The Lung-Swimbladder Issue: A Simple Case of Homology – Or Not?

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... (Darwin, 1859, chapter Simpson (1953) and by Mayr in his article on novelty (1960) Stephen Jay Gould has stressed that Darwin's explanation is "not only wrong, but backwards" (Gould, 2002(Gould, , p. 1224) The hypothesis favoured by Gould (that lung is the ancestral and swimbladder the derived condition) was actually already the dominant one in the Modern Synthesis era, as noted by Mayr and Simpson (Simpson, 1953, p. 192, note 11;Mayr, 1960, p. 352). The case is still debated today The main competing hypothesis is the independent derivation of both organs from a respiratory pharynx (Lambertz and Perry, 2015;Perry and Sander, 2004 the subsidiary function gradually becomes the chief function, the total function becomes quite different, and the consequence of the whole process is the transformation of the organ (Dohrn 1875, p60 cited by Russel, 1916). ...
Full text available on request or in open access at Evolutionary novelty, the origin of new characters such as the turtle shell or the flower, is a fundamental problem for an evolutionary view of life. Accordingly, it is a central research topic in contemporary biology involving input from several biological disciplines and explanations at several levels of organization. I study the evolution of research on novelty from the 1950s to the present. The problem of novelty has recently been appropriated by evolutionary developmental biology or evo-devo, a synthesis of evolutionary and developmental biology that started emerging in the 1980s following technological advances and discoveries in developmental genetics. I focus instead on three neglected dimensions of the problem of novelty: the functional-historical approach to the problem, research on novelty in the late Modern Synthesis era (1950-1980) and novelty in plants. My argument runs against the view of some scientists and historians, often tied to evo-devo, who oppose structuralist and functionalist approaches in biology and who claim that the origin of novelty is a structuralist problem. I advocate an approach to novelty that ties together structural and functional dimensions and show how some research programs of the last eighty years implemented different versions of this approach.
... Although parsimony is a key tool in phylogenetic reconstructions , it alone ultimately is in and of itself not a definitive indicator for the homology of traits, and additional organismic considerations are required (see e.g. Wagner, 2014; Lambertz and Perry, 2015 ). Indeed, the ossification sequence and mode of the sternum formation in enantiornithine birds, the dominant clade of Mesozoic birds (e.g., Zhou, 2004 ) here represented by Eopengornis , Longipteryx and Bohaiornis, is different from that of its sister taxon, the ornithuromorphs, which also include the modern birds (Zheng et al., 2012). ...
This paper summarizes the main morphological tracts exhibited by lungs and gas bladders in fishes. The origin and organ location, the presence of a glottal region, the inner architecture, the characteristics of the exchange barrier and the presence of pulmonary arteries have been reviewed in the two types of air-breathing organs. With the exception of the dorsal (bladders) or ventral (lungs) origin from the posterior pharynx, none of the morphological traits analyzed can be considered specific for either lungs or gas bladders. This is exemplified by analysis of the morphology of the lung of the Dipnoii and Polypteriformes and of the bladder of the Lepisosteiformes. All of them are obligate air-breathers and show a lung-like (pulmonoid) air-breathing organ. However, while the lungfish lung and the bladder of the Lepisosteiformes occupy a dorsal position and are highly trabeculated, the polypterid lung occupies a ventral position and shows a smooth inner surface. Structural and ultrastructural differences are also highlighted. Noticeably, a large part of the inner surface area of the lung of the Australian lungfish is covered by a ciliated epithelium. A restricted respiratory surface area may help to explain the incapability of this species to aestivate. The respiratory bladder of basal teleosts displays a more complex morphology than that observed in more primitive species. The bladder of basal teleosts may appear divided into respiratory and non-respiratory portions, exhibit intricate shapes, invade adjacent structures and gain additional functions. The increase in morphological and functional complexity appears to prelude the loss of the respiratory functions.
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The coelacanth, Latimeria chalumnae Smith, 1939 [1] (Sarcopterygii: Actinistia), together with the closely related L. menadoensis Pouyaud et al., 1999 [2], remains the only living representative of one of the most basally-branching primary radiations of lobe-finned fishes (Sarcopterygii). Even though extant species cannot be considered 'primitive' due to the inherent logic of phylogenetic theory, the coelacanth nonetheless is invaluable for understanding evolutionary transformations in basal sarcopterygians as it can help in the determination of character polarity. The appearance of one novelty during early vertebrate evolution that had major implications for the success of a huge number of species is the origin of lungs. The conventional interpretation is that lungs evolved in basal bony fishes (Osteichthyes or Osteognathostomata), were maintained in the lobe-finned fishes, and eventually were transformed into a swimbladder among the ray-finned fishes (Actinopterygii) (e.g. [3]). However, the currently available data do not rule out separate origins of lungs and swimbladders from a common 'respiratory pharynx', even though this would require a slightly less parsimonious course of evolution [4,5]. The coelacanth is a key species in addressing this question and for this reason the data recently provided by Cupello and colleagues [6] are a very welcome addition to the discussion. Here, I would like to add a few points pertinent to lung evolution that appear to be a consequence of these exciting data. One of the most interesting aspects of the coelacanth is that it apparently exhibits an unpaired structure of putative homology with lungs [6-8]. In the Polypteriformes (bichir and reed fish), the lungs are paired [5,9,10], as are those of the lungfishes (Dipnoi) [11], except the Australian lungfish, Neoceratodus forsteri (Krefft, 1870)….
Increased organismic complexity in metazoans was achieved via the specialization of certain parts of the body involved in different faculties (structure-function complexes). One of the most basic metabolic demands of animals in general is a sufficient supply of all tissues with oxygen. Specialized structures for gas exchange (and transport) consequently evolved many times and in great variety among bilaterians. This review focuses on some of the latest advancements that morphological research has added to our understanding of how the respiratory apparatus of the primarily terrestrial vertebrates (amniotes) works and how it evolved. Two main components of the respiratory apparatus, the lungs as the "exchanger" and the ventilatory apparatus as the "active pump," are the focus of this paper. Specific questions related to the exchanger concern the structure of the lungs of the first amniotes and the efficiency of structurally simple snake lungs in health and disease, as well as secondary functions of the lungs in heat exchange during the evolution of sauropod dinosaurs. With regard to the active pump, I discuss how the unique ventilatory mechanism of turtles evolved and how understanding the avian ventilatory strategy affects animal welfare issues in the poultry industry.
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We show-in contrast to the traditional textbook contention-that the first amniote lungs were complex, multichambered organs and that the single-chambered lungs of lizards and snakes represent a secondarily simplified rather than the plesiomorphic condition. We combine comparative anatomical and embryological data and show that shared structural principles of multichamberedness are recognizable in amniotes including all lepidosaurian taxa. Sequential intrapulmonary branching observed during early organogenesis becomes obscured during subsequent growth, resulting in a secondarily simplified, functionally single-chambered lung in lepidosaurian adults. Simplification of pulmonary structure maximized the size of the smallest air spaces and eliminated biophysically compelling surface tension problems that were associated with miniaturization evident among stem lepidosaurmorphs. The remaining amniotes, however, retained the multichambered lungs, which allowed both large surface area and high pulmonary compliance, thus initially providing a strong selective advantage for efficient respiration in terrestrial environments. Branched, multichambered lungs instead of simple, sac-like organs were part and parcel of the respiratory apparatus of the first amniotes and pivotal for their success on dry land, with the sky literally as the limit. © 2015 The Author(s) Published by the Royal Society. All rights reserved.
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Life originated in anoxia, but many organisms came to depend upon oxygen for survival, independently evolving diverse respiratory systems for acquiring oxygen from the environment. Ambient oxygen tension (PO2 ) fluctuated through the ages in correlation with biodiversity and body size, enabling organisms to migrate from water to land and air and sometimes in the opposite direction. Habitat expansion compels the use of different gas exchangers, for example, skin, gills, tracheae, lungs, and their intermediate stages, that may coexist within the same species; coexistence may be temporally disjunct (e.g., larval gills vs. adult lungs) or simultaneous (e.g., skin, gills, and lungs in some salamanders). Disparate systems exhibit similar directions of adaptation: toward larger diffusion interfaces, thinner barriers, finer dynamic regulation, and reduced cost of breathing. Efficient respiratory gas exchange, coupled to downstream convective and diffusive resistances, comprise the “oxygen cascade”—step-down of PO2 that balances supply against toxicity. Here, we review the origin of oxygen homeostasis, a primal selection factor for all respiratory systems, which in turn function as gatekeepers of the cascade. Within an organism’s lifespan, the respiratory apparatus adapts in various ways to upregulate oxygen uptake in hypoxia and restrict uptake in hyperoxia. In an evolutionary context, certain species also become adapted to environmental conditions or habitual organismic demands. We, therefore, survey the comparative anatomy and physiology of respiratory systems from invertebrates to vertebrates, water to air breathers, and terrestrial to aerial inhabitants. Through the evolutionary directions and variety of gas exchangers, their shared features and individual compromises may be appreciated.
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Homology can have different meanings for different kinds of biologists. A phylogenetic view holds that homology, defined by common ancestry, is rigorously identified through phylogenetic analysis. Such homologies are taxic homologies (=synapomorphies). A second interpretation, "biological homology" emphasizes common ancestry through the continuity of genetic information underlying phenotypic traits, and is favored by some developmental geneticists. A third kind of homology, deep homology, was recently defined as "the sharing of the genetic regulatory apparatus used to build morphologically and phylogenetically disparate features." Here we explain the commonality among these three versions of homology. We argue that biological homology, as evidenced by a conserved gene regulatory network giving a trait its "essential identity" (a Character Identity Network or "ChIN") must also be a taxic homology. In cases where a phenotypic trait has been modified over the course of evolution such that homology (taxic) is obscured (e.g. jaws are modified gill arches), a shared underlying ChIN provides evidence of this transformation. Deep homologies, where molecular and cellular components of a phenotypic trait precede the trait itself (are phylogenetically deep relative to the trait), are also taxic homologies, undisguised. Deep homologies inspire particular interest for understanding the evolutionary assembly of phenotypic traits. Mapping these deeply homologous building blocks on a phylogeny reveals the sequential steps leading to the origin of phenotypic novelties. Finally, we discuss how new genomic technologies will revolutionize the comparative genomic study of non-model organisms in a phylogenetic context, necessary to understand the evolution of phenotypic traits.
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The fish swimbladder is a unique organ in vertebrate evolution and it functions for regulating buoyancy in most teleost species. It has long been postulated as a homolog of the tetrapod lung, but the molecular evidence is scarce. In order to understand the molecular function of swimbladder as well as its relationship with lungs in tetrapods, transcriptomic analyses of zebrafish swimbladder were carried out by RNA-seq. Gene ontology classification showed that genes in cytoskeleton and endoplasmic reticulum were enriched in the swimbladder. Further analyses depicted gene sets and pathways closely related to cytoskeleton constitution and regulation, cell adhesion, and extracellular matrix. Several prominent transcription factor genes in the swimbladder including hoxc4a, hoxc6a, hoxc8a and foxf1 were identified and their expressions in developing swimbladder during embryogenesis were confirmed. By comparison of enriched transcripts in the swimbladder with those in human and mouse lungs, we established the resemblance of transcriptome of the zebrafish swimbladder and mammalian lungs. Based on the transcriptomic data of zebrafish swimbladder, the predominant functions of swimbladder are in its epithelial and muscular tissues. Our comparative analyses also provide molecular evidence of the relatedness of the fish swimbladder and mammalian lung.
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The vertebrate liver, pancreas and lung arise in close proximity from the multipotent foregut endoderm. Tissue-explant experiments uncovered instructive signals emanating from the neighbouring lateral plate mesoderm, directing the endoderm towards specific organ fates. This suggested that an intricate network of signals is required to control the specification and differentiation of each organ. Here, we show that sequential functions of Wnt2bb and Wnt2 control liver specification and proliferation in zebrafish. Their combined specific activities are essential for liver specification, as their loss of function causes liver agenesis. Conversely, excess wnt2bb or wnt2 induces ectopic liver tissue at the expense of pancreatic and anterior intestinal tissues, revealing the competence of intestinal endoderm to respond to hepatogenic signals. Epistasis experiments revealed that the receptor frizzled homolog 5 (fzd5) mediates part of the broader hepatic competence of the alimentary canal. fzd5 is required for early liver formation and interacts genetically with wnt2 as well as wnt2bb. In addition, lack of both ligands causes agenesis of the swim bladder, the structural homolog of the mammalian lung. Thus, tightly regulated spatiotemporal expression of wnt2bb, wnt2 and fzd5 is central to coordinating early liver, pancreas and swim bladder development from a multipotent foregut endoderm.
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Lung, cardiovascular system, liver and kidney are some examples for organs that develop ramified three-dimensional networks of epithelial tubes. The tube morphology affects flow rates of transported materials, such as liquids and gases. Therefore, it is important to understand how tube morphology is controlled. In Drosophila melanogaster many evolutionarily conserved genetic pathways have been shown to be involved in airway patterning. Recent studies identified a number of conserved mechanisms that drive Drosophila airway maturation, such as controlling tube size, barrier formation and lumen clearance. Genetically highly ordered branching modes previously have been found, also for mouse lung development. The understanding of tube patterning, outgrowth, ramification and maturation also is of clinical relevance, since many factors are evolutionarily conserved and may have similar functions in humans. This meeting report highlights novel findings concerning tube development in the fruit fly (D. melanogaster), the zebrafish (Danio rerio) and the laboratory mouse (Mus musculus).
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Primary ciliary dyskinesia (PCD; MIM 242650) is an autosomal recessive disorder of ciliary dysfunction with extensive genetic heterogeneity. PCD is characterized by bronchiectasis and upper respiratory tract infections, and half of the patients with PCD have situs inversus (Kartagener syndrome). We characterized the transcript and the genomic organization of the axonemal heavy chain dynein type 11 (DNAH11) gene, the human homologue of murine Dnah11 or lrd, which is mutated in the iv/iv mouse model with situs inversus. To assess the role of DNAH11, which maps on chromosome 7p21, we searched for mutations in the 82 exons of this gene in a patient with situs inversus totalis, and probable Kartagener syndrome associated with paternal uniparental disomy of chromosome 7 (patUPD7). We identified a homozygous nonsense mutation (R2852X) in the DNAH11 gene. This patient is remarkable because he is also homozygous for the F508del allele of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Sequence analysis of the DNAH11 gene in an additional 6 selected PCD sibships that shared DNAH11 alleles revealed polymorphic variants and an R3004Q substitution in a conserved position that might be pathogenic. We conclude that mutations in the coding region of DNAH11 account for situs inversus totalis and probably a minority of cases of PCD.
Homology-a similar trait shared by different species and derived from common ancestry, such as a seal's fin and a bird's wing-is one of the most fundamental yet challenging concepts in evolutionary biology. This groundbreaking book provides the first mechanistically based theory of what homology is and how it arises in evolution. Günter Wagner, one of the preeminent researchers in the field, argues that homology, or character identity, can be explained through the historical continuity of character identity networks-that is, the gene regulatory networks that enable differential gene expression. He shows how character identity is independent of the form and function of the character itself because the same network can activate different effector genes and thus control the development of different shapes, sizes, and qualities of the character. Demonstrating how this theoretical model can provide a foundation for understanding the evolutionary origin of novel characters, Wagner applies it to the origin and evolution of specific systems, such as cell types; skin, hair, and feathers; limbs and digits; and flowers. The first major synthesis of homology to be published in decades, Homology, Genes, and Evolutionary Innovation reveals how a mechanistically based theory can serve as a unifying concept for any branch of science concerned with the structure and development of organisms, and how it can help explain major transitions in evolution and broad patterns of biological diversity.
We report here on the macroscopic, light microscopic, and electron microscopic structure of the gas bladder (GB) of the spotted gar, Lepisosteus oculatus. The GB opens into the pharynx, dorsal to the opening of the oesophagus, through a longitudinal slit bordered by two glottal ridges. Caudal to the ridges, the GB is an elongated sac divided into a central duct and right and left lobes. The lobes are formed by a cranio-caudal sequence of large air spaces that open into the central duct. The structure of the GB is that of a membranous sac supported by a system of septa arising from the walls of a central duct. The septa contain variable amounts of striated and smooth muscle might function to maintain the bladder shape and in providing contractile capabilities. The presence of muscle cells, nerves, and neuroepithelial cells in the wall of the GB strongly suggests that GB function is tightly regulated. The central duct and the apical surface of the thickest septa are covered by mucociliated epithelium. Most of the rest of the inner bladder surface is covered by a respiratory epithelium which contains goblet cells and a single type of pneumocyte. These two cell types produce surfactant. The respiratory barrier contains thick areas with fibrillar material and cell prolongations, and thin areas that only contain basement membrane material between the capillary wall and the respiratory epithelium. Lungs and GBs share many anatomical and histological features. There appears to be no clear criterion for structural distinction between these two types of respiratory organs. J. Morphol., 2014. © 2014 Wiley Periodicals, Inc.
The presence of an air‐filled organ (AO), either lungs or a swimbladder, is a defining character of the Osteichthyes (bony vertebrates, including tetrapods). Despite the functional and structural diversity of AOs, it was not previously known whether the same group of developmental regulatory genes are involved in the early development of both lungs and swimbladders. This study demonstrates that a suite of genes (Nkx2.1, FoxA2, Wnt7b, GATA6), previously reported to be co‐expressed only in the tetrapod lung, is also co‐expressed in the zebrafish swimbladder. We document the expression pattern of these genes in the adult and developing zebrafish swimbladder and compare the expression patterns to those in the mouse lung. Early‐acting genes involved in endoderm specification are expressed in the same relative location and stage of AO development in both taxa (FoxA2 and GATA6), but the order of onset and location of expression are not completely conserved for the later acting genes (Nkx2.1 and Wnt7b). Co‐expression of this suite of genes in both tetrapod lungs and swimbladders of ray‐finned fishes is more likely due to common ancestry than independent co‐option, because these genes are not known to be co‐expressed anywhere except in the AOs of Osteichthyes. Any conserved gene product interactions may comprise a character identity network (ChIN) for the osteichthyan AO.
The application of homology varies depending on the data being examined. This volume represents a state-of-the-art treatment of the different applications of this unifying concept. Chapters deal with homology on all levels, from molecules to behavior, and are authored by leading contributors to systematics, natural history, and evolutionary, developmental, and comparative biology. This paperback reprint of the original hardbound edition continues to commemorate the 150th anniversary of Sir Richard Owens seminal paper distinguishing homology from analogy. Special features include: * Commemoration of the 150th anniversary of Sir Richard Owens seminal paper distinguishing homology from analogy * Contributors who are renowned leaders in comparative biology * Coverage that is both comprehensive and interdisciplinary.
Biological Systematics has changed dramatically during the past 60 years from a handicraft or art to an accepted branch of science proper, due to the work of Willi Hennig, who was born in 1913. The scientific method of reconstructing phylogenetic relationships of organisms bases on Hennig’s approach, the “Phylogenetic Systematics”. The method is now so widely accepted and applied that it can firmly be regarded a paradigm, named ‘cladistics’. In contrast, the life and personality of its founder is remarkably little known in the scientific community. The present book offers a detailed biography of Willi Hennig, and traces the roots of his thinking from his schooldays until his death in 1976. Some outstanding academic teachers and friends of his are introduced, too. The book offers an insight into the historical development of a ‘scientific revolution’, and highlights the life and the work of a ‘cautious revolutioniser’ in a Germany of dictatorship, war, and separation.
Cover illustration. Gas bladders of ray-finned fishes have long been regarded an evolutionary modification of lungs. Critical evidence for this hypothesized homology is whether pulmonary arteries supply the gas bladder as well as the lungs. In this issue of the Journal of Morphology, Longo et al. present a study (pp. 687–703) of the pattern of major arteries supplying lungs and gas bladders in ray finned-fish and lungfish. The cover image shows the 3D-reconstruction of the anterior arteries from a micro-CT scan of a sturgeon (Acipenser transmontanus), including the heart and gills (yellow). Injection with radiopaque barium prior to scanning facilitated visualization of the arterial vasculature (yellow, orange, and light red) and led to the discovery of vestigial pulmonary arteries in sturgeon and their close relatives, paddlefish. Image created in Avizo Fire 7.1 by Mark Riccio and edited by Sarah Longo.
The swimbladder of Pangusius sutchi first appears on the dorsal surface of the oesophagus at about 5 days after hatching. The swimbladder has double chambers when it is separated by a medial septum at 8–10 days. Alveoli start to develop and function in air-breathing at 12–14 days. Their number is increased by subdivision, and the respiratory portion grows towards the centre. Morphometric analysis shows that the swimbladder increases in respiratory surface, volume and surface area: volume ratio during development. On a histological basis, the development of the swimbladder is divided into three distinct periods: a blind tube, a double chamber and an alveolus period. It is characteristic that the flat epithelial cell arises from a primordial cuboidal cell and that a double capillary system is arranged in the interalveolar septa. Multilamellar bodies appear and a blood-air barrier is established when the swimbladder becomes functional.
The sonic hedgehog (Shh) pathway plays indispensable roles in the morphogenesis of mouse epithelial linings of the oral cavity and respiratory and digestive tubes. However, no enhancers that regulate regional Shh expression within the epithelial linings have been identified so far. In this study, comparison of genomic sequences across mammalian species and teleost fishes revealed three novel conserved non-coding sequences (CNCSs) that cluster in a region 600 to 900 kb upstream of the transcriptional start site of the mouse Shh gene. These CNCSs drive regional transgenic lacZ reporter expression in the epithelial lining of the oral cavity, pharynx, lung and gut. Together, these enhancers recapitulate the endogenous Shh expression domain within the major epithelial linings. Notably, genomic arrangement of the three CNCSs shows co-linearity that mirrors the order of the epithelial expression domains along the anteroposterior body axis. The results suggest that the three CNCSs are epithelial lining-specific long-range Shh enhancers, and that their actions partition the continuous epithelial linings into three domains: ectoderm-derived oral cavity, endoderm-derived pharynx, and respiratory and digestive tubes of the mouse. Targeted deletion of the pharyngeal epithelium specific CNCS results in loss of endogenous Shh expression in the pharynx and postnatal lethality owing to hypoplasia of the soft palate, epiglottis and arytenoid. Thus, this long-range enhancer is indispensable for morphogenesis of the pharyngeal apparatus.
Do new anatomical structures arise de novo, or do they evolve from pre-existing structures? Advances in developmental genetics, palaeontology and evolutionary developmental biology have recently shed light on the origins of some of the structures that most intrigued Charles Darwin, including animal eyes, tetrapod limbs and giant beetle horns. In each case, structures arose by the modification of pre-existing genetic regulatory circuits established in early metazoans. The deep homology of generative processes and cell-type specification mechanisms in animal development has provided the foundation for the independent evolution of a great variety of structures.
A recessive mutation was identified in a family of transgenic mice that resulted in a reversal of left-right polarity (situs inversus) in 100 percent of the homozygous transgenic mice tested. Sequences that flanked the transgenic integration site were cloned and mapped to mouse chromosome 4, between the Tsha and Hxb loci. During early embryonic development, the direction of postimplantation turning, one of the earliest manifestations of left-right asymmetry, was reversed in homozygous transgenic embryos. This insertional mutation identifies a gene that controls embryonic turning and visceral left-right polarity.
Lungs are the characteristic air-filled organs (AO) of the Polypteriformes, lungfish and tetrapods, whereas the swimbladder is ancestral in all other bony fish. Lungs are paired ventral derivatives of the pharynx posterior to the gills. Their respiratory blood supply is the sixth branchial artery and the venous outflow enters the heart separately from systemic and portal blood at the sinus venosus (Polypteriformes) or the atrium (lungfish), or is delivered to a separate left atrium (tetrapods). The swimbladder, on the other hand, is unpaired, and arises dorsally from the posterior pharynx. It is employed in breathing in Ginglymodi (gars), Halecomorphi (bowfin) and in basal teleosts. In most cases, its respiratory blood supply is homologous to that of the lung, but the vein drains to the cardinal veins. Separate intercardiac channels for oxygenated and deoxygenated blood are lacking. The question of the homology of lungs and swimbladders and of breathing mechanisms remains open. On the whole, air ventilatory mechanisms in the actinopterygian lineage are similar among different groups, including Polypteriformes, but are distinct from those of lungfish and tetrapods. However, there is extreme variation within this apparent dichotomy. Furthermore, the possible separate origin of air breathing in actinopterygian and 'sarcopterygian' lines is in conflict with the postulated much more ancient origin of vertebrate air-breathing organs. New studies on the isolated brainstem preparation of the gar (Lepisosteus osseus) show a pattern of efferent activity associated with a glottal opening that is remarkably similar to that seen in the in-vitro brainstem preparation of frogs and tadpoles. Given the complete lack of evidence for AO in chondrichthyans, and the isolated position of placoderms for which buoyancy organs of uncertain homology have been demonstrated, it is likely that homologous pharyngeal AO arose in the ancestors of early bony fish, and was pre-dated by behavioral mechanisms for surface (water) breathing. The primitive AO may have been the posterior gill pouches or even the modified gills themselves, served by the sixth branchial artery. Further development of the dorsal part may have led to the respiratory swimbladder, whereas the paired ventral parts evolved into lungs.
Convergence is an important evolutionary phenomenon often attributed solely to natural selection acting in similar environments. The frequency of mutation and number of ways a phenotypic trait can be generated genetically, however, may also affect the probability of convergence. Here we report both a high frequency of loss of gas bladder (swim bladder) mutations in zebrafish and widespread convergent loss of gas bladders among teleost fishes. The phenotypes of 22 of 27 recessive lethal mutations, carried by a sample of 26 wild-caught zebrafish, involve loss or noninflation of the gas bladder. Nine of these bladderless mutations showed no other obvious phenotypic abnormalities other than the lack of an inflated gas bladder. At least 19 of the 22 bladderless mutations are genetically distinct, as shown by unique morphology or complementation. Although we were not able to obtain eggs for all 21 required crosses to demonstrate definitively that the remaining three mutations are different from all other bladderless mutations, all available evidence suggests that these mutants are also distinct. At least 79 of 425 families of extant teleosts include one or more species lacking a gas bladder as adults. Analysis of the trait's phylogenetic distribution shows that the gas bladder has been lost at least 30-32 times independently. Although adaptive explanations for gas bladder loss are convincing, a developmental bias toward bladderless phenotypes may also have contributed to the widespread convergence of this trait among teleosts. If gas bladder development in teleosts is as vulnerable to genetic perturbation as it is in zebrafish, then perhaps a supply of bladderless phenotypes has been readily available to natural selection under conditions for which it is advantageous not to have a gas bladder. In this way, developmental bias and selection can work together to produce widespread convergence.
Homology is an essential idea of biology, referring to the historical continuity of characters, but it is also conceptually highly elusive. The main difficulty is the apparently loose relationship between morphological characters and their genetic basis. Here I propose that it is the historical continuity of gene regulatory networks rather than the expression of individual homologous genes that underlies the homology of morphological characters. These networks, here referred to as 'character identity networks', enable the execution of a character-specific developmental programme.
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Mutations in the DNAH11 (axonemal heavy chain dynein type 11) gene cause one form of situs inversus totalis and most likely primary ciliary dyskinesia
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Bartolini, L., J.-L. Blouin, Y. Pan, C. Gehrig, A.K. Maiti, N. Scamuffa, C. Rossier, M. Jorissen, M. Armengot, M. Meeks, H.M. Mitchison, E.M.K. Chung, C.D. Delozier-Blanchet, W.J. Craigen and S.E. Antonarakis. 2002. Mutations in the DNAH11 (axonemal heavy chain dynein type 11) gene cause one form of situs inversus totalis and most likely primary ciliary dyskinesia. Proc. Natl. Acad. Sci. USA 99: 10282-10286.
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