Fin development in a cartilaginous fish and the origin of vertebrate limbs

The sensory shark: high-quality morphological, genomic and transcriptomic data for the small-spotted catshark Scyliorhinus canicula reveal the molecular bases of sensory organ evolution in jawed vertebrates

Cartilaginous fishes (chimaeras and elasmobranchs -sharks, skates and rays) hold a key phylogenetic position to explore the origin and diversifications of jawed vertebrates. Here, we report and integrate reference genomic, transcriptomic and morphological data in the small-spotted catshark Scyliorhinus canicula to shed light on the evolution of sensory organs. We first characterise general aspects of the catshark genome, confirming the high conservation of genome organisation across cartilaginous fishes, and investigate population genomic signatures. Taking advantage of a dense sampling of transcriptomic data, we also identify gene signatures for all major organs, including chondrichthyan specializations, and evaluate expression diversifications between paralogs within major gene families involved in sensory functions. Finally, we combine these data with 3D synchrotron imaging and in situ gene expression analyses to explore chondrichthyan-specific traits and more general evolutionary trends of sensory systems. This approach brings to light, among others, novel markers of the ampullae of Lorenzini electro-sensory cells, a duplication hotspot for crystallin genes conserved in jawed vertebrates, and a new metazoan clade of the Transient-receptor potential (TRP) family. These resources and results, obtained in an experimentally tractable chondrichthyan model, open new avenues to integrate multiomics analyses for the study of elasmobranchs and jawed vertebrates.

Paired fins in vertebrate evolution and ontogeny

The origin of paired appendages became one of the most important adaptations of vertebrates, allowing them to lead active lifestyles and explore a wide range of ecological niches. The basic form of paired appendages in evolution is the fins of fishes. The problem of paired appendages has attracted the attention of researchers for more than 150 years. During this time, a number of theories have been proposed, mainly based on morphological data, two of which, the Balfour-Thacher-Mivart lateral fold theory and Gegenbaur's gill arch theory, have not lost their relevance. So far, however, none of the proposed ideas has been supported by decisive evidence. The study of the evolutionary history of the appearance and development of paired appendages lies at the intersection of several disciplines and involves the synthesis of paleontological, morphological, embryological, and genetic data. In this review, we attempt to summarize and discuss the results accumulated in these fields and to analyze the theories put forward regarding the prerequisites and mechanisms that gave rise to paired fins and limbs in vertebrates.

Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle

The origin of vertebrate paired appendages is one of the most investigated and debated examples of evolutionary novelty1-7. Paired appendages are widely considered as key innovations that enabled new opportunities for controlled swimming and gill ventilation and were prerequisites for the eventual transition from water to land. The past 150 years of debate8-10 has been shaped by two contentious theories4,5: the ventrolateral fin-fold hypothesis9,10 and the archipterygium hypothesis8. The latter proposes that fins and girdles evolved from an ancestral gill arch. Although studies in animal development have revived interest in this idea11-13, it is apparently unsupported by fossil evidence. Here we present palaeontological support for a pharyngeal basis for the vertebrate shoulder girdle. We use computed tomography scanning to reveal details of the braincase of Kolymaspis sibirica14, an Early Devonian placoderm fish from Siberia, that suggests a pharyngeal component of the shoulder. We combine these findings with refreshed comparative anatomy of placoderms and jawless outgroups to place the origin of the shoulder girdle on the sixth branchial arch. These findings provide a novel framework for understanding the origin of the pectoral girdle. Our evidence clarifies the location of the presumptive head-trunk interface in jawless fishes and explains the constraint on branchial arch number in gnathostomes15. The results revive a key aspect of the archipterygium hypothesis and help reconcile it with the ventrolateral fin-fold model.

Imaging With the Past: Revealing the Complexity of Chimaeroid Pelvic Musculature Anatomy and Development

Chondrichthyans are now widely adopted as models for examining the development and evolution of the stem gnathostome body plan. The fins of some cartilaginous fish are recognized for their plesiomorphic form and mode of muscular development, i.e., epithelial extension. Despite detailed molecular and descriptive examinations of these developmental mechanisms, there has been little contemporary examination of the ontogeny and morphology of the musculature in chondrichthyans including that of the paired fins. This gap represents a need for further examination of the developmental morphology of these appendicular musculatures to gain insight into their evolution in gnathostomes. The elephant shark is a Holocephalan, the sister group of all other chondrichthyans (Holocephali: Callorhinchus milii). Here, we use nano-CT imaging and 3D reconstructions to describe the development of the pelvic musculature of a growth series of elephant shark embryos. We also use historical descriptions from the nineteenth century and traditional dissection methods to describe the adult anatomy. This combined approach, using traditional methods and historical knowledge with modern imaging techniques, has enabled a more thorough examination of the anatomy and development of the pelvic musculature revealing that chimaeroid musculatures are more complex than previously thought. These data, when compared to extant and extinct sister taxa, are essential for interpreting and reconstructing fossil musculatures as well as understanding the evolution of paired fins.

Limb positioning and initiation: An evolutionary context of pattern and formation

Before limbs or fins, can be patterned and grow they must be initiated. Initiation of the limb first involves designating a portion of lateral plate mesoderm along the flank as the site of the future limb. Following specification, a myriad of cellular and molecular events interact to generate a bud that will grow and form the limb. The past three decades has provided a wealth of understanding on how those events generate the limb bud and how variations in them result in different limb forms. Comparatively, much less attention has been given to the earliest steps of limb formation and what impacts altering the position and initiation of the limb have had on evolution. Here, we first review the processes and pathways involved in these two phases of limb initiation, as determined from amniote model systems. We then broaden our scope to examine how variation in the limb initiation module has contributed to biological diversity in amniotes. Finally, we review what is known about limb initiation in fish and amphibians, and consider what mechanisms are conserved across vertebrates.

Evolution of Matrix Gla and Bone Gla Protein Genes in Jawed Vertebrates

Matrix Gla protein (Mgp) and bone Gla protein (Bgp) are vitamin-K dependent proteins that bind calcium in their γ-carboxylated versions in mammals. They are recognized as positive (Bgp) or negative (Mgp and Bgp) regulators of biomineralization in a number of tissues, including skeletal tissues of bony vertebrates. The Mgp/Bgp gene family is poorly known in cartilaginous fishes, which precludes the understanding of the evolution of the biomineralization toolkit at the emergence of jawed vertebrates. Here we took advantage of recently released genomic and transcriptomic data in cartilaginous fishes and described the genomic loci and gene expression patterns of the Mgp/Bgp gene family. We identified three genes, Mgp1, Mgp2, and Bgp, in cartilaginous fishes instead of the single previously reported Mgp gene. We describe their genomic loci, resulting in a dynamic evolutionary scenario for this gene family including several events of local (tandem) duplications, but also of translocation events, along jawed vertebrate evolution. We describe the expression patterns of Mgp1, Mgp2, and Bgp in embryonic stages covering organogenesis in the small-spotted catshark Scyliorhinus canicula and present a comparative analysis with Mgp/Bgp family members previously described in bony vertebrates, highlighting ancestral features such as early embryonic, soft tissues, and neuronal expressions, but also derived features of cartilaginous fishes such as expression in fin supporting fibers. Our results support an ancestral function of Mgp in skeletal mineralization and a later derived function of Bgp in skeletal development that may be related to the divergence of bony vertebrates.

The Developmental Phases of Zebrafish Myogenesis

The development and growth of vertebrate axial muscle have been studied for decades at both the descriptive and molecular level. The zebrafish has provided an attractive model system for investigating both muscle patterning and growth due to its simple axial musculature with spatially separated fibre types, which contrasts to complex muscle groups often deployed in amniotes. In recent years, new findings have reshaped previous concepts that define how final teleost muscle form is established and maintained. Here, we summarise recent findings in zebrafish embryonic myogenesis with a focus on fibre type specification, followed by an examination of the molecular mechanisms that control muscle growth with emphasis on the role of the dermomyotome-like external cell layer. We also consider these data sets in a comparative context to gain insight into the evolution of axial myogenic patterning systems within the vertebrate lineage.

Fin-fold development in paddlefish and catshark and implications for the evolution of the autopod

The evolutionary origin of the autopod involved a loss of the fin-fold and associated dermal skeleton with a concomitant elaboration of the distal endoskeleton to form a wrist and digits. Developmental studies, primarily from teleosts and amniotes, suggest a model for appendage evolution in which a delay in the AER-to-fin-fold conversion fuelled endoskeletal expansion by prolonging the function of AER-mediated regulatory networks. Here, we characterize aspects of paired fin development in the paddlefish Polyodon spathula (a non-teleost actinopterygian) and catshark Scyliorhinus canicula (chondrichthyan) to explore aspects of this model in a broader phylogenetic context. Our data demonstrate that in basal gnathostomes, the autopod marker HoxA13 co-localizes with the dermoskeleton component And1 to mark the position of the fin-fold, supporting recent work demonstrating a role for HoxA13 in zebrafish fin ray development. Additionally, we show that in paddlefish, the proximal fin and fin-fold mesenchyme share a common mesodermal origin, and that components of the Shh/LIM/Gremlin/Fgf transcriptional network critical to limb bud outgrowth and patterning are expressed in the fin-fold with a profile similar to that of tetrapods. Together these data draw contrast with hypotheses of AER heterochrony and suggest that limb-specific morphologies arose through evolutionary changes in the differentiation outcome of conserved early distal patterning compartments.

Similarity of morphological composition and developmental patterning in paired fins of the elephant shark

Jawed vertebrates, or gnathostomes, have two sets of paired appendages, pectoral and pelvic fins in fishes and fore- and hindlimbs in tetrapods. As for paired limbs, paired fins are purported serial homologues, and the advent of pelvic fins has been hypothesized to have resulted from a duplication of the developmental mechanisms present in the pectoral fins, but re-iterated at a posterior location. Developmental similarity of gene expression between pectoral and pelvic fins has been documented in chondrichthyans, but a detailed morphological description of the progression of paired fin development for this group is still lacking. We studied paired fin development in an ontogenetic series of a phylogenetically basal chondrichthyan, the elephant shark Callorhinchus milii. A strong similarity in the morphology and progression of chondrification between the pectoral and pelvic fins was found, which could be interpretated as further evidence of serial homology in paired fins, that could have arisen by duplication. Furthermore, this high degree of morphological and developmental similarity suggests the presence of morphological and developmental modules within paired fins, as observed in paired limbs. This is the first time morphological and developmental modules are described for the paired fins of chimaeras.

Alterations in anterior-posterior patterning and its accompanying changes along the proximal-distal axis during the fin-to-limb transition

The evolution from fins to limbs was one of the most successful innovations for vertebrates, allowing them to vastly expand their behaviors and habitats. Fossil records suggest that morphological changes occurred not only along the proximal-distal axis included appearance of the autopod, but also occurred along the anterior-posterior axis included reductions in the size and number of basal bones and digits. This review focuses on recent progress in developmental and genetic studies aimed at elucidating the mechanisms underlying alteration of anterior-posterior patterning and its accompanying changes along the proximal-distal axis during the fin-to-limb transition.

Engrailed controls epaxial-hypaxial muscle innervation and the establishment of vertebrate three-dimensional mobility

Chordates are characterised by contractile muscle on either side of the body that promotes movement by side-to-side undulation. In the lineage leading to modern jawed vertebrates (crown group gnathostomes), this system was refined: body muscle became segregated into distinct dorsal (epaxial) and ventral (hypaxial) components that are separately innervated by the medial and hypaxial motors column, respectively, via the dorsal and ventral ramus of the spinal nerves. This allows full three-dimensional mobility, which in turn was a key factor in their evolutionary success. How the new gnathostome system is established during embryogenesis and how it may have evolved in the ancestors of modern vertebrates is not known. Vertebrate Engrailed genes have a peculiar expression pattern as they temporarily demarcate a central domain of the developing musculature at the epaxial-hypaxial boundary. Moreover, they are the only genes known with this particular expression pattern. The aim of this study was to investigate whether Engrailed genes control epaxial-hypaxial muscle development and innervation. Investigating chick, mouse and zebrafish as major gnathostome model organisms, we found that the Engrailed expression domain was associated with the establishment of the epaxial-hypaxial boundary of muscle in all three species. Moreover, the outgrowing epaxial and hypaxial nerves orientated themselves with respect to this Engrailed domain. In the chicken, loss and gain of Engrailed function changed epaxial-hypaxial somite patterning. Importantly, in all animals studied, loss and gain of Engrailed function severely disrupted the pathfinding of the spinal motor axons, suggesting that Engrailed plays an evolutionarily conserved role in the separate innervation of vertebrate epaxial-hypaxial muscle.

Muscle development in the shark Scyliorhinus canicula: implications for the evolution of the gnathostome head and paired appendage musculature

BACKGROUND

The origin of jawed vertebrates was marked by profound reconfigurations of the skeleton and muscles of the head and by the acquisition of two sets of paired appendages. Extant cartilaginous fish retained numerous plesiomorphic characters of jawed vertebrates, which include several aspects of their musculature. Therefore, myogenic studies on sharks are essential in yielding clues on the developmental processes involved in the origin of the muscular anatomy.

RESULTS

Here we provide a detailed description of the development of specific muscular units integrating the cephalic and appendicular musculature of the shark model, Scyliorhinus canicula. In addition, we analyze the muscle development across gnathostomes by comparing the developmental onset of muscle groups in distinct taxa. Our data reveal that appendicular myogenesis occurs earlier in the pectoral than in the pelvic appendages. Additionally, the pectoral musculature includes muscles that have their primordial developmental origin in the head. This culminates in a tight muscular connection between the pectoral girdle and the cranium, which founds no parallel in the pelvic fins. Moreover, we identified a lateral to ventral pattern of formation of the cephalic muscles, that has been equally documented in osteichthyans but, in contrast with these gnathostomes, the hyoid muscles develop earlier than mandibular muscle in S. canicula.

CONCLUSION

Our analyses reveal considerable differences in the formation of the pectoral and pelvic musculatures in S. canicula, reinforcing the idea that head tissues have contributed to the formation of the pectoral appendages in the common ancestor of extant gnathostomes. In addition, temporal differences in the formation of some cranial muscles between chondrichthyans and osteichthyans might support the hypothesis that the similarity between the musculature of the mandibular arch and of the other pharyngeal arches represents a derived feature of jawed vertebrates.

Progressive Loss of Function in a Limb Enhancer during Snake Evolution

The evolution of body shape is thought to be tightly coupled to changes in regulatory sequences, but specific molecular events associated with major morphological transitions in vertebrates have remained elusive. We identified snake-specific sequence changes within an otherwise highly conserved long-range limb enhancer of Sonic hedgehog (Shh). Transgenic mouse reporter assays revealed that the in vivo activity pattern of the enhancer is conserved across a wide range of vertebrates, including fish, but not in snakes. Genomic substitution of the mouse enhancer with its human or fish ortholog results in normal limb development. In contrast, replacement with snake orthologs caused severe limb reduction. Synthetic restoration of a single transcription factor binding site lost in the snake lineage reinstated full in vivo function to the snake enhancer. Our results demonstrate changes in a regulatory sequence associated with a major body plan transition and highlight the role of enhancers in morphological evolution. PAPERCLIP.

Regulatory evolution of Tbx5 and the origin of paired appendages

The diversification of paired appendages has been a major factor in the evolutionary radiation of vertebrates. Despite its importance, an understanding of the origin of paired appendages has remained elusive. To address this problem, we focused on T-box transcription factor 5 (Tbx5), a gene indispensable for pectoral appendage initiation and development. Comparison of gene expression in jawless and jawed vertebrates reveals that the Tbx5 expression in jawed vertebrates is derived in having an expression domain that extends caudal to the heart and gills. Chromatin profiling, phylogenetic footprinting, and functional assays enabled the identification of a Tbx5 fin enhancer associated with this apomorphic pattern of expression. Comparative functional analysis of reporter constructs reveals that this enhancer activity is evolutionarily conserved among jawed vertebrates and is able to rescue the finless phenotype of tbx5a mutant zebrafish. Taking paleontological evidence of early vertebrates into account, our results suggest that the gain of apomorphic patterns of Tbx5 expression and regulation likely contributed to the morphological transition from a finless to finned condition at the base of the vertebrate lineage.

The fin-to-limb transition as the re-organization of a Turing pattern

A Turing mechanism implemented by BMP, SOX9 and WNT has been proposed to control mouse digit patterning. However, its generality and contribution to the morphological diversity of fins and limbs has not been explored. Here we provide evidence that the skeletal patterning of the catshark Scyliorhinus canicula pectoral fin is likely driven by a deeply conserved Bmp–Sox9–Wnt Turing network. In catshark fins, the distal nodular elements arise from a periodic spot pattern of Sox9 expression, in contrast to the stripe pattern in mouse digit patterning. However, our computer model shows that the Bmp–Sox9–Wnt network with altered spatial modulation can explain the Sox9 expression in catshark fins. Finally, experimental perturbation of Bmp or Wnt signalling in catshark embryos produces skeletal alterations which match in silico predictions. Together, our results suggest that the broad morphological diversity of the distal fin and limb elements arose from the spatial re-organization of a deeply conserved Turing mechanism.

Mouse digit patterning is controlled by a Turing network of Bmp, Sox9, and Wnt. Here, Onimaru et al. show that fin patterning in the catshark, Scyliorhinus canicula, is controlled by the same network with a different spatial organization; thus, the Turing network is deeply conserved in limb development.

Developmental Mechanism of Limb Field Specification along the Anterior-Posterior Axis during Vertebrate Evolution

In gnathostomes, limb buds arise from the lateral plate mesoderm at discrete positions along the body axis. Specification of these limb-forming fields can be subdivided into several steps. The lateral plate mesoderm is regionalized into the anterior lateral plate mesoderm (ALPM; cardiac mesoderm) and the posterior lateral plate mesoderm (PLPM). Subsequently, Hox genes appear in a nested fashion in the PLPM and provide positional information along the body axis. The lateral plate mesoderm then splits into the somatic and splanchnic layers. In the somatic layer of the PLPM, the expression of limb initiation genes appears in the limb-forming region, leading to limb bud initiation. Furthermore, past and current work in limbless amphioxus and lampreys suggests that evolutionary changes in developmental programs occurred during the acquisition of paired fins during vertebrate evolution. This review presents these recent advances and discusses the mechanisms of limb field specification during development and evolution, with a focus on the role of Hox genes in this process.

An ancient dental gene set governs development and continuous regeneration of teeth in sharks

The evolution of oral teeth is considered a major contributor to the overall success of jawed vertebrates. This is especially apparent in cartilaginous fishes including sharks and rays, which develop elaborate arrays of highly specialized teeth, organized in rows and retain the capacity for life-long regeneration. Perpetual regeneration of oral teeth has been either lost or highly reduced in many other lineages including important developmental model species, so cartilaginous fishes are uniquely suited for deep comparative analyses of tooth development and regeneration. Additionally, sharks and rays can offer crucial insights into the characters of the dentition in the ancestor of all jawed vertebrates. Despite this, tooth development and regeneration in chondrichthyans is poorly understood and remains virtually uncharacterized from a developmental genetic standpoint. Using the emerging chondrichthyan model, the catshark (Scyliorhinus spp.), we characterized the expression of genes homologous to those known to be expressed during stages of early dental competence, tooth initiation, morphogenesis, and regeneration in bony vertebrates. We have found that expression patterns of several genes from Hh, Wnt/β-catenin, Bmp and Fgf signalling pathways indicate deep conservation over ~450 million years of tooth development and regeneration. We describe how these genes participate in the initial emergence of the shark dentition and how they are redeployed during regeneration of successive tooth generations. We suggest that at the dawn of the vertebrate lineage, teeth (i) were most likely continuously regenerative structures, and (ii) utilised a core set of genes from members of key developmental signalling pathways that were instrumental in creating a dental legacy redeployed throughout vertebrate evolution. These data lay the foundation for further experimental investigations utilizing the unique regenerative capacity of chondrichthyan models to answer evolutionary, developmental, and regenerative biological questions that are impossible to explore in classical models.

Molecular mechanisms underlying the exceptional adaptations of batoid fins

Extreme novelties in the shape and size of paired fins are exemplified by extinct and extant cartilaginous and bony fishes. Pectoral fins of skates and rays, such as the little skate (Batoid, Leucoraja erinacea), show a strikingly unique morphology where the pectoral fin extends anteriorly to ultimately fuse with the head. This results in a morphology that essentially surrounds the body and is associated with the evolution of novel swimming mechanisms in the group. In an approach that extends from RNA sequencing to in situ hybridization to functional assays, we show that anterior and posterior portions of the pectoral fin have different genetic underpinnings: canonical genes of appendage development control posterior fin development via an apical ectodermal ridge (AER), whereas an alternative Homeobox (Hox)-Fibroblast growth factor (Fgf)-Wingless type MMTV integration site family (Wnt) genetic module in the anterior region creates an AER-like structure that drives anterior fin expansion. Finally, we show that GLI family zinc finger 3 (Gli3), which is an anterior repressor of tetrapod digits, is expressed in the posterior half of the pectoral fin of skate, shark, and zebrafish but in the anterior side of the pelvic fin. Taken together, these data point to both highly derived and deeply ancestral patterns of gene expression in skate pectoral fins, shedding light on the molecular mechanisms behind the evolution of novel fin morphologies.

Shared Enhancer Activity in the Limbs and Phallus and Functional Divergence of a Limb-Genital cis-Regulatory Element in Snakes

The amniote phallus and limbs differ dramatically in their morphologies but share patterns of signaling and gene expression in early development. Thus far, the extent to which genital and limb transcriptional networks also share cis-regulatory elements has remained unexplored. We show that many limb enhancers are retained in snake genomes, suggesting that these elements may function in non-limb tissues. Consistent with this, our analysis of cis-regulatory activity in mice and Anolis lizards reveals that patterns of enhancer activity in embryonic limbs and genitalia overlap heavily. In mice, deletion of HLEB, an enhancer of Tbx4, produces defects in hindlimbs and genitalia, establishing the importance of this limb-genital enhancer for development of these different appendages. Further analyses demonstrate that the HLEB of snakes has lost hindlimb enhancer function while retaining genital activity. Our findings identify roles for Tbx4 in genital development and highlight deep similarities in cis-regulatory activity between limbs and genitalia.

A shift in anterior-posterior positional information underlies the fin-to-limb evolution

The pectoral fins of ancestral fishes had multiple proximal elements connected to their pectoral girdles. During the fin-to-limb transition, anterior proximal elements were lost and only the most posterior one remained as the humerus. Thus, we hypothesised that an evolutionary alteration occurred in the anterior–posterior (AP) patterning system of limb buds. In this study, we examined the pectoral fin development of catshark (Scyliorhinus canicula) and revealed that the AP positional values in fin buds are shifted more posteriorly than mouse limb buds. Furthermore, examination of Gli3 function and regulation shows that catshark fins lack a specific AP patterning mechanism, which restricts its expression to an anterior domain in tetrapods. Finally, experimental perturbation of AP patterning in catshark fin buds results in an expansion of posterior values and loss of anterior skeletal elements. Together, these results suggest that a key genetic event of the fin-to-limb transformation was alteration of the AP patterning network.

DOI:http://dx.doi.org/10.7554/eLife.07048.001

Humans, mice, and other animals with four limbs belong to a group of land-dwelling animals known as the tetrapods. This group of animals evolved from ancient fish and one crucial adaptation to life on land involved the modification of fins to form limbs. The front pair of limbs (the ‘arms’) evolved from the ‘pectoral’ fins of the ancient fish. These fins contain numerous bones that fan out from a set of bones called the pectoral girdle. However, most of the bones nearer the front side (the thumb side in the human limb) were lost in the ancestors of tetrapods as they moved onto land. Only the bone nearest the back remained as the ‘humerus’, which forms the upper part of the limb (i.e., the upper arm of humans).

In the embryos of mice and other animals, the limbs develop from structures called limb buds. For the limb to develop properly, the cells in the limb bud need to receive specific instructions that depend on their position in the bud. A protein called Gli3R provides cells with information about their position along the ‘anterior–posterior’ (or thumb-to-little finger) axis of the bud. This protein regulates several genes that are involved in limb development, and this results in different genes being expressed in cells along the anterior–posterior axis. For example, Alx4 is only expressed in a small area at the anterior end of the bud, while Hand2 expression is found in a large area towards the posterior part.

Gli3R is also found in a fish called the catshark, but it is not clear how it controls the formation of fins. Onimaru et al. show that the pattern of gene expression in the catshark fin bud is different to that of the mouse limb bud. For example, Alx4 is expressed in a larger area of the fin bud that extends further towards the posterior, while Hand2 is only found in a much smaller area at the posterior end of the bud. The experiments also suggest that Gli3R is active in a much larger area of the fin bud than in the limb bud.

Next, Onimaru et al. used a drug on the catshark embryos to increase the activity of another protein that can inhibit Gli3R. The fin buds of these shark had anterior shift in several gene expression domains, and the fins that formed were missing several anterior bones and had only a single bone connected to the pectoral girdle. Onimaru et al.'s findings suggest that during the evolution of the tetrapods, there may have been a shift in the anterior–posterior patterning of the fin bud to form a limb. An important area for future work will be to use genome-wide studies to study the fin/limb buds of other species.

DOI:http://dx.doi.org/10.7554/eLife.07048.002

Connectivity of vertebrate genomes: Paired-related homeobox (Prrx) genes in spotted gar, basal teleosts, and tetrapods

Teleost fish are important models for human biology, health, and disease. Because genome duplication in a teleost ancestor (TGD) impacts the evolution of teleost genome structure and gene repertoires, we must discriminate gene functions that are shared and ancestral from those that are lineage-specific in teleosts or tetrapods to accurately apply inferences from teleost disease models to human health. Generalizations must account both for the TGD and for divergent evolution between teleosts and tetrapods after the likely two rounds of genome duplication shared by all vertebrates. Progress in sequencing techniques provides new opportunities to generate genomic and transcriptomic information from a broad range of phylogenetically informative taxa that facilitate detailed understanding of gene family and gene function evolution. We illustrate here the use of new sequence resources from spotted gar (Lepisosteus oculatus), a rayfin fish that diverged from teleosts before the TGD, as well as RNA-Seq data from gar and multiple teleost lineages to reconstruct the evolution of the Paired-related homeobox (Prrx) transcription factor gene family, which is involved in the development of mesoderm and neural crest-derived mesenchyme. We show that for Prrx genes, the spotted gar genome and gene expression patterns mimic mammals better than teleosts do. Analyses force the seemingly paradoxical conclusion that regulatory mechanisms for the limb expression domains of Prrx genes existed before the evolution of paired appendages. Detailed evolutionary analyses like those reported here are required to identify fish species most similar to the human genome to optimally connect fish models to human gene functions in health and disease.

Evolving Hox activity profiles govern diversity in locomotor systems

The emergence of limb-driven locomotor behaviors was a key event in the evolution of vertebrates and fostered the transition from aquatic to terrestrial life. We show that the generation of limb-projecting lateral motor column (LMC) neurons in mice relies on a transcriptional autoregulatory module initiated via transient activity of multiple genes within the HoxA and HoxC clusters. Repression of this module at thoracic levels restricts expression of LMC determinants, thus dictating LMC position relative to the limbs. This suppression is mediated by a key regulatory domain that is specifically found in the Hoxc9 proteins of appendage-bearing vertebrates. The profile of Hoxc9 expression inversely correlates with LMC position in land vertebrates and likely accounts for the absence of LMC neurons in limbless species such as snakes. Thus, modulation of both Hoxc9 protein function and Hoxc9 gene expression likely contributed to evolutionary transitions between undulatory and ambulatory motor circuit connectivity programs.

Body wall development in lamprey and a new perspective on the origin of vertebrate paired fins

Classical hypotheses regarding the evolutionary origin of paired appendages propose transformation of precursor structures (gill arches and lateral fin folds) into paired fins. During development, gnathostome paired appendages form as outgrowths of body wall somatopleure, a tissue composed of somatic lateral plate mesoderm (LPM) and overlying ectoderm. In amniotes, LPM contributes connective tissue to abaxial musculature and forms ventrolateral dermis of the interlimb body wall. The phylogenetic distribution of this character is uncertain because lineage analyses of LPM have not been generated in anamniotes. We focus on the evolutionary history of the somatopleure to gain insight into the tissue context in which paired fins first appeared. Lampreys diverged from other vertebrates before the acquisition of paired fins and provide a model for investigating the preappendicular condition. We present vital dye fate maps that suggest the somatopleure is eliminated in lamprey as the LPM is separated from the ectoderm and sequestered to the coelomic linings during myotome extension. We also examine the distribution of postcranial mesoderm in catshark and axolotl. In contrast to lamprey, our findings support an LPM contribution to the trunk body wall of these taxa, which is similar to published data for amniotes. Collectively, these data lead us to hypothesize that a persistent somatopleure in the lateral body wall is a gnathostome synapomorphy, and the redistribution of LPM was a key step in generating the novel developmental module that ultimately produced paired fins. These embryological criteria can refocus arguments on paired fin origins and generate hypotheses testable by comparative studies on the source, sequence, and extent of genetic redeployment.

TBX4 mutations (small patella syndrome) are associated with childhood-onset pulmonary arterial hypertension

Background

Childhood-onset pulmonary arterial hypertension (PAH) is rare and differs from adult-onset disease in clinical presentation, with often unexplained mental retardation and dysmorphic features (MR/DF). Mutations in the major PAH gene, BMPR2, were reported to cause PAH in only 10–16% of childhood-onset patients. We aimed to identify more genes associated with childhood-onset PAH.

Methods

We studied 20 consecutive cases with idiopathic or heritable PAH. In patients with accompanying MR/DF (n=6) array-comparative genomic hybridisation analysis was performed, with the aim of finding common deletion regions containing candidate genes for PAH. Three patients had overlapping deletions of 17q23.2. TBX2 and TBX4 were selected from this area as candidate genes and sequenced in all 20 children. After identifying TBX4 mutations in these children, we subsequently sequenced TBX4 in a cohort of 49 adults with PAH. Because TBX4 mutations are known to cause small patella syndrome (SPS), all patients with newly detected TBX4 mutations were screened for features of SPS. We also screened a third cohort of 23 patients with SPS for PAH.

Results

TBX4 mutations (n=3) or TBX4-containing deletions (n=3) were detected in 6 out of 20 children with PAH (30%). All living patients and two parents with TBX4 mutations appeared to have previously unrecognised SPS. In the adult PAH-cohort, one TBX4 mutation (2%) was detected. Screening in the cohort of (predominantly adult) SPS patients revealed no PAH.

Conclusions

These data indicate that TBX4 mutations are associated with childhood-onset PAH, but that the prevalence of PAH in adult TBX4 mutation carriers is low.

Development of the head and trunk mesoderm in the dogfish, Scyliorhinus torazame: II. Comparison of gene expression between the head mesoderm and somites with reference to the origin of the vertebrate head

The vertebrate mesoderm differs distinctly between the head and trunk, and the evolutionary origin of the head mesoderm remains enigmatic. Although the presence of somite-like segmentation in the head mesoderm of model animals is generally denied at molecular developmental levels, the appearance of head cavities in elasmobranch embryos has not been explained, and the possibility that they may represent vestigial head somites once present in an amphioxus-like ancestor has not been ruled out entirely. To examine whether the head cavities in the shark embryo exhibit any molecular signatures reminiscent of trunk somites, we isolated several developmentally key genes, including Pax1, Pax3, Pax7, Pax9, Myf5, Sonic hedgehog, and Patched2, which are involved in myogenic and chondrogenic differentiation in somites, and Pitx2, Tbx1, and Engrailed2, which are related to the patterning of the head mesoderm, from an elasmobranch species, Scyliorhinus torazame. Observation of the expression patterns of these genes revealed that most were expressed in patterns that resembled those found in amniote embryos. In addition, the head cavities did not exhibit an overt similarity to somites; that is, the similarity was no greater than that of the unsegmented head mesoderm in other vertebrates. Moreover, the shark head mesoderm showed an amniote-like somatic/visceral distinction according to the expression of Pitx2, Tbx1, and Engrailed2. We conclude that the head cavities do not represent a manifestation of ancestral head somites; rather, they are more likely to represent a derived trait obtained in the lineage of gnathostomes.

The evolutionary history of the development of the pelvic fin/hindlimb

The arms and legs of man are evolutionarily derived from the paired fins of primitive jawed fish. Few evolutionary changes have attracted as much attention as the origin of tetrapod limbs from the paired fins of ancestral fish. The hindlimbs of tetrapods are derived from the pelvic fins of ancestral fish. These evolutionary origins can be seen in the examination of shared gene and protein expression patterns during the development of pelvic fins and tetrapod hindlimbs. The pelvic fins of fish express key limb positioning, limb bud induction and limb outgrowth genes in a similar manner to that seen in hindlimb development of higher vertebrates. We are now at a point where many of the key players in the development of pelvic fins and vertebrate hindlimbs have been identified and we can now readily examine and compare mechanisms between species. This is yielding fascinating insights into how the developmental programme has altered during evolution and how that relates to anatomical change. The role of pelvic fins has also drastically changed over evolutionary history, from playing a minor role during swimming to developing into robust weight-bearing limbs. In addition, the pelvic fins/hindlimbs have been lost repeatedly in diverse species over evolutionary time. Here we review the evolution of pelvic fins and hindlimbs within the context of the changes in anatomical structure and the molecular mechanisms involved.

Molecular and evolutionary basis of limb field specification and limb initiation

Specification of limb field and initiation of limb development involve multiple steps, each of which is tightly regulated both spatially and temporally. Recent developmental analyses on various vertebrates have provided insights into the molecular mechanisms that specify limb field and have revealed several genetic interactions of signals involved in limb initiation processes. Furthermore, new approaches to the study of the developmental mechanisms of the lateral plate mesoderm of amphioxus and lamprey embryos have given us clues to understand the evolutionary scenarios that led to the acquisition of paired appendages during evolution. This review highlights such recent findings and discusses the mechanisms of limb field specification and limb bud initiation during development and evolution.

500,000 fish phenotypes: The new informatics landscape for evolutionary and developmental biology of the vertebrate skeleton

The rich phenotypic diversity that characterizes the vertebrate skeleton results from evolutionary changes in regulation of genes that drive development. Although relatively little is known about the genes that underlie the skeletal variation among fish species, significant knowledge of genetics and development is available for zebrafish. Because developmental processes are highly conserved, this knowledge can be leveraged for understanding the evolution of skeletal diversity. We developed the Phenoscape Knowledgebase (KB; http://kb.phenoscape.org) to yield testable hypotheses of candidate genes involved in skeletal evolution. We developed a community anatomy ontology for fishes and ontology-based methods to represent complex free-text character descriptions of species in a computable format. With these tools, we populated the KB with comparative morphological data from the literature on over 2,500 teleost fishes (mainly Ostariophysi) resulting in over 500,000 taxon phenotype annotations. The KB integrates these data with similarly structured phenotype data from zebrafish genes (http://zfin.org). Using ontology-based reasoning, candidate genes can be inferred for the phenotypes that vary across taxa, thereby uniting genetic and phenotypic data to formulate evo-devo hypotheses. The morphological data in the KB can be browsed, sorted, and aggregated in ways that provide unprecedented possibilities for data mining and discovery.

Islet1 regulates establishment of the posterior hindlimb field upstream of the Hand2-Shh morphoregulatory gene network in mouse embryos

How divergent genetic systems regulate a common pathway during the development of two serial structures, forelimbs and hindlimbs, is not well understood. Specifically, HAND2 has been shown to regulate Shh directly to initiate its expression in the posterior margin of the limb mesenchyme. Although the Hand2-Shh morphoregulatory system operates in both the forelimb and hindlimb bud, a recent analysis suggested that its upstream regulation is different in the forelimb and hindlimb bud. A combination of all four Hox9 genes is required for Hand2 expression in the forelimb-forming region; however, it remains elusive what genetic system regulates the Hand2-Shh pathway in the hindlimb-forming region. By conditional inactivation of Islet1 in the hindlimb-forming region using the Hoxb6Cre transgene, we show that Islet1 is required for establishing the posterior hindlimb field, but not the forelimb field, upstream of the Hand2-Shh pathway. Inactivation of Islet1 caused the loss of posterior structures in the distal and proximal regions, specifically in the hindlimb. We found that Hand2 expression was downregulated in the hindlimb field and that Shh expression was severely impaired in the hindlimb bud. In the Hoxb6Cre; Islet1 mutant pelvis, the proximal element that is formed in a Shh-independent manner, displayed complementary defects in comparison with Pitx1(-/-) hindlimbs. This suggests that Islet1 and Pitx1 function in parallel during girdle development in hindlimbs, which is in contrast with the known requirement for Tbx5 in girdle development in forelimbs. Our studies have identified a role for Islet1 in hindlimb-specific development and have revealed Islet1 functions in two distinct processes: regulation upstream of the Hand2-Shh pathway and contributions to girdle development.

Identity and fate of Tbx4-expressing cells reveal developmental cell fate decisions in the allantois, limb, and external genitalia

T-box gene Tbx4 is critical for the formation of the umbilicus and the initiation of the hindlimb. Previous studies show broad expression in the allantois, hindlimb, lung and proctodeum. We have examined the expression of Tbx4 in detail and used a Tbx4-Cre line to trace the fates of Tbx4-expressing cells. Tbx4 expression and lineage reveal that various distinct appendages, such as the allantois, hindlimb, and external genitalia, all arise from a single mesenchymal expression domain. Additionally, although Tbx4 is associated primarily with the hindlimb, we find two forelimb expression domains. Most notably, we find that, despite the requirement for Tbx4 in allantoic vasculogenesis, the presumptive endothelial cells of the allantois do not express Tbx4 and lineage tracing reveals that the umbilical vasculature never expresses Tbx4. These results suggest that endothelial lineages are segregated before the onset of vasculogenesis, and demonstrate a role for the peri-vascular tissue in vasculogenesis.

Body plan convergence in the evolution of skates and rays (Chondrichthyes: Batoidea)

Skates, rays and allies (Batoidea) comprise more than half of the species diversity and much of the morphological disparity among chondrichthyan fishes, the sister group to all other jawed vertebrates. While batoids are morphologically well characterized and have an excellent fossil record, there is currently no consensus on the interrelationships of family-level taxa. Here we construct a resolved, robust and time-calibrated batoid phylogeny using mitochondrial genomes, nuclear genes, and fossils, sampling densely across taxa. Data partitioning schemes, biases in the sequence data, and the relative informativeness of each fossil are explored. The molecular phylogeny is largely congruent with morphology crownward in the tree, but the branching orders of major batoid groups are mostly novel. Body plan convergence appears to be widespread in batoids. A depressed, rounded pectoral disk supported to the snout tip by fin radials, common to skates and stingrays, is indicated to have been derived independently by each group, while the long, spiny rostrum of sawfishes similarly appears to be convergent with that of sawsharks, which are not batoids. The major extant batoid lineages are inferred to have arisen relatively rapidly from the Late Triassic into the Jurassic, with long stems followed by subsequent radiations in each group around the Cretaceous/Tertiary boundary. The fossil record indicates that batoids were affected with disproportionate severity by the end-Cretaceous extinction event.

Development and evolution of the lateral plate mesoderm: comparative analysis of amphioxus and lamprey with implications for the acquisition of paired fins

Possession of paired appendages is regarded as a novelty that defines crown gnathostomes and allows sophisticated behavioral and locomotive patterns. During embryonic development, initiation of limb buds in the lateral plate mesoderm involves several steps. First, the lateral plate mesoderm is regionalized into the cardiac mesoderm (CM) and the posterior lateral plate mesoderm (PLPM). Second, in the PLPM, Hox genes are expressed in a collinear manner to establish positional values along the anterior-posterior axis. The developing PLPM splits into somatic and splanchnic layers. In the presumptive limb field of the somatic layer, expression of limb initiation genes appears. To gain insight into the evolutionary sequence leading to the emergence of paired appendages in ancestral vertebrates, we examined the embryonic development of the ventral mesoderm in the cephalochordate amphioxus Branchiostoma floridae and of the lateral plate mesoderm in the agnathan lamprey Lethenteron japonicum, and studied the expression patterns of cognates of genes known to be expressed in these mesodermal layers during amniote development. We observed that, although the amphioxus ventral mesoderm posterior to the pharynx was not regionalized into CM and posterior ventral mesoderm, the lateral plate mesoderm of lampreys was regionalized into CM and PLPM, as in gnathostomes. We also found nested expression of two Hox genes (LjHox5i and LjHox6w) in the PLPM of lamprey embryos. However, histological examination showed that the PLPM of lampreys was not separated into somatic and splanchnic layers. These findings provide insight into the sequential evolutionary changes that occurred in the ancestral lateral plate mesoderm leading to the emergence of paired appendages.

Overview of the transcriptome profiles identified in hagfish, shark, and bichir: current issues arising from some nonmodel vertebrate taxa

Because of their crucial phylogenetic positions, hagfishes, sharks, and bichirs are recognized as key taxa in our understanding of vertebrate evolution. The expression patterns of the regulatory genes involved in developmental patterning have been analyzed in the context of evolutionary developmental studies. However, in a survey of public sequence databases, we found that the large-scale sequence data for these taxa are still limited. To address this deficit, we used conventional Sanger DNA sequencing and a next-generation sequencing technology based on 454 GS FLX sequencing to obtain expressed sequence tags (ESTs) of the Japanese inshore hagfish (Eptatretus burgeri; 161,482 ESTs), cloudy catshark (Scyliorhinus torazame; 165,819 ESTs), and gray bichir (Polypterus senegalus; 34,336 ESTs). We deposited the ESTs in a newly constructed database, designated the "Vertebrate TimeCapsule." The ESTs include sequences from genes that can be effectively used in evolutionary developmental studies; for instance, several encode cartilaginous extracellular matrix proteins, which are central to an understanding of the ways in which evolutionary processes affected the skeletal elements, whereas others encode regulatory genes involved in craniofacial development and early embryogenesis. Here, we discuss how hagfishes, sharks, and bichirs contribute to our understanding of vertebrate evolution, we review the current status of the publicly available sequence data for these three taxa, and we introduce our EST projects and newly developed database.

Teratogenic effects of thalidomide: molecular mechanisms

Fifty years ago, prescription of the sedative thalidomide caused a worldwide epidemic of multiple birth defects. The drug is now used in the treatment of leprosy and multiple myeloma. However, its use is limited due to its potent teratogenic activity. The mechanism by which thalidomide causes limb malformations and other developmental defects is a long-standing question. Multiple hypotheses exist to explain the molecular mechanism of thalidomide action. Among them, theories involving oxidative stress and anti-angiogenesis have been widely supported. Nevertheless, until recently, the direct target of thalidomide remained elusive. We identified a thalidomide-binding protein, cereblon (CRBN), as a primary target for thalidomide teratogenicity. Our data suggest that thalidomide initiates its teratogenic effects by binding to CRBN and inhibiting its ubiquitin ligase activity. In this review, we summarize the biology of thalidomide, focusing on the molecular mechanisms of its teratogenic effects. In addition, we discuss the questions still to be addressed.

Evolution of vertebrate appendicular structures: Insight from genetic and palaeontological data

The new body of evidence from fossils and comparative-developmental analysis of subset of appendicular patterning genes has revealed that limb elements seen in tetrapods are assembled in fish fin over evolutionary time. However, despite of deep homology in basic structure and underlying developmental system, there remains a large morphological gap between distal elements of tetrapod limb and distal fin skeleton of tetrapodomorph fish. Understanding the genetic basis of major transformations in distal-limb morphology is the next challenge for evolutionary developmental biologists. Here by integrating data from fossils, comparative-developmental and genetic studies, models are proposed describing the evolution of cis-regulatory elements as a basis for diversification of appendicular architecture. Instead of emphasizing the subset of developmental genes, for instance Hoxd genes, the focus here is on the significance of elucidating cis-regulatory elements for multiple other key molecular players of limb/fin development and genetic/molecular interactions among them, for a better understanding of the developmental and genetic basis of limb evolution.

Developmental origins of precocial forelimbs in marsupial neonates

Marsupial mammals are born in an embryonic state, as compared with their eutherian counterparts, yet certain features are accelerated. The most conspicuous of these features are the precocial forelimbs, which the newborns use to climb unaided from the opening of the birth canal to the teat. The developmental mechanisms that produce this acceleration are unknown. Here we show that heterochronic and heterotopic changes early in limb development contribute to forelimb acceleration. Using Tbx5 and Tbx4 as fore- and hindlimb field markers, respectively, we have found that, compared with mouse, both limb fields arise notably early during opossum development. Patterning of the forelimb buds is also accelerated, as Shh expression appears early relative to the outgrowth of the bud itself. In addition, the forelimb fields and forelimb myocyte allocation are increased in size and number, respectively, and migration of the spinal nerves into the forelimb bud has been modified. This shift in the extent of the forelimb field is accompanied by shifts in Hox gene expression along the anterior-posterior axis. Furthermore, we found that both fore- and hindlimb fields arise gradually during gastrulation and extension of the embryonic axis, in contrast to the appearance of the limb fields in their entirety in all other known cases. Our results show a surprising evolutionary flexibility in the early limb development program of amniotes and rule out the induction of the limb fields by mature structures such as the somites or mesonephros.

Conservation of shh cis-regulatory architecture of the coelacanth is consistent with its ancestral phylogenetic position

Background

The modern coelacanth (Latimeria) is the extant taxon of a basal sarcopterygian lineage and sister group to tetrapods. Apart from certain apomorphic traits, its morphology is characterized by a high degree of retention of ancestral vertebrate structures and little morphological change. An insight into the molecular evolution that may explain the unchanged character of Latimeria morphology requires the analysis of the expression patterns of developmental regulator genes and their cis-regulatory modules (CRMs).

Results

We describe the comparative and functional analysis of the sonic hedgehog (shh) genomic region of Latimeria menadoensis. Several putative enhancers in the Latimeria shh locus have been identified by comparisons to sarcopterygian and actinopterygian extant species. Specific sequence conservation with all known actinopterygian enhancer elements has been detected. However, these elements are selectively missing in more recently diverged actinopterygian and sarcopterygian species. The functionality of the putative Latimeria enhancers was confirmed by reporter gene expression analysis in transient transgenic zebrafish and chick embryos.

Conclusions

Latimeria shh CRMs represent the ancestral set of enhancers that have emerged before the split of lobe-finned and ray-finned fishes. In contrast to lineage-specific losses and differentiations in more derived lineages, Latimeria shh enhancers reveal low levels of sequence diversification. High overall sequence conservation of shh conserved noncoding elements (CNE) is consistent with the general trend of high levels of conservation of noncoding DNA in the slowly evolving Latimeria genome.

Allometric growth of the trunk leads to the rostral shift of the pelvic fin in teleost fishes

The pelvic fin position among teleost fishes has shifted rostrally during evolution, resulting in diversification of both behavior and habitat. We explored the developmental basis for the rostral shift in pelvic fin position in teleost fishes using zebrafish (abdominal pelvic fins) and Nile tilapia (thoracic pelvic fins). Cell fate mapping experiments revealed that changes in the distribution of lateral plate mesodermal cells accompany the trunk-tail protrusion. Presumptive pelvic fin cells are originally located at the body wall adjacent to the anterior limit of hoxc10a expression in the spinal cord, and their position shifts rostrally as the trunk grows. We then showed that the differences in pelvic fin position between zebrafish and Nile tilapia were not due to changes in expression or function of gdf11. We also found that hox-independent motoneurons located above the pelvic fins innervate into the pelvic musculature. Our results suggest that there is a common mechanism among teleosts and tetrapods that controls paired appendage positioning via gdf11, but in teleost fishes the position of prospective pelvic fin cells on the yolk surface shifts as the trunk grows. In addition, teleost motoneurons, which lack lateral motor columns, innervate the pelvic fins in a manner independent of the rostral-caudal patterns of hox expression in the spinal cord.

Human intronic enhancers control distinct sub-domains of Gli3 expression during mouse CNS and limb development

Background

The zinc-finger transcription factor GLI3 is an important mediator of Sonic hedgehog signaling and crucial for patterning of many aspects of the vertebrate body plan. In vertebrates, the mechanism of SHH signal transduction and its action on target genes by means of activating or repressing forms of GLI3 have been studied most extensively during limb development and the specification of the central nervous system. From these studies it has emerged, that Gli3 expression must be subject to a tight spatiotemporal regulation. However, the genetic mechanisms and the cis-acting elements controlling the expression of Gli3 remained largely unknown.

Results

Here, we demonstrate in chicken and mouse transgenic embryos that human GLI3-intronic conserved non-coding sequence elements (CNEs) autonomously control individual aspects of Gli3 expression. Their combined action shows many aspects of a Gli3-specific pattern of transcriptional activity. In the mouse limb bud, different CNEs enhance Gli3-specific expression in evolutionary ancient stylopod and zeugopod versus modern skeletal structures of the autopod. Limb bud specificity is also found in chicken but had not been detected in zebrafish embryos. Three of these elements govern central nervous system specific gene expression during mouse embryogenesis, each targeting a subset of endogenous Gli3 transcription sites. Even though fish, birds, and mammals share an ancient repertoire of gene regulatory elements within Gli3, the functions of individual enhancers from this catalog have diverged significantly. During evolution, ancient broad-range regulatory elements within Gli3 attained higher specificity, critical for patterning of more specialized structures, by abolishing the potential for redundant expression control.

Conclusion

These results not only demonstrate the high level of complexity in the genetic mechanisms controlling Gli3 expression, but also reveal the evolutionary significance of cis-acting regulatory networks of early developmental regulators in vertebrates.