hand2 and Dlx genes specify dorsal, intermediate and ventral domains within zebrafish pharyngeal arches

Developmental dissociation in morphological evolution of the stickleback opercle

Oceanic threespine sticklebacks have repeatedly and independently evolved new morphologies upon invasions of freshwater habitats. A consistent derived feature of the freshwater form across populations and geography is a shape change of the opercle, a large early developing facial bone. We show that the principal multivariate axis describing opercle shape development from the young larva to the full adult stage of oceanic fish matches the principal axis of evolutionary change associated with relocation from the oceanic to freshwater habitat. The opercle phenotype of freshwater adults closely resembles the phenotype of the bone in juveniles. Thus, evolution to the freshwater condition is in large part by truncation of development; the freshwater fish do not achieve the full ancestral adult bone shape. Additionally, the derived state includes dissociated ontogenetic changes. Dissociability may reflect an underlying modular pattern of opercle development, and facilitate flexibility of morphological evolution.

fras1 shapes endodermal pouch 1 and stabilizes zebrafish pharyngeal skeletal development

Lesions in the epithelially expressed human gene FRAS1 cause Fraser syndrome, a complex disease with variable symptoms, including facial deformities and conductive hearing loss. The developmental basis of facial defects in Fraser syndrome has not been elucidated. Here we show that zebrafish fras1 mutants exhibit defects in facial epithelia and facial skeleton. Specifically, fras1 mutants fail to generate a late-forming portion of pharyngeal pouch 1 (termed late-p1) and skeletal elements adjacent to late-p1 are disrupted. Transplantation studies indicate that fras1 acts in endoderm to ensure normal morphology of both skeleton and endoderm, consistent with well-established epithelial expression of fras1. Late-p1 formation is concurrent with facial skeletal morphogenesis, and some skeletal defects in fras1 mutants arise during late-p1 morphogenesis, indicating a temporal connection between late-p1 and skeletal morphogenesis. Furthermore, fras1 mutants often show prominent second arch skeletal fusions through space occupied by late-p1 in wild type. Whereas every fras1 mutant shows defects in late-p1 formation, skeletal defects are less penetrant and often vary in severity, even between the left and right sides of the same individual. We interpret the fluctuating asymmetry in fras1 mutant skeleton and the changes in fras1 mutant skeletal defects through time as indicators that skeletal formation is destabilized. We propose a model wherein fras1 prompts late-p1 formation and thereby stabilizes skeletal formation during zebrafish facial development. Similar mechanisms of stochastic developmental instability might also account for the high phenotypic variation observed in human FRAS1 patients.

A temporal chromatin signature in human embryonic stem cells identifies regulators of cardiac development

Directed differentiation of human embryonic stem cells (ESCs) into cardiovascular cells provides a model for studying molecular mechanisms of human cardiovascular development. Although it is known that chromatin modification patterns in ESCs differ markedly from those in lineage-committed progenitors and differentiated cells, the temporal dynamics of chromatin alterations during differentiation along a defined lineage have not been studied. We show that differentiation of human ESCs into cardiovascular cells is accompanied by programmed temporal alterations in chromatin structure that distinguish key regulators of cardiovascular development from other genes. We used this temporal chromatin signature to identify regulators of cardiac development, including the homeobox gene MEIS2. Using the zebrafish model, we demonstrate that MEIS2 is critical for proper heart tube formation and subsequent cardiac looping. Temporal chromatin signatures should be broadly applicable to other models of stem cell differentiation to identify regulators and provide key insights into major developmental decisions.

Retinoic acid expands the evolutionarily reduced dentition of zebrafish

Zebrafish lost anterior teeth during evolution but retain a posterior pharyngeal dentition that requires retinoic acid (RA) cell-cell signaling for its development. The purposes of this study were to test the sufficiency of RA to induce tooth development and to assess its role in evolution. We found that exposure of embryos to exogenous RA induces a dramatic anterior expansion of the number of pharyngeal teeth that later form and shifts anteriorly the expression patterns of genes normally expressed in the posterior tooth-forming region, such as pitx2 and dlx2b. After RA exposure, we also observed a correlation between cartilage malformations and ectopic tooth induction, as well as abnormal cranial neural crest marker gene expression. Additionally, we observed that the RA-induced zebrafish anterior teeth resemble in pattern and number the dentition of fish species that retain anterior pharyngeal teeth such as medaka but that medaka do not express the aldh1a2 RA-synthesizing enzyme in tooth-forming regions. We conclude that RA is sufficient to induce anterior ectopic tooth development in zebrafish where teeth were lost in evolution, potentially by altering neural crest cell development, and that changes in the location of RA synthesis correlate with evolutionary changes in vertebrate dentitions.

New perspectives on pharyngeal dorsoventral patterning in development and evolution of the vertebrate jaw

Patterning of the vertebrate facial skeleton involves the progressive partitioning of neural-crest-derived skeletal precursors into distinct subpopulations along the anteroposterior (AP) and dorsoventral (DV) axes. Recent evidence suggests that complex interactions between multiple signaling pathways, in particular Endothelin-1 (Edn1), Bone Morphogenetic Protein (BMP), and Jagged-Notch, are needed to pattern skeletal precursors along the DV axis. Rather than directly determining the morphology of individual skeletal elements, these signals appear to act through several families of transcription factors, including Dlx, Msx, and Hand, to establish dynamic zones of skeletal differentiation. Provocatively, this patterning mechanism is largely conserved from mouse and zebrafish to the jawless vertebrate, lamprey. This implies that the diversification of the vertebrate facial skeleton, including the evolution of the jaw, was driven largely by modifications downstream of a conversed pharyngeal DV patterning program.

Ectodermal-derived Endothelin1 is required for patterning the distal and intermediate domains of the mouse mandibular arch

Morphogenesis of the vertebrate head relies on proper dorsal-ventral (D-V) patterning of neural crest cells (NCC) within the pharyngeal arches. Endothelin-1 (Edn1)-induced signaling through the endothelin-A receptor (Ednra) is crucial for cranial NCC patterning within the mandibular portion of the first pharyngeal arch, from which the lower jaw arises. Deletion of Edn1, Ednra or endothelin-converting enzyme in mice causes perinatal lethality due to severe craniofacial birth defects. These include homeotic transformation of mandibular arch-derived structures into more maxillary-like structures, indicating a loss of NCC identity. All cranial NCCs express Ednra whereas Edn1 expression is limited to the overlying ectoderm, core paraxial mesoderm and pharyngeal pouch endoderm of the mandibular arch as well as more caudal arches. To define the developmental significance of Edn1 from each of these layers, we used Cre/loxP technology to inactivate Edn1 in a tissue-specific manner. We show that deletion of Edn1 in either the mesoderm or endoderm alone does not result in cellular or molecular changes in craniofacial development. However, ectodermal deletion of Edn1 results in craniofacial defects with concomitant changes in the expression of early mandibular arch patterning genes. Importantly, our results also both define for the first time in mice an intermediate mandibular arch domain similar to the one defined in zebrafish and show that this region is most sensitive to loss of Edn1. Together, our results illustrate an integral role for ectoderm-derived Edn1 in early arch morphogenesis, particularly in the intermediate domain.

Multiple enhancers associated with ACAN suggest highly redundant transcriptional regulation in cartilage

The chondroitin sulfate proteoglycan core protein aggrecan is the major protein constituent of cartilage aside from collagen, and is largely responsible for its distinctive mechanical properties. Aggrecan is required both for proper cartilage formation in development and maintenance of mature cartilage. Prominent ACAN transcription is a conserved feature of vertebrate cartilage, although little is known about its specific transcriptional regulation. We examined the genomic interval containing human ACAN for transcriptional enhancers directing expression to cartilage, using a functional assay in transgenic zebrafish. We tested 24 conserved non-coding sequences, representing ~6% of the total sequence in the interval, and identified eleven independently capable of regulating reporter gene expression in cartilage. These enhancers were widely spaced, from >100kb upstream of the gene to within the first intron. While the majority displayed broad cartilage expression in zebrafish larvae, several were restricted to a subset of cartilage cells in the craniofacial skeleton. In older fish, the enhancers displayed differential activity; some maintained expression, either in all cartilage or preferentially in articular cartilage at the joints, while others were not active. This remarkable degree of overlapping regulatory control has been highly conserved; we identified clear orthologues of six enhancers at the chicken ACAN locus, arranged in the same order relative to the gene. These were also functional in directing expression to cartilage in transgenic zebrafish. Several enhancers contain potential binding sites for Sox9, consistent with its described role as an upstream regulator of ACAN expression. However, others lacked Sox9 consensus binding sites, implicating additional pathways and transcription factors as regulators of ACAN expression in cartilage, either in development or adult tissue. Our identification of these enhancer sequences is the necessary first step in detailed examination of the upstream regulators of ACAN expression.

Hh signaling regulates patterning and morphogenesis of the pharyngeal arch-derived skeleton

The proper function of the craniofacial skeleton requires the proper shaping of many individual skeletal elements. Neural crest cells generate much of the craniofacial skeleton and morphogenesis of skeletal elements occurs in transient, reiterated structures termed pharyngeal arches. The shape of individual elements depends upon intrinsic patterning within the neural crest as well as extrinsic signals to the neural crest from adjacent tissues within the arches. Hedgehog (Hh) signaling is known to play roles in craniofacial development, yet its involvement in intrinsic and extrinsic patterning of the craniofacial skeleton is still not well understood. Here, we show that morphogenetic movements of the pharyngeal arches and patterning of the neural crest require Hh signaling. Loss of Hh signaling, in smoothened (smo) mutants, disrupts the expression of some Dlx genes as well as other markers of dorsal/ventral patterning of the neural crest. Transplantation of wild-type neural crest cells into smo mutants rescues this defect, demonstrating that the neural crest requires reception of Hh signals for proper patterning. Despite the rescue, morphogenesis of the facial skeleton is not fully recovered. Through transplant analyses, we find two additional requirements for Hh signaling. The endoderm requires the reception of Hh signals for proper morphogenetic movements of the pharyngeal arches and the neural crest require the reception of Hh signaling for the activity of a reverse signal that maintains sonic hedgehog expression in the endoderm. Collectively, these results demonstrate that Hh signaling is essential to establish intrinsic and extrinsic patterning information for the craniofacial skeleton.

Histone deacetylase-4 is required during early cranial neural crest development for generation of the zebrafish palatal skeleton

Background

Histone deacetylase-4 (Hdac4) is a class II histone deacetylase that inhibits the activity of transcription factors. In humans, HDAC4 deficiency is associated with non-syndromic oral clefts and brachydactyly mental retardation syndrome (BDMR) with craniofacial abnormalities.

Results

We identify hdac4 in zebrafish and characterize its function in craniofacial morphogenesis. The gene is present as a single copy, and the deduced Hdac4 protein sequence shares all known functional domains with human HDAC4. The zebrafish hdac4 transcript is widely present in migratory cranial neural crest (CNC) cells of the embryo, including populations migrating around the eye, which previously have been shown to contribute to the formation of the palatal skeleton of the early larva. Embryos injected with hdac4 morpholinos (MO) have reduced or absent CNC populations that normally migrate medial to the eye. CNC-derived palatal precursor cells do not recover at the post-migratory stage, and subsequently we found that defects in the developing cartilaginous palatal skeleton correlate with reduction or absence of early CNC cells. Palatal skeletal defects prominently include a shortened, clefted, or missing ethmoid plate, and are associated with a shortening of the face of young larvae.

Conclusions

Our results demonstrate that Hdac4 is a regulator of CNC-derived palatal skeletal precursors during early embryogenesis. Cleft palate resulting from HDAC4 mutations in human patients may result from defects in a homologous CNC progenitor cell population.

A timeline of pharyngeal endoskeletal condensation and differentiation in the shark, Scyliorhinus canicula, and the paddlefish, Polyodon spathula

The lesser-spotted dogfish (Scyliorhinus canicula) and the North American paddlefish (Polyodon spathula) are two emerging model systems for the study of vertebrate craniofacial development. Notably, both of these taxa have retained plesiomorphic aspects of pharyngeal endoskeletal organization, relative to more commonly used models of vertebrate craniofacial development (e.g. zebrafish, chick and mouse), and are therefore well suited to inform the pharyngeal endoskeletal patterning mechanisms that functioned in the last common ancestor of jawed vertebrates. Here, we present a histological overview of the condensation and chondrogenesis of the most prominent endoskeletal elements of the jaw, hyoid and gill arches - the palatoquadrate/Meckel's cartilage, the hyomandibula/ceratohyal, and the epi-/ceratobranchial cartilages, respectively - in embryonic series of S. canicula and P. spathula. Our observations provide a provisional timeline and anatomical framework for further molecular developmental and functional investigations of pharyngeal endoskeletal differentiation and patterning in these phylogenetically informative taxa.

Hedgehog-dependent proliferation drives modular growth during morphogenesis of a dermal bone

In the developing skeleton, dermal bone morphogenesis includes the balanced proliferation, recruitment and differentiation of osteoblast precursors, yet how bones acquire unique morphologies is unknown. We show that Hedgehog (Hh) signaling mediates bone shaping during early morphogenesis of the opercle (Op), a well characterized dermal bone of the zebrafish craniofacial skeleton. ihha is specifically expressed in a local population of active osteoblasts along the principal growing edge of the bone. Mutational studies show that Hh signaling by this osteoblast population is both necessary and sufficient for full recruitment of pre-osteoblasts into the signaling population. Loss of ihha function results in locally reduced proliferation of pre-osteoblasts and consequent reductions in recruitment into the osteoblast pool, reduced bone edge length and reduced outgrowth. Conversely, hyperactive Hh signaling in ptch1 mutants causes opposite defects in proliferation and growth. Time-lapse microscopy of early Op morphogenesis using transgenically labeled osteoblasts demonstrates that ihha-dependent bone development is not only region specific, but also begins exactly at the onset of a second phase of morphogenesis, when the early bone begins to reshape into a more complex form. These features strongly support a hypothesis that dermal bone development is modular, with different gene sets functioning at specific times and locations to pattern growth. The Hh-dependent module is not limited to this second phase of bone growth: during later larval development, the Op is fused along the dysmorphic edge to adjacent dermal bones. Hence, patterning within a module may include adjacent regions of functionally related bones and might require that signaling pathways function over an extended period of development.

Examination of a palatogenic gene program in zebrafish

Human palatal clefting is debilitating and difficult to rectify surgically. Animal models enhance our understanding of palatogenesis and are essential in strategies designed to ameliorate palatal malformations in humans. Recent studies have shown that the zebrafish palate, or anterior neurocranium, is under similar genetic control to the amniote palatal skeleton. We extensively analyzed palatogenesis in zebrafish to determine the similarity of gene expression and function across vertebrates. By 36 hours postfertilization (hpf) palatogenic cranial neural crest cells reside in homologous regions of the developing face compared with amniote species. Transcription factors and signaling molecules regulating mouse palatogenesis are expressed in similar domains during palatogenesis in zebrafish. Functional investigation of a subset of these genes, fgf10a, tgfb2, pax9, and smad5 revealed their necessity in zebrafish palatogenesis. Collectively, these results suggest that the gene regulatory networks regulating palatogenesis may be conserved across vertebrate species, demonstrating the utility of zebrafish as a model for palatogenesis.

Identification and characterization of the zebrafish pharyngeal arch-specific enhancer for the basic helix-loop-helix transcription factor Hand2

The development of the vertebrate jaw relies on a network of transcription factors that patterns the dorsal-ventral axis of the pharyngeal arches. Recent findings in both mouse and zebrafish illustrate that the basic-helix-loop-helix transcription factor, Hand2, is crucial in this patterning process. While Hand2 has functionally similar roles in these two species, little is known about the regulatory sequences controlling hand2 expression in zebrafish. Using bioinformatics and Tol2-mediated transgenesis, we have generated zebrafish transgenic reporter lines in which either the mouse or zebrafish arch-specific hand2 enhancer direct expression of a fluorescent reporter. We find that both the mouse and zebrafish enhancers drive early reporter expression in a hand2-specific pattern in the ventral pharyngeal arches of zebrafish embryos. These lines provide useful tools to follow ventral arch cells during vertebrate jaw development while also allowing dissection of hand2 transcriptional regulation during this process.

Dcc regulates asymmetric outgrowth of forebrain neurons in zebrafish

The guidance receptor DCC (deleted in colorectal cancer) ortholog UNC-40 regulates neuronal asymmetry development in Caenorhabditis elegans, but it is not known whether DCC plays a role in the specification of neuronal polarity in vertebrates. To examine the roles of DCC in neuronal asymmetry regulation in vertebrates, we studied zebrafish anterior dorsal telencephalon (ADt) neuronal axons. We generated transgenic zebrafish animals expressing the photo-convertible fluorescent protein Kaede in ADt neurons and then photo-converted Kaede to label specifically the ADt neuron axons. We found that ADt axons normally project ventrally. Knock down of Dcc function by injecting antisense morpholino oligonucleotides caused the ADt neurons to project axons dorsally. To examine the axon projection pattern of individual ADt neurons, we labeled single ADt neurons using a forebrain-specific promoter to drive fluorescent protein expression. We found that individual ADt neurons projected axons dorsally or formed multiple processes after morpholino knock down of Dcc function. We further found that knock down of the Dcc ligand, Netrin1, also caused ADt neurons to project axons dorsally. Knockdown of Neogenin1, a guidance receptor closely related to Dcc, enhanced the formation of aberrant dorsal axons in embryos injected with Dcc morpholino. These experiments provide the first evidence that Dcc regulates polarized axon initiation and asymmetric outgrowth of forebrain neurons in vertebrates.

Trps1 is necessary for normal temporomandibular joint development

Mutation of the human TRPS1 gene leads to trichorhinophalangeal syndrome (TRPS), which is characterized by an abnormal development of various organs including the craniofacial skeleton. Trps1 has recently been shown to be expressed in the jaw joints of zebrafish; however, whether Trps1 is expressed in the mammalian temporomandibular joint (TMJ), or whether it is necessary for TMJ development is unknown. We have analyzed (1) the expression pattern of Trps1 during TMJ development in mice and (2) TMJ development in Trps1 knockout animals. Trps1 is expressed in the maxillo-mandibular junction at embryonic day (E) 11.5. At E15.5, expression is restricted to the developing condylar cartilage and to the surrounding joint disc progenitor cells. In Trps1 knockout mice, the glenoid fossa of the temporal bone forms relatively normally but the condylar process is extremely small and the joint disc and cavities do not develop. The initiation of condyle formation is slightly delayed in the mutants at E14.5; however, at E18.5, the flattened chondrocyte layer is narrowed and most of the condylar chondrocytes exhibit precocious chondrocyte maturation. Expression of Runx2 and its target genes is expanded toward the condylar apex in the mutants. These observations underscore the indispensable role played by Trps1 in normal TMJ development in supporting the differentiation of disc and synoviocyte progenitor cells and in coordinating condylar chondrocyte differentiation.

Using transgenic reporters to visualize bone and cartilage signaling during development in vivo

Green fluorescent protein was first used as a marker of protein expression in vivo 18 years ago, heralding the beginning of what became known as the Green Revolution. Since then, there has been an explosion in the number of transgenic lines in existence, and these transgenic tools are now being applied to skeletal research. Advances in transgenesis are also leading to increasing use of new model organisms for studying skeletogenesis. Such new models include the small teleosts zebrafish and medaka, which due to their optical translucency offer imaging possibilities in the live animals. In this review, we will introduce a number of recent advances in genetic engineering and transgenesis and the new genetic tools that are currently being developed. We will provide examples of how zebrafish and medaka transgenic lines are helping us to understand the behavior of skeletal cells in vivo. Finally, we will discuss future prospects for the application of transgenic technology to skeletal research.

Combinatorial roles for BMPs and Endothelin 1 in patterning the dorsal-ventral axis of the craniofacial skeleton

Bone morphogenetic proteins (BMPs) play crucial roles in craniofacial development but little is known about their interactions with other signals, such as Endothelin 1 (Edn1) and Jagged/Notch, which pattern the dorsal-ventral (DV) axis of the pharyngeal arches. Here, we use transgenic zebrafish to monitor and perturb BMP signaling during arch formation. With a BMP-responsive transgene, Tg(Bre:GFP), we show active BMP signaling in neural crest (NC)-derived skeletal precursors of the ventral arches, and in surrounding epithelia. Loss-of-function studies using a heat shock-inducible, dominant-negative BMP receptor 1a [Tg(hs70I:dnBmpr1a-GFP)] to bypass early roles show that BMP signaling is required for ventral arch development just after NC migration, the same stages at which we detect Tg(Bre:GFP). Inhibition of BMP signaling at these stages reduces expression of the ventral signal Edn1, as well as ventral-specific genes such as hand2 and dlx6a in the arches, and expands expression of the dorsal signal jag1b. This results in a loss or reduction of ventral and intermediate skeletal elements and a mis-shapen dorsal arch skeleton. Conversely, ectopic BMP causes dorsal expansion of ventral-specific gene expression and corresponding reductions/transformations of dorsal cartilages. Soon after NC migration, BMP is required to induce Edn1 and overexpression of either signal partially rescues ventral skeletal defects in embryos deficient for the other. However, once arch primordia are established the effects of BMPs become restricted to more ventral and anterior (palate) domains, which do not depend on Edn1. This suggests that BMPs act upstream and in parallel to Edn1 to promote ventral fates in the arches during early DV patterning, but later acquire distinct roles that further subdivide the identities of NC cells to pattern the craniofacial skeleton.

Gremlin 2 regulates distinct roles of BMP and Endothelin 1 signaling in dorsoventral patterning of the facial skeleton

Patterning of the upper versus lower face involves generating distinct pre-skeletal identities along the dorsoventral (DV) axes of the pharyngeal arches. Whereas previous studies have shown roles for BMPs, Endothelin 1 (Edn1) and Jagged1b-Notch2 in DV patterning of the facial skeleton, how these pathways are integrated to generate different skeletal fates has remained unclear. Here, we show that BMP and Edn1 signaling have distinct roles in development of the ventral and intermediate skeletons, respectively, of the zebrafish face. Using transgenic gain-of-function approaches and cell-autonomy experiments, we find that BMPs strongly promote hand2 and msxe expression in ventral skeletal precursors, while Edn1 promotes the expression of nkx3.2 and three Dlx genes (dlx3b, dlx5a and dlx6a) in intermediate precursors. Furthermore, Edn1 and Jagged1b pattern the intermediate and dorsal facial skeletons in part by inducing the BMP antagonist Gremlin 2 (Grem2), which restricts BMP activity to the ventral-most face. We therefore propose a model in which later cross-inhibitory interactions between BMP and Edn1 signaling, in part mediated by Grem2, separate an initially homogenous ventral region into distinct ventral and intermediate skeletal precursor domains.

Netrin signaling breaks the equivalence between two identified zebrafish motoneurons revealing a new role of intermediate targets

Background

We previously showed that equivalence between two identified zebrafish motoneurons is broken by interactions with identified muscle fibers that act as an intermediate target for the axons of these motoneurons. Here we investigate the molecular basis of the signaling interaction between the intermediate target and the motoneurons.

Principal Findings

We provide evidence that Netrin 1a is an intermediate target-derived signal that causes two equivalent motoneurons to adopt distinct fates. We show that although these two motoneurons express the same Netrin receptors, their axons respond differently to Netrin 1a encountered at the intermediate target. Furthermore, we demonstrate that when Netrin 1a is knocked down, more distal intermediate targets that express other Netrins can also function to break equivalence between these motoneurons.

Significance

Our results suggest a new role for intermediate targets in breaking neuronal equivalence. The data we present reveal that signals encountered during axon pathfinding can cause equivalent neurons to adopt distinct fates. Such signals may be key in diversifying a neuronal population and leading to correct circuit formation.

Downregulation of Dlx5 and Dlx6 expression by Hand2 is essential for initiation of tongue morphogenesis

Lower jaw development is a complex process in which multiple signaling cascades establish a proximal-distal organization. These cascades are regulated both spatially and temporally and are constantly refined through both induction of normal signals and inhibition of inappropriate signals. The connective tissue of the tongue arises from cranial neural crest cell-derived ectomesenchyme within the mandibular portion of the first pharyngeal arch and is likely to be impacted by this signaling. Although the developmental mechanisms behind later aspects of tongue development, including innervation and taste acquisition, have been elucidated, the early patterning signals driving ectomesenchyme into a tongue lineage are largely unknown. We show here that the basic helix-loop-helix transcription factor Hand2 plays key roles in establishing the proximal-distal patterning of the mouse lower jaw, in part through establishing a negative-feedback loop in which Hand2 represses Dlx5 and Dlx6 expression in the distal arch ectomesenchyme following Dlx5- and Dlx6-mediated induction of Hand2 expression in the same region. Failure to repress distal Dlx5 and Dlx6 expression results in upregulation of Runx2 expression in the mandibular arch and the subsequent formation of aberrant bone in the lower jaw along with proximal-distal duplications. In addition, there is an absence of lateral lingual swelling expansion, from which the tongue arises, resulting in aglossia. Hand2 thus appears to establish a distal mandibular arch domain that is conducive for lower jaw development, including the initiation of tongue mesenchyme morphogenesis.

Ancestral and derived attributes of the dlx gene repertoire, cluster structure and expression patterns in an African cichlid fish

Background

Cichlid fishes have undergone rapid, expansive evolutionary radiations that are manifested in the diversification of their trophic morphologies, tooth patterning and coloration. Understanding the molecular mechanisms that underlie the cichlids' unique patterns of evolution requires a thorough examination of genes that pattern the neural crest, from which these diverse phenotypes are derived. Among those genes, the homeobox-containing Dlx gene family is of particular interest since it is involved in the patterning of the brain, jaws and teeth.

Results

In this study, we characterized the dlx genes of an African cichlid fish, Astatotilapia burtoni, to provide a baseline to later allow cross-species comparison within Cichlidae. We identified seven dlx paralogs (dlx1a, -2a, -4a, -3b, -4b, -5a and -6a), whose orthologies were validated with molecular phylogenetic trees. The intergenic regions of three dlx gene clusters (dlx1a-2a, dlx3b-4b, and dlx5a-6a) were amplified with long PCR. Intensive cross-species comparison revealed a number of conserved non-coding elements (CNEs) that are shared with other percomorph fishes. This analysis highlighted additional lineage-specific gains/losses of CNEs in different teleost fish lineages and a novel CNE that had previously not been identified. Our gene expression analyses revealed overlapping but distinct expression of dlx orthologs in the developing brain and pharyngeal arches. Notably, four of the seven A. burtoni dlx genes, dlx2a, dlx3b, dlx4a and dlx5a, were expressed in the developing pharyngeal teeth.

Conclusion

This comparative study of the dlx genes of A. burtoni has deepened our knowledge of the diversity of the Dlx gene family, in terms of gene repertoire, expression patterns and non-coding elements. We have identified possible cichlid lineage-specific changes, including losses of a subset of dlx expression domains in the pharyngeal teeth, which will be the targets of future functional studies.

Evidence for the prepattern/cooption model of vertebrate jaw evolution

The appearance of jaws was a turning point in vertebrate evolution because it allowed primitive vertebrates to capture and process large, motile prey. The vertebrate jaw consists of separate dorsal and ventral skeletal elements connected by a joint. How this structure evolved from the unjointed gill bar of a jawless ancestor is an unresolved question in vertebrate evolution. To understand the developmental bases of this evolutionary transition, we examined the expression of 12 genes involved in vertebrate pharyngeal patterning in the modern jawless fish lamprey. We find nested expression of Dlx genes, as well as combinatorial expression of Msx, Hand and Gsc genes along the dorso-ventral (DV) axis of the lamprey pharynx, indicating gnathostome-type pharyngeal patterning evolved before the appearance of the jaw. In addition, we find that Bapx and Gdf5/6/7, key regulators of joint formation in gnathostomes, are not expressed in the lamprey first arch, whereas Barx, which is absent from the intermediate first arch in gnathostomes, marks this domain in lamprey. Taken together, these data support a new scenario for jaw evolution in which incorporation of Bapx and Gdf5/6/7 into a preexisting DV patterning program drove the evolution of the jaw by altering the identity of intermediate first-arch chondrocytes. We present this "Pre-pattern/Cooption" model as an alternative to current models linking the evolution of the jaw to the de novo appearance of sophisticated pharyngeal DV patterning.