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Coalescence
Kinds of Coalescence Terms modified by Coalescence Selected AbstractsCOALESCENCE IN A METAPOPULATION WITH RECURRENT LOCAL EXTINCTION AND RECOLONIZATIONEVOLUTION, Issue 5 2003John R. Pannell Abstract Many species exist as metapopulations in balance between local population extinction and recolonization. The effect of these processes on average population differentiation, within-deme diversity, and specieswide diversity has been considered previously. In this paper, coalescent simulations of Slatkin's propagule-pool and migrant-pool models are used to characterize the distribution of neutral genetic diversity within demes (,s), diversity in the metapopulation a whole (TTT), the ratio FST= (,t,,S)/,T, Tajima's D statistic, and several ratios of gene-tree branch lengths. Using these distributions, power to detect differences in key metapopulation parameter values is determined under contrasting sampling regimes. The results indicate that it will be difficult to use sequence data from a single locus to detect a history of extinctions and recolonizations in a metapopulation because of high genealogical variance, the loss of diversity due to reductions in effective population size, and the fact that a genealogy of lineages from different demes under Slatkin's model differs from a neutral coalescent only in its time scale. Genetic indices of gene-tree shape that capture the effects of extinction/recolonization on both external branches and the length of the genealogy as a whole will provide the best indication of metapopulation dynamics if several lineages are sampled from several different demes. [source] COALESCENCE VERSUS COMPETITION: FIELD AND LABORATORY STUDIES OF INTRA- AND INTERSPECIFIC ENCOUNTERS AMONG COALESCING SEAWEEDSJOURNAL OF PHYCOLOGY, Issue 2000B. Santelices Classical ecological theory predicts that whenever growing individuals share a common and limiting resource, such as substratum in mid-intertidal and shallow subtidal habitats, preemptive competition will occur determining species abundance and distribution patterns. However, conspecificity of several ecologically dominant Rhodophyta may coalesce when grown in laboratory cultures. The extent at which intraspecific coalescence occurs in the field and whether the process may also happens during interspecific encounters remain to be determined. If intra- and interspecific coalescence effectively occurs, then coexistence through coalescence rises as an alternative to competition among red-algal dominated intertidal and shallow subtidal communities. Populations of Mazzaella laminarioides and Nothogenia fastigiata living in mid-intertidal, semi-exposed rocky habitats in Central Chile are being used to test the above ideas. Intra- and interspecific encounters occur in the field throughout the year. Coalescence does occur among conspecific partners but it has not been detected in interspecific encounters. Rather, a thick interface of compressed cells, necrotic tissues and cyanobacterial nodules is formed between the two contacting partners. In addition, observations of laboratory cultures indicate that spore germination, germling survival and differentiation of erect axes in bispecific cultures may be reduced when compared to single-species controls. Interspecific differences in growth and differentiation rates appear as the mechanisms explaining a lack of coalescence and negative effects during interspecific contacts. On the other hand, the existence of conspecific coalescence in the field suggests this process should be considered as a real alternative to intraspecific competition among coalescing Rhodophyta. [source] Preparing to move: Assembly of the MSP amoeboid motility apparatus during spermiogenesis in AscarisCYTOSKELETON, Issue 4 2005Maria Antonia Rodriguez Abstract We exploited the rapid, inducible conversion of non-motile Ascaris spermatids into crawling spermatozoa to examine the pattern of assembly of the MSP motility apparatus that powers sperm locomotion. In live sperm, the first detectable motile activity is the extension of spikes and, later, blebs from the cell surface. However, examination of cells by EM revealed that the formation of surface protrusions is preceded by assembly of MSP filament tails on the membranous organelles in the peripheral cytoplasm. These organelle-associated filament meshworks assemble within 30 sec after induction of spermiogenesis and persist until the membranous organelles are sequestered into the cell body when the lamellipod extends. The filopodia-like spikes, which are packed with bundles of filaments, extend and retract rapidly but last only a few seconds before giving way to, or converting into, blebs. Coalescence of these blebs, each supported by a dense mesh of filaments, often initiates lamellipod extension, which culminates in the formation of the robust, dynamic MSP fiber complexes that generate sperm motility. The same membrane phosphoprotein that orchestrates assembly of the fiber complexes at the leading edge of the lamellipod of mature sperm is also found at all sites of filament assembly during spermiogenesis. The orderly progression of steps that leads to construction of a functional motility apparatus illustrates the precise spatio-temporal control of MSP filament assembly in the developing cell and highlights the remarkable similarity in organization and plasticity shared by the MSP cytoskeleton and the actin filament arrays in conventional crawling cells. Cell Motil. Cytoskeleton 60:191,199, 2005 © 2005 Wiley-Liss, Inc. [source] Nanowire-Templated Lateral Epitaxial Growth of Low-Dislocation Density Nonpolar a -Plane GaN on r -Plane SapphireADVANCED MATERIALS, Issue 23 2009Qiming Li Coalescence of a vertically aligned GaN nanowire array on r -plane sapphire, a technique called nanowire-templated lateral epitaxial growth, is used to grow low-dislocation density a -plane GaN. The resulting film is connected to the lattice-mismatched substrate by nanowires, which facilitates dramatic strain relaxation and leads to a significant reduction in defects. [source] Coalescence of deformable granules in wet granulation processesAICHE JOURNAL, Issue 3 2000L. X. Liu In this work, the coalescence of deformable granules in wet granulation processes is modelled. The model accounts for both the mechanical properties of the granules and the effect of the liquid layer at the granule surface. It is an extension to the model of Ennis et al. (1991) to include the possibility of granule plastic deformation during collisions. The model is written in dimensionless groups such as viscous and deformation Stokes numbers and the ratio of granule dynamic yield strength to granule Young's modulus (Yd/E*). These variables are bulk parameters of the powder-binder mixture and also functions of the process intensity. The model gives the conditions for two types of coalescence,type I and type II. Type I coalescence occurs when granules coalesce by viscous dissipation in the surface liquid layer before their surfaces touch. Type II coalescence occurs when granules are slowed to a halt during rebound, after their surfaces have made contact. The model explains some of the trends observed in the literature, are preliminary validation of the coalescence criterion with drum granulation data is encouraging. An extension is also made to the case of surface dry granules, where liquid is squeezed to the surface during granule deformation. [source] COALESCENCE VERSUS COMPETITION: FIELD AND LABORATORY STUDIES OF INTRA- AND INTERSPECIFIC ENCOUNTERS AMONG COALESCING SEAWEEDSJOURNAL OF PHYCOLOGY, Issue 2000B. Santelices Classical ecological theory predicts that whenever growing individuals share a common and limiting resource, such as substratum in mid-intertidal and shallow subtidal habitats, preemptive competition will occur determining species abundance and distribution patterns. However, conspecificity of several ecologically dominant Rhodophyta may coalesce when grown in laboratory cultures. The extent at which intraspecific coalescence occurs in the field and whether the process may also happens during interspecific encounters remain to be determined. If intra- and interspecific coalescence effectively occurs, then coexistence through coalescence rises as an alternative to competition among red-algal dominated intertidal and shallow subtidal communities. Populations of Mazzaella laminarioides and Nothogenia fastigiata living in mid-intertidal, semi-exposed rocky habitats in Central Chile are being used to test the above ideas. Intra- and interspecific encounters occur in the field throughout the year. Coalescence does occur among conspecific partners but it has not been detected in interspecific encounters. Rather, a thick interface of compressed cells, necrotic tissues and cyanobacterial nodules is formed between the two contacting partners. In addition, observations of laboratory cultures indicate that spore germination, germling survival and differentiation of erect axes in bispecific cultures may be reduced when compared to single-species controls. Interspecific differences in growth and differentiation rates appear as the mechanisms explaining a lack of coalescence and negative effects during interspecific contacts. On the other hand, the existence of conspecific coalescence in the field suggests this process should be considered as a real alternative to intraspecific competition among coalescing Rhodophyta. [source] Microstructure in Silicon Nitride Containing ,-Phase Seeding: III, Grain Growth and CoalescenceJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2001Horng-Hwa Lu The mechanical properties of Si3N4 materials depend mainly on the microstructure, which originates during the densification process. The microscopic evidence indicates that ,-Si3N4 seeds incorporated in the starting powders play an important role in microstructural development, especially in the heterogeneous grain growth of ,-Si3N4 grains during sintering. The growth of ,-grains is initiated from the ,-seeds, resulting in a core/shell microstructure. The presence of Moiré fringes and dislocations is attributed to misfit strain and compositional differences between the core and the shell. Coalescence can occur at the final stage of sintering. [source] Coagulation of Carboxylic Acid-Functionalized LatexesMACROMOLECULAR SYMPOSIA, Issue 1 2008Adélia Santos Abstract In the present work, the stability of particles produced by emulsion polymerization and stabilized by carboxylic acid groups was studied from turbidity measurements. To achieve this, a number of copolymerization runs was carried out under different reaction conditions, including the use of different carboxylic monomers. Partitioning analyses using conductimetric and potentiometric titrations were performed in order to assess the distribution of carboxylic monomers among the main phases of the produced latexes. Additionally, the stability and coalescence of particles were measured by turbidimetry in a diluted latex considering either the presence or not of the anionic surfactant sodium lauryl sulfate. Coalescence of particles was provoked in the latex samples at different temperatures by addition of an aliquot of a concentrated solution of electrolyte. The influence of surfactant, temperature and type of carboxylic acid group on the particle stability was investigated. [source] Coalescence and breakage processesMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 9 2002Christoph Walker We extend a model for coalescence and breakage of liquid,liquid dispersions proposed by Fasano/Rosso. The main feature is that the experimentally observed fact of a maximal droplet mass is taken into account. Our model includes spontaneous breakage as well as collisional fragmentation. Existence and uniqueness of solutions is proved and the long-time behaviour is investigated. Copyright © 2002 John Wiley & Sons, Ltd. [source] Recent invasion of the tropical Atlantic by an Indo-Pacific coral reef fishMOLECULAR ECOLOGY, Issue 13 2005LUIZ A. ROCHA Abstract The last tropical connection between Atlantic and Indian,Pacific habitats closed c. 2 million years ago (Ma), with the onset of cold-water upwelling off southwestern Africa. Yet comparative morphology indicates more recent connections in several taxa, including reef-associated gobies (genus Gnatholepis). Coalescence and phylogenetic analyses of mtDNA cytochrome b sequences demonstrate that Gnatholepis invaded the Atlantic during an interglacial period ,145 000 years ago (d = 0.0054), colonizing from the Indian Ocean to the western Atlantic, and subsequently to the central (,100 000 years ago) and eastern Atlantic (,30 000 years ago). Census data show a contemporary range expansion in the northeastern Atlantic linked to global warming. [source] Binary coalescence of air bubbles in viscous liquids in presence of non-ionic surfactantTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2008K. Giribabu Abstract Coalescence of air bubbles is important in gas,liquid reactors and food processing operations. Bubbles can be stabilized by using non-ionic surfactants. Binary coalescence of air bubbles in ethylene glycol and aqueous glycerol solutions were studied in this work in presence of Span 80. A novel set-up was developed to study long coalescence times. Coalescence time was observed to follow broad stochastic distributions in all systems. The distributions were fitted with a stochastic model developed earlier. The surface tension of ethylene glycol and glycerol solutions was measured at various concentrations of Span 80. These data were fitted using a surface equation of state derived from the Langmuir isotherm. The effect of surfactant concentration on coalescence time was explained in terms of the surface excess of the surfactant and the repulsive force generated at the air,liquid interface. The results from this work illustrate the stochastic nature of bubble coalescence in viscous liquids. This work also demonstrates how non-ionic surfactants can stabilize bubbles in such liquids. La coalescence des bulles d'air est importante dans les réacteurs gaz-liquide et les opérations de l'industrie alimentaire. Les bulles peuvent être stabilisées en utilisant des surfactants non ioniques. La coalescence binaire de bulles d'air dans des solutions aqueuses d'éthylène glycol et de glycérol a été étudiée dans ce travail en présence de Span 80. Un nouveau montage a été mis au point pour caractériser les temps de coalescence longs. Le temps de coalescence a été observé afin de suivre les distributions de modèle stochastique dans tous les systèmes. Les distributions ont été calées à un modèle stochastique mis au point antérieurement. La tension de surface des solutions d'éthylène glycol et de glycérol a été mesurée à différentes concentrations de Span 80. Ces données ont été calées à l'aide d'une équation d'état de surface calculée à partir de l'isotherme de Langmuir. L'effet de la concentration de surfactant sur le temps de coalescence est expliqué par l'excès de surface du surfactant et la force répulsive créée à l'interface air-liquide. Les résultats de ce travail illustrent la nature stochastique de la coalescence des bulles dans les liquides visqueux. Ce travail démontre également comment les surfactants non ioniques peuvent stabiliser les bulles dans de tels liquides. [source] Combined PIV/PTV-Measurements for the Analysis of Bubble Interactions and Coalescence in a Turbulent FlowTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2003Dirk Bröder Abstract In order to allow more reliable modeling of coalescence processes in turbulent bubbly flows, detailed experiments in a double loop reactor were performed. Narrow and essentially monomodal bubble size distributions in the range of 2 to 4 mm were created. For simultaneous measurements of bubble size, bubble velocity and liquid velocity a combined system of PIV and PTV was developed and applied. It was possible to determine bubble size distributions and mean, as well as fluctuating velocities for both phases. The spatial changes of the bubble size distribution, due to the influence of bubble coalescence, was analyzed and coalescence rates were calculated. Afin d'améliorer la fiabilité de la modélisation des procédés de coalescence dans les écoulements bouillonnants turbulents, des expériences poussées ont été menées dans un réacteur à double boucle. Des distributions de taille des bulles étroites, essentiellement monodisperses, dans la gamme de 2 à 4 mm, ont été créées. Pour les mesures simultanées de la taille des bulles, de la vitesse des bulles et de la vitesse du liquide, on a mis au point et appliqué un système combiné de PIV et PTV. Il a été possible de déterminer les distributions de taille des bulles et la moyenne mais également les vitesses fluctuantes pour les deux phases. Les changements spatiaux de la distribution de la taille des bulles dus à l'influence de la coalescence des bulles ont été analysés et les vitesses de coalescence ont été calculées. [source] Tuneable Control of Interfacial Rheology and Emulsion CoalescenceCHEMPHYSCHEM, Issue 5 2009Andrew S. Malcolm Abstract Breaking point: Switchable peptide surfactants are used to demonstrate that the extent of cross-linking in an interfacial surfactant layer can control the rate of emulsion coalescence. Pictured is the rupture of an aqueous thin film where the peptide layer lacks sufficient strength to prevent hole formation, but nonetheless dramatically slows the rate of hole expansion. [source] splatche: a program to simulate genetic diversity taking into account environmental heterogeneityMOLECULAR ECOLOGY RESOURCES, Issue 1 2004M. Currat Abstract We present a program called splatche (SPatiaL And Temporal Coalescences in Heterogeneous Environments) to simulate the molecular diversity of samples of genes in an environmentally heterogeneous world. Simulations are performed by, first, simulating the colonization of the world using environmental information to constrain migrations and local densities. These simulated densities and migration rates recorded over time and space are then used to simulate genetic diversity under a coalescent framework. The program thus virtually allows the translation of ecological information into molecular diversity, a novel approach that can be used to study the effect of climatic change on genetic diversity. [source] On slow protein crystal nucleation: cluster-cluster aggregation on diffusional encountersCRYSTAL RESEARCH AND TECHNOLOGY, Issue 1 2009C. N. NanevArticle first published online: 7 NOV 200 Abstract With a view to experimental results on protein crystal nucleation the effects of cluster coalescence are probed semi-quantitatively. The steric association restriction, which stems from the patchy surface of the protein molecules, explains both experimentally measured low crystal nucleation rate and coalescence limitations for crystalline clusters of protein molecules. The conclusion is that due to its action, and the impact of rotational diffusion, the coalescence of critical (and/or supercritical) clusters should be rejected as a conceivable alternative for explaining the slow nucleation of protein crystals. Besides, the analysis of cluster-cluster aggregation on diffusional encounters may be of more general interest; it may be helpful by considering the coalescence of structured bio-nano-particles. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Recovering True Selves in the Electro-Spiritual Field of Universal LoveCULTURAL ANTHROPOLOGY, Issue 2 2004Nickola Pazderic ABSTRACT In Taiwan, the quasi-religious practice of Heqi reveals a complex relationship between the neoliberal demand for success; conceptions of energy and love; technologies of audio transmission; reception, and recording; and the production of modern selves. A transnational coalescence of psychoanalysis and Heqi as both theory and practice produces modern, properly cultured subjects fully in tune with the prevailing demands of global capitalism. Furthermore, these therapies and their explanatory discourses reflect, as much as they describe, globally salient audio technologies (such as radio). [source] Development of lymphatic vessels in mouse embryonic and early postnatal heartsDEVELOPMENTAL DYNAMICS, Issue 10 2008Juszy, Micha Abstract We aimed to study the spatiotemporal pattern of lymphatic system formation in the embryonic and early postnatal mouse hearts. The first sign of the development of lymphatics are Lyve-1,positive cells located on the subepicardial area. Strands of Lyve-1,positive cells occur first along the atrioventricular sulcus of the diaphragmatic surface and then along the great arteries. Lumenized tubules appear, arranged in rows or in a lattice. They are more conspicuous in dorsal atrioventricular junction, along the major venous and coronary artery branches and at the base of the aorta and the pulmonary trunk extending toward the heart apex. At later stages, some segments of the lymphatic vessels are partially surrounded by smooth muscle cells. Possible mechanisms of lymphangiogenesis are: addition of Lyve-1,positive cells to the existing tubules, elongation of the lymphatic lattice, sprouting and coalescence of tubules. We discuss the existence of various subpopulations of endothelial cells among the Lyve-1,positive cells. Developmental Dynamics 237:2973,2986, 2008. © 2008 Wiley-Liss, Inc. [source] MafA transcription factor identifies the early ret-expressing sensory neuronsDEVELOPMENTAL NEUROBIOLOGY, Issue 7 2010Laure Lecoin Abstract Dorsal root ganglia proceed from the coalescence of cell bodies of sensory neurons, which have migrated dorsoventrally from the delaminating neural crest. They are composed of different neuronal subtypes with specific sensory functions, including nociception, thermal sensation, proprioception, and mechanosensation. In contrast to proprioceptors and thermonociceptors, little is known about the molecular mechanisms governing the early commitment and later differentiation into mechanosensitive neurons. This is mainly due to the absence of specific molecular markers for this particular cell type. Using knockout mice, we identified the bZIP transcription factor MafA as the first specific marker of a subpopulation of "early c-ret" positive neurons characterized by medium-to-large diameters. This marker will allow further functional characterization of these neurons. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70:485,497, 2010 [source] Earth hummocks (thúfur): new insights to their thermal characteristics and development in eastern Lesotho, southern AfricaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 5 2005Stefan W. Grab Abstract The aspect-controlled variations in soil freezing within earth hummocks of eastern Lesotho (southern Africa) are analysed. Ground thermal data were measured for an earth hummock from late autumn to early spring in 1995 and 1996, using TinytalkÔ data loggers. During 1995, ground temperatures were recorded at 15 and 20 cm depth on the hummock north, east, south and west aspects, whilst in 1996 temperatures were recorded at 1 cm, 5 cm and 10 cm on the north and south aspects. The data from 1995 indicate that soil freezing commences on the hummock southern aspects and gradually progresses towards the western and northern aspects, whilst the eastern aspect remained unfrozen throughout winter. The data from 1996 indicate that a thick snow cover almost nullifies the temperature differences between the hummock northern and southern aspects. However, given the relative absence of snow during contemporary winters, freeze intensity and duration is longest on the hummock southern and western aspects, which helps explain earth hummock deformation (elongation and coalescence) in a southwesterly direction on slope gradients ,3°. Copyright © 2005 John Wiley & Sons, Ltd. [source] Channels, wetlands and islands in the Okavango Delta, Botswana, and their relation to hydrological and sedimentological processesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 1 2004T. Gumbricht Abstract The Okavango wetland in northern Botswana is one of the world's largest inland deltas. The delta is a dynamic environment with shifting channel routes, causing growth and decay of ,anking wetlands, and giving birth to islands. Primary island nuclei are formed by ,uvial processes and bioengineering, and subsequently grow into secondary larger islands of irregular shape by clastic and chemical sedimentation, and later by coalescence. This article presents classi,cations and quantitative estimations of channels, wetlands and islands of the Okavango Delta. Islands were classi,ed dependent on composition, pattern of composition, shape and juxtaposition. 90 per cent of all islands in the entire wetland were identi,ed, with a classi,cation accuracy of 60 to 85 per cent. Smaller islands of the nucleus types dominate the upper parts of the delta, whereas larger secondary islands are more common in the distal part, a re,ection of the age of the islands. Islands in the entry valley of the delta, the Panhandle, are larger in the top end , the primary region of recent clastic sedimentation. The overall size distribution of islands in the delta, however, shows no clumps, indicating that island growth is a uniform process over time and space. The total area ,ooded at least every decade is approximately 14 000 km2, of which 9000 km2 is classi,ed as actual wetland. Channel meandering decreases from the Panhandle to the distal part of the delta, with the abandoned Thaoge channel as an exception. Occurrence of ,uvially formed islands in the distal delta indicates that the water ,ow and area of inundation must once have been much larger. Copyright © 2004 John Wiley & Sons, Ltd. [source] Droplet fusion by alternating current (AC) field electrocoalescence in microchannelsELECTROPHORESIS, Issue 19 2005Max Chabert Abstract We present a system for the electrocoalescence of microfluidic droplets immersed in an immiscible solvent, where the undeformed droplet diameters are comparable to the channel diameter. The electrodes are not in direct contact with the carrier liquid or the droplets, thereby minimizing the risk of cross-contamination between different coalescence events. Results are presented for the coalescence of buffered aqueous droplets in both quiescent and flowing fluorocarbon streams, and on-flight coalescence is demonstrated. The capillary-based system presented here is readily amenable to further miniaturization to any lab-on-a-chip application where the conductivity of the droplets is much greater than the conductivity of the stream containing them, and should aid in the further application of droplet microreactors to biological analyses. [source] NMR Quantification of Tautomeric Populations in Biogenic Purine BasesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 9 2009Bartl Abstract Purine bases such as purine, adenine, hypoxanthine, and mercaptopurine are known to exist in several tautomeric forms. Characterization of their tautomeric equilibria is important not only for predicting the regioselectivity of their N -alkylation reactions, but also for gaining knowledge of the patterns with which these compounds of significant biological activity form hydrogen bonds with their biological targets. The tautomeric equilibria of purine and some purine derivatives in methanol and N,N -dimethylformamide solutions were investigated by low-temperature 1H and 13C NMR spectroscopy. The N(7)H and N(9)H tautomeric forms were quantified by integrating the individual 1H NMR signals at low temperatures. The Gibbs free energy differences were calculated and the effects of substitution on the N(7)H/N(9)H ratio discussed. A previously published theoretically predicted mechanism of the tautomeric exchange is compared with our measurements in deuteriated solvents. The influence of concentration on the temperature of coalescence indicates that supramolecular clusters play a significant role in this proton transfer process. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] THE COMBINED EFFECTS OF RIVERS AND REFUGIA GENERATE EXTREME CRYPTIC FRAGMENTATION WITHIN THE COMMON GROUND SKINK (SCINCELLA LATERALIS)EVOLUTION, Issue 2 2010Nathan D. Jackson Rivers can act as both islands of mesic refugia for terrestrial organisms during times of aridification and barriers to gene flow, though evidence for long-term isolation by rivers is mixed. Understanding the extent to which riverine barrier effects can be heightened for populations trapped in mesic refugia can help explain maintenance and generation of diversity in the face of Pleistocene climate change. Herein, we implement phylogenetic and population genetic approaches to investigate the phylogeographic structure and history of the ground skink, Scincella lateralis, using mtDNA and eight nuclear loci. We then test several predictions of a river,refugia model of diversification. We recover 14 well-resolved mtDNA lineages distributed east,west along the Gulf Coast with a subset of lineages extending northward. In contrast, ncDNA exhibits limited phylogenetic structure or congruence among loci. However, multilocus population structure is broadly congruent with mtDNA patterns and suggests that deep coalescence rather than differential gene flow is responsible for mtDNA,ncDNA discordance. The observed patterns suggest that most lineages originated from population vicariance due to riverine barriers strengthened during the Plio,Pleistocene by a climate-induced coastal distribution. Diversification due to rivers is likely a special case, contingent upon other environmental or biological factors that reinforce riverine barrier effects. [source] ESTIMATING A GEOGRAPHICALLY EXPLICIT MODEL OF POPULATION DIVERGENCEEVOLUTION, Issue 3 2007L. Lacey Knowles Patterns of genetic variation can provide valuable insights for deciphering the relative roles of different evolutionary processes in species differentiation. However, population-genetic models for studying divergence in geographically structured species are generally lacking. Since these are the biogeographic settings where genetic drift is expected to predominate, not only are population-genetic tests of hypotheses in geographically structured species constrained, but generalizations about the evolutionary processes that promote species divergence may also be potentially biased. Here we estimate a population-divergence model in montane grasshoppers from the sky islands of the Rocky Mountains. Because this region was directly impacted by Pleistocene glaciation, both the displacement into glacial refugia and recolonization of montane habitats may contribute to differentiation. Building on the tradition of using information from the genealogical relationships of alleles to infer the geography of divergence, here the additional consideration of the process of gene-lineage sorting is used to obtain a quantitative estimate of population relationships and historical associations (i.e., a population tree) from the gene trees of five anonymous nuclear loci and one mitochondrial locus in the broadly distributed species Melanoplus oregonensis. Three different approaches are used to estimate a model of population divergence; this comparison allows us to evaluate specific methodological assumptions that influence the estimated history of divergence. A model of population divergence was identified that significantly fits the data better compared to the other approaches, based on per-site likelihood scores of the multiple loci, and that provides clues about how divergence proceeded in M. oregonensis during the dynamic Pleistocene. Unlike the approaches that either considered only the most recent coalescence (i.e., information from a single individual per population) or did not consider the pattern of coalescence in the gene genealogies, the population-divergence model that best fits the data was estimated by considering the pattern of gene lineage coalescence across multiple individuals, as well as loci. These results indicate that sampling of multiple individuals per population is critical to obtaining an accurate estimate of the history of divergence so that the signal of common ancestry can be separated from the confounding influence of gene flow,even though estimates suggest that gene flow is not a predominant factor structuring patterns of genetic variation across these sky island populations. They also suggest that the gene genealogies contain information about population relationships, despite the lack of complete sorting of gene lineages. What emerges from the analyses is a model of population divergence that incorporates both contemporary distributions and historical associations, and shows a latitudinal and regional structuring of populations reminiscent of population displacements into multiple glacial refugia. Because the population-divergence model itself is built upon the specific events shaping the history of M. oregonensis, it provides a framework for estimating additional population-genetic parameters relevant to understanding the processes governing differentiation in geographically structured species and avoids the problems of relying on overly simplified and inaccurate divergence models. The utility of these approaches, as well as the caveats and future improvements, for estimating population relationships and historical associations relevant to genetic analyses of geographically structured species are discussed. [source] CALIBRATING A MOLECULAR CLOCK FROM PHYLOGEOGRAPHIC DATA: MOMENTS AND LIKELIHOOD ESTIMATORSEVOLUTION, Issue 10 2003Michael J. Hickerson Abstract We present moments and likelihood methods that estimate a DNA substitution rate from a group of closely related sister species pairs separated at an assumed time, and we test these methods with simulations. The methods also estimate ancestral population size and can test whether there is a significant difference among the ancestral population sizes of the sister species pairs. Estimates presented in the literature often ignore the ancestral coalescent prior to speciation and therefore should be biased upward. The simulations show that both methods yield accurate estimates given sample sizes of five or more species pairs and that better likelihood estimates are obtained if there is no significant difference among ancestral population sizes. The model presented here indicates that the larger than expected variation found in multitaxa datasets can be explained by variation in the ancestral coalescence and the Poisson mutation process. In this context, observed variation can often be accounted for by variation in ancestral population sizes rather than invoking variation in other parameters, such as divergence time or mutation rate. The methods are applied to data from two groups of species pairs (sea urchins and Alpheus snapping shrimp) that are thought to have separated by the rise of Panama three million years ago. [source] GENETIC EVIDENCE ON THE DEMOGRAPHY OF SPECIATION IN ALLOPATRIC DOLPHIN SPECIESEVOLUTION, Issue 4 2002Matthew P. Hare Abstract Under a neutral model, the stochastic lineage sorting that leads to gene monophyly proceeds slowly in large populations. Therefore, in many recent species with large population size, the genome will have mixed support for monophyly unless historical bottlenecks have accelerated coalescence. We use genealogical patterns in mitochondrial DNA and in introns of four nuclear loci to test for historical bottlenecks during the speciation and divergence of two temperate Lagenorhynchus dolphin species isolated by tropical Pacific waters (an antitropical distribution). Despite distinct morphologies, foraging behaviors, and mitochondrial DNAs, these dolphin species are polyphyletic at all four nuclear loci. The abundance of shared polymorphisms between these sister taxa is most consistent with the maintenance of large effective population sizes (5.09 × 104 to 10.9 × 104) during 0.74,1.05 million years of divergence. A variety of population size histories are possible, however. We used gene tree coalescent probabilities to explore the rejection region for historical bottlenecks of different intensity given best estimates of effective population size under a strict isolation model of divergence. In L. obliquidens the data are incompatible with a colonization propagule of an effective size of 10 or fewer individuals. Although the ability to reject less extreme historical bottlenecks will require data from additional loci, the intermixed genealogical patterns observed between these dolphin sister species are highly probable only under an extended history of large population size. If similar demographic histories are inferred for other marine antitropical taxa, a parsimonious model for the Pleistocene origin of these distributions would not involve rare breaches of a constant dispersal barrier by small colonization propagules. Instead, a history of large population size in L. obliquidens and L. obscurus contributes to growing biological and environmental evidence that the equatorial barrier became permeable during glacial/interglacial cycles, leading to vicariant isolation of antitropical populations. [source] X-ray Microtomography Analysis of Dynamic Damage in Tantalum,ADVANCED ENGINEERING MATERIALS, Issue 6 2006J. Bontaz-Carion With the plate-impact technique, ductile spallation experiments are carried out on Ta at impact velocities 570, 1039 and 1078 m/s. Samples are soft-recovered and analyzed by X-Ray microtomography at the European Synchrotron Radiation Facility. The porosity along the shock direction, and the distribution of the pore volumes are extracted from tri-dimensional reconstructions of the damaged samples. Quantitative assessment of pore coalescence is made via an inverse power-law model for the distribution. The decay exponent we associate to coalescence is of order 1.6 in the sample impacted at 1039 m/s. [source] Numerical modelling of fatigue crack initiation and growth of martensitic steelsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2010N. JEZERNIK ABSTRACT This paper presents a numerical simulation of micro-crack initiation that is based on Tanaka-Mura micro-crack nucleation model. Three improvements were added to this model. First, multiple slip bands where micro-cracks may occur are used in each grain. Second improvement deals with micro-crack coalescence by extending existing micro-cracks along grain boundaries and connecting them into a macro-crack. The third improvement handles segmented micro-crack generation, where a micro-crack is not nucleated in one step like in Tanaka-Mura model, but is instead generated in multiple steps. High cycle fatigue testing was also performed and showed reasonably good correlation of proposed model to experimental results. Because numerical model was directed at simulating fatigue properties of thermally cut steel, edge properties of test specimens were additionally inspected in terms of surface roughness and micro-structural properties. [source] Creep failure mechanisms of a Ti-6Al-4V thick plateFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2001F. J. Seco The creep failure operating mechanisms of a 17-mm thick plate of a Ti,6Al,4V alloy in various heat treating conditions have been investigated. Specimens in the as-received, mill-annealed, condition (50 min maintenance at 720 °C and air cooled as the final step of the thermomechanical process) showed the lowest creep resistance and their metallographic analysis revealed that the temperature-activated dislocation climb was the mechanism responsible for the failure and that observed holes were generated by plastic deformation, rather than by creep cavitation. Conversely, maximum times to failure were recorded in beta-annealed specimens (1030 °C for 30 min, air cooled and aged for 2 h at 730 °C). The fracture surfaces of these broken specimens exhibited an intergranular morphology that was attributed to grain boundary sliding along the former beta grains. Finally, alpha,beta field-annealed samples (940 °C 4 h, and furnace cooled to 700 °C) possessed intermediate lives between those of mill-annealed and beta-annealed specimens and the failure operating mechanism was diffusional creep by the nucleation and coalescence of the creep cavities generated at the alpha,beta interfaces and the triple points. [source] Colloidal Films That Mimic CiliaADVANCED FUNCTIONAL MATERIALS, Issue 18 2010Fang Liu Abstract Cilia are wavy hair-like structures that extend outward from surfaces of various organisms. They are classified into two general categories, primary cilia, which exhibit sensing attributes, and motile cilia, which exert mechanical forces. A new poly(2-(N,N -dimethylamino)ethyl methacrylate- co -n-butyl acrylate- co - N,N -(dimethylamino) azobenzene acrylamide) (p(DMAEMA/nBA/DMAAZOAm) copolymer is prepared using colloidal synthesis, which, upon coalescence, form films capable of generating surfaces with cilia-like features. While film morphological features allow the formation of wavy whiskers, the chemical composition of the copolymer facilitates chemical, thermal, and electromagnetic responses manifested by simultaneous shape and color changes as well as excitation wavelength dependent fluorescence. These studies demonstrate that synthetically produced polymeric films can exhibit combined thermal, chemical, and electromagnetic sensing leading to locomotive and color responses, which may find numerous applications in sensing devices, intelligent actuators, defensive mechanisms, and others. [source] | |||||||||||||||||||||||||||||||||||||||||||||||||