Evolution of paired fins and the lateral somitic frontier

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.

Morphology and phylogenetic significance of the pelvic articular region in elasmobranchs (Chondrichthyes)

The morphology of paired fins is commonly overlooked in morphological studies, particularly the pelvic girdle and fins. Consequently, previous phylogenetic studies incorporating morphological data used few skeletal characters from this complex. In this paper, the phylogenetic significance of pelvic articular characters for elasmobranchs is discussed in light of the morphological variation observed in 130 species, the most comprehensive study exploring the morphology of the pelvic girdle done so far. The 10 morphological characters proposed herein for the pelvic articulation were incorporated into a molecular matrix of NADH2 sequences and submitted to an analysis of maximum parsimony employing extended implied weighting. The most stable tree was selected based on the distortion coefficients, SPR distances (subtree pruning and regrafting) and fit values. Some of the striking synapomorphies recovered within elasmobranchs include the presence of an articular surface for the first enlarged pelvic radial supporting Elasmobranchii and the pelvic articular region for the basipterygium extending from the posterolatral margin of the pelvic girdle over its lateral surface in Echinorhinus + Hexanchiformes. Additionally, the proposed characters and their distributions are discussed considering the relationships recovered and also compared with previous morphological and molecular phylogenetic hypotheses.

Galeaspid anatomy and the origin of vertebrate paired appendages

Paired fins are a major innovation1,2 that evolved in the jawed vertebrate lineage after divergence from living jawless vertebrates3. Extinct jawless armoured stem gnathostomes show a diversity of paired body-wall extensions, ranging from skeletal processes to simple flaps4. By contrast, osteostracans (a sister group to jawed vertebrates) are interpreted to have the first true paired appendages in a pectoral position, with pelvic appendages evolving later in association with jaws5. Here we show, on the basis of articulated remains of Tujiaaspis vividus from the Silurian period of China, that galeaspids (a sister group to both osteostracans and jawed vertebrates) possessed three unpaired dorsal fins, an approximately symmetrical hypochordal tail and a pair of continuous, branchial-to-caudal ventrolateral fins. The ventrolateral fins are similar to paired fin flaps in other stem gnathostomes, and specifically to the ventrolateral ridges of cephalaspid osteostracans that also possess differentiated pectoral fins. The ventrolateral fins are compatible with aspects of the fin-fold hypothesis for the origin of vertebrate paired appendages6-10. Galeaspids have a precursor condition to osteostracans and jawed vertebrates in which paired fins arose initially as continuous pectoral-pelvic lateral fins that our computed fluid-dynamics experiments show passively generated lift. Only later in the stem lineage to osteostracans and jawed vertebrates did pectoral fins differentiate anteriorly. This later differentiation was followed by restriction of the remaining field of fin competence to a pelvic position, facilitating active propulsion and steering.

Expression of meis and hoxa11 in dipnoan and teleost fins provides new insights into the evolution of vertebrate appendages

Background

The concerted activity of Meis and Hoxa11 transcription factors is essential for the subdivision of tetrapod limbs into proximo-distal (PD) domains; however, little is know about the evolution of this patterning mechanism. Here, we aim to study the expression of meis and hoxa11 orthologues in the median and paired rayed fins of zebrafish and in the lobed fins of the Australian lungfish.

Results

First, a late phase of expression of meis1.1 and hoxa11b in zebrafish dorsal and anal fins relates with segmentation of endochondral elements in proximal and distal radials. Second, our zebrafish in situ hybridization results reveal spatial and temporal changes between pectoral and pelvic fins. Third, in situ analysis of meis1, meis3 and hoxa11 genes in Neoceratodus pectoral fins identifies decoupled domains of expression along the PD axis.

Conclusions

Our data raise the possibility that the origin of stylopod and zeugopod lies much deeper in gnathostome evolution and that variation in meis and hoxa11 expression has played a substantial role in the transformation of appendage anatomy. Moreover, these observations provide evidence that the Meis/Hoxa11 profile considered a hallmark of stylopod/zeugopod patterning is present in Neoceratodus.

Electronic supplementary material

The online version of this article (10.1186/s13227-018-0099-9) contains supplementary material, which is available to authorized users.

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.

Fin modules: an evolutionary perspective on appendage disparity in basal vertebrates

BACKGROUND

Fishes are extremely speciose and also highly disparate in their fin configurations, more specifically in the number of fins present as well as their structure, shape, and size. How they achieved this remarkable disparity is difficult to explain in the absence of any comprehensive overview of the evolutionary history of fish appendages. Fin modularity could provide an explanation for both the observed disparity in fin configurations and the sequential appearance of new fins. Modularity is considered as an important prerequisite for the evolvability of living systems, enabling individual modules to be optimized without interfering with others. Similarities in developmental patterns between some of the fins already suggest that they form developmental modules during ontogeny. At a macroevolutionary scale, these developmental modules could act as evolutionary units of change and contribute to the disparity in fin configurations. This study addresses fin disparity in a phylogenetic perspective, while focusing on the presence/absence and number of each of the median and paired fins.

RESULTS

Patterns of fin morphological disparity were assessed by mapping fin characters on a new phylogenetic supertree of fish orders. Among agnathans, disparity in fin configurations results from the sequential appearance of novel fins forming various combinations. Both median and paired fins would have appeared first as elongated ribbon-like structures, which were the precursors for more constricted appendages. Among chondrichthyans, disparity in fin configurations relates mostly to median fin losses. Among actinopterygians, fin disparity involves fin losses, the addition of novel fins (e.g., the adipose fin), and coordinated duplications of the dorsal and anal fins. Furthermore, some pairs of fins, notably the dorsal/anal and pectoral/pelvic fins, show non-independence in their character distribution, supporting expectations based on developmental and morphological evidence that these fin pairs form evolutionary modules.

CONCLUSIONS

Our results suggest that the pectoral/pelvic fins and the dorsal/anal fins form two distinct evolutionary modules, and that the latter is nested within a more inclusive median fins module. Because the modularity hypotheses that we are testing are also supported by developmental and variational data, this constitutes a striking example linking developmental, variational, and evolutionary modules.

From body scale ontogeny to species ontogeny: Histological and morphological assessment of the Late Devonian acanthodian Triazeugacanthus affinis from Miguasha, Canada

Growth series of Palaeozoic fishes are rare because of the fragility of larval and juvenile specimens owing to their weak mineralisation and the scarcity of articulated specimens. This rarity makes it difficult to describe early vertebrate growth patterns and processes in extinct taxa. Indeed, only a few growth series of complete Palaeozoic fishes are available; however, they allow the growth of isolated elements to be described and individual growth from these isolated elements to be inferred. In addition, isolated and in situ scales are generally abundant and well-preserved, and bring information on (1) their morphology and structure relevant to phylogenetic relationships and (2) individual growth patterns and processes relative to species ontogeny. The Late Devonian acanthodian Triazeugacanthusaffinis from the Miguasha Fossil-Lagerstätte preserves one of the best known fossilised ontogenies of early vertebrates because of the exceptional preservation, the large size range, and the abundance of complete specimens. Here, we present morphological, histological, and chemical data on scales from juvenile and adult specimens (scales not being formed in larvae). Histologically, Triazeugacanthus scales are composed of a basal layer of acellular bone housing Sharpey’s fibers, a mid-layer of mesodentine, and a superficial layer of ganoine. Developmentally, scales grow first through concentric addition of mesodentine and bone around a central primordium and then through superposition of ganoine layers. Ontogenetically, scales form first in the region below the dorsal fin spine, then squamation spreads anteriorly and posteriorly, and on fin webs. Phylogenetically, Triazeugacanthus scales show similarities with acanthodians (e.g. “box-in-box” growth), chondrichthyans (e.g. squamation pattern), and actinopterygians (e.g. ganoine). Scale histology and growth are interpreted in the light of a new phylogenetic analysis of gnathostomes supporting acanthodians as stem chondrichthyans.

The dlx5a/dlx6a genes play essential roles in the early development of zebrafish median fin and pectoral structures

The Dlx5 and Dlx6 genes encode homeodomain transcription factors essential for the proper development of limbs in mammalian species. However, the role of their teleost counterparts in fin development has received little attention. Here, we show that dlx5a is an early marker of apical ectodermal cells of the pectoral fin buds and of the median fin fold, but also of cleithrum precursor cells during pectoral girdle development. We propose that early median fin fold establishment results from the medial convergence of dlx5a-expressing cells at the lateral edges of the neural keel. Expression analysis also shows involvement of dlx5a during appendage skeletogenesis. Using morpholino-mediated knock down, we demonstrate that disrupted dlx5a/6a function results in pectoral fin agenesis associated with misexpression of bmp4, fgf8a, and1 and msx genes. In contrast, the median fin fold presents defects in mesenchymal cell migration and actinotrichia formation, whereas the initial specification seems to occur normally. Our results demonstrate that the dlx5a/6a genes are essential for the induction of pectoral fin outgrowth, but are not required during median fin fold specification. The dlx5a/6a knock down also causes a failure of cleithrum formation associated with a drastic loss of runx2b and col10a1 expression. The data indicate distinct requirements for dlx5a/6a during median and pectoral fin development suggesting that initiation of unpaired and paired fin formation are not directed through the same molecular mechanisms. Our results refocus arguments on the mechanistic basis of paired appendage genesis during vertebrate evolution.

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.

Expression of the Lhx genes apterous and lim1 in an errant polychaete: implications for bilaterian appendage evolution, neural development, and muscle diversification

BACKGROUND

Arthropod and vertebrate appendages appear to have evolved via parallel co-option of a plesiomorphic gene regulatory network. Our previous work implies that annelids evolved unrelated appendage-forming mechanisms; we therefore found no support for homology of parapodia and arthropodia at the level of the whole appendage. We expand on that study here by asking whether expression of the LIM homeobox (Lhx) genes apterous and lim1 in the annelid Neanthes arenaceodentata supports homology of the dorsal branches as well as the proximodistal axes of parapodia and arthropodia. In addition, we explore whether the neural expression of apterous and lim1 in Neanthes supports the putative ancestral function of the Lhx gene family in regulating the differentiation and maintenance of neuronal subtypes.

RESULTS

Both genes exhibit continuous expression in specific portions of the developing central nervous system, from hatching to at least the 13-chaetiger stage. For example, nerve cord expression occurs in segmentally iterated patterns consisting of diffuse sets of many lim1-positive cells and comparatively fewer, clustered pairs of apterous-positive cells. Additionally, continuous apterous expression is observed in presumed neurosecretory ganglia of the posterior brain, while lim1 is continuously expressed in stomatogastric ganglia of the anterior brain. apterous is also expressed in the jaw sacs, dorsal parapodial muscles, and a presumed pair of cephalic sensory organs, whereas lim1 is expressed in multiple pharyngeal ganglia, the segmental peripheral nervous system, neuropodial chaetal sac muscles, and parapodial ligules.

CONCLUSIONS

The early and persistent nervous system expression of apterous and lim1 in Neanthes juveniles supports conservation of Lhx function in bilaterian neural differentiation and maintenance. Our results also suggest that diversification of parapodial muscle precursors involves a complementary LIM code similar to those generating distinct neuronal identities in fly and mouse nerve cords. Expression of apterous and lim1 in discrete components of developing parapodia is intriguing but does not map to comparable expression of these genes in developing arthropod appendages. Thus, annelid and arthropod appendage development apparently evolved, in part, via distinct co-option of the neuronal regulatory architecture. These divergent patterns of apterous and lim1 activity seemingly reflect de novo origins of parapodia and arthropodia, although we discuss alternative hypotheses.

Evolution and development of the vertebrate neck

Muscles of the vertebrate neck include the cucullaris and hypobranchials. Although a functional neck first evolved in the lobe-finned fishes (Sarcopterygii) with the separation of the pectoral/shoulder girdle from the skull, the neck muscles themselves have a much earlier origin among the vertebrates. For example, lampreys possess hypobranchial muscles, and may also possess the cucullaris. Recent research in chick has established that these two muscles groups have different origins, the hypobranchial muscles having a somitic origin but the cucullaris muscle deriving from anterior lateral plate mesoderm associated with somites 1-3. Additionally, the cucullaris utilizes genetic pathways more similar to the head than the trunk musculature. Although the latter results are from experiments in the chick, cucullaris homologues occur in a variety of more basal vertebrates such as the sharks and zebrafish. Data are urgently needed from these taxa to determine whether the cucullaris in these groups also derives from lateral plate mesoderm or from the anterior somites, and whether the former or the latter represent the basal vertebrate condition. Other lateral plate mesoderm derivatives include the appendicular skeleton (fins, limbs and supporting girdles). If the cucullaris is a definitive lateral plate-derived structure it may have evolved in conjunction with the shoulder/limb skeleton in vertebrates and thereby provided a greater degree of flexibility to the heads of predatory vertebrates.

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.

Fossil fishes from china provide first evidence of dermal pelvic girdles in osteichthyans

BACKGROUND

The pectoral and pelvic girdles support paired fins and limbs, and have transformed significantly in the diversification of gnathostomes or jawed vertebrates (including osteichthyans, chondrichthyans, acanthodians and placoderms). For instance, changes in the pectoral and pelvic girdles accompanied the transition of fins to limbs as some osteichthyans (a clade that contains the vast majority of vertebrates--bony fishes and tetrapods) ventured from aquatic to terrestrial environments. The fossil record shows that the pectoral girdles of early osteichthyans (e.g., Lophosteus, Andreolepis, Psarolepis and Guiyu) retained part of the primitive gnathostome pectoral girdle condition with spines and/or other dermal components. However, very little is known about the condition of the pelvic girdle in the earliest osteichthyans. Living osteichthyans, like chondrichthyans (cartilaginous fishes), have exclusively endoskeletal pelvic girdles, while dermal pelvic girdle components (plates and/or spines) have so far been found only in some extinct placoderms and acanthodians. Consequently, whether the pectoral and pelvic girdles are primitively similar in osteichthyans cannot be adequately evaluated, and phylogeny-based inferences regarding the primitive pelvic girdle condition in osteichthyans cannot be tested against available fossil evidence.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report the first discovery of spine-bearing dermal pelvic girdles in early osteichthyans, based on a new articulated specimen of Guiyu oneiros from the Late Ludlow (Silurian) Kuanti Formation, Yunnan, as well as a re-examination of the previously described holotype. We also describe disarticulated pelvic girdles of Psarolepis romeri from the Lochkovian (Early Devonian) Xitun Formation, Yunnan, which resemble the previously reported pectoral girdles in having integrated dermal and endoskeletal components with polybasal fin articulation.

CONCLUSIONS/SIGNIFICANCE

The new findings reveal hitherto unknown similarity in pectoral and pelvic girdles among early osteichthyans, and provide critical information for studying the evolution of pelvic girdles in osteichthyans and other gnathostomes.

An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates

Almost all gnathostomes or jawed vertebrates (including osteichthyans, chondrichthyans, 'acanthodians' and most placoderms) possess paired pectoral and pelvic fins. To date, it has generally been believed that antiarch placoderms (extinct armoured jawed fishes from the Silurian-Devonian periods) lacked pelvic fins. The putative absence of pelvic fins is a key character bearing on the monophyly or paraphyly of placoderms. It also has far-reaching implications for studying the sequence of origin of pelvic girdles versus that of movable jaws in the course of vertebrate evolution. Parayunnanolepis xitunensis represents the only example of a primitive antiarch with extensive post-thoracic preservation, and its original description has been cited as confirming the primitive lack of pelvic fins in early antiarchs. Here, we present a revised description of Parayunnanolepis and offer the first unambiguous evidence for the presence of pelvic girdles in antiarchs. As antiarchs are placed at the base of the gnathostome radiation in several recent studies, our finding shows that all jawed vertebrates (including antiarch placoderms) primitively possess both pectoral and pelvic fins and that the pelvic fins did not arise within gnathostomes at a point subsequent to the origin of jaws.

Distinct modes of vertebrate hypaxial muscle formation contribute to the teleost body wall musculature

The formation of the body wall musculature in vertebrates is assumed to be initiated by direct ventral extension of the somites/myotomes. This contrasts to the formation of limb muscles and muscles involved in feeding or respiration/ventilation, which are founded by migratory muscle precursors (MMPs) distant to the somites. Here, we present evidence from morphology and expression of molecular markers proposing that the formation of the two muscle layers of the teleost body wall involves both of the above mechanisms: (1) MMPs from somites 5 and 6 found an independent muscle primordium-the so-called posterior hypaxial muscle (PHM)-which subsequently gives rise to the most anterior two segments of the medial obliquus inferioris (OI) muscle. (2) Direct epithelial extension of the hypaxial myotomes generates the OI segments from somite 7 caudalward and the entire lateral obliquus superioris (OS) muscle. The findings are discussed in relation to the evolution of hypaxial myogenic patterning including functional considerations. We hypothesise that the potential of the most anterior somites to generate migratory muscle precursors is a general vertebrate feature that has been differently utilised in the evolution in vertebrate groups.