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At least 1,207 records · Page 67Linked to original sources
PubMed2006

A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales.

Extant baleen whales (Cetacea, Mysticeti) are all large filter-feeding marine mammals that lack teeth as adults, instead possessing baleen, and feed on small marine animals in bulk. The early evolution of these superlative mammals, and their unique feeding method, has hitherto remained enigmatic. Here, I report a new toothed mysticete from the Late Oligocene of Australia that is more archaic than any previously described. Unlike all other mysticetes, this new whale was small, had enormous eyes and lacked derived adaptations for bulk filter-feeding. Several morphological features suggest that this mysticete was a macrophagous predator, being convergent on some Mesozoic marine reptiles and the extant leopard seal (Hydrurga leptonyx). It thus refutes the notions that all stem mysticetes were filter-feeders, and that the origins and initial radiation of mysticetes was linked to the evolution of filter-feeding. Mysticetes evidently radiated into a variety of disparate forms and feeding ecologies before the evolution of baleen or filter-feeding. The phylogenetic context of the new whale indicates that basal mysticetes were macrophagous predators that did not employ filter-feeding or echolocation, and that the evolution of characters associated with bulk filter-feeding was gradual.

Animals↗
PubMed2003

Axonal injury in multiple sclerosis.

The pivotal role of axons in the pathophysiology and pathogenesis of multiple sclerosis (MS) is increasingly becoming the focus of our attention. Axonal injury, considered at one time to be a late phenomenon, is now recognized as an early occurrence in the inflammatory lesions of MS. There is converging evidence from histopathologic, as well as magnetic resonance imaging and magnetic resonance spectroscopy studies, that axons play a crucial and dynamic role during the evolution of MS pathology and the development of clinical disability. It has been repeatedly demonstrated that neurologic functional impairment correlates best with axonal, rather than myelin, injury. The pathophysiology of axonal injury remains speculative. Although generally considered to be sequelae of demyelination, it is possible that axonal injury in MS is indeed a primary event. The discovery that axonal injury can be reversible has provided an impetus to institute early therapy. The finding that irreversible axonal transection occurs in early lesions has underscored now, more than ever before, the need to curtail inflammation and the need to institute early treatment with disease-modifying agents. The axon will undoubtedly remain the focus of our attention regarding research on MS now and in the future.

Axons↗
PubMed2006

The cAMP response element binding protein (CREB) as an integrative HUB selector in metazoans: clues from the hydra model system.

In eukaryotic cells, a multiplicity of extra-cellular signals can activate a unique signal transduction system that at the nuclear level will turn on a variety of target genes, eliciting thus diverse responses adapted to the initial signal. How distinct signals can converge on a unique signalling pathway that will nevertheless produce signal-specific responses provides a theoretical paradox that can be traced back early in evolution. In bilaterians, the CREB pathway connects diverse extra-cellular signals via cytoplasmic kinases to the CREB transcription factor and the CBP co-activator, regulating according to the context, cell survival, cell proliferation, cell differentiation, pro-apoptosis, long-term memory, hence achieving a "hub" function for cellular and developmental processes. In hydra, the CREB pathway is highly conserved and activated during early head regeneration through RSK-dependent CREB phosphorylation. We show here that the CREB transcription factor and the RSK kinase are co-expressed in all three hydra cell lineages including dividing interstitial stem cells, proliferating nematoblasts, proliferating spermatogonia and spermatocytes, differentiating and mature neurons as well as ectodermal and endodermal myoepithelial cells. In addition, CREB gene expression is specifically up-regulated during early regeneration and early budding. When the CREB function was chemically prevented, the early post-amputation induction of the HyBraI gene was no longer observed and head regeneration was stacked. Thus, in hydra, the CREB pathway appears already involved in multiple tasks, such as reactivation of developmental programs in an adult context, self-renewal of stem cells, proliferation of progenitors and neurogenesis. Consequently, the hub function played by the CREB pathway was established early in animal evolution and might have contributed to the formation of an efficient oral pole through the integration of the neurogenic and patterning functions.

Animals↗
PubMed2004

Evolutionary convergence of alternative splicing in ion channels.

In Drosophila melanogaster and humans, members of three different ion-channel gene families share tandem exon duplications, which are alternatively spliced. In this article, I demonstrate that the duplication events that give rise to these mutually exclusive exons are unlikely to be ancestral but have probably occurred independently in different lineages. These events provide remarkable examples of evolutionary convergence in alternative splicing. The result has important implications for the analysis of regulation of alternative splicing using comparative genomics and our understanding of molecular evolution.

Alternative Splicing↗
PubMed1998

The future of genetic epidemiology.

Genetic epidemiology is a hybrid discipline whose ultimate aim is to identify and to characterize population-level factors that contribute to disease. Genetic epidemiologists often pursue this aim through the design and implementation of studies that simultaneously invoke principles in population genetics, epidemiology, molecular biology and biostatistics. However, traditional (and much contemporary) research in genetic epidemiology has barely tapped the potential that these disciplines have to work together. It is our view that future genetic epidemiology inquiry will benefit greatly from stronger integration of these disciplines and is likely to converge on themes in fields as diverse as demography, classical population and evolutionary genetics, pharmacoepidemiology, and ecology. The ultimate focus of this research will be evolution and maintenance of disease within and across populations.

Genetics, Population↗
PubMed2004

Posterior ischemic optic neuropathy: case report of a rare complication after general surgery.

BACKGROUND: Posterior ischemic optic neuropathy (PION) is a rare cause of bilateral severe and bilateral visual loss. Three mechanisms for posterior ischemic optic neuropathy are recognized: arteritic, non-arteritic, and peri- or postoperative. We report a case of bilateral postoperative posterior ischemic optic neuropathy. HISTORY AND SIGNS: A previously healthy 51-year-old man complained of the sudden onset of visual loss and unformed visual hallucinations in both eyes, 24 hours after an uneventful lumbar spine surgery. Duration of surgery was 12 hours with 2.5 liters of blood loss. At the time of visual loss, anemia was 74 g/L. Ophthalmic examination revealed decreased visual acuity to count fingers and normal optic disc without swelling or hemorrhages in both eyes. THERAPY AND OUTCOME: Despite blood transfusion 2 days after visual loss, evolution was stable. One month later, visual acuity was 0.15 RE and 0.2 LE with severe dyschromatopsia. Pupils reacted sluggishly to light but briskly to convergence. Optic discs were diffusely pale. DISCUSSION: Peri- and postoperative posterior ischemic optic neuropathy is a rare but devastating complication of surgery performed at distance from the visual pathways and its vascularization. Risk factors include: spine surgery, peri-operative hypotension, anemia, facial edema. Early recognition of posterior ischemic optic neuropathy and rapid correction of hypotension and/or anemia might improve outcome of posterior ischemic optic neuropathy, which usually is poor.

Anemia↗
PubMed2006

Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda).

Living archosaurs comprise birds (dinosaurs) and crocodylians (suchians). The morphological diversity of birds and stem group dinosaurs is tremendous and well-documented. Suchia, the archosaurian group including crocodylians, is generally considered more conservative. Here, we report a new Late Triassic suchian archosaur with unusual, highly specialized features that are convergent with ornithomimid dinosaurs. Several derived features of the skull and postcranial skeleton are identical to conditions in ornithomimids. Such cases of extreme convergence in multiple regions of the skeleton in two distantly related vertebrate taxa are rare. This suggests that these archosaurs show iterative patterns of morphological evolution. It also suggests that this group of suchians occupied the adaptive zone that was occupied by ornithomimosaurs later in the Mesozoic.

Adaptation, Physiological↗
PubMed1999

Conformational attractors on the Ramachandran map.

Frequency distributions of protein backbone dihedral angles phi and psi have been analyzed systematically for their apparent correlation with various crystallographic parameters, including the resolution at which the protein structures had been determined, the R factor and the free R factor, and the results have been displayed in novel differential Ramachandran maps. With improved sensitivity compared with conventionally derived heuristic Ramachandran maps, such differential maps automatically reveal conformational 'attractors' to which phi/psi distributions converge as the crystallographic resolution improves, as well as conformations tied specifically to low-resolution structures. In particular, backbone angular combinations associated with residues in alpha--helical conformation show a pronounced consolidation with substantially narrowed phi/psi distributions at higher (better) resolution. Convergence to distinct conformational attractors was also observed for all other secondary-structural types and random-coil conformations. Similar resolution-dependent phi/psi evolutions were obtained for different crystallographic refinement packages, documenting the absence of any significant artificial biases in the refinement programs investigated here. A comparison of differential Ramachandran maps derived for the R factor and the free R factor as independent parameters proved the better suitability of the free R factor for structure-quality assessment. The resolution-based differential Ramachandran map is available as a reference for comparison with actual protein structural data under WebMol, a Java-based structure viewing and analysis program (http://www. cmpharm.ucsf.edu/cgi-bin/webmol.pl).

Crystallography↗
PubMed1993

The evolution of chemoreception in squamate reptiles: a phylogenetic approach.

Recent advances in the field of squamate reptile chemoreception have been paralleled by the growth and preeminence of cladistics in the field of systematics, but for the most part, workers in the former have failed to incorporate the conceptual and informational advances of the latter. In this paper, I attempt a preliminary rapprochement by combining the methods of phylogenetic systematics and current hypotheses of squamate relationships with an overview of squamate chemosensory biology. This purely phylogenetic approach leads to a number of falsifiable generalizations about the evolution of chemoreception in squamates: 1) Evolution of this system is conservative rather than plastic, reflecting to a large extent suprafamilial attributes rather than adaptation to local conditions; 2) Anguimorphs are highly chemosensory and teiids show convergence with this group; 3) Tongue-flicking, a bifurcated tongue tip, a vomeronasal (VNO) mushroom body, and a complete circular muscle system in the tongue are a correlated character complex associated with the attainment, in squamates, of a direct VNO-oral connection and the loss of a VNO-nasal connection; 4) There is little support for a visual-chemosensory dichotomy within Squamata; 5) Gekkotans are allied with Autarchoglossa, both phylogenetically and in terms of chemosensory biology; 6) Iguania are highly variable in chemosensory development; iguanids represent the primitive iguanian condition, while agamids and chamaeleonids have secondarily reduced or lost their chemosensory abilities; 7) Apparent contradictions in chemosensory behavior among iguanids probably represent intrafamilial divergence; 8) Ecological correlates within Iguanidae and other taxa might be spurious, resulting from historical factors unrelated to the adaptations in question; 9) The mechanical demands of lingual food prehension have constrained chemosensory evolution in Iguania; chemosensory evolution within Scleroglossa was permitted by the liberation of the tongue from this ancestral role.

Amphibians↗
PubMed1992

Eukaryotic DNA replication.

The past decade has witnessed an exciting evolution in our understanding of eukaryotic DNA replication at the molecular level. Progress has been particularly rapid within the last few years due to the convergence of research on a variety of cell types, from yeast to human, encompassing disciplines ranging from clinical immunology to the molecular biology of viruses. New eukaryotic DNA replicases and accessory proteins have been purified and characterized, and some have been cloned and sequenced. In vitro systems for the replication of viral DNA have been developed, allowing the identification and purification of several mammalian replication proteins. In this review we focus on DNA polymerases alpha and delta and the polymerase accessory proteins, their physical and functional properties, as well as their roles in eukaryotic DNA replication.

Animals↗
PubMed1976

[Microscopic anatomy and muscle architecture of the commissure of the lips in the newborn infant and the adult].

The muscular structure of the commissura of the buccal orifice in humans confers on it a special significance. It consists of two layers each apparently having a well-defined function. Deep layer. Quite distinct at the embryonic stage, it develops rapidly in the new-born. It consists of the internal orbicular muscle, or Poirier's sphincter oris, and the two buccinator muscles. This layer, of functional importance for digestion, is formed by a central sphincter surrounded by two cavitary muscles. In the new-born, it forms a real suction apparatus. From this point of view, the internal orbicular and the two buccinator muscles are the equivalent respectively of the circular and longitudinal fibres of the muscular digestive tunic. This layer is more cavitary and sphincteral than platysmal. Superficial layer. Platysmal, muscular it is dependent on the cephalic superficial fascia, it is mimetic. This layer is made up of platysmal muscles converging on the commissura. The labial muscular network of the platysmal muscles forms the external orbicular. This layer, which is rudimentary in the new-born, develops during growth alongside relationships (mimic-phonation). It appears complete in adults. Facial superposition and commissural fusion of the deep and superficial layers confer a dual function of the buccal orifice: --digestive by means of the deep sphinctero-cavitary layer, --mimetic by means of the superficial platysmal layer. Wilst growth is taking place, the digestive layer in fact assumes mimetic function and the platysmal layer may intervene at will in the functioning of the digestive sphincter. The commissura labiorum is the anatomical site at which the cutaneous and digestive coats converge. Briefly, the buccal orifice is a digestive sphincter complete with mimetic function. Has not the face become a cortical organ in the course of evolution?

Adult↗
PubMed2000

Topological evolution of dynamical networks: global criticality from local dynamics.

We evolve network topology of an asymmetrically connected threshold network by a simple local rewiring rule: quiet nodes grow links, active nodes lose links. This leads to convergence of the average connectivity of the network towards the critical value K(c) = 2 in the limit of large system size N. How this principle could generate self-organization in natural complex systems is discussed for two examples: neural networks and regulatory networks in the genome.

Algorithms↗
PubMed1985

Evolution of crystallins: expression of lens-specific proteins in the blind mammals mole (Talpa europaea) and mole rat (Spalax ehrenbergi).

The mole (Talpa europaea; Insectivora) and the mole rat (Spalax ehrenbergi; Rodentia) both have degenerated eyes as a convergent adaptation to subterranean life. The rudimentary eye lenses of these blind mammals no longer function in a visual process. The crystallin genes, which display a lens-specific expression pattern, were studied in these blind mammals and in related species with normal eyes by hybridizing their genomic DNAs with probes obtained from cDNA clones for alpha A-, alpha B-, and beta Bp-crystallins from calf and gamma 3-crystallin from the rat. For all crystallin genes examined, the hybridization signals of mole and mole rat genomic DNA were comparable, respectively, with those of shrew and of rat and mouse, normal-vision representatives of the orders Insectivora and Rodentia. The expression of the crystallins at the protein level was tested by using antiserum specific for alpha-crystallin in immunofluorescence reactions on lens sections of mole and mole rat eyes and by using antisera against the beta- and gamma-crystallins on sections of the mole eye. All antisera gave positive fluorescence reactions exclusively with lens tissue of these blind mammals, indicating that the crystallins are still normally expressed despite the fact that these lenses have had no function in a visual process in these mammals for at least many million years. These findings apparently imply that some unknown selective advantage has conserved the crystallin genes and their expression after the loss of normal function of the lenses.

Adaptation, Physiological↗
PubMed1998

Putting more genetics into genetic algorithms.

The majority of current genetic algorithms (GAs), while inspired by natural evolutionary systems, are seldom viewed as biologically plausible models. This is not a criticism of GAs, but rather a reflection of choices made regarding the level of abstraction at which biological mechanisms are modeled, and a reflection of the more engineering-oriented goals of the evolutionary computation community. Understanding better and reducing this gap between GAs and genetics has been a central issue in an interdisciplinary project whose goal is to build GA-based computational models of viral evolution. The result is a system called Virtual Virus (VIV). VIV incorporates a number of more biologically plausible mechanisms, including a more flexible genotype-to-phenotype mapping. In VIV the genes are independent of position, and genomes can vary in length and may contain noncoding regions, as well as duplicative or competing genes. Initial computational studies with VIV have already revealed several emergent phenomena of both biological and computational interest. In the absence of any penalty based on genome length, VIV develops individuals with long genomes and also performs more poorly (from a problem-solving viewpoint) than when a length penalty is used. With a fixed linear length penalty, genome length tends to increase dramatically in the early phases of evolution and then decrease to a level based on the mutation rate. The plateau genome length (i.e., the average length of individuals in the final population) generally increases in response to an increase in the base mutation rate. When VIV converges, there tend to be many copies of good alternative genes within the individuals. We observed many instances of switching between active and inactive genes during the entire evolutionary process. These observations support the conclusion that noncoding regions serve as scratch space in which VIV can explore alternative gene values. These results represent a positive step in understanding how GAs might exploit more of the power and flexibility of biological evolution while simultaneously providing better tools for understanding evolving biological systems.

Algorithms↗
PubMed1976

Structural origins of mammalian albumin.

The amino acid sequence and disulfide bridges of bovine serum albumin reveal nine double loops formed by the bridges. The pattern and size of loops and connecting segments between loops indicate that a basic repeating unit (domain) consists of "large double loop-short connecting segment-small double loop-long connecting segment-large double loop-connecting segment between "domains" and repeats exactly three times. Thus, albumin arose by duplication of the primordial single domain gene, followed some time later by a half-gene duplication to give the ancestor of the present triple domain structure. These duplications are estimated to have occurred about 700 million years ago, based on the differences between domain (75 to 82%), the difference between bovine and human albumins (20%),and reported time estimates for globin and immunoglobulin evolution. Still farther back in time, the single domain arose by triplication of a gene of a primordial subdomain of about 77 amino acid residues, consisting of one large double loop with a segment on each end. The structure resulting from this triplication consisted of three large loops and three long connecting segments. Later, a gene deletion for part of a connecting segment and part of the middle large loop occurred to produce the large-small-large loop pattern. A proposed three dimensional structure for the "subdomain" shows spatial and sequence similarity with the G-H helical regions of myoglobin or hemoglobin, suggesting that it arose by duplication and separation of a gene for a C-terminal segment of a primitive globin. The CYS residues were introduced as an adaptive, convergent evolutionary event after separation from globin, but before duplication of the subdomain gene.

Amino Acid Sequence↗
PubMed1986

Character displacement and niche shift analyzed using consumer-resource models of competition.

This paper analyzes the adaptive responses to competition (both character displacement and niche shift) in a two consumer-two resource model. The model includes density dependence that is unrelated to the resources that are explicit in the model. This could be due to another resource dimension, parasites, or interference competition. Competitors adapt by changing their relative consumption rate constants on the two resource types. This model can result in mutually divergent, parallel, or mutually convergent displacement of competitors. Parallel displacement may entail net divergence, net convergence, or no net change. Parallel change with net convergence is most likely when the competitors have similar constraints on the possible values of consumption rate constants, unequal allopatric abundances, and significant intraspecific density dependence. Numerical calculations of displacements are presented for several models and the effect of a number of different possible alterations of the model are discussed. The evolution of resource handling and processing efficiency, and displacement in the presence of additional selective pressures on the character are considered in detail. The results have implications for questions about maximization of population size, the relationship of character displacement and the competition coefficient, and "null" models in the study of competition. Differences between this and previous theoretical works are discussed. It is argued that conditions allowing parallel or convergent displacement are not biologically unlikely, and possible examples are discussed. Data on resource partitioning seem to be more consistent with the results reached here than with previous theory.

Animals↗
PubMed2006

Sex determination: ways to evolve a hermaphrodite.

Most species of the nematode genus Caenorhabditis reproduce through males and females; C. elegans and C. briggsae, however, produce self-fertile hermaphrodites instead of females. These transitions to hermaphroditism evolved convergently through distinct modifications of germline sex determination mechanisms.

Animals↗
PubMed2003

Redundant representations in evolutionary computation.

This paper discusses how the use of redundant representations influences the performance of genetic and evolutionary algorithms. Representations are redundant if the number of genotypes exceeds the number of phenotypes. A distinction is made between synonymously and non-synonymously redundant representations. Representations are synonymously redundant if the genotypes that represent the same phenotype are very similar to each other. Non-synonymously redundant representations do not allow genetic operators to work properly and result in a lower performance of evolutionary search. When using synonymously redundant representations, the performance of selectorecombinative genetic algorithms (GAs) depends on the modification of the initial supply. We have developed theoretical models for synonymously redundant representations that show the necessary population size to solve a problem and the number of generations goes with O(2(kr)/r), where kr is the order of redundancy and r is the number of genotypic building blocks (BB) that represent the optimal phenotypic BB. As a result, uniformly redundant representations do not change the behavior of GAs. Only by increasing r, which means overrepresenting the optimal solution, does GA performance increase. Therefore, non-uniformly redundant representations can only be used advantageously if a-priori information exists regarding the optimal solution. The validity of the proposed theoretical concepts is illustrated for the binary trivial voting mapping and the real-valued link-biased encoding. Our empirical investigations show that the developed population sizing and time to convergence models allow an accurate prediction of the empirical results.

Algorithms↗