Paired Appendages Research Articles

Evolution of vertebrate postcranial complexity: axial skeleton regionalization and paired appendages in a Devonian jawless fish

AbstractOne of the major events in vertebrate evolution involves the transition from jawless (agnathan) to jawed (gnathostome) vertebrates, including a variety of cranial and postcranial innovations. It has long been assumed that characters such as the pelvic girdles and fins, male intromittent organs independent from the pelvic girdles, as well as a regionalized axial skeleton first appeared in various basal gnathostome groups if not at the origin of gnathostomes. Here we describe the first occurrence of pelvic girdles and intromittent organs in the Late Devonian jawless anaspid‐like fish Euphanerops longaevus Woodward (Miguasha Lagerstätte, eastern Canada), associated with a morphologically differentiated region of the axial skeleton. Morphological differentiation of the axial skeleton is also described for the first time in an extant jawless fish, the sea lamprey Petromyzon marinus Linnaeus. Our data indicate that regionalization of the axial skeleton occurred earlier in vertebrate evolutionary history than previously appreciated. This regionalization is coupled with modifications of the appendicular skeleton in Euphanerops. These new observations combined with a new phylogenetic analysis of early vertebrates provide a more precise understanding of how the appendicular and axial skeletons developed and evolved within vertebrate evolutionary history.

Late Ediacaran trackways produced by bilaterian animals with paired appendages.

Ediacaran trace fossils provide key paleontological evidence for the evolution of early animals and their behaviors. Thus far, however, this fossil record has been limited to simple surface trails and relatively shallow burrows. We report possible trackways, preserved in association with burrows, from the terminal Ediacaran Shibantan Member (ca. 551 to ca. 541 million years ago) in the Yangtze Gorges area of South China. These trace fossils represent the earliest known trackways. They consist of two rows of imprints arranged in poorly organized series or repeated groups. These trackways may have been produced by bilaterian animals with paired appendages, although the phylum-level phylogenetic affinity of the trace makers remains unknown. It is possible that the trackways and associated burrows were produced by the same trace maker, indicating a complex behavior involving both walking and burrowing. Together, these trackways and burrows mark the arrival of a new era characterized by an increasing geobiological footprint of bilaterian animals.

Waptia fieldensis Walcott, a mandibulate arthropod from the middle Cambrian Burgess Shale.

Waptia fieldensis Walcott, 1912 is one of the iconic animals from the middle Cambrian Burgess Shale biota that had lacked a formal description since its discovery at the beginning of the twentieth century. This study, based on over 1800 specimens, finds that W. fieldensis shares general characteristics with pancrustaceans, as previous authors had suggested based mostly on its overall aspect. The cephalothorax is covered by a flexible, bivalved carapace and houses a pair of long multisegmented antennules, palp-bearing mandibles, maxillules, and four pairs of appendages with five-segmented endopods—the anterior three pairs with long and robust enditic basipods, the fourth pair with proximal annulations and lamellae. The post-cephalothorax has six pairs of lamellate and fully annulated appendages which appear to be extensively modified basipods rather than exopods. The front part of the body bears a pair of stalked eyes with the first ommatidia preserved in a Burgess Shale arthropod, and a median ‘labral’ complex flanked by lobate projections with possible affinities to hemi-ellipsoid bodies. Waptia confirms the mandibulate affinity of hymenocarines, retrieved here as part of an expanded Pancrustacea, thereby providing a novel perspective on the evolutionary history of this hyperdiverse group. We construe that Waptia was an active swimming predator of soft prey items, using its anterior appendages for food capture and manipulation, and also potentially for clinging to epibenthic substrates.

Motor innervation pattern of labral muscles of Locusta migratoria.

The current study investigates the motor innervation pattern of labral muscles in the adult locust and tries to interpret the results in the light of the hypothesis that the labrum phylogenetically developed by the fusion of paired appendages associated with the intercalary segment. Using Neurobiotin™ as a retrograde neuronal tracer, specific motor nerves or individual labral muscles were stained. Results show that the labral muscles receive innervation from tritocerebrum and suboesophageal ganglion. The axons of many motor neurons use three different pathways to cross the midline in the periphery to innervate ipsi- and contralateral muscles. Intracellular recordings from fibers of individual muscles and simultaneous recordings from motor neurons imply that the labral muscles lack inhibitory innervation. The location of motor neurons in both tritocerebrum and suboesophageal ganglion supports the notion that the labrum is innervated by the so-called intercalary segment. That many of the efferent axons cross the midline in the periphery might be explained by the hypothesis that the labrum derives from a fusion of appendages.

A conserved Shh cis-regulatory module highlights a common developmental origin of unpaired and paired fins.

Despite their evolutionary, developmental, and functional importance the origin of vertebrate paired appendages remains uncertain. In mice, a single enhancer termed ZRS is solely responsible for Shh expression in limbs. Here, zebrafish and mouse transgenic assays trace the functional equivalence of ZRS across the gnathostome phylogeny. CRISPR/Cas9-mediated deletion of the medaka-ZRS and enhancer assays reveal the existence of ZRS shadow enhancers in both teleost and human genomes. Deletion of both ZRS and shadow ZRS abolish shh expression and completely truncate pectoral fin formation. Strikingly, deletion of ZRS results in an almost complete ablation of the dorsal fin. This finding indicates that a ZRS-Shh regulatory module is shared by paired and median fins, and that paired fins likely emerged by the co‐option of developmental programs established in the median fins of stem gnathostomes. Shh function was later reinforced in pectoral fin development with the recruitment of shadow enhancers, conferring additional robustness.

The Ancient Origins of Neural Substrates for Land Walking

Walking is the predominant locomotor behavior expressed by land-dwelling vertebrates, but it is unknown when the neural circuits that are essential for limb control first appeared. Certain fish species display walking-like behaviors, raising the possibility that the underlying circuitry originated in primitive marine vertebrates. We show that the neural substrates of bipedalism are present in the little skate Leucoraja erinacea, whose common ancestor with tetrapods existed ∼420 million years ago. Leucoraja exhibits core features of tetrapod locomotor gaits, including left-right alternation and reciprocal extension-flexion of the pelvic fins. Leucoraja also deploys a remarkably conserved Hox transcription factor-dependent program that is essential for selective innervation of fin/limb muscle. This network encodes peripheral connectivity modules that are distinct from those used in axial muscle-based swimming and has apparently been diminished in most modern fish. These findings indicate that the circuits that are essential for walking evolved through adaptation of a genetic regulatory network shared by all vertebrates with paired appendages. VIDEO ABSTRACT.

Mandibulate convergence in an armoured Cambrian stem chelicerate

BackgroundChelicerata represents a vast clade of mostly predatory arthropods united by a distinctive body plan throughout the Phanerozoic. Their origins, however, with respect to both their ancestral morphological features and their related ecologies, are still poorly understood. In particular, it remains unclear whether their major diagnostic characters were acquired early on, and their anatomical organization rapidly constrained, or if they emerged from a stem lineage encompassing an array of structural variations, based on a more labile “panchelicerate” body plan.ResultsIn this study, we reinvestigated the problematic middle Cambrian arthropod Habelia optata Walcott from the Burgess Shale, and found that it was a close relative of Sanctacaris uncata Briggs and Collins (in Habeliida, ord. nov.), both retrieved in our Bayesian phylogeny as stem chelicerates. Habelia possesses an exoskeleton covered in numerous spines and a bipartite telson as long as the rest of the body. Segments are arranged into three tagmata. The prosoma includes a reduced appendage possibly precursor to the chelicera, raptorial endopods connected to five pairs of outstandingly large and overlapping gnathobasic basipods, antennule-like exopods seemingly dissociated from the main limb axis, and, posteriorly, a pair of appendages morphologically similar to thoracic ones. While the head configuration of habeliidans anchors a seven-segmented prosoma as the chelicerate ground pattern, the peculiar size and arrangement of gnathobases and the presence of sensory/tactile appendages also point to an early convergence with the masticatory head of mandibulates.ConclusionsAlthough habeliidans illustrate the early appearance of some diagnostic chelicerate features in the evolution of euarthropods, the unique convergence of their cephalons with mandibulate anatomies suggests that these traits retained an unusual variability in these taxa. The common involvement of strong gnathal appendages across non-megacheiran Cambrian taxa also illustrates that the specialization of the head as the dedicated food-processing tagma was critical to the emergence of both lineages of extant euarthropods—Chelicerata and Mandibulata—and implies that this diversification was facilitated by the expansion of durophagous niches.

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.

Developmental and Evolutionary Perspectives on the Origin and Diversification of Arthropod Appendages.

Jointed, segmented appendages are a key innovation of arthropods. The subsequent diversification of these appendages, both along the body axis and across taxa, has contributed to the evolutionary success of arthropods. Both developmental and fossil data are informative for understanding how these transitions occurred. Comparative analyses help to pinpoint the developmental novelties that distinguish arthropod appendages from the lobopodous appendages of other panarthropods, and that distinguish different appendage types. The fossil record of stem group arthropods is diverse and preserves intermediate steps in these evolutionary transitions, including some that cannot be directly inferred based on extant taxa. These lead to hypotheses that can be tested with comparative developmental data, as well as to reinterpretations of developmental results. One developmental novelty of arthropods is the reiterated deployment of the joint formation network, which divides the appendages into segments. The fossil record raises questions about how this joint formation network was first deployed, given the contrasting morphologies of appendages in stem group versus extant arthropods. The fossil record supports a character tree for appendage diversification showing progressive individuation of appendages in an anterior-to-posterior sequence. However, to date, developmental evidence provides at best limited support for this character tree. Recent interpretations of the fossil record suggest that the labrum of extant arthropods is a greatly reduced protocerebral appendage pair; this hypothesis is consistent with the extensive shared developmental patterning of the labrum and jointed appendages. Reciprocal illumination from fossils and developmental patterning in a phylogenetic context both makes sense of some results and helps motivates questions for future research.

Parasite-mediated selection promotes small body size in yellow dung flies

Evidence for selective disadvantages of large body size remains scarce in general. Previous phenomenological studies of the yellow dung fly Scathophaga stercoraria have demonstrated strong positive sexual and fecundity selection on male and female size. Nevertheless, the body size of flies from a Swiss study population has declined by almost 10% from 1993 to 2009. Given substantial heritability of body size, this negative evolutionary response of a trait that is measurably positively selected suggests important selective factors being missed. We took advantage of a periodic epidemic outbreak of the fungus Entomophthora scatophagae to assess selection exerted by this fatal parasite. Fungal infection varied over the season from ca. 50% in the cooler and more humid spring and autumn to almost 0% in summer. The probability of dying from fungal infection increased with adult body size. Females never laid any eggs after infection, so there was no fungus effect on female fecundity beyond its impact on mortality. Large males showed the typical mating advantage in the field, but this pattern of positive sexual selection was nullified by fungal infection. Mean fluctuating asymmetry of paired appendages (legs, wings) did not affect the viability, fecundity or mating success of yellow dung flies in the field. Our study demonstrates rare size-selective parasite-mediated disadvantages of large adult body size in the field. Reduced ability to combat parasites such as Entomophthora may be an immunity cost of large body size in dung flies, although the hypothesized trade-off between fluctuating asymmetry, a presumed indicator of developmental instability and environmental stress, and immunocompetence was not found here.

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

BackgroundThe 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.ResultsHere 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.ConclusionOur 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.

Dorsal fin development in flounder, Paralichthys olivaceus: Bud formation and its cellular origin

The development of the median fin has not been investigated extensively in teleosts, although in other fishes it has been proposed that it involves the same genetic programs operating in the paired appendages. Adult median fins develop from the larval bud; therefore an investigation of fin bud formation and its cellular origin is essential to understanding the maturation mechanisms. In Paralichthys olivaceus, skeletogenesis proceeds from an anterior to posterior direction providing a good opportunity to study the formation of dorsal fin bud. An apical ectodermal ridge appeared at the basal stratum of the presumptive dorsal fin was first observed at 3 days post hatching. Then the apical ectodermal fold formed as the bud outgrew in 6 days post-hatch larvae. The bud continued to grow, breaking through the dorsal fin fold in 9 days post-hatch larvae. At 13 days post-hatch, the bud grew beyond the edge of the fin fold and formed into the four future rays. Molecular markers of cell type showed the existence of neural crest cells, scleroblasts and sclerotomes in the dorsal fin bud. The earliest gene expression in the dorsal fin bud was Hoxd10 at 3 days post-hatch larvae, then Hoxd9, Hoxd11 and Hoxd12. This indicates Hoxd10 might be a candidate molecular marker of the bud formation site. Some key molecular markers for paired appendage development, such as FGF8, Wnt7, and Shh were expressed at the apical ectodermal ridge and later the apical ectodermal fold. Moreover, the form of the dorsal fin bud could be inhibited by Hh pathway inhibitor, further indicating that common basic molecular mechanisms might be utilized by median fins.

Thismia inconspicua (Thismiaceae), a new mycoheterotrophic species from Borneo

A new species belonging to the mycoheterotrophic genus Thismia is described and illustrated. Thismia inconspicua was found in a lowland mixed dipterocarp forest in Ulu Temburong National Park, Brunei Darussalam. It is characterized by its sepia-brown perianth with free equal lobes with very short terminal appendages, two pairs of appendages on connective apices, perianth tube displaced from the ovary axis and short stem. DNA sequence data from commonly studied nuclear and mitochondrial loci are provided. Phylogenetic analysis confirmed the close relationship with other members of section Thismia, subsection Odoardoa. An updated determination key of Thismia species of Borneo is included.

Enigmas from the past: M'Intosh's (1885) annelidicolous copepodsfrom the voyage of H.M.S. Challenger.

The type material of four monotypic genera, Leaniricola, Oestrella, Praxillinicola and Trophoniphila (Copepoda, Cyclopoida), described by M'Intosh (1885) from deep water polychaete hosts collected during the H.M.S. Challenger expedition, is re-examined. Leaniricola rotundata M'Intosh (1885) is removed from its floating status as species inquirenda in the Nereicolidae and fixed as the type of a new family, Leaniricolidae fam. nov., based on the presence of an oral cone and massive, three-dimensionally expanded, mandibular gnathobases which are used to anchor the parasite in the parapodial integument of its host. The ectoparasitic Praxillinicola kroyeri (M'Intosh, 1885), previously treated as a species inquirenda in the Clausiidae, cannot be placed in any of the currently recognized poecilostome families and is here fixed as the type of a new family, Praxillinicolidae fam. nov. Females are characterized by unique bilobate antennules, contained within anterior sockets shared with the reduced antennae, and by paired labral hooks, both of which probably serve as auxiliary attachment organs. Despite its highly transformed body P. kroyeri has retained the plesiomorphic condition of the female genital system, with the gonopores positioned dorsolaterally and the paired copulatory pores lying close together on the midventral surface. The position of the mesoparasitic Trophoniphila bradyi M'Intosh, 1885 in the Bradophilidae is confirmed based on the presence of a median copulatory pore in the female, the disproportionately large egg sacs, the shape of the ectosoma and host utilization. Both sexes of Bradophila pygmaea Levinsen, 1878 are re-examined and the male is redescribed in detail. Flabellicola Gravier, 1918a is treated as a genus incertae sedis in the Bradophilidae. The anal prominence, previously reported in the female of Oestrella (= Melinnacheres) levinseni M'Intosh, 1885 (Saccopsidae), proved upon re-examination to be one of the paired genital apertures. The third and fourth pairs of appendages reported in male saccopsids are here interpreted as the maxillae and maxillipeds, respectively.

Secondary Evolution of Aquatic Propulsion in Higher Vertebrates: Validation and Prospect.

Re-invasion of the aquatic environment by terrestrial vertebrates resulted in the evolution of species expressing a suite of adaptations for high-performance swimming. Examination of swimming by secondarily aquatic vertebrates provides opportunities to understand potential selection pressures and mechanical constraints, which may have directed the evolution of these aquatic species. Mammals and birds realigned the body and limbs for cursorial movements and flight, respectively, from the primitive tetrapod configuration. This realignment produced multiple solutions for aquatic specializations and swimming modes. Initially, in the evolution of aquatic mammals and birds, swimming was accomplished by using paired appendages in a low-efficiency, drag-based paddling mode. This mode of swimming arose from the modification of neuromotor patterns associated with gaits characteristic of terrestrial and aerial locomotion. The evolution of advanced swimming modes occurred in concert with changes in buoyancy control for submerged swimming, and a need for increased aquatic performance. Aquatic mammals evolved three specialized lift-based modes of swimming that included caudal oscillation, pectoral oscillation, and pelvic oscillation. Based on modern analogs, a biomechanical model was developed to explain the evolution of specialized aquatic mammals and their transitional forms. Subsequently, fossil aquatic mammals were described that validated much of the model. However, for birds, which were adapted for aerial flight, fossil evidence has been less forthcoming to explain the transition to aquatic capabilities. A biomechanical model is proposed for birds to describe the evolution of specialized lift-based foot and wing swimming. For both birds and mammals, convergence in morphology and propulsive mechanics is dictated by the need to increase speed, reduce drag, improve thrust output, enhance efficiency, and control maneuverability in the aquatic environment.

Deep phylogenomics of a tandem-repeat galectin regulating appendicular skeletal pattern formation.

BackgroundA multiscale network of two galectins Galectin-1 (Gal-1) and Galectin-8 (Gal-8) patterns the avian limb skeleton. Among vertebrates with paired appendages, chondrichthyan fins typically have one or more cartilage plates and many repeating parallel endoskeletal elements, actinopterygian fins have more varied patterns of nodules, bars and plates, while tetrapod limbs exhibit tandem arrays of few, proximodistally increasing numbers of elements. We applied a comparative genomic and protein evolution approach to understand the origin of the galectin patterning network. Having previously observed a phylogenetic constraint on Gal-1 structure across vertebrates, we asked whether evolutionary changes of Gal-8 could have critically contributed to the origin of the tetrapod pattern.ResultsTranslocations, duplications, and losses of Gal-8 genes in Actinopterygii established them in different genomic locations from those that the Sarcopterygii (including the tetrapods) share with chondrichthyans. The sarcopterygian Gal-8 genes acquired a potentially regulatory non-coding motif and underwent purifying selection. The actinopterygian Gal-8 genes, in contrast, did not acquire the non-coding motif and underwent positive selection.ConclusionThese observations interpreted through the lens of a reaction-diffusion-adhesion model based on avian experimental findings can account for the distinct endoskeletal patterns of cartilaginous, ray-finned, and lobe-finned fishes, and the stereotypical limb skeletons of tetrapods.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0729-6) contains supplementary material, which is available to authorized users.

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.

Evolution of Serial Patterns in the Vertebrate Pharyngeal Apparatus and Paired Appendages via Assimilation of Dissimilar Units

Evolution of serially similar structures has attracted interest since the infancy of comparative morphology and embryology. A long-standing assumption is that the serial patterning reflects ancestral metamerism, which persists in preconceived character polarity from a primitive state of polyisomerism (a series of identical or similar units) to a derived state of anisomerism (a series of differentially specialized parts). We test this assumption against an alternative character polarity — from anisomeric to polyisomeric — in the vertebrate pharyngeal apparatus and paired appendages. We show that, contrary to what is usually assumed, serial similarity represents a derived state in both pharyngeal apparatus and paired appendages: the distinctly patterned structures secondarily assimilated each other. Acquisitions of serial similarity in the pharyngeal apparatus and paired appendages straddle major evolutionary events such as the origin of the jaw and fish-tetrapod transitions. We suggest that: (a) the origin of the jaw coincided with extension of the serial pharyngeal patterning onto the mandibular region; and (b) the pectoral and pelvic appendages have independent origins and their distal portions acquired serial similarities later during the fin-limb transitions.

Developmental origin of the clavicle, and its implications for the evolution of the neck and the paired appendages in vertebrates.

In fish, the pectoral appendage is adjacent to the head, but during vertebrate evolution a long neck region emerged via caudal relocation of the pectoral appendage. The pectoral appendage is comprised of endochondral portions, such as the humerus and the scapula, and a dermal portion, such as the clavicle, that contributes to the shoulder girdle. In the search for clues to the mechanism of the caudal relocation of the pectoral appendage, the cell lineage of the rostral lateral plate mesoderm was analyzed in chickens. It was found that, despite the long neck region in chickens, the origin of the clavicle attached to the head mesoderm ranged between 1 and 14 somite levels. Because the pectoral limb bud and the endochondral pectoral appendage developed on 15-20 and 15-24 somite levels, respectively, the clavicle-forming region corresponds to the embryonic neck, which suggests that the relocation would have been executed by the expansion of the source of the clavicle. The rostral portion of the clavicle-forming region overlaps the source of the cucullaris muscle, embraces the pharyngeal arches caudally, and can be experimentally replaced with the head mesoderm to form the cucullaris muscle, which implies that the mesodermal portion could have been the head mesodermand that the clavicle would have developed at the head/trunk boundary. The link between the head mesoderm and the presumptive clavicle appears to have been the developmental constraint needed to create the evolutionarily conserved musculoskeletal connectivities characterizing the gnathostome neck. In this sense, the dermal girdle of the ganathostomes would represent the wall of the branchial chamber into which the endochondral pectoral appendage appears to have attached since its appearance in evolution.

A shared role for sonic hedgehog signalling in patterning chondrichthyan gill arch appendages and tetrapod limbs.

Chondrichthyans (sharks, skates, rays and holocephalans) possess paired appendages that project laterally from their gill arches, known as branchial rays. This led Carl Gegenbaur to propose that paired fins (and hence tetrapod limbs) originally evolved via transformation of gill arches. Tetrapod limbs are patterned by asonic hedgehog(Shh)-expressing signalling centre known as the zone of polarising activity, which establishes the anteroposterior axis of the limb bud and maintains proliferative expansion of limb endoskeletal progenitors. Here, we use loss-of-function, label-retention and fate-mapping approaches in the little skate to demonstrate that Shh secretion from a signalling centre in the developing gill arches establishes gill arch anteroposterior polarity and maintains the proliferative expansion of branchial ray endoskeletal progenitor cells. These findings highlight striking parallels in the axial patterning mechanisms employed by chondrichthyan branchial rays and paired fins/limbs, and provide mechanistic insight into the anatomical foundation of Gegenbaur's gill arch hypothesis.