Developmental processes, developmental sequences and early vertebrate phylogeny

Germ layers, the neural crest and emergent organization in development and evolution

Discovered in chick embryos by Wilhelm His in 1868 and named the neural crest by Arthur Milnes Marshall in 1879, the neural crest cells that arise from the neural folds have since been shown to differentiate into almost two dozen vertebrate cell types and to have played major roles in the evolution of such vertebrate features as bone, jaws, teeth, visceral (pharyngeal) arches, and sense organs. I discuss the discovery that ectodermal neural crest gave rise to mesenchyme and the controversy generated by that finding; the germ layer theory maintained that only mesoderm could give rise to mesenchyme. A second topic of discussion is germ layers (including the neural crest) as emergent levels of organization in animal development and evolution that facilitated major developmental and evolutionary change. The third topic is gene networks, gene co-option, and the evolution of gene-signaling pathways as key to developmental and evolutionary transitions associated with the origin and evolution of the neural crest and neural crest cells.

Mechanotransduction in Blood and Lymphatic Vascular Development and Disease

The blood and lymphatic vasculatures are hierarchical networks of vessels, which constantly transport fluids and, therefore, are exposed to a variety of mechanical forces. Considering the role of mechanotransduction is key for fully understanding how these vascular systems develop, function, and how vascular pathologies evolve. During embryonic development, for example, initiation of blood flow is essential for early vascular remodeling, and increased interstitial fluid pressure as well as initiation of lymph flow is needed for proper development and maturation of the lymphatic vasculature. In this review, we introduce specific mechanical forces that affect both the blood and lymphatic vasculatures, including longitudinal and circumferential stretch, as well as shear stress. In addition, we provide an overview of the role of mechanotransduction during atherosclerosis and secondary lymphedema, which both trigger tissue fibrosis.

HETEROCHRONY AND DEVELOPMENTAL INNOVATION: EVOLUTION OF FEMALE GAMETOPHYTE ONTOGENY IN GNETUM, A HIGHLY APOMORPHIC SEED PLANT

Seed plant female gametophytes are focal points for the evolutionary modification of development. From a structural perspective, the most divergent female gametophytes among all seed plants are found in Gnetum, a clade within Gnetales. Coenocytic organization at sexual maturity, absence of defined egg cells (free nuclei are fertilized), lack of centripetal cellularization, and postfertilization development of embryo-nourishing tissues are features of the female gametophytes of Gnetum unparalleled among seed plants. Although the female gametophyte of Gnetum retains the three basic phases of somatic development common to female gametophytes of plesiomorphic seed plants (free nuclear development, cellularization, cellular growth), the timing of fertilization has been accelerated relative to the rate of somatic development. As a consequence, the female gametophyte of Gnetum matures sexually (is fertilized) at a juvenile (compared with the ancestral somatic ontogeny) and free nuclear stage of somatic development, thereby precluding differentiation of egg cells. Unlike progenetic animals, where truncation of somatic ontogeny evolves in tandem with acceleration in the timing of sexual maturation, the female gametophyte of Gnetum completes the entire ancestral somatic ontogeny after precocious sexual maturation. This results in the evolution of postfertilization development of embryo-nourishing female gametophyte tissues, a phenomenon unique among seed plants. Nonheterochronic developmental innovations have also played important roles in the evolution of the female gametophyte of Gnetum. Centripetal cellularization, which is always associated with the phase change from coenocytic to cellular organization among plesiomorphic seed plant female gametophytes, is lacking in Gnetum. Instead, during early phases of development, apomorphic free nuclear organization is coupled with a highly anomalous pattern of cellularization. Stage-specific innovations during early development in the female gametophyte of Gnetum do not affect plesiomorphic aspects of later phases of development. Thus, a complex array of heterochronic and nonheterochronic developmental innovations have played critical roles in the ontogenetic evolution of the highly apomorphic female gametophyte of Gnetum.

Left-right asymmetry of the gnathostome skull: its evolutionary, developmental, and functional aspects

Much of the gnathostome (jawed vertebrate) evolutionary radiation was dependent on the ability to sense and interpret the environment and subsequently act upon this information through utilization of a specialized mode of feeding involving the jaws. While the gnathostome skull, reflective of the vertebrate baüplan, typically is bilaterally symmetric with right (dextral) and left (sinistral) halves essentially representing mirror images along the midline, both adaptive and abnormal asymmetries have appeared. Herein we provide a basic primer on studies of the asymmetric development of the gnathostome skull, touching briefly on asymmetry as a field of study, then describing the nature of cranial development and finally underscoring evolutionary and functional aspects of left-right asymmetric cephalic development.

Pattern and polarity in the development and evolution of the gnathostome jaw: both conservation and heterotopy in the branchial arches of the shark, Scyliorhinus canicula

The acquisition of jaws constitutes a landmark event in vertebrate evolution, one that in large part potentiated their success and diversification. Jaw development and patterning involves an intricate spatiotemporal series of reciprocal inductive and responsive interactions between the cephalic epithelia and the cranial neural crest (CNC) and cephalic mesodermal mesenchyme. The coordinated regulation of these interactions is critical for both the ontogenetic registration of the jaws and the evolutionary elaboration of variable jaw morphologies and designs. Current models of jaw development and evolution have been built on molecular and cellular evidence gathered mostly in amniotes such as mice, chicks and humans, and augmented by a much smaller body of work on the zebrafish. These have been partnered by essential work attempting to understand the origins of jaws that has focused on the jawless lamprey. Chondrichthyans (cartilaginous fish) are the most distant group to amniotes within extant gnathostomes, and comprise the crucial clade uniting amniotes and agnathans; yet despite their critical phylogenetic position, evidence of the molecular and cellular underpinnings of jaw development in chondrichthyans is still lacking. Recent advances in genome and molecular developmental biology of the lesser spotted dogfish shark, Scyliorhinus canicula, make it ideal for the molecular study of chondrichthyan jaw development. Here, following the 'Hinge and Caps' model of jaw development, we have investigated evidence of heterotopic (relative changes in position) and heterochronic (relative changes in timing) shifts in gene expression, relative to amniotes, in the jaw primordia of S. canicula embryos. We demonstrate the presence of clear proximo-distal polarity in gene expression patterns in the shark embryo, thus establishing a baseline molecular baüplan for branchial arch-derived jaw development and further validating the utility of the 'Hinge and Caps' model in comparative studies of jaw development and evolution. Moreover, we correlate gene expression patterns with the absence of a lambdoidal junction (formed where the maxillary first arch meets the frontonasal processes) in chondrichthyans, further highlighting the importance of this region for the development and evolution of jaw structure in advanced gnathostomes.

Mechanical strain enhances survivability of collagen micronetworks in the presence of collagenase: implications for load-bearing matrix growth and stability

There has been great interest in understanding the methods by which collagen-based load-bearing tissue is constructed, grown and maintained in vertebrate animals. To date, the responsibility for this process has largely been placed with mesenchymal fibroblastic cells that are thought to fully control the morphology of load-bearing extracellular matrix (ECM). However, given clear limitations in the ability of fibroblastic cells to precisely place or remove single collagen molecules to sculpt tissue, we have hypothesized that the material itself must play a critical role in the determination of the form of structural ECM. We here demonstrate directly, using live, dynamic, differential interference contrast imaging, that mechanically strained networks of collagen fibrils, exposed to collagenase (Clostridium histolyticum), degrade preferentially. Specifically, unstrained fibrils are removed 'quickly', while strained fibrils persist significantly longer. The demonstration supports the idea that collagen networks are mechanosensitive in that they are stabilized by mechanical strain. Thus, collagen molecules (together with their complement enzymes) may comprise the basis of a smart, load-adaptive, structural material system. This concept has the potential to drastically simplify the assumed role of the fibroblast, which would need only to provide ECM molecules and mechanical force to sculpt collagenous tissue.

Comparison of Mandibular Phenotypic and Genetic Integration between Baboon and Mouse

In this study we compare patterns of mandibular integration between mice and baboons using both phenotypic and quantitative genetic data. Specifically, we test how well each species fits with the mosaic model of mandibular integration suggested by Atchley and Hall (Biol Rev Camb Philos Soc 66:101-157, 1991) based on developmental modules. We hypothesize that patterns of integration will be similar for mice and baboons and that both species will show strong integration within developmental modules and weaker integration between modules. Corresponding landmark data were collected from the hemi-mandibles of an advanced intercross mouse sample (N = 1239) and mandibles from a baboon sample of known pedigree from the Southwest Foundation for Biomedical Research (N = 430). We used four methods of analysis to quantify and compare the degree of mandibular integration between species including two methods based on a priori assumptions, and two a posteriori analyses. We found that patterns of integration are broadly similar for baboon and mouse mandibles, with both species displaying a modular pattern of integration. While there is a general trend of similarity in integration patterns between species, there were some marked differences. Mice are strongly correlated among distances within the coronoid process and the incisive alveolar region, whereas baboons are strongly integrated within the condylar process. We discuss the potential evolutionary implications of the similar patterns of integration between these species with an emphasis on the role of modularity.

Tweaking the hinge and caps: testing a model of the organization of jaws

Historically, examinations of gnathostome skulls have indicated that for essentially the entirety of their existence, jaws have been characterized by a high degree of fidelity to an initial basic structural design that will then go on to manifest an amazing array of end-point phenotypes. These two traits-bauplan fidelity and elaboration of design-are inter-connected and striking, and beg a number of questions, including: Are all jaws made in the same manner and if not how not? To begin to tackle such questions, we herein operationally define jaws as two appositional, hinged cranial units for which polarity and potential modularity are characteristics, and then address what is necessary for them to form, including delineating both the sources of cells and tissues that will formally yield the jaws as well as what informs their ontogeny (e.g., sources of positional information and factors directing the interpretation of developmental cues). Following on this, we briefly describe a predictive, testable model of jaw development (the "Hinge and Caps" model) and present evidence that the Satb2+cell population in the developing jaw primordia of mice defines a developmentally and evolutionarily significant jaw module such as would be predicted by the model.

21st century neontology and the comparative development of the vertebrate skull

Classic neontology (comparative embryology and anatomy), through the application of the concept of homology, has demonstrated that the development of the gnathostome (jawed vertebrate) skull is characterized both by a fidelity to the gnathostome bauplan and the exquisite elaboration of final structural design. Just as homology is an old concept amended for modern purposes, so are many of the questions regarding the development of the skull. With due deference to Geoffroy-St. Hilaire, Cuvier, Owen, Lankester et al., we are still asking: How are bauplan fidelity and elaboration of design maintained, coordinated, and modified to generate the amazing diversity seen in cranial morphologies? What establishes and maintains pattern in the skull? Are there universal developmental mechanisms underlying gnathostome autapomorphic structural traits? Can we detect and identify the etiologies of heterotopic (change in the topology of a developmental event), heterochronic (change in the timing of a developmental event), and heterofacient (change in the active capacetence, or the elaboration of capacity, of a developmental event) changes in craniofacial development within and between taxa? To address whether jaws are all made in a like manner (and if not, then how not), one needs a starting point for the sake of comparison. To this end, we present here a "hinge and caps" model that places the articulation, and subsequently the polarity and modularity, of the upper and lower jaws in the context of cranial neural crest competence to respond to positionally located epithelial signals. This model expands on an evolving model of polarity within the mandibular arch and seeks to explain a developmental patterning system that apparently keeps gnathostome jaws in functional registration yet tractable to potential changes in functional demands over time. It relies upon a system for the establishment of positional information where pattern and placement of the "hinge" is driven by factors common to the junction of the maxillary and mandibular branches of the first arch and of the "caps" by the signals emanating from the distal-most first arch midline and the lamboidal junction (where the maxillary branch meets the frontonasal processes). In this particular model, the functional registration of jaws is achieved by the integration of "hinge" and "caps" signaling, with the "caps" sharing at some critical level a developmental history that potentiates their own coordination. We examine the evidential foundation for this model in mice, examine the robustness with which it can be applied to other taxa, and examine potential proximate sources of the signaling centers. Lastly, as developmental biologists have long held that the anterior-most mesendoderm (anterior archenteron roof or prechordal plate) is in some way integral to the normal formation of the head, including the cranial skeletal midlines, we review evidence that the seminal patterning influences on the early anterior ectoderm extend well beyond the neural plate and are just as important to establishing pattern within the cephalic ectoderm, in particular for the "caps" that will yield medial signaling centers known to coordinate jaw development.

Reassessing the Dlx code: the genetic regulation of branchial arch skeletal pattern and development

The branchial arches are meristic vertebrate structures, being metameric both between each other within the rostrocaudal series along the ventrocephalic surface of the embryonic head and within each individual arch: thus, just as each branchial arch must acquire a unique identity along the rostrocaudal axis, each structure within the proximodistal axis of an arch must also acquire a unique identity. It is believed that regional specification of metameric structures is controlled by the nested expression of related genes resulting in a regional code, a principal that is though to be demonstrated by the regulation of rostrocaudal axis development in animals exerted by the nested HOM-C/Hox homeobox genes. The nested expression pattern of the Dlx genes within the murine branchial arch ectomesenchyme has more recently led to the proposal of a Dlx code for the regional specification along the proximodistal axis of the branchial arches (i.e. it establishes intra-arch identity). This review re-examines this hypothesis, and presents new work on an allelic series of Dlx loss-of-function mouse mutants that includes various combinations of Dlx1, Dlx2, Dlx3, Dlx5 and Dlx6. Although we confirm fundamental aspects of the hypothesis, we further report a number of novel findings. First, contrary to initial reports, Dlx1, Dlx2 and Dlx1/2 heterozygotes exhibit alterations of branchial arch structures and Dlx2-/- and Dlx1/2-/- mutants have slight alterations of structures derived from the distal portions of their branchial arches. Second, we present evidence for a role for murine Dlx3 in the development of the branchial arches. Third, analysis of compound Dlx mutants reveals four grades of mandibular arch transformations and that the genetic interactions of cis first-order (e.g. Dlx5 and Dlx6), trans second-order (e.g. Dlx5 and Dlx2) and trans third-order paralogues (e.g. Dlx5 and Dlx1) result in significant and distinct morphological differences in mandibular arch development. We conclude by integrating functions of the Dlx genes within the context of a hypothesized general mechanism for the establishment of pattern and polarity in the first branchial arch of gnathostomes that includes regionally secreted growth factors such as Fgf8 and Bmp and other transcription factors such as Msx1, and is consistent both with the structure of the conserved gnathostome jaw bauplan and the elaboration of this bauplan to meet organismal end-point designs.

Developing animals flout prominent assumptions of ecological physiology

Every field of biology has its assumptions, but when they grow to be dogma, they can become constraining. This essay presents data-based challenges to several prominent assumptions of developmental physiologists. The ubiquity of allometry is such an assumption, yet animal development is characterized by rate changes that are counter to allometric predictions. Physiological complexity is assumed to increase with development, but examples are provided showing that complexity can be greatest at intermediate developmental stages. It is assumed that organs have functional equivalency in embryos and adults, yet embryonic structures can have quite different functions than inferred from adults. Another assumption challenged is the duality of neural control (typically sympathetic and parasympathetic), since one of these two regulatory mechanisms typically considerably precedes in development the appearance of the other. A final assumption challenged is the notion that divergent phylogeny creates divergent physiologies in embryos just as in adults, when in fact early in development disparate vertebrate taxa show great quantitative as well as qualitative similarity. Collectively, the inappropriateness of these prominent assumptions based on adult studies suggests that investigation of embryos, larvae and fetuses be conducted with appreciation for their potentially unique physiologies.

Evolution of skeletal type e-c coupling: a novel means of controlling calcium delivery

The functional separation between skeletal and cardiac muscles, which occurs at the threshold between vertebrates and invertebrates, involves the evolution of separate contractile and control proteins for the two types of striated muscles, as well as separate mechanisms of contractile activation. The functional link between electrical excitation of the surface membrane and activation of the contractile material (known as excitation–contraction [e–c] coupling) requires the interaction between a voltage sensor in the surface membrane, the dihydropyridine receptor (DHPR), and a calcium release channel in the sarcoplasmic reticulum, the ryanodine receptor (RyR). Skeletal and cardiac muscles have different isoforms of the two proteins and present two structurally and functionally distinct modes of interaction.

We use structural clues to trace the evolution of the dichotomy from a single, generic type of e–c coupling to a diversified system involving a novel mechanism for skeletal muscle activation. Our results show that a significant structural transition marks the protochordate to the Craniate evolutionary step, with the appearance of skeletal muscle–specific RyR and DHPR isoforms.

Study on the common teratogenic pathway elicited by the fungicides triazole-derivatives

Triazole-derivatives alter the pharyngeal apparatus morphogenesis of rodent embryos cultured in vitro. The hindbrain segmentation and the rhombencephalic neural crest cell (NCCs) migration are altered by Fluconazole exposure in vitro. The aim of the present work is to identify if a common pathogenic pathway is detectable also for other molecules of this class of compounds. 9.5 days post coitum (d.p.c.) old rat embryos were exposed in vitro to the teratogenic concentrations of Flusilazole, Triadimefon and Triadimenol and cultured for 24, 48 or 60 h. The expression and localisation of Hox-b1 and Krox-20 proteins (used as markers for hindbrain segmentation) were evaluated after 24 h of culture. The localisation and distribution of NCC was evaluated after 24, 30 and 48 h of culture. The morphology of the embryos was analysed after 48 h, while the branchial nerve structures were evaluated after 60 h of culture. Hindbrain segmentation and NCC migration alteration as well as pharyngeal arch and cranial nerve abnormalities were detected after exposure of the tested molecules. A common severe teratogenic intrinsic property for the tested molecules of this chemical class has been found, acting through alteration of the normal hindbrain developmental pattern.

Consideration of the neural crest and its skeletal derivatives in the context of novelty/innovation

I examine the neural crest and skeletal tissues derived from neural crest cells in the context of novelty/innovation by asking whether the neural crest is a novel tissue and whether the evolutionary origin of the neural crest required innovative developmental processes. As a vertebrate autapomorphy, the neural crest is a novel structure. I equate novelty with innovation and take a hierarchical approach. Some other workers separate the two, using novelty for new structures not found in an ancestor and not homologous with a feature in an ancestor, and innovation for the new processes required to generate the novel structure. While development clearly evolves, I do not separate those processes that result in the production of novel features from those that lead to change in existing structures, whether that change is a transition or transformation from one homologous feature to another (fins-->tetrapod limbs or locomotory appendages-->crustacean maxilliped feeding appendages). The existence of novelties causes us to consider the concept of latent homology. Neural crest cells form cartilage, dentine and bone. Cartilage is found in invertebrates and so is not a vertebrate innovation. No invertebrate cartilage mineralizes in vivo, although some can be induced to mineralize in vitro. Mineralization of cartilage in vivo is a vertebrate innovation. Dentine is a novel tissue that only forms from neural crest cells. Bone is a vertebrate innovation but not one exclusive to the neural crest. The developmental processes responsible for the neural crest and for these skeletal tissues did not arise de novo with the vertebrates. Novelty/innovation results from tinkering with existing processes, from the flexibility that arises from modifications of existing gene networks, and from the selective advantage provided by gene duplications or modifications.

Comparative analysis of neural crest cell death, migration, and function during vertebrate embryogenesis

Cranial neural crest cells are a multipotent, migratory population that generates most of the cartilage, bone, connective tissue and peripheral nervous system in the vertebrate head. Proper neural crest cell patterning is essential for normal craniofacial morphogenesis and is highly conserved among vertebrates. Neural crest cell patterning is intimately connected to the early segmentation of the neural tube, such that neural crest cells migrate in discrete segregated streams. Recent advances in live embryo imaging have begun to reveal the complex behaviour of neural crest cells which involve intricate cell-cell and cell-environment interactions. Despite the overall similarity in neural crest cell migration between distinct vertebrates species there are important mechanistic differences. Apoptosis for example, is important for neural crest cell patterning in chick embryos but not in mouse, frog or fish embryos. In this paper we highlight the potential evolutionary significance of such interspecies differences in jaw development and evolution. Developmental Dynamics 229:14-29, 2004.

Modularity of the angiosperm female gametophyte and its bearing on the early evolution of endosperm in flowering plants

The monosporic seven-celled/eight-nucleate Polygonum-type female gametophyte has long served as a focal point for discussion of the origin and subsequent evolution of the angiosperm female gametophyte. In Polygonum-type female gametophytes, two haploid female nuclei are incorporated into the central cell, and fusion of a sperm cell with the binucleate central cell produces a triploid endosperm with a complement of two maternal and one paternal genomes, characteristic of most angiosperms. We document the development of a four-celled/four-nucleate female gametophyte in Nuphar polysepala (Engelm.) and infer its presence in many other ancient lineages of angiosperms. The central cell of the female gametophyte in these taxa contains only one haploid nucleus; thus endosperm is diploid and has a ratio of one maternal to one paternal genome. Based on comparisons among flowering plants, we conclude that the angiosperm female gametophyte is constructed of modular developmental subunits. Each module is characterized by a common developmental pattern: (1) positioning of a single nucleus within a cytoplasmic domain (pole) of the female gametophyte; (2) two free-nuclear mitoses to yield four nuclei within that domain; and (3) partitioning of three uninucleate cells adjacent to the pole such that the fourth nucleus is confined to the central region of the female gametophyte (central cell). Within the basal angiosperm lineages Nymphaeales and Illiciales, female gametophytes are characterized by a single developmental module that produces a four-celled/four-nucleate structure with a haploid uninucleate central cell. A second pattern, typical of Amborella and the overwhelming majority of eumagnoliids, monocots, and eudicots, involves the early establishment of two developmental modules that produce a seven-celled/eight-nucleate female gametophyte with two haploid nuclei in the central cell. Comparative analysis of ontogenetic sequences suggests that the seven-celled female gametophyte (two modules) evolved by duplication and ectopic expression of an ancestral Nuphar-like developmental module within the chalazal domain of the female gametophyte. These analyses indicate that the first angiosperm female gametophytes were composed of a single developmental module, which upon double fertilization yielded a diploid endosperm. Early in angiosperm history this basic module was duplicated, and resulted in a seven-celled/eight-nucleate female gametophyte, which yielded a triploid endosperm with the characteristic 2:1 maternal to paternal genome ratio.

Molecular dissection of craniofacial development using zebrafish

The zebrafish, Danio rerio, is a small, freshwater teleost that only began to be used as a vertebrate genetic model by the late George Streisinger in the early 1980s. The strengths of the zebrafish complement genetic studies in mice and embryological studies in avians. Its advantages include high fecundity, externally fertilized eggs and transparent embryos that can be easily manipulated, inexpensive maintenance, and the fact that large-scale mutagenesis screens can be performed. Here we review studies that have used the zebrafish as a model for craniofacial development. Lineage studies in zebrafish have defined the origins of the cranial skeleton at the single-cell level and followed the morphogenetic behaviors of these cells in skeletal condensations. Furthermore, genes identified by random mutational screening have now revealed genetic pathways controlling patterning of the jaw and other pharyngeal arches, as well as the midline of the skull, that are conserved between fish and humans. We discuss the potential impact of specialized mutagenesis screens and the future applications of this versatile, vertebrate developmental model system in the molecular dissection of craniofacial development.

Haeckel's ABC of evolution and development

One of the central, unresolved controversies in biology concerns the distribution of primitive versus advanced characters at different stages of vertebrate development. This controversy has major implications for evolutionary developmental biology and phylogenetics. Ernst Haeckel addressed the issue with his Biogenetic Law, and his embryo drawings functioned as supporting data. We re-examine Haeckel's work and its significance for modern efforts to develop a rigorous comparative framework for developmental studies. Haeckel's comparative embryology was evolutionary but non-quantitative. It was based on developmental sequences, and treated heterochrony as a sequence change. It is not always clear whether he believed in recapitulation of single characters or entire stages. The Biogenetic Law is supported by several recent studies -- if applied to single characters only. Haeckel's important but overlooked alphabetical analogy of evolution and development is an advance on von Baer. Haeckel recognized the evolutionary diversity in early embryonic stages, in line with modern thinking. He did not necessarily advocate the strict form of recapitulation and terminal addition commonly attributed to him. Haeckel's much-criticized embryo drawings are important as phylogenetic hypotheses, teaching aids, and evidence for evolution. While some criticisms of the drawings are legitimate, others are more tendentious. In opposition to Haeckel and his embryo drawings, Wilhelm His made major advances towards developing a quantitative comparative embryology based on morphometrics. Unfortunately His's work in this area is largely forgotten. Despite his obvious flaws, Haeckel can be seen as the father of a sequence-based phylogenetic embryology.

Specification of jaw subdivisions by Dlx genes

The success of vertebrates was due in part to the acquisition and modification of jaws. Jaws are principally derived from the branchial arches, embryonic structures that exhibit proximodistal polarity. To investigate the mechanisms that specify the identity of skeletal elements within the arches, we examined mice lacking expression of Dlx5 and Dlx6, linked homeobox genes expressed distally but not proximally within the arches. Dlx5/6-/- mutants exhibit a homeotic transformation of lower jaws to upper jaws. We suggest that nested Dlx expression in the arches patterns their proximodistal axes. Evolutionary acquisition and subsequent refinement of jaws may have been dependent on modification of Dlx expression.

Cloning of zebrafish T-box genes tbx15 and tbx18 and their expression during embryonic development

Members of the T-box (tbx) gene family encode developmentally regulated transcription factors, several of which are implicated in human hereditary diseases. We have cloned the paralogous genes tbx15 and tbx18 in zebrafish and have characterised their expression in detail. tbx15 is expressed in paraxial head mesenchyme and its derivatives, the extraocular and jaw musculature and the posterior neurocranium. Further areas of tbx15 expression are in the anterior somitic mesoderm, in periocular mesenchyme and in the pectoral fin mesenchyme throughout larval development. Areas of strong tbx18 expression are found in the developing somitic and presomitic mesoderm, in the heart and in pectoral fin mesenchyme, as well as the ventral neuroectoderm and the developing palate. Both genes exhibit particular differences in expression compared to their murine orthologs.

Sequence heterochrony and the evolution of development

One of the most persistent questions in comparative developmental biology concerns whether there are general rules by which ontogeny and phylogeny are related. Answering this question requires conceptual and analytic approaches that allow biologists to examine a wide range of developmental events in well-structured phylogenetic contexts. For evolutionary biologists, one of the most dominant approaches to comparative developmental biology has centered around the concept of heterochrony. However, in recent years the focus of studies of heterochrony largely has been limited to one aspect, changes in size and shape. I argue that this focus has restricted the kinds of questions that have been asked about the patterns of developmental change in phylogeny, which has narrowed our ability to address some of the most fundamental questions about development and evolution. Here I contrast the approaches of growth heterochrony with a broader view of heterochrony that concentrates on changes in developmental sequence. I discuss a general approach to sequence heterochrony and summarize newly emerging methods to analyze a variety of kinds of developmental change in explicit phylogenetic contexts. Finally, I summarize a series of studies on the evolution of development in mammals that use these new approaches.

Rainbow trout glucose transporter (OnmyGLUT1): functional assessment in Xenopus laevis oocytes and expression in fish embryos

Recently, we reported the cloning of a putative glucose transporter (OnmyGLUT1) from rainbow trout embryos. In this paper, we describe the functional characteristics of OnmyGLUT1 and its expression during embryonic development of rainbow trout. Transport of d-glucose was analysed in Xenopus laevis oocytes following microinjection of mRNA transcribed in vitro. These experiments confirmed that OnmyGLUT1 is a facilitative Na+-independent transporter. Assessment of substrate selectivity, sensitivity to cytochalasin B and phloretin and kinetic parameters showed that the rainbow trout glucose transporter was similar to a carp transporter and to mammalian GLUT1. Embryonic expression of OnmyGLUT1 was studied using whole-mount in situ hybridization. Ubiquitous distribution of transcripts was observed until the early phase of somitogenesis. During the course of organogenesis, somitic expression decreased along the rostro-caudal axis, finally ceasing in the mature somites. The OnmyGLUT1 transcripts were detected in the neural crest during the whole study period. Transcripts were also found in structures that are likely to originate from the neural crest cells (gill arches, pectoral fins, upper jaw, olfactory organs and primordia of mouth lips). Hexose transport activity was detected at all developmental stages after blastulation. Cytochalasin B blocked the accumulation of phosphorylated 2-deoxy-d-glucose by dissociated embryonic cells, suggesting an important role for transport in glucose metabolism.

Structure and developmental expression of the ascidian TRP gene: insights into the evolution of pigment cell-specific gene expression

The tyrosinase family in vertebrates consists of three related melanogenic enzymes: tyrosinase, tyrosinase-related protein-1 (TRP-1), and TRP-2. These proteins control melanin production in pigment cells and play a crucial role in determining vertebrate coloration. We have isolated a gene from the ascidian Halocynthia roretzi which encodes a tyrosinase-related protein (HrTRP) with 45-49% identity with vertebrate TRP-1 and TRP-2. The expression of the HrTRP gene in pigment lineage a8.25 cells starts at the early-mid gastrula stage, which coincides with the stage when these cells are determined as pigment precursor cells; therefore, it provides the earliest pigment lineage-specific marker, which enables us to trace the complete cell lineage leading to two pigment cells in the larval brain. In addition, the expression pattern of the HrTRP gene appears to share similar characteristics with the mouse TRP-2 gene although structurally the HrTRP gene is more closely related to mammalian TRP-1 genes. Based on these observations and on results from molecular phylogenetic and hybridization analyses, we suggest that triplication of the tyrosinase family occurred during the early radiation of chordates. Initially, duplication of an ancestral tyrosinase gene produced a single TRP gene before the urochordate and cephalochordate-vertebrate divergence, and a subsequent duplication of the ancestral TRP gene in the vertebrate lineage gave rise to two TRP genes before the emergence of teleost fishes. Evolution of the melanin synthetic pathway and possible phylogenetic relationships among chordate pigment cells that accommodate the metabolic process are discussed. Dev Dyn 1999;215:225-237.

Neural crest potential for tooth development in a urodele amphibian: developmental and evolutionary significance

Tooth development in urodele amphibians occurs from a restricted region of anterior cranial neural crest. An in vitro culture system was used to test the odontogenic potential of more caudal regions of neural crest, including an "intermediate region" of neural folds which has never previously been tested for either fate or potential. Explants of different axial levels of neural crest with stomodaeal ectoderm and endoderm demonstrated that odontogenic potential extends not only further caudally than the axial level fated to produce teeth, but also beyond that with potential to produce cartilage. Our results show that chondrogenic potential is found only within the most rostral portion of the intermediate region, but that odontogenic potential extends to its most caudal limit. This separation of skeletogenic cell lineages in the neural crest necessitates a reevaluation of the designations of "cranial" and "trunk" and a reconsideration of the evolutionary implications of developmentally distinct crest-derived mesenchyme populations. The proposal that odontogenic potential extends into the trunk neural crest may be explained as conserved from a phylogenetically older, more extensive skeletogenic ability which produced the exoskeleton of more basal vertebrates.

A homeotic transformation is generated in the rostral branchial region of the head by disruption of Hoxa-2, which acts as a selector gene

The Hoxa-2 gene was disrupted by homologous recombination. Homozygous mutant mice died at birth. Defects were found in the branchial region of the head, which corresponds to the Hoxa-2 rostral expression domain. While rhombomeric and neural crest cell (NCC) segmentation was not affected, mesenchymal NCC derivatives of the second arch were lacking, and second arch mesenchymal NCC identity was changed to first arch identity, resulting in homeotic transformation of second to first arch skeletal elements. These results reveal the existence of a skeletogenic ground pattern program common to at least the mesenchymal NCC that originated from rhombomeres 2 and 4. The appearance of an atavistic reptilian pterygoquadrate element in Hoxa-2 mutants suggests that this ground pattern is intermediate between reptiles and mammals. The ground pattern program appears to be modified in the mouse first arch by a Hox-independent process, whereas Hoxa-2 acts as a selector gene in the second arch.

Early development of the actinopterygian head. I. External development and staging of the paddlefish Polyodon spathula

A large sample of embryological material of the North American paddlefish Polyodon spathula (Acipenseriformes: Polyodontidae) confirms that early development in Polyodon is very similar to that reported for the sister group of Polyodontidae, the sturgeons (Acipenseridae). Polyodon illustrates many basic aspects of acipenseriform (and actinopterygian) head development that have not been adequately described. In this paper, we provide an overview of external features of cranial development using scanning electron microscopy. The observations are correlated with staging schemes previously proposed for paddlefishes and other acipenseriforms. Events that occur after the start of neurulation (stage 19) to the start of feeding (stage 46) are emphasized. New information on the structure and folding of the mandibular and hyoid segments permits an understanding of the early development of the pharyngeal region. In addition, we offer new descriptions of the hatching gland, the olfactory organ, the sensory barbel, and the initiation of paddle outgrowth. We also comment on the mode of origin of the hypophysis, and refute the notion that it is derived from the lips of the anterior neuropore as suggested in older literature. This information sets the stage for future comparative and experimental studies of the embryology of basal actinopterygians. © 1992 Wiley-Liss, Inc.

Development and evolutionary origins of vertebrate skeletogenic and odontogenic tissues

This review deals with the following seven aspects of vertebrate skeletogenic and odontogenic tissues. 1. The evolutionary sequence in which the tissues appeared amongst the lower craniate taxa. 2. The topographic association between skeletal (cartilage, bone) and dental (dentine, cement, enamel) tissues in the oldest vertebrates of each major taxon. 3. The separate developmental origin of the exo- and endoskeletons. 4. The neural-crest origin of cranial skeletogenic and odontogenic tissues in extant vertebrates. 5. The neural-crest origin of trunk dermal skeletogenic and odontogenic tissues in extant vertebrates. 6. The developmental processes that control differentiation of skeletogenic and odontogenic tissues in extant vertebrates. 7. Maintenance of developmental interactions regulating skeletogenic/odontogenic differentiation across vertebrate taxa. We derive twelve postulates, eight relating to the earliest vertebrate skeletogenic and odontogenic tissues and four relating to the development of these tissues in extant vertebrates and extrapolate the developmental data back to the evolutionary origin of vertebrate skeletogenic and odontogenic tissues. The conclusions that we draw from this analysis are as follows. 8. The dermal exoskeleton of thelodonts, heterostracans and osteostracans consisted of dentine, attachment tissue (cement or bone), and bone. 9. Cartilage (unmineralized) can be inferred to have been present in heterostracans and osteostracans, and globular mineralized cartilage was present in Eriptychius, an early Middle Ordovician vertebrate unassigned to any established group, but assumed to be a stem agnathan. 10. Enamel and possibly also enameloid was present in some early agnathans of uncertain affinities. The majority of dentine tubercles were bare. 11. The contemporaneous appearance of cellular and acellular bone in heterostracans and osteostracans during the Ordovician provides no clue as to whether one is more primitive than the other. 12. We interpret aspidin as being developmentally related to the odontogenic attachment tissues, either closer to dentine or a form of cement, rather than as derived from bone. 13. Dentine is present in the stratigraphically oldest (Cambrian) assumed vertebrate fossils, at present some only included as Problematica, and is cladistically primitive, relative to bone. 14. The first vertebrate exoskeletal skeletogenic ability was expressed as denticles of dentine. 15. Dentine, the bone of attachment associated with dentine, the basal bone to which dermal denticles are fused and cartilage of the Ordovician agnathan dermal exoskeleton were all derived from the neural crest and not from mesoderm. Therefore the earliest vertebrate skeletogenic/odontogenic tissues were of neural-crest origin.(ABSTRACT TRUNCATED AT 400 WORDS)