Understanding how the chordate body plan originated and evolved is still controversial. The discovery by Spemann and Mangold in 1924 of the vertebrate organizer and its inductive properties in patterning the AP and DV axis was followed by a long gap until the 1960s when scientists started characterizing the molecular events responsible for such inductions. However, the evolutionary origin of the organizer itself remained obscure until very recently; did it appear together with the origin and radiation of vertebrates, or was (...) it a chordate affair? A recent study by Yu and collaborators,1 which analyses the expression of several organizer‐specific genes in amphioxus together with recent phylogenetic data that reversed the position of invertebrate extant chordates (e.g. urochordates and cephalochordates), indicates that the organizer probably appeared in early chordates. It likely had separate signalling centres generating BMP and Wnt signalling gradients along the DV and AP axis. The organizer was then lost in the urochordate lineage, most probably as an adaptation to a rapid and determinate development. BioEssays 29:619–624, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

A recent paper by Shu et al.(1) reinterprets the fossil Vetulicola and related forms, all from the Lower Cambrian, as basal deuterostomes, assigning them their own phylum, Vetulicolia. Their conclusion is based on the presence of structures resembling gill slits and a trunk‐like region that shows evidence of segmentation. This report summarizes the fossil evidence for their interpretation and evaluates a possible alternative, that vetulicolians may instead be tunicate‐like chordates. Implications for our understanding of the nature of the primitive (...) deuterostome (and chordate) body plan are discussed. BioEssays 24:208–211, 2002. © 2002 Wiley Periodicals, Inc.; DOI 10.1002/bies.10064. (shrink)

The origin of chordates and the consequent genesis of vertebrates were major events in natural history. The amphioxus (lancelet) is now recognised as the closest extant relative to the stem chordate and is the only living invertebrate that retains a vertebrate‐like development and body plan through its lifespan, despite more than 500 million years of independent evolution from the stem vertebrate. The inspiring data coming from its recently sequenced genome confirms that amphioxus has a prototypical chordate genome with respect (...) to gene content and structure, and even chromosomal organisation. Pushed by joint efforts of amphioxus researchers, amphioxus is now entering a new era, namely its maturation as a laboratory model, through the availability of a large amount of molecular data and the advent of experimental manipulation of the embryo. These two facts may well serve to illuminate the hidden secrets of the genetic changes that generated, among other vertebrates, ourselves. (shrink)

The internal organs of all vertebrates are asymmetrically organised across the left–right axis. The development of this asymmetry is controlled by a molecular pathway that includes the signalling molecule Nodal and the transcription factor Pitx2, proteins encoded by genes that are predominantly expressed on the left side of all vertebrate embryos studied to date. Vertebrates share Phylum Chordata with two other groups of animals, amphioxus and the urochordates (including ascidians). Both these taxa develop left–right asymmetries, and recent studies have begun (...) to address the degree of conservation of nodal and Pitx2 in this process. Pitx2 is a member of the Pitx homeobox gene family, and in both amphioxus and ascidians Pitx gene expression is predominantly left sided. These studies suggest that left–right asymmetry in all chordates is regulated by a conserved developmental pathway, and that this pathway evolved before the separation of the lineages leading to living chordates. BioEssays 24:1004–1011, 2002. © 2002 Wiley‐Periodicals, Inc. (shrink)

In vertebrate development, the HOX genes act to specify cell identity along much of the anterior‐posterior axis of the embryonic central nervous system. In all vertebrates examined to date, the vitamin A metabolite retinoic acid is implicated in the patterning of the anterior posterior axis and the induction of HOX gene expression. Two recent papers have extended the study of retinoic acid induction of HOX genes to the closest relatives of the vertebrates, amphioxus and tunicates(1,2). In both these species, exogenous (...) retinoic acid is able to induce ectopic expression of HOX 1 genes in the anterior central nervous system. This suggests that retinoic acid control of anterior‐posterior axis formation and HOX induction is not specific to vertebrates. However, in the more distantly related echinoderms and arthropods, retinoic acid does not seem to act in the same way. Thus the role of retinoic acid in anterior‐posterior axis specification may be a chordate innovation, perhaps linked to the evolution of another chordate character, the dorsal neural tube. (shrink)

Hindbrain development is orchestrated by a vertebrate gene regulatory network that generates segmental patterning along the anterior–posterior axis via Hox genes. Here, we review analyses of vertebrate and invertebrate chordate models that inform upon the evolutionary origin and diversification of this network. Evidence from the sea lamprey reveals that the hindbrain regulatory network generates rhombomeric compartments with segmental Hox expression and an underlying Hox code. We infer that this basal feature was present in ancestral vertebrates and, as an evolutionarily constrained (...) developmental state, is fundamentally important for patterning of the vertebrate hindbrain across diverse lineages. Despite the common ground plan, vertebrates exhibit neuroanatomical diversity in lineage‐specific patterns, with different vertebrates revealing variations of Hox expression in the hindbrain that could underlie this diversification. Invertebrate chordates lack hindbrain segmentation but exhibit some conserved aspects of this network, with retinoic acid signaling playing a role in establishing nested domains of Hox expression. (shrink)

Amphioxus represents the most basally divergent group in chordates and probably the best extant proxy to the ancestor of all chordates including vertebrates. The amphioxus, or lancelets, are benthic filter feeding marine animals and their interest as a model in research is due to their phylogenetic position and their anatomical and genetic stasis throughout their evolutionary history. From the first works in the 19th century to the present day, enormous progress is made mainly favored by technical development at (...) different levels, from spawning induction and husbandry techniques, through techniques for studies of gene function or of the role of different signalling pathways through embryonic development, to functional genomics techniques. Together, these advances foretell a plethora of interesting developments in the world of research with the amphioxus model. Here, the author review the discovery and development of amphioxus as a superb model organism in evolutionary and evolutionary‐developmental biology. (shrink)

Achieving regeneration of the central nervous system (CNS) is a major challenge for regenerative medicine. The inability of mammals to regrow a severed CNS contrasts with the amazing regenerative powers of their deuterostome kin, the echinoderms. Rapid CNS regeneration from a specialized autotomy plane in echinoderms presents a highly tractable and suitable non‐model system for regenerative biology and evolution. Starfish arm autotomy triggers mass cell migration and local proliferation, facilitating rapid CNS regeneration. Many regeneration events in nature are preceded by (...) autotomy and there are striking parallels between autotomy and regeneration in starfish and lizards. Comparison of these systems holds promise to provide insight into regeneration deficiency in higher vertebrates and to uncover evolutionarily conserved deuterostome‐chordate regenerative processes. This will help identify mechanisms that may be present but inactive in higher vertebrates to address the problem of their poor regenerative capacities and the challenge to achieve CNS repair and regrowth. (shrink)

In vertebrates it is often found that if one considers a group of genes clustered on a certain chromosome, then the homologues of those genes often form another cluster on a different chromosome. There are four explanations, not necessarily mutually exclusive, to explain how such homologous clusters appeared. Homologous clusters are expected at a low probability even if genes are distributed at random. The duplication of a subset of the genome might create homologous clusters, as would a duplication of the (...) entire genome. Alternatively, it may be adaptive for certain combinations of genes to cluster, although clearly the genes must have duplicated prior to rearrangement into clusters. Molecular phylogenetics provides a means to examine the origins of homologous clusters, although it is difficult to discriminate between the different explanations using current data. However, with more extensive sequencing and mapping of vertebrate genomes, especially those of the early diverging chordates, it should soon become possible to resolve the origins of homologous clusters. BioEssays 21:697–703, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

Many of the features that distinguish the vertebrates from other chordates are derived from the neural crest, and it has long been argued that the emergence of this multipotent embryonic population was a key innovation underpinning vertebrate evolution. More recently, however, a number of studies have suggested that the evolution of the neural crest was less sudden than previously believed. This has exposed the fact that neural crest, as evidenced by its repertoire of derivative cell types, has evolved through (...) vertebrate evolution. In this light, attempts to derive a typological definition of neural crest, in terms of molecular signatures or networks, are unfounded. We propose a less restrictive, embryological definition of this cell type that facilitates, rather than precludes, investigating the evolution of neural crest. While the evolutionary origin of neural crest has attracted much attention, its subsequent evolution has received almost no attention and yet it is more readily open to experimental investigation and has greater relevance to understanding vertebrate evolution. Finally, we provide a brief outline of how the evolutionary emergence of neural crest potentiality may have proceeded, and how it may be investigated. BioEssays 30:530–541, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

The origin of the notochord is one of the key remaining mysteries of our evolutionary ancestry. Here, we present a multi‐level comparison of the chordate notochord to the axochord, a paired axial muscle spanning the ventral midline of annelid worms and other invertebrates. At the cellular level, comparative molecular profiling in the marine annelids P. dumerilii and C. teleta reveals expression of similar, specific gene sets in presumptive axochordal and notochordal cells. These cells also occupy corresponding positions in a conserved (...) anatomical topology and undergo similar morphogenetic movements. At the organ level, a detailed comparison of bilaterian musculatures reveals that most phyla form axochord‐like muscles, suggesting that such a muscle was already present in urbilaterian ancestors. Integrating comparative evidence at the cell and organ level, we propose that the notochord evolved by modification of a ventromedian muscle followed by the assembly of an axial complex supporting swimming in vertebrate ancestors. (shrink)

Recent advances in molecular biology and microanatomy have supported homologies of body parts between vertebrates and extant invertebrate chordates, thus providing insights into the body plan of the proximate ancestor of the vertebrates. For example, this ancestor probably had a relatively complex brain and a precursor of definitive neural crest. Additional insights into early vertebrate evolution have come from recent discoveries of Lower Cambrian soft body fossils of Haikouichthys and Myllokunmingia (almost certainly vertebrates, possibly related to modern lampreys) and (...) Yunnanozoon and Haikouella (evidently stem-group vertebrates). The earliest vertebrates had an unequivocally marine origin, probably evolved mineralised pharyngeal denticles before the dermal skeleton, and evidently utilised elastic recoil of the visceral arch skeleton for suction feeding. Moreover, the new data emphasise that the advent of definitive neural crest was supremely important for the evolutionary origin of the vertebrates. BioEssays 23:142–151, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

Every living entity requires the capacity to defend against viruses in some form. From bacteria to plants to arthropods, cells retain the capacity to capture genetic material, process it in a variety of ways, and subsequently use it to generate pathogen-specific small RNAs. These small RNAs can then be used to provide specificity to an otherwise non-specific nuclease, generating a potent antiviral system. While small RNA-based defenses in chordates are less utilized, the protein-based antiviral invention in this phylum appears (...) to have derived from components of the same ancestral small RNA machinery. Based on recent evidence, it would seem that RNase III nucleases have been reiteratively repurposed over billions of years to provide cells with the capacity to recognize and destroy unwanted genetic material. Here we describe an overview of what is known on this subject and provide a model for how these defenses may have evolved. RNase III nuclease domains are widely used in an array of RNA metabolic functions as well as antiviral defenses. Here, we review their current usage as well as provide insight into how these domains may have shaped all major antiviral pathways over the course of evolution. (shrink)

The claim of consistent hemispheric specialisations across classes of chordates is undermined by the absence of population-based directional asymmetry of paw/hand use in rodents and primates. No homologue of the cerebral torque from right frontal to left occipital has been established in a nonhuman species. The null hypothesis that the torque is the sapiens-specific neural basis of language has not been disproved.

The questions raised for the study of marine invertebrate larvae have implications for the evolution of development, the life histories of animals, and life in the sea more generally. These questions began to coalesce in the 19th century around two main factors. The first was the discovery of marine larvae. Through careful observation, investigators detected and confirmed that the development of animals exhibited stages surprisingly different from the previously known adults and adult-like juveniles. Famous examples include the demonstration that barnacles (...) were arthropods because of a shared larval form (Thompson 1830), the characterization of bilateral echinoderm larvae by Johannes Müller (summarized in Huxley 1851), and the unexpected chordate affinities of tunicates. Subsequent studies also demonstrated that adults with similar morphological features could have radically different larval forms and life histories. (shrink)

The retina pigment epithelium (RPE) is a highly specialised epithelium that serves as a multifunctional and indispensable component of the vertebrate eye. Although a great deal of attention has been paid to its transdifferentiation capabilities and its ancillary functions in neural retina development, little is known about the molecular mechanisms that specify the RPE itself. Recent advances in our understanding of the genetic network that controls the progressive specification of the eye anlage in vertebrates have provided some of the initial (...) cues to the mechanisms responsible for RPE patterning. Here, we have outlined many recent findings that suggest that a limited number of transcription factors, including Otx2, Mitf and Pax6 and a few signalling cascades, are the elements required for the onset of RPE specification in vertebrates. Furthermore, using this information and the data available on the specification of the pigmented cells of primitive chordates, we have ventured some hypotheses on the origin of RPE cells during evolution. BioEssays 26:766–777, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

The urochordates, whose stem groups may have included the direct predecessors of the chordate line, serve as an excellent model group of organisms for a variety of scientific disciplines. One taxon, the botryllid ascidian, has emerged as the model system for studying allorecognition; this work has concentrated on the cosmopolitan species Botryllus schlosseri. Studies analyzing self–nonself recognition in this colonial marine organism point to three levels of allorecognition, each associated with different outcomes. The first level controls natural allogeneic rejections and (...) fusions, in which blood‐shared chimeras are formed. The second level leads to morphological resorption of partners within chimeras while the third allows the development of somatic and germ cell parasitic events. Recent studies on multi‐chimeric entities formed in allogeneic fusions reveal evolutionary links between allorecognition, stem cell biology and ecology. Thus, the Botryllus system generates perspectives from different biological disciplines to yield a unique life history portrait. BioEssays 24:730–740, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

Even today, a century after the publication of the “Origin of Species”, current zoological literature often reveals an insufficient grasp of the implications of the now generally accepted view that it is natural selection that confers direction on the evolutionary process.This is, in part, due to a reaction against oversimplified teleology and against Lamarckism. In rejecting Lamarck's thesis that the activities of an animal directly affect its hereditary characters it is frequently assumed that this implies that such activities are irrelevant (...) to the study of evolution. This is a non-sequitur, for activities may affect evolution not directly, through heredity, but indirectly, by influencing the direction of the selective forces impinging on the organism. Reasons are given for concluding that changes in habits and behaviour frequently precede structural change and, in fact, determine the direction of the latter.As an example of a concept which deserves a more evolutionary treatment than it commonly receives, the subject of neoteny is considered. It appears that failure to think in terms of the selective forces involved has frequently led to error. An analysis in functional terms is made of the theory that the chordates represent secondarily glorified sea-squirts and it is concluded that this idea is unlikely to be correct. The characteristics of man that are commonly considered to be neotenous are also discussed. It is concluded that a facile and unanalytical application of the label “neoteny” to many of the evolutionary changes involved has tended to obscure rather than to clarify their significance: the neotenous features of man are not those that have been of primary importance in human evolution, but are secondary to the acquisition of new characters which are not to be found in either the adult or the young stages of any other primate.Aujourd'hui, un siècle après l'apparition de l'Originedes Espèces, la conviction est presque générale que la sélection naturelle dirige le procès de l'évolution. Néanmoins, les publications zoologiques actuelles révèlent souvent une compréhension insuffisante de ce qui en résulte.Ceci doit en partie être considéré comme réaction contre une téléologie simpliste et contre le Lamarckisme. En rejetant la thèse de Lamarck, que les activités d'un animal ont une influence directe sur ses caractéristiques héréditaires, on admet fréquemment que de pareilles activités n'ont rien à voir avec l'étude le l'évolution. Cependant, une telle conclusion n'est pas justifiée, car les activités peuvent influer sur l'évolution non pas directement, par la voie de l'hérédité, mais indirectement en agissant sur la direction des forces sélectives que l'organisme rencontre. Des raisons ont été données qui mènent à conclure que des changements dans les habitudes et dans le comportement précèdent maintes fois le changement structural et même déterminent la direction de celui-ci.Comme exemple d'un concept qui mérite un traitement plus évolutionniste qu'il ne reçoive en général, la néoténie a été examinée. Il paraît que l'on a fait beaucoup d'erreurs en ne tenant pas compte des forces sélectives engagées. Une analyse au pointde-vue de la fonction a été appliquée à la théorie que les chordées ne sont que des ascidies néoténiques transfigurées par après, et il s'ensuit que cette idée n'est probablement pas correcte. Aussi les caractéristiques de l'homme généralement regardées comme neoténiques ont été discutées. Il en résulte que, en désignant légèrement et sans analyse plusieurs changements évolutionnaires comme néoténie, on a plutôt obsurci qu'éclairci leur signification. Ce ne sont pas les traits néoteniques de l'homme qui ont joué le rôle principal dans l'évolution humaine; au contraire, ils n'apparaissent qu'après l'acquisition de quelques nouvelles caractéristiques qui ne se trouvent ni dans l'adulte ni dans les phases plus jeunes des autres primates.Sogar heutzutage, ein Jahrhundert nach dem Erscheinen der „Origin of Species”, zeigt die jetzige zoologische Literatur oft eine unzulängliche Einsicht in die Folgerungen, zu denen die jetzt allgemein akzeptierte Annahme leitet, dass die natürliche Selektion die Richtung des Evolutionsprozesses bedingt.Dies ist teilweise zu sehen als eine Reaktion gegen eine einseitige Teleologie und gegen das Lamarckismus. Bei der Ablehnung der TheseLamarck's, dass die Aktivitäten eines Tieres einen direkten Einfluss ausüben auf seine erbliche Konstitution, nimmt man oft an, dass dies bedeutet, dass derartige Aktivitäten demnach nichts zur Sache tun bei dem Studium der Evolution. Zu einer derartigen Konklusion ist man jedoch nicht berechtigt, denn, obwohl die Aktivitäten die Evolution nicht direkt beeinflussen können mittels der Erblichkeit, so können sie dies jedoch indirekt, weil sie die Richtung der selektiven Kräfte, welche auf das Organismus einwirken, bestimmen. Es werden Argumente gegeben, welche die Konklusion rechtfertigten, dass Änderungen in Gewohnheit und Verhalten oft Strukturänderungen vorhergehen und tatsächlich die Richtung dieser letztgenannten bestimmen. Die Neotenie wirdt als ein Beispiel eines Begriff betrachtet, welcher eine mehr evolutionäre Behandlung verdient als bisher geschehen ist. Es ergibt sich dass man viele Irrtümer hat gemacht, indem man den betreffenden selektiven Kräften keine Rechnung getragen hat. Eine Analyse der Theorie, welche die Chordaten als neotenische Ascidien betrachtet, wird von einem funktionellen Gesichtspunkt aus durchgeführt. Daraus scheint der Schluss berechtigt, dass diese Theorie falsch ist. Auch werden die menschlichen Kennzeichen, welche gewöhnlich als neotenische betrachtet werden, besprochen. Die Konklusion lautet, dass, wenn man leichtfertig und ohne genügende Analyse den Zettel „Neotenie“ für mancherlei evolutive Änderungen anwendet, man die richtige Bedeutung dieser Erscheinung vielmehr verdunkelt als aufklärt: nicht die neotenische Kennzeichen des Menschen sind am wichtigsten in seiner Evolution; in Gegenteil sie erscheinen nur sekundär, nachdem die neuen Kennzeichen, welche sich weder in den Erwachsenen noch in den jüngeren Stadien von den anderen Primaten finden lassen, erschienen sind. (shrink)

The multiplicity of deubiquitinating enzymes (DUBs) encoded by vertebrate genomes is partly attributable to whole genome duplication events that occurred early in chordate evolution. By surveying the literature for the largest family of DUBs (the ubiquitin-specific proteases), extensive functional redundancy for duplicated genes has been confirmed as opposed to singletons. Dramatically conflicting results have been reported for loss of function studies conducted through RNA interference as opposed to inactivating mutations, but the contradictory findings can be reconciled by a recently proposed (...) compensatory mechanism involving nonsense-mediated RNA degradation. Duplicated genes are often inactivated to become pseudogenes, and it is proposed that such is the fate of the USP15 gene of zebrafish, a commonly used model system. As it is reviewed here, these observations have implications not only for the interpretation of model system phenotypes but also for therapeutic interventions designed to target DUBs. (shrink)

In the maximal form of indirect development found in many taxa of marine invertebrates, embryonic cell lineages of fixed fate and limited division capacity give rise to the larval structures. The adult arises from set‐aside cells in the larva that are held out from the early embryonic specification processes, and that retain extensive proliferative capacity. We review the locations and fates of set‐aside cells in two protostomes, a lophophorate and a deuterostome. The distinct adult body plans of many phyla develop (...) from homologous set‐aside cells within homologous larvae. We argue that the stocks from which these phyla arose utilized these respective larvae, and the diversity of their adult body plans reflects diverse pattern formation processes executed in their set‐aside cell populations. Chordates and arthropods develop directly, but share adult characters with indirectly developing phyla. Thus the deuterostome and protostome stocks that were ancestral to chordates and arthropods, respectively, also utilized maximal indirect development. (shrink)

Colonial ascidians offer opportunities to investigate how developmental events are integrated to generate the animal form, since they can develop similar individuals (oozooids from eggs, blastozooids from pluripotent somatic cells) through very different reproductive processes, i.e. embryogenesis and blastogenesis. Moreover, thanks to their key phylogenetic position, they can help in the understanding of the molecular mechanisms of morphogenesis and their evolution in chordates. We review organogenesis of the ascidian neural complex comparing embryos and buds in terms of topology, developmental (...) mechanisms and terminology. We propose a new interpretation of bud territories, and reconsider nervous system development based on recent results suggesting that ascidians have vertebrate placodal and neural‐crest‐like cells. Comparing embryonic and blastogenic development in Botryllus schlosseri, we propose that the bud has territories with a placodal potentiality, suggesting that chordate ancestors possessed neurogenic placodes, and that the genetic pathways regulating neurogenic placode formation were co‐opted for new developmental processes, such as blastogenesis. BioEssays 28: 902–912, 2006. © 2006 Wiley periodicals, Inc. (shrink)

The brain ventricular system is a series of connected cavities, filled with cerebrospinal fluid (CSF), that forms within the vertebrate central nervous system (CNS). The hollow neural tube is a hallmark of the chordate CNS, and a closed neural tube is essential for normal development. Development and function of the ventricular system is examined, emphasizing three interdigitating components that form a functional system: ventricle walls, CSF fluid properties, and activity of CSF constituent factors. The cellular lining of the ventricle both (...) can produce and is responsive to CSF. Fluid properties and conserved CSF components contribute to normal CNS development. Anomalies of the CSF/ventricular system serve as diagnostics and may cause CNS disorders, further highlighting their importance. This review focuses on the evolution and development of the brain ventricular system, associated function, and connected pathologies. It is geared as an introduction for scholars with little background in the field. (shrink)

Genetic analysis of Drosophila has shown that a morphogenetic gradient of the Transforming Growth Factor‐β family member dpp patterns the embryonic dorsalventral axis. Molecular and embryological evidence from Xenopus has strongly suggested a similar role for Bmp‐4, the dpp homolog, in patterning the dorsalventral axis of chordates. A recent report has now identified mutations in two genes, dino and swirl, that disrupt dorsal‐ventral patterning in the zebrafish Danio rerio(1). Characterization of these mutations parallels findings from Drosophila, thus establishing a (...) genetic framework for the analysis of dorsalventral patterning in a vertebrate. (shrink)

The last decade has seen a dramatic increase in studies on the development, function and evolution of asymmetries in vertebrates, including amphibians. Here we discuss current knowledge of behavioral and anatomical asymmetries in amphibians. Behavioral laterality in the response of both adult and larval anurans to presumed predators and competitors is strong and may be related, respectively, to laterality in the telencephalon of adults and the Mauthner neurons of tadpoles. These behavior lateralities, however, do not seem to correlate with visceral (...) asymmetries in the same animals. We briefly compare what is known about the evolution and development of asymmetry in the structure and function of amphibians with what is known about asymmetries in other chordate and non‐chordate groups. Available data suggest that the majority of asymmetries in amphibians fall into two independent groups: (1) related to situs viscerum and (2) of a neurobehavioral nature. We find little evidence linking these two groups, which implies different developmental regulatory pathways and independent evolutionary histories for visceral and telencephalic lateralizations. Studies of animals other than standard model species are essential to test hypotheses about the evolution of laterality in amphibians and other chordates. BioEssays 26:512–522, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

Early animal embryos are patterned by localized egg cytoplasmic factors and cell interactions. In invertebrate chordate ascidians, larval tail muscle originates from the posterior marginal zone of the early embryo. It has recently been demonstrated that maternal macho‐1 mRNA encoding transcription factor acts as a localized muscle determinant. Other mesodermal tissues such as notochord and mesenchyme are also derived from the vegetal marginal zone. In contrast, formation of these tissues requires induction from endoderm precursors at the 32‐cell stage. FGF–Ras–MAPK signaling (...) is involved in the induction of both tissues. The responsiveness for induction to notochord or mesenchyme depends on the inheritance of localized egg cytoplasmic factors. Previous studies also point to critical roles of directed signaling in polarization of induced cells and in subsequent asymmetric divisions resulting in the formation of two daughter cells with distinct fates. One cell adopts an induced fate, while the other assumes a default fate. A simple model of mesoderm patterning in ascidian embryos is proposed in comparison with that of vertebrates. BioEssays 24:613–624, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

This paper tries to trace the different evolutional stages the egg of primitive chordates had to pass through to reach the complex state of the bird egg . It tries to ascribe to the evolutional stages the successive appearance of transitional particularities which characterise the egg and the ontogeny of birds.The different forms of individual development in birds are classified in 7 groups, proceeding from primitive to more advanced types. The type of Galliformes, especially of the Megapodidae, is shown (...) to be very probably the most primitive one, while the Passeriformes show the highest degree of advanced specialisation.Proceeding from the primitive stages of chordates to the more advanced stages of brids a steadily growing participation of the maternal organism in the development of the young animal is stated . Corresponding to this participation we find the young in a more and more intimate dependance of the mother — a dependance that unites the young and his parents in a “system” just as complicated as that built up by the young mammal and its mother. The comparative study allows us to trace the progressive evolution of such a system in avian phylogeny.Ce travail essaie d'établir les stades de l'évolution par lesquels l'oeuf des cordés primitifs a dû successivement passer pour atteindre l'état complexe réalisé dans l'oeuf des oiseaux . Nous essayons ici d'attribuer aux stades évolutifs l'apparition successive de particularités transitoires qui, dans leur totalité, caractérisent l'oeuf et l'ontogénèse des oiseaux.Les différentes formes de l'ontogénèse des oiseaux sont groupées en 7 catégories, allant d'un mode reconnu primitif à un autre mode évolué. Les Gallinacés, en particulier les Mégapodides, nous paraissent se rapprocher le plus du stade primitif; les Passereaux semblent représenter le stade le plus avancé.En passant des stades primitifs des cordés aux stades plus évolués des oiseaux, on constate une participation progressive de la mère à l'ontogénèse de l'espèce . De cette participation résulte une dépendance de plus en plus étroite du jeune animal — dépendance qui unit les parents et leur progéniture en un „système” supérieur dépassant l'individu seul et qui n'est pas moins complexe que celui formé par la mère et l'enfant des mammifères. L'étude comparative permet actuellement de suivre l'évolution progressive d'un tel système dans la phylogénèse des oiseaux. (shrink)

This paper addresses the question of whether animals have souls and the ability to experience God after death within the limitations of their nature. Plausible explanations for the natural origin of life and for the development of subsequent complexity are increasingly being advanced by molecular biologists. Christian tradition and scholasticism teach that the human body is animated by the soul which is the agent of vital activities. This teaching is incompatible with the claim for a natural origin for life. At (...) some stage in the evolution of chordates and cephalopods a sense of self-awareness and an ability to distinguish between pleasure and the absence of pleasure would have arisen permitting the potential for ensoulment. The premise that evolution was gradual but ensoulment was discontinuous predicates the irrational conclusion that for one generation the parents were animals without souls and their children humans, made in the image of God, and with souls. Biological gradualism is incompatible with a sudden ensoulment dichotomy both in the evolutionary history of humans and for a maturing foetus, human or animal. Gradualism must apply to both body and soul. A Christian interpretation of physicalism, however, provides an alternative to dualism and resolves the paradoxes and difficulties relating to animals. (shrink)

In the last few years, an understanding has emerged of the developmental mechanism for the consistent internal left–right structure, termed situs, that characterises vertebrate anatomy. This involves largely vertebrate‐conserved (i.e. ‘phylotypic’) gene expression cascades that encode ‘leftness’ and ‘rightness’ in appropriate tissues either side of the embryo's midline soon after gastrulation. Recent evidence indicates that the initial, directional symmetry breaking that initiates these cascades utilises mechanisms that are conserved or at least closely related in different vertebrate types. I describe a (...) scenario whereby the capacity for directional modification of an otherwise bilateral body plan can be viewed as an adaptive innovation rather closely connected with vertebrate origins, enabling optimal ‘design’ for very active lifestyles. But an alternative scenario, while retaining the view that situs and indeed other vertebrate functional lateralisations are deeply adaptive, proposes that they originated in the co‐optation of left–right developmental information inherited from a very early stage in metazoan diversification. It is proposed that a remote chordate ancestor lost its original or ‘ur‐bilaterian’ symmetry to pass through an altogether non‐symmetrical stage, and that the vertebrate dorsoventral midline plane is not descended from that original one. I review the considerable evidence in favour of this scenario, and discuss its wider implications for directional asymmetries across the Metazoa. BioEssays 26:413–421, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

An important mechanism for the evolution of phenotypic complexity, diversity and innovation, and the origin of novel gene functions is the duplication of genes and entire genomes. Recent phylogenomic studies suggest that, during the evolution of vertebrates, the entire genome was duplicated in two rounds (2R) of duplication. Later, ∼350 mya, in the stem lineage of ray‐finned (actinopterygian) fishes, but not in that of the land vertebrates, a third genome duplication occurred—the fish‐specific genome duplication (FSGD or 3R), leading, at least (...) initially, to up to eight copies of the ancestral deuterostome genome. Therefore, the sarcopterygian (lobe‐finned fishes and tetrapods) genome possessed originally only half as many genes compared to the derived fishes, just like the most‐basal and species‐poor lineages of extant fishes that diverged from the fish stem lineage before the 3R duplication. Most duplicated genes were secondarily lost, yet some evolved new functions. The genomic complexity of the teleosts might be the reason for their evolutionary success and astounding biological diversity. BioEssays 27:937–945, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

James Valentine's "On the Origin of Phyla" is divided into three main sections: "Evidence of the Origins of Metazoan Phyla," "The Metazoan Phyla," and "The Evolution of the Phyla." The second section is the zoological core of the book, a more or less conventional treatment of major animal taxa, arranged in chain-of-being fashion from sponges to cnidarians to "worms" of many kinds, and so onward to arthropods, echinoderms, chordates, and all others in between. Philosophically inclined readers will be most (...) interested in the first and third sections of the volume, which treat phylogenetic principles and macroevolution respectively. These sections are rich in detail, but Valentine adopts something of a rear-guard position on many issues, arguing in favor of a number of concepts that most systematists have now abandoned. The publisher's advertisement describes "On the Origin of Phyla" as "one of the classic scientific texts of the twentieth century," and for us in the 21st century, that may be the best way to regard it. It will be of greatest value to zoologists who want a good compendium of current comparative data, and to historians and philosophers of science who have an interest in the complex development of phylogenetic "tree-thinking" in systematic biology. (shrink)