neuronal progenitors.

[Pluripotency and induced nuclear reprogramming in vertebrates: new perspectives]

Pluripotency is a transitory state during vertebrate development. A pluripotent cell can theoretically acquire all cell fates of the organism. During ontogenetic dynamics, loss of pluripotency is associated with a progressive acquisition of a specific genetic program, which is determined both by instructions received and by cell position in the whole organism. Pluripotent embryonic stem cells can be isolated and cultured in vitro indefinitely. Using mammalian embryonic stem cells (ESCs), it has been possible to identify the factors involved in the establishment and maintenance of pluripotency state. In this review, we will describe recent scientific advances in the understanding of pluripotency, the molecular actors involved in such a regulation and their functional conservation during evolution. We shall focus on new concepts, obtained from the study of vertebrate model organisms, to shed light on the cell transition from pluripotency to differentiated state, and shall recapitulate fundamental and clinical applications of pluripotent cells, of "somatic cell nuclear transfer" (SCNT), of induced nuclear reprogramming in vitro and future perspectives of in vivo applications. Our results, in the xenopus, concerning the first in vivo induced nuclear reprogramming might open new perspectives about the understanding of cell plasticity in an integrated context. Our analyses sought to encourage new and alternative clinical approaches to achieve in situ tissue regeneration.

Evolution of new characters after whole genome duplications: insights from amphioxus

Additional copies of genes resulting from two whole genome duplications at the base of the vertebrates have been suggested as enabling the evolution of vertebrate-specific structures such as neural crest, a midbrain/hindbrain organizer and neurogenic placodes. These structures, however, did not evolve entirely de novo, but arose from tissues already present in an ancestral chordate. This review discusses the evolutionary history of co-option of old genes for new roles in vertebrate development as well as the relative contributions of changes in cis-regulation and in protein structure. Particular examples are the FoxD, FGF8/17/18 and Pax2/5/8 genes. Comparisons with invertebrate chordates (amphioxus and tunicates) paint a complex picture with co-option of genes into new structures occurring both after and before the whole genome duplications. In addition, while cis-regulatory changes are likely of primary importance in evolution of vertebrate-specific structures, changes in protein structure including alternative splicing are non-trivial.

A genome-wide view of transcription factor gene diversity in chordate evolution: less gene loss in amphioxus?

Previous studies of gene diversity in the homeobox superclass have shown that the Florida amphioxus Branchiostoma floridae has undergone remarkably little gene family loss. Here we use a combined BLAST and HMM search strategy to assess the family level diversity of four other transcription factor superclasses: the Paired/Pax genes, Tbx genes, Fox genes and Sox genes. We apply this across genomes from five chordate taxa, including B. floridae and Ciona intestinalis, plus two outgroup taxa. Our results show scattered gene family loss. However, as also found for homeobox genes, B. floridae has retained all ancient Pax, Tbx, Fox and Sox gene families that were present in the common ancestor of living chordates. We conclude that, at least in terms of transcription factor gene complexity, the genome of amphioxus has experienced remarkable stasis compared to the genomes of other chordates.

Unravelling the evolution of neural stem cells in arthropods: notch signalling in neural stem cell development in the crustacean Daphnia magna

The genetic regulatory networks controlling major developmental processes seem to be conserved in bilaterians regardless of an independent or a common origin of the structures. This has been explained by the employment of a genetic toolkit that was repeatedly used during bilaterian evolution to build the various forms and body plans. However, it is not clear how genetic networks were incorporated into the formation of novel structures and how homologous genes can regulate the disparate morphological processes. Here we address this question by analysing the role of Notch signalling, which is part of the bilaterian toolkit, in neural stem cell evolution in arthropods. Within arthropods neural stem cells have evolved in the last common ancestor of insects and crustaceans (Tetraconata). We analyse here for the first time the role of Notch signalling in a crustacean, the branchiopod Daphnia magna, and show that it is required in neural stem cells for regulating the time of neural precursor production and for binary cell fate decisions in the ventral neuroectoderm. The function of Notch signalling has diverged in the ventral neuroectoderm of insects and crustaceans accompanied by changes in the morphogenetic processes. In the crustacean, Notch controlled mechanisms of neuroblast regulation have evolved that are surprisingly similar to vertebrates and thus present a remarkable case of parallel evolution. These new data on a representative of crustaceans complete the arthropod data set on Notch signalling in the nervous system and allow for reconstructing how the Notch signalling pathway has been co-opted from pre-existing structures to the development of the evolving neural stem cells in the Tetraconata ancestor.

Early development of cephalochordates (amphioxus)

The Phylum Chordata includes three groups--Vertebrata, Tunicata, and Cephalochordata. In cephalochordates, commonly called amphioxus or lancelets, which are basal in the Chordata, the eggs are small and relatively non-yolky. As in vertebrates, cleavage is indeterminate with cell fates determined gradually as development proceeds. The oocytes are attached to the ovarian follicle at the animal pole, where the oocyte nucleus is located. The cytoplasm at the opposite side of the egg, the vegetal pole, contains the future germ plasm or pole plasm, which includes determinants of the germline. After fertilization, additional asymmetries are established by movements of the egg and sperm nuclei, resulting in a concentration of mitochondria at one side of the animal hemisphere. This may be related to establishment of the dorsal/ventral axis. Patterning along the embryonic axes is mediated by secreted signaling proteins. Dorsal identity is specified by Nodal/Vg1 signaling, while during the gastrula stage, opposition between Nodal/Vg1 and BMP signaling establishes dorsal/anterior (i.e., head) and ventral/posterior (i.e., trunk/tail) identities, respectively. Wnt/β-catenin signaling specifies posterior identity while retinoic acid signaling specifies positions along the anterior/posterior axis. These signals are further modulated by a number of secreted antagonists. This fundamental patterning mechanism is conserved, with some modifications, in vertebrates.

A study of neural-related microRNAs in the developing amphioxus

BACKGROUND

MicroRNAs are small noncoding RNAs regulating expression of protein coding genes at post-transcriptional level and controlling several biological processes. At present microRNAs have been identified in various metazoans and seem also to be involved in brain development, neuronal differentiation and subtypes specification. An approach to better understand the role of microRNAs in animal gene expression is to determine temporal and tissue-specific expression patterns of microRNAs in different model organisms. Therefore, we have investigated the expression of six neural related microRNAs in amphioxus, an organism having an important phylogenetic position in terms of understanding the origin and evolution of chordates.

RESULTS

In amphioxus, all the microRNAs we examined are expressed in specific regions of the CNS, and some of them are correlated with specific cell types. In addition, miR-7, miR-137 and miR-184 are also expressed in endodermal and mesodermal tissues. Several potential targets expressed in the nervous system of amphioxus have been identified by computational prediction and some of them are coexpressed with one or more miRNAs.

CONCLUSION

We identified six miRNAs that are expressed in the nervous system of amphioxus in a variety of patterns. miR-124 is found in both differentiating and mature neurons, miR-9 in differentiated neurons, miR-7, miR-137 and miR-184 in restricted CNS regions, and miR-183 in cells of sensory organs. Therefore, such amphioxus miRNAs may play important roles in regional patterning and/or specification of neuronal cell types.

Sry, more than testis determination?

The Sry locus on the mammalian Y chromosome is the developmental switch responsible for testis determination. Inconsistent with this important function, the Sry locus is transcribed in adult males at times and in tissues not involved with testis determination. Sry is expressed in multiple tissues of the peripheral and central nervous system. Sry is derived from Sox3 and is similar to other SOXB family loci. The SOXB loci are responsible for nervous system development. Sry has been demonstrated to modulate the catecholamine pathway, so it should have functional consequences in the central and peripheral nervous system. The nervous system expression and potential function are consistent with Sry as a SOXB family member. In mammals, Sox3 is X-linked and undergoes dosage compensation in females. The expression of Sry in adult males allows for a type of sexual differentiation independent of circulating gonadal hormones. A quantitative difference in Sox3 plus Sry expression in males vs. females could drive changes in the transcriptome of these cells, differentiating male and female cells. Sry expression and its transcriptional effects should be considered when investigating sexual dimorphic phenotypes.

Steps towards a centralized nervous system in basal bilaterians: insights from neurogenesis of the acoel Symsagittifera roscoffensis

Due to its proposed basal position in the bilaterian Tree of Life, Acoela may hold the key to our understanding of the evolution of a number of bodyplan features including the central nervous system. In order to contribute novel data to this discussion we investigated the distribution of α-tubulin and the neurotransmitters serotonin and RFamide in juveniles and adults of the sagittiferid Symsagittifera roscoffensis. In addition, we present the expression pattern of the neuropatterning gene SoxB1. Adults and juveniles exhibit six serotonergic longitudinal neurite bundles and an anterior concentration of serotonergic sensory cells. While juveniles show an "orthogon-like" arrangement of longitudinal neurite bundles along the anterior-posterior axis, it appears more diffuse in the posterior region of adults. Commissures between the six neurite bundles are present only in the anterior body region of adults, while irregularly distributed individual neurites, often interconnected by serotonergic nerve cells, are found in the posterior region. Anti-RFamide staining shows numerous individual neurites around the statocyst. The orthogon-like nervous system of S. roscoffensis is confirmed by α-tubulin immunoreactivity. In the region of highest neurotransmitter density (i.e., anterior), the HMG-box gene SrSoxB1, a transcription factor known to be involved in neurogenesis in other bilaterians, is expressed in juvenile specimens. Accordingly, SoxB1 expression in S. roscoffensis follows the typical pattern of higher bilaterians that have a brain. Thus, our data support the notion that Urbilateria already had the genetic toolkit required to form brain-like neural structures, but that its morphological degree of neural concentration was still low.

Identification and expression of an elastase homologue in Branchiostoma belcheri with implications to the origin of vertebrate pancreas

Elastases have been identified in a variety of organisms ranging from bacteria to insects to mammals, yet little is known to date about them in amphioxus, a model animal for insights into the origin and evolution of vertebrates. In this study we demonstrate the presence of an elastase homologue, named BbElas, in Branchiostoma belcheri. The recombinant BbElas hydrolyses the elastase specific substrate N-succinyl-Ala-Ala-Ala p-nitroanilide, which can be inhibited by the serine proteinase inhibitor PMSF, the elastase-specific inhibitor elastatinal and the cysteine proteinase inhibitor PCMB. Phylogenetic analysis shows that BbElas represents the archetype of vertebrate elastases, hinting at the clues that the different isoforms of vertebrate elastases are originated from an ancestral gene like BbElas. Our results also suggest that the mid-gut in amphioxus is to homologous vertebrate pancreas, a novel proposal which deserves further study.

Characterization of SoxB2 and SoxC genes in amphioxus (Branchiostoma belcheri): implications for their evolutionary conservation

Most Sox genes directly affect cell fate determination and differentiation. In this study, we isolated two Sox genes: SoxB2 and SoxC from amphioxus (Branchiostoma belcheri), the closest living invertebrate relative of the vertebrates. Alignments of SoxB2 and SoxC protein sequences and their vertebrate homologs show high conservation of their HMG domains. Phylogenic analysis shows that amphioxus SoxB2 and SoxC fall out of the vertebrate branches, suggesting that vertebrate homologs might arise from gene duplications during evolution. The two genes possess similar spatial and temporal expression patterns during embryogenesis and in adults. They are both maternally inherited. During neurulation, they are expressed in the neural ectoderm and archenterons. In adults, they are expressed not only in the nerve cord, but also in the gut, midgut diverticulum, gill and oocytes. These results suggest that amphioxus SoxB2 and SoxC might co-function and have conserved functions in the nervous system and gonads as their vertebrate homologs.

Chordate roots of the vertebrate nervous system: expanding the molecular toolkit

The vertebrate brain is highly complex with millions to billions of neurons. During development, the neural plate border region gives rise to the neural crest, cranial placodes and, in anamniotes, to Rohon-Beard sensory neurons, whereas the boundary region of the midbrain and hindbrain develops organizer properties. Comparisons of developmental gene expression and neuroanatomy between vertebrates and the basal chordate amphioxus, which has only thousands of neurons and lacks a neural crest, most placodes and a midbrain-hindbrain organizer, indicate that these vertebrate features were built on a foundation already present in the ancestral chordate. Recent advances in genomics have provided insights into the elaboration of the molecular toolkit at the invertebrate-vertebrate transition that may have facilitated the evolution of these vertebrate characteristics.

Evidence for stasis and not genetic piracy in developmental expression patterns of Branchiostoma lanceolatum and Branchiostoma floridae, two amphioxus species that have evolved independently over the course of 200 Myr

Cephalochordates, the most basal extant group in the phylum Chordata, are represented chiefly by about 20 species of the genus Branchiostoma, commonly called amphioxus or lancelets. In recent years, insights into the evolutionary origin of the vertebrates have been gained from molecular genetic studies during the development of three of these amphioxus species (Branchiostoma floridae in North America, Branchiostoma lanceolatum in Europe, and Branchiostoma belcheri in East Asia). In spite of an estimated divergence time of 100-200 Myr among these species, all three are remarkably similar morphologically, and students of amphioxus have tacitly assumed that such resemblances arise during ontogeny from nearly identical networks of developmental genes. We felt that this assumption needed to be reexamined because instances are known--even in comparisons of closely related species--where characters seeming homologous on the basis of morphology actually develop under the control of conspicuously divergent genetic programs (a phenomenon termed "genetic piracy"). In the present work, we tested the hypothesis that morphological similarities reflect strict conservation of developmentally important genes' expression patterns in order to assess whether the developmental genetics of different amphioxus species show evidence of genetic piracy. To these ends, we cloned 18 genes implicated in different developmental functions in B. lanceolatum and compared their gene expression patterns with the known expression patterns of their orthologous genes in B. floridae. We show that, for the most part, conservation of gene expression parallels that of morphology in these two species. We also identified some differences in gene expression, likely reflecting experimental sensitivity, with the exception of Pax1/9, which may result from true developmental specificities in each amphioxus species. Our results demonstrate that morphological conservation reflects stasis in developmental gene expression patterns and find no evidence for genetic piracy. Thus, different species of amphioxus appear to be very similar, not only morphologically, but also in the genetic programs directing the development of their structural features. Moreover, we provide the first catalogue of gene expression data for the European species, B. lanceolatum.

Sox genes in the coral Acropora millepora: divergent expression patterns reflect differences in developmental mechanisms within the Anthozoa

Background

Sox genes encode transcription factors that function in a wide range of developmental processes across the animal kingdom. To better understand both the evolution of the Sox family and the roles of these genes in cnidarians, we are studying the Sox gene complement of the coral, Acropora millepora (Class Anthozoa).

Results

Based on overall domain structures and HMG box sequences, the Acropora Sox genes considered here clearly fall into four of the five major Sox classes. AmSoxC is expressed in the ectoderm during development, in cells whose morphology is consistent with their assignment as sensory neurons. The expression pattern of the Nematostella ortholog of this gene is broadly similar to that of AmSoxC, but there are subtle differences – for example, expression begins significantly earlier in Acropora than in Nematostella. During gastrulation, AmSoxBb and AmSoxB1 transcripts are detected only in the presumptive ectoderm while AmSoxE1 transcription is restricted to the presumptive endoderm, suggesting that these Sox genes might play roles in germ layer specification. A third type B Sox gene, AmSoxBa, and a Sox F gene AmSoxF also have complex and specific expression patterns during early development. Each of these genes has a clear Nematostella ortholog, but in several cases the expression pattern observed in Acropora differs significantly from that reported in Nematostella.

Conclusion

These differences in expression patterns between Acropora and Nematostella largely reflect fundamental differences in developmental processes, underscoring the diversity of mechanisms within the anthozoan Sub-Class Hexacorallia (Zoantharia).

Evolution of the neural crest

The recent advances in studies of the neural crest in vertebrates and the analysis of basal chordates using molecular and embryological approaches have demonstrated that at least part of the genetic programs and the cellular behavior were in place in nonvertebrate chordates before the neural crest evolved. Nevertheless, both the missing aspects and the close similarities found could explain why basal chordates lack a bona fide neural crest population, even though some migratory neurons and pigment cells have been recently identified in ascidians and amphioxus.

The amphioxus SoxB family: implications for the evolution of vertebrate placodes

Cranial placodes are regions of thickened ectoderm that give rise to sense organs and ganglia in the vertebrate head. Homologous structures are proposed to exist in urochordates, but have not been found in cephalochordates, suggesting the first chordates lacked placodes. SoxB genes are expressed in discrete subsets of vertebrate placodes. To investigate how placodes arose and diversified in the vertebrate lineage we isolated the complete set of SoxB genes from amphioxus and analyzed their expression in embryos and larvae. We find that while amphioxus possesses a single SoxB2 gene, it has three SoxB1 paralogs. Like vertebrate SoxB1 genes, one of these paralogs is expressed in non-neural ectoderm destined to give rise to sensory cells. When considered in the context of other amphioxus placode marker orthologs, amphioxus SoxB1 expression suggests a diversity of sensory cell types utilizing distinct placode-type gene programs was present in the first chordates. Our data supports a model for placode evolution and diversification whereby the full complement of vertebrate placodes evolved by serial recruitment of distinct sensory cell specification programs to anterior pre-placodal ectoderm.

Comparative analysis of neurotrophin receptors and ligands in vertebrate neurons: tools for evolutionary stability or changes in neural circuits?

To better understand the role of multiple neurotrophin ligands and their receptors in vertebrate brain evolution, we examined the distribution of trk neurotrophin receptors in representatives of several vertebrate classes. Trk receptors are largely expressed in homologous neuronal populations among different species/classes of vertebrates. In many neurons, trkB and trkC receptors are co-expressed. TrkB and trkC receptors are primarily found in neurons with more restricted, specialized dendritic and axonal fields that are thought to be involved in discriminative or 'analytical' functions. The neurotrophin receptor trkA is expressed predominantly in neurons with larger, overlapping dendritic fields with more heterogeneous connections ('integrative' or 'modulatory' systems) such as nociceptive and sympathetic autonomic nervous system, locus coeruleus and cholinergic basal forebrain. Surveys of trk receptor expression and function in the peripheral nervous system of different vertebrate classes reveal trends ranging from dependency on a single neurotrophin to a more complex dependency on increasing numbers of neurotrophins and their receptors, for example, in taste and inner ear innervation. Gene deletion studies in mice provide evidence for a complex regulation of neuronal survival of sensory ganglion cells by different neurotrophins. Although expression of neurotrophins and their receptors is predominantly conserved in most circuits, increasing diversity of neurotrophin ligands and their receptors and a more complex dependency of neurons on neurotrophins might have facilitated the formation of at least some new neuronal entities.

A retinoic acid-Hox hierarchy controls both anterior/posterior patterning and neuronal specification in the developing central nervous system of the cephalochordate amphioxus

Retinoic acid (RA) mediates both anterior/posterior patterning and neuronal specification in the vertebrate central nervous system (CNS). However, the molecular mechanisms downstream of RA are not well understood. To investigate these mechanisms, we used the invertebrate chordate amphioxus, in which the CNS, although containing only about 20,000 neurons in adults, like the vertebrate CNS, has a forebrain, midbrain, hindbrain, and spinal cord and is regionalized by RA-signaling. Here we show, first, that domains of genes with expression normally limited to diencephalon and midbrain are generally not affected by altered RA-signaling, second, that contrary to previous reports, not only Hox1, 3, and 4, but also Hox2 and Hox6 are collinearly expressed in the amphioxus CNS, and third, that collinear expression of all these Hox genes is controlled by RA-signaling. Finally, we show that Hox1 is involved in mediating both the role of RA-signaling in regionalization of the hindbrain and in specification of hindbrain motor neurons. Thus, morpholino knock-down of the single amphioxus Hox1 mimics the effects of treatments with an RA-antagonist. This analysis establishes RA-dependent regulation of collinear Hox expression as a feature common to the chordate CNS and indicates that the RA-Hox hierarchy functions both in proper anterior/posterior patterning of the developing CNS and in specification of neuronal identity.

A gene catalogue of the amphioxus nervous system

The elaboration of extremely complex nervous systems is a major success of evolution. However, at the dawn of the post-genomic era, few data have helped yet to unravel how a nervous system develops and evolves to complexity. On the evolutionary road to vertebrates, amphioxus occupies a key position to tackle this exciting issue. Its “simple” nervous system basically consists of a dorsal nerve cord and a diffuse net of peripheral neurons, which contrasts greatly with the complexity of vertebrate nervous systems. Notwithstanding, increasing data on gene expression has faced up this simplicity by revealing a mounting level of cryptic complexity, with unexpected levels of neuronal diversity, organisation and regionalisation of the central and peripheral nervous systems. Furthermore, recent gene expression data also point to the high neurogenic potential of the epidermis of amphioxus, suggestive of a skin-brain track for the evolution of the vertebrate nervous system. Here I attempt to catalogue and synthesise current gene expression data in the amphioxus nervous system. From this global point of view, I suggest scenarios for the evolutionary origin of complex features in the vertebrate nervous system, with special emphasis on the evolutionary origin of placodes and neural crest, and postulate a pre-patterned migratory pathway of cells, which, in the epidermis, may represent an intermediate state towards the deployment of one of the most striking innovative features of vertebrates: the neural crest and its derivatives.

Comparative genomic and expression analysis of group B1soxgenes in zebrafish indicates their diversification during vertebrate evolution

Group B1 Sox genes encode HMG domain transcription factors that play major roles in neural development. We have identified six zebrafish B1 sox genes, which include pan-vertebrate sox1a/b, sox2, and sox3, and also fish-specific sox19a/b. SOX19A/B proteins show a transcriptional activation potential that is similar to other B1 SOX proteins. The expression of sox19a and sox3 begins at approximately the 1,000-cell stage during embryogenesis and becomes confined to the future ectoderm by the shield stage. This is reminiscent of the epiblastic expression of Sox2 and/or Sox3 in amniotes. As development progresses, these six B1 sox genes display unique expression patterns that overlap distinctly from one region to another. sox19a expression is widespread in the early neuroectoderm, resembling pan-neural Sox2 expression in amniotes, whereas zebrafish sox2 shows anterior-restricted expression. Comparative genomics suggests that sox19a/b and mammalian Sox15 (group G) have an orthologous relationship and that the B1/G Sox genes arose from a common ancestral gene through two rounds of genome duplication. It seems likely, therefore, that each B1/G Sox gene has gained a distinct expression profile and function during vertebrate evolution.

Nuclear beta-catenin promotes non-neural ectoderm and posterior cell fates in amphioxus embryos

In vertebrate development, Wnt/beta-catenin signaling has an early role in specification of dorsal/anterior identity and a late one in posterior specification. To understand the evolution of these roles, we cloned beta-catenin from the invertebrate chordate amphioxus. The exon/intron organization of beta-catenin is highly conserved between amphioxus and other animals including a cnidarian, but not Drosophila. In development, amphioxus beta-catenin is concentrated in all nuclei from the 16-cell stage until the onset of gastrulation when it becomes undetectable in presumptive mesendoderm. Li(+), which up-regulates Wnt/beta-catenin signaling, had no detectable effect on axial patterning when applied before the late blastula stage, suggesting that a role for beta-catenin in specification of dorsal/anterior identity may be a vertebrate innovation. From the mid-gastrula through the neurula stage, the highest levels of nuclear beta-catenin are around the blastopore. In the early neurula, beta-catenin is down-regulated in the neural plate, but remains high in adjacent non-neural ectoderm. Embryos treated with Li(+) at the late blastula stage are markedly posteriorized and lack a neural plate. These results suggest that in amphioxus, as in vertebrates, down-regulation of Wnt/beta-catenin signaling in the neural plate is necessary for maintenance of the neuroectoderm and that a major evolutionarily conserved role of Wnt/beta-catenin signaling is to specify posterior identity and pattern the anterior/posterior axis.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

Mutations affecting the formation of posterior lateral line system in Medaka, Oryzias latipes

We performed a systematic screen for mutations affecting the trajectory of axons visualized by immunohistochemical staining of Medaka embryos with anti-acetylated tubulin antibody. Among the mutations identified, yanagi (yan) and kazura (kaz) mutations caused specific defects in projection of the posterior lateral line (PLL) nerve. In yan and kaz mutant embryos, the PLL nerve main bundle was misrouted ventrally and dorsally or anteriorly. Medaka semaphorin3A, sdf1, and cxcr4 cDNA fragments were cloned to allow analysis of these mutants. There were no changes in semaphorin3A or sdf1 expression in mutant embryos, suggesting that the tissues expressing semaphorin3A or sdf1 that are involved in PLL nerve guidance are present in these mutant embryos. Double staining revealed that the mislocated PLL primordium and growth cone of the ectopically projected PLL nerve were always colocalized in both yan and kaz mutant embryos, suggesting that migration of PLL primordia and PLL nerve growth cones are not uncoupled in these mutants. Although homozygous yan larvae showed incomplete migration of the PLL primordium along the anteroposterior axis, ventral proneuromast migration was complete, suggesting that ventral migration of the proneuromast does not require the signaling affected in yan mutants. In addition to the PLL system, the distribution of primordial germ cells (PGCs) was also affected in both yan and kaz mutant embryos, indicating that yan and kaz genes are required for the migration of both PLL primordia and PGCs. Genetic linkage analysis indicated that kaz is linked to cxcr4, but yan is not linked to sdf1 or cxcr4. These mutations will provide genetic clues to investigate the molecular mechanism underlying formation of the PLL system.

SOX genes and neural progenitor identity

Resident among the highly structured adult nervous system, a few cells, referred to as neural progenitors or stem cells, maintain the ability to self-renew or differentiate. From the time of their specification during neural induction and throughout the building of the nervous system, neural progenitor cells preserve their broad developmental potential and replicative capacity to be able to produce the vast array of neuronal and glial cell types of the mature nervous system as, and when, required. Recently, considerable attention has been focused on identifying the molecular mechanisms responsible for maintaining neural progenitor or stem cell fate throughout ontogeny. The expression of a subset of SOX transcription factors is initiated concomitant with the acquisition of neural progenitor identity and is then maintained in the entire progenitor population of the developing and adult nervous system. Strikingly, studies in the central and peripheral nervous system of chick and mouse have revealed that SOX factors are key regulators of neural progenitor identity, promoting self-renewal in a context-dependent manner by sustaining the undifferentiated state of progenitor cells and maintaining their ability to either proliferate or differentiate.

Non-neural ectoderm is really neural: evolution of developmental patterning mechanisms in the non-neural ectoderm of chordates and the problem of sensory cell homologies

In chordates, the ectoderm is divided into the neuroectoderm and the so-called non-neural ectoderm. In spite of its name, however, the non-neural ectoderm contains numerous sensory cells. Therefore, the term "non-neural" ectoderm should be replaced by "general ectoderm." At least in amphioxus and tunicates and possibly in vertebrates as well, both the neuroectoderm and the general ectoderm are patterned anterior/posteriorly by mechanisms involving retinoic acid and Hox genes. In amphioxus and tunicates the ectodermal sensory cells, which have a wide range of ciliary and microvillar configurations, are mostly primary neurons sending axons to the CNS, although a minority lack axons. In contrast, vertebrate mechanosensory cells, called hair cells, are all secondary neurons that lack axons and have a characteristic eccentric cilium adjacent to a group of microvilli of graded lengths. It has been highly controversial whether the ectodermal sensory cells in the oral siphons of adult tunicates are homologous to vertebrate hair cells. In some species of tunicates, these cells appear to be secondary neurons, and microvillar and ciliary configurations of some of these cells approach those of vertebrate hair cells. However, none of the tunicate cells has all the characteristics of a hair cell, and there is a high degree of variation among ectodermal sensory cells within and between different species. Thus, similarities between the ectodermal sensory cells of any one species of tunicate and craniate hair cells may well represent convergent evolution rather than homology.

Amphioxus molecular biology: insights into vertebrate evolution and developmental mechanisms

The cephalochordate amphioxus is the best available proxy for the last common invertebrate ancestor of the vertebrates. During the last decade, the developmental genetics of amphioxus have been extensively examined for insights into the evolutionary origin and early evolution of the vertebrates. Comparisons between expression domains of homologous genes in amphioxus and vertebrates have strengthened proposed homologies between specific body parts. Molecular genetic studies have also highlighted parallels in the developmental mechanisms of amphioxus and vertebrates. In both groups, a similar nested pattern of Hox gene expression is involved in rostrocaudal patterning of the neural tube, and homologous genes also appear to be involved in dorsoventral neural patterning. Studies of amphioxus molecular biology have also hinted that the protochordate ancestor of the vertebrates included cell populations that modified their developmental genetic pathways during early vertebrate evolution to yield definitive neural crest and neurogenic placodes. We also discuss how the application of expressed sequence tag and gene-mapping approaches to amphioxus have combined with developmental studies to advance our understanding of chordate genome evolution. We conclude by considering the potential offered by the sequencing of the amphioxus genome, which was completed in late 2004.

Keeping sensory cells and evolving neurons to connect them to the brain: molecular conservation and novelties in vertebrate ear development

The evolution of the mechanosensory cellular module and the molecular details that regulate its development has included morphological modifications of these cells as well as the formation of larger assemblies of mechanosensory cell aggregates among metazoans. This has resulted in a wide diversity of mechanosensory organs. The wide morphological diversity of organs, including the associated morphological modifications of the mechanosensory cells, suggests parallel evolution of these modules and their associated organs. This morphological diversity is in stark contrast to the molecular conservation of developmental modules across phyla. These molecular data suggest that the evolution of mechanosensory transduction might have preceded that of distinct cellular differentiation. However, once a molecular network governing development of specialized cells involved in mechanosensory transduction evolved, that molecular network was preserved across phyla. Present data suggest that at least the common ancestor of triploblastic organisms, perhaps even the common diploblastic ancestor of bilaterian metazoans, had molecular and cellular specializations for mechanosensation. It is argued that the evolution of multicellular organs dedicated to specific aspects of mechanosensation, such as gravity and sound perception, are evolutionary transformations that build on this conserved molecular network for cellular specialization, but reflect distinct morphological solutions. We propose that the sensory neurons, connecting the craniate ear with the brain, are a derived feature of craniates, and possibly chordates, that came about through diversification of the lineage forming mechanosensory cells during development. This evolutionarily late event suggests a heterochronic shift, so that sensory neurons develop in mammals prior to mechanosensory hair cells. However, sensory neuron development is connected to hair cell development, likely in a clonal relationship. The theme of cellular conservation is reiterated in two examples of chordate otic diversification: the evolution of the horizontal canal system and the evolution of the basilar papilla/cochlea. It is suggested that here again, cellular multiplication and formation of a special epithelium predates the functional transformation to an 'organ' system for horizontal angular acceleration and sound pressure reception, respectively. Overall, evolution of the vertebrate ear needs to be understood as an interplay between and utilization of two gene networks or modules. One is at the level of the molecularly and developmentally conserved mechanosensory cellular module. The other is an increased complexity in the morphology of both adult mechanosensory cells and organs by the addition of end-stage and novel features and associated gene networks to detect specific aspects of mechanosensory stimuli.

Expression of AmphiCoe, an amphioxus COE/EBF gene, in the developing central nervous system and epidermal sensory neurons

The COE/EBF gene family marks a subset of prospective neurons in the vertebrate central and peripheral nervous system, including neurons deriving from some ectodermal placodes. Since placodes are often considered unique to vertebrates, we have characterised an amphioxus COE/EBF gene with the aim of using it as a marker to examine the timing and location of peripheral neuron differentiation. A single COE/EBF family member, AmphiCoe, was isolated from the amphioxus Branchiostoma floridae. AmphiCoe lies basal to the vertebrate COE/EBF genes in molecular phylogenetic analysis, suggesting that the duplications that formed the vertebrate COE/EBF family were specific to the vertebrate lineage. AmphiCoe is expressed in the central nervous system and in a small number of scattered ectodermal cells on the flanks of neurulae stage embryos. These cells become at least largely recessed beneath the ectoderm. Scanning electron microscopy was used to examine embryos in which the ectoderm had been partially peeled away. This revealed that these cells have neuronal morphology, and we infer that they are the precursors of epidermal primary sensory neurons. These characters lead us to suggest that differentiation of some ectodermal cells into sensory neurons with a tendency to sink beneath the embryonic surface represents a primitive feature that has become incorporated into placodes during vertebrate evolution.

Gene-regulatory interactions in neural crest evolution and development

In this review, we outline the gene-regulatory interactions driving neural crest development and compare these to a hypothetical network operating in the embryonic ectoderm of the cephalochordate amphioxus. While the early stages of ectodermal patterning appear conserved between amphioxus and vertebrates, later activation of neural crest-specific factors at the neural plate border appears to be a vertebrate novelty. This difference may reflect co-option of genetic pathways which conferred novel properties upon the evolving vertebrate neural plate border, potentiating the evolution of definitive neural crest.

Expression of a SoxB and a Wnt2/13 gene during the development of the mollusc Patella vulgata

We cloned and analysed the expression of a SoxB gene ( PvuSoxB) in the marine mollusc, Patella vulgata. Like its orthologues in deuterostomes, after an early broad ectodermal distribution, PvuSoxB expression only persists in cells competent to form neural structures. In the post-gastrulation larva, PvuSoxB is expressed in the prospective neuroectoderm in the head and in the trunk. No expression can be seen dorsally, around the mouth and the anus, or along the ventral midline. We also report the expression of a Wnt2/13 orthologue ( PvuWnt2) in Patella. After gastrulation, PvuWnt2 is expressed in the posterior part of the mouth, along the ventral midline and around the anus. This expression seems to be complementary to that of PvuSoxB in the trunk. We suggest the existence of a fundamental subdivision of the Patella trunk ectoderm into midline (mouth, midline, anus) and more lateral structures.

Fossil sister group of craniates: predicted and found

This study investigates whether the recently described Cambrian fossil Haikouella (and the very similar Yunnanozoon) throws light on the longstanding problem of the origin of craniates. In the first rigorous cladistic analysis of the relations of this animal, we took 40 anatomical characters from Haikouella and other taxa (hemichordates, tunicates, cephalochordates, conodont craniates and other craniates, plus protostomes as the outgroup) and subjected these characters to parsimony analysis. The characters included several previously unrecognized traits of Haikouella, such as upper lips resembling those of larval lampreys, the thick nature of the branchial bars, a mandibular branchial artery but no mandibular branchial bar, muscle fibers defining the myomeres, a dark fibrous sheath that defines the notochord, conclusive evidence for paired eyes, and a large hindbrain and diencephalon in the same positions as in the craniate brain. The cladistic analysis produced this tree: (protostomes, hemichordates (tunicates, (cephalochordates, (Haikouella, (conodonts + other craniates))))), with the "Haikouella + craniate" clade supported by bootstrap values that ranged from 81-96%, depending on how the analysis was structured. Thus, Haikouella is concluded to be the sister group of the craniates. Alternate hypotheses that unite Haikouella with hemichordates or cephalochordates, or consider it a basal deuterostome, received little or no support. Although it is the sister group of craniates, Haikouella is skull-less and lacks an ear, but it does have neural-crest derivatives in its branchial bars. Its craniate characters occur mostly in the head and pharynx; its widely spaced, robust branchial bars indicate it ventilated with branchiomeric muscles, not cilia. Despite its craniate mode of ventilation, Haikouella was not a predator but a suspension feeder, as shown by its cephalochordate-like endostyle, and tentacles forming a screen across the mouth. Haikouella was compared to pre-craniates predicted by recent models of craniate evolution and was found to fit these predictions closely. Specifically, it fits Northcutt and Gans' prediction that the change from ciliary to muscular ventilation preceded the change from suspension feeding to predatory feeding; it fits Butler's claim that vision was the first craniate sense to start elaborating; it is consistent with the ideas of Donoghue and others about the ancestor of conodont craniates; and, most strikingly, it resembles Mallatt's prediction of the external appearance of the ancestral craniate head. By contrast, Haikouella does not fit the widespread belief that ancestral craniates resembled hagfishes, because it has no special hagfish characters. Overall, Haikouella agrees so closely with recent predictions about pre-craniates that we conclude that the difficult problem of craniate origins is nearly solved.