Academic literature on the topic 'Gene flow barrier'
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Journal articles on the topic "Gene flow barrier"
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Tavares, Hugo, Annabel Whibley, David L. Field, Desmond Bradley, Matthew Couchman, Lucy Copsey, Joane Elleouet, et al. "Selection and gene flow shape genomic islands that control floral guides." Proceedings of the National Academy of Sciences 115, no. 43 (October 8, 2018): 11006–11. http://dx.doi.org/10.1073/pnas.1801832115.
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Genomes of closely-related species or populations often display localized regions of enhanced relative sequence divergence, termed genomic islands. It has been proposed that these islands arise through selective sweeps and/or barriers to gene flow. Here, we genetically dissect a genomic island that controls flower color pattern differences between two subspecies of Antirrhinum majus, A.m.striatum and A.m.pseudomajus, and relate it to clinal variation across a natural hybrid zone. We show that selective sweeps likely raised relative divergence at two tightly-linked MYB-like transcription factors, leading to distinct flower patterns in the two subspecies. The two patterns provide alternate floral guides and create a strong barrier to gene flow where populations come into contact. This barrier affects the selected flower color genes and tightly-linked loci, but does not extend outside of this domain, allowing gene flow to lower relative divergence for the rest of the chromosome. Thus, both selective sweeps and barriers to gene flow play a role in shaping genomic islands: sweeps cause elevation in relative divergence, while heterogeneous gene flow flattens the surrounding “sea,” making the island of divergence stand out. By showing how selective sweeps establish alternative adaptive phenotypes that lead to barriers to gene flow, our study sheds light on possible mechanisms leading to reproductive isolation and speciation.
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BARTON, N. H. "The effect of a barrier to gene flow on patterns of geographic variation." Genetics Research 90, no. 1 (February 2008): 139–49. http://dx.doi.org/10.1017/s0016672307009081.
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SummaryExplicit formulae are given for the effects of a barrier to gene flow on random fluctuations in allele frequency; these formulae can also be seen as generating functions for the distribution of coalescence times. The formulae are derived using a continuous diffusion approximation, which is accurate over all but very small spatial scales. The continuous approximation is confirmed by comparison with the exact solution to the stepping stone model. In both one and two spatial dimensions, the variance of fluctuations in allele frequencies increases near the barrier; when the barrier is very strong, the variance doubles. However, the effect on fluctuations close to the barrier is much greater when the population is spread over two spatial dimensions than when it occupies a linear, one-dimensional habitat: barriers of strength comparable with the dispersal range (B≈σ) can have an appreciable effect in two dimensions, whereas only barriers with strength comparable with the characteristic scale (B\! \approx\! L \equals \sigma \sol \sqrt {2 \mu}\hskip2) are significant in one dimension (μ is the rate of mutation or long-range dispersal). Thus, in a two-dimensional population, barriers to gene flow can be detected through their effect on the spatial pattern of genetic marker alleles.
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Joannon, B., C. Lavigne, H. Lecoq, and C. Desbiez. "Barriers to Gene Flow Between Emerging Populations of Watermelon mosaic virus in Southeastern France." Phytopathology® 100, no. 12 (December 2010): 1373–79. http://dx.doi.org/10.1094/phyto-04-10-0118.
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Since 1999, “emerging” (EM) strains of Watermelon mosaic virus (WMV) have been detected in cucurbit crops of southeastern France, probably as a result of recent introductions. Population genetic approaches were used to study the structure of EM isolates in southeastern France and to identify factors involved in their spatial distribution. A population clustering method (SAMOVA) and a maximum-difference algorithm (Monmonier's algorithm) were combined to visualize and quantify barriers to gene flow between populations. Both methods yielded similar results and two main barriers were identified. A North/South oriented barrier may be related to physical obstacles to gene flow (Rhône River, presence of an area with few cucurbit crops). Although the barrier was very strong, some “crossing” events were detected. A second barrier, oriented Northwest to Southeast, was not correlated with obvious geographical features. The two methods used here are complementary and confirm the limited spread of WMV-EM isolates. This approach can be useful in epidemiology studies to characterize the structure of viral populations and identify barriers to gene flow.
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Laetsch, Dominik R., Gertjan Bisschop, Simon H. Martin, Simon Aeschbacher, Derek Setter, and Konrad Lohse. "Demographically explicit scans for barriers to gene flow using gIMble." PLOS Genetics 19, no. 10 (October 10, 2023): e1010999. http://dx.doi.org/10.1371/journal.pgen.1010999.
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Identifying regions of the genome that act as barriers to gene flow between recently diverged taxa has remained challenging given the many evolutionary forces that generate variation in genetic diversity and divergence along the genome, and the stochastic nature of this variation. Progress has been impeded by a conceptual and methodological divide between analyses that infer the demographic history of speciation and genome scans aimed at identifying locally maladaptive alleles i.e. genomic barriers to gene flow. Here we implement genomewide IM blockwise likelihood estimation (gIMble), a composite likelihood approach for the quantification of barriers, that bridges this divide. This analytic framework captures background selection and selection against barriers in a model of isolation with migration (IM) as heterogeneity in effective population size (Ne) and effective migration rate (me), respectively. Variation in both effective demographic parameters is estimated in sliding windows via pre-computed likelihood grids. gIMble includes modules for pre-processing/filtering of genomic data and performing parametric bootstraps using coalescent simulations. To demonstrate the new approach, we analyse data from a well-studied pair of sister species of tropical butterflies with a known history of post-divergence gene flow: Heliconius melpomene and H. cydno. Our analyses uncover both large-effect barrier loci (including well-known wing-pattern genes) and a genome-wide signal of a polygenic barrier architecture.
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de Abreu Moreira, Patrícia, and G. Wilson Fernandes. "Is the São Francisco River a geographic barrier to gene flow in trees of Handroanthus ochraceus?" Journal of Tropical Ecology 29, no. 3 (April 19, 2013): 243–50. http://dx.doi.org/10.1017/s0266467413000217.
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Abstract:Many landscape features represent geographic barriers to gene flow, and promote genetic discontinuity. Rivers are effective barriers. However, most studies on this subject have focused on animals and only a few have focused on plants. We studied the genetic structure and gene flow of the tropical tree Handroanthus ochraceus (Bignoniaceae) on both banks of the São Francisco River in a Brazilian seasonally dry tropical forest. The São Francisco is located in eastern Brazil and is 600 m wide at the study site. Our hypothesis was that the river is a geographic barrier to gene flow of H. ochraceus trees. We sampled two populations on the left bank and one population on the right bank. We used seven microsatellites to genotype 212 individual plants. All populations had low polymorphism and genetic diversity, but high inbreeding. There was no genetic differentiation among populations and, consequently, the estimated gene flow was high for all pairs of populations. The genetic relatedness among individuals from populations of the same margin did not differ from the relatedness among individuals from populations of opposite margins. Hence, the São Francisco River is not an effective geographic barrier to gene flow among H. ochraceus populations.
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Hu, Xin-Sheng. "Mating system as a barrier to gene flow." Evolution 69, no. 5 (May 2015): 1158–77. http://dx.doi.org/10.1111/evo.12660.
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Gross, Liza. "Autoimmunity: A Barrier to Gene Flow in Plants?" PLoS Biology 5, no. 9 (September 4, 2007): e262. http://dx.doi.org/10.1371/journal.pbio.0050262.
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Lessios, H. A. "A sea water barrier to coral gene flow." Molecular Ecology 21, no. 22 (October 29, 2012): 5390–92. http://dx.doi.org/10.1111/mec.12037.
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Gayden, Tenzin, Annabel Perez, Patrice J. Persad, Areej Bukhari, Shilpa Chennakrishnaiah, Tanya Simms, Trisha Maloney, Kristina Rodriguez, and Rene J. Herrera. "The Himalayas: Barrier and conduit for gene flow." American Journal of Physical Anthropology 151, no. 2 (April 12, 2013): 169–82. http://dx.doi.org/10.1002/ajpa.22240.
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Christmas, Matthew J., Julia C. Jones, Anna Olsson, Ola Wallerman, Ignas Bunikis, Marcin Kierczak, Valentina Peona, et al. "Genetic Barriers to Historical Gene Flow between Cryptic Species of Alpine Bumblebees Revealed by Comparative Population Genomics." Molecular Biology and Evolution 38, no. 8 (April 6, 2021): 3126–43. http://dx.doi.org/10.1093/molbev/msab086.
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Abstract Evidence is accumulating that gene flow commonly occurs between recently diverged species, despite the existence of barriers to gene flow in their genomes. However, we still know little about what regions of the genome become barriers to gene flow and how such barriers form. Here, we compare genetic differentiation across the genomes of bumblebee species living in sympatry and allopatry to reveal the potential impact of gene flow during species divergence and uncover genetic barrier loci. We first compared the genomes of the alpine bumblebee Bombus sylvicola and a previously unidentified sister species living in sympatry in the Rocky Mountains, revealing prominent islands of elevated genetic divergence in the genome that colocalize with centromeres and regions of low recombination. This same pattern is observed between the genomes of another pair of closely related species living in allopatry (B. bifarius and B. vancouverensis). Strikingly however, the genomic islands exhibit significantly elevated absolute divergence (dXY) in the sympatric, but not the allopatric, comparison indicating that they contain loci that have acted as barriers to historical gene flow in sympatry. Our results suggest that intrinsic barriers to gene flow between species may often accumulate in regions of low recombination and near centromeres through processes such as genetic hitchhiking, and that divergence in these regions is accentuated in the presence of gene flow.
Dissertations / Theses on the topic "Gene flow barrier"
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Onparn, N. "Effectiveness of selfing as a barrier to gene flow in the Mimulus guttatus complex." Thesis, University of Exeter, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.398968.
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Barreto, Felipe S. "Assortative mating as a barrier to gene flow in a coral reef fish species flock /." Electronic version (PDF), 2003. http://dl.uncw.edu/etd/2003/barretof/felipebarreto.pdf.
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Flor, Matthias. "Unidirectional CI and the consequences of Wolbachia for gene flow and reinforcement." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2011. http://dx.doi.org/10.18452/16312.
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Die intrazellulären Parasiten der Bakteriengattung Wolbachia sind weit verbreitet im Phylum der Arthropoden. In vielen Wirten lösen sie eine Paarungsinkompatibilität zwischen nicht infizierten Weibchen und infizierten Männchen aus. Die mögliche Rolle dieser zytoplasmatischen Inkompatibilität in Artbildungsprozessen der Wirtsorganismen wird seit langer Zeit diskutiert. In dieser Arbeit analysieren wir häufig angeführte Kritikpunkte einer solchen Rolle mit Hilfe von mathematischen Modellen, in denen Infektionsdynamik von Wolbachia und Populationsgenetik der Wirte kombiniert werden. Die einzelnen Teile befassen sich mit dem Folgenden: (i) Wir untersuchen die Stabilität von Infektionsmustern in Wirts-Metapopulationen, indem wir kritische Migrationsraten herleiten. (ii) Zur Abschätzung des Einflusses der zytoplasmatischen Inkompatibilität auf den Genfluss zwischen Populationen berechnen wir effektive Migrationsraten. (iii) Wir bestimmen die Bedingungen, die die Verstärkung von Reproduktionsbarrieren durch die Evolution von weiblichen Paarungspräferenzen begünstigen. Schließlich (iv) wenden wir unsere Modelle auf einen realen Artbildungsprozess zweier Drosophila-Arten in Nordamerika an, diskutieren auftretende Probleme und unterbreiten Vorschläge für weiterführende Forschung. Zusammenfassend implizieren unsere Ergebnisse, dass Wolbachien häufig mit der Entstehung neuer Wirtsarten verknüpft sein können, allerdings in den meisten Fällen nur, indem sie als einer von mehreren Faktoren zur reproduktiven Isolation beitragen. Eine Verstärkung sexueller Isolation wird nur unter speziellen Bedingungen bewirkt.The intracellular bacterial parasites of the genus Wolbachia are widespread among arthropod species. In many hosts, they induce a reproductive incompatibility between uninfected females and infected males. The potential role of this cytoplasmic incompatibility in speciation processes of the bacteria''s hosts has long been debated. In this thesis, we analyze common criticisms of such a role by means of mathematical models, combining Wolbachia infection dynamics and host population genetics. In particular, we are concerned with the following: (i) In order to measure the stability of infection patterns within host metapopulations, we derive critical migration rates. (ii) We evaluate the impact of cytoplasmic incompatibility on gene flow between populations by calculating effective migration rates. (iii) We determine the conditions that favor the evolution of female mating preferences through reinforcement. Finally, (iv) we apply our models to a particular real-world speciation process of two sibling Drosophila species in North America, discuss emerging problems, and suggest future directions of research. In summary, our results implicate that Wolbachia might be a frequent factor in host speciation, but usually only by contributing to overall reproductive isolation among other factors. Reinforcement of premating isolation is selected for only under stringent conditions.
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Figueira, Cláudia Alexandra Rodrigues Marques. "Comparison of different methods to detect genetic barriers in a small mammal population." Master's thesis, Universidade de Aveiro, 2016. http://hdl.handle.net/10773/15871.
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Mestrado em Ecologia AplicadaHabitat fragmentation and the consequently the loss of connectivity between populations can reduce the individuals interchange and gene flow, increasing the chances of inbreeding, and the increase the risk of local extinction. Landscape genetics is providing more and better tools to identify genetic barriers.. To our knowledge, no comparison of methods in terms of consistency has been made with observed data and species with low dispersal ability. The aim of this study is to examine the consistency of the results of five methods to detect barriers to gene flow in a Mediterranean pine vole population Microtus duodecimcostatus: F-statistics estimations, Non-Bayesian clustering, Bayesian clustering, Boundary detection and Simple/Partial Mantel tests. All methods were consistent in detecting the stream as a non-genetic barrier. However, no consistency in results among the methods were found regarding the role of the highway as a genetic barrier. Fst, Bayesian clustering assignment test and Partial Mantel test identifyed the highway as a filter to individual interchange. The Mantel tests were the most sensitive method. Boundary detection method (Monmonier’s Algorithm) and Non-Bayesian approaches did not detect any genetic differentiation of the pine vole due to the highway. Based on our findings we recommend that the genetic barrier detection in low dispersal ability populations should be analyzed with multiple methods such as Mantel tests, Bayesian clustering approaches because they show more sensibility in those scenarios and with boundary detection methods by having the aim of detect drastic changes in a variable of interest between the closest individuals. Although simulation studies highlight the weaknesses and the strengths of each method and the factors that promote some results, tests with real data are needed to increase the effectiveness of genetic barrier detection.
A fragmentação do habitat e a consequente perda da conectividade entre populações pode reduzir o intercâmbio de indivíduos e consequentemente o fluxo genético, aumentando as hipóteses de ocorrer consanguinidade e consequentemente aumentar o risco de extinção local. A disciplina da genética da paisagem fornece cada vez mais e melhores ferramentas para detectar barreiras genéticas. No entanto, não se conhecem até à data, comparações de métodos em termos de consistência de resultados com dados observados e espécies com reduzida capacidade de dispersão. O objectivo deste estudo é avaliar a consistência dos resultados de cinco métodos de análise do papel da auto-estrada e de um rio como barreira ao fluxo genético numa população de rato-cego-mediterrânico Microtus duodecimcostatus: estimativas do Festatistico, método de aglomeração não-Bayesianos, métodos de aglomeração Bayesianos, método de detecção de fronteiras (algoritmo Monmonier) e o teste Mantel simples e parcial. Todos os métodos testados foram consistentes em considerar o rio como uma não barreira genética ao rato-cego-mediterrânico. No entanto, não houve consistência nos resultados quanto ao papel da autoestrada como barreira genética. As estimativas do F-estatistico, os métodos de aglomeração Bayesianos e o teste de Mantel parcial que mostram que a autoestrada pode estar a funcionar como um filtro ao movimento dos indivíduos entre os dois lados da estrutura. Os métodos de deteção de fronteiras (algoritmo Monmonier) e de aglomeração não-Bayesiano não detectaram diferenciação genética nas populações de rato-cego-mediterrâneo devido à estrada. Com base nos nossos resultados nós recomendamos a aplicação dos testes de Mantel, os métodos de aglomeração Bayesianos e dos métodos de detecção de fronteiras para esclarecer o papel dos atributos da paisagem como barreiras genéticas uma vez que, todos foram capazes de detectar barreiras mas não obtiveram resultados similares. Apesar dos estudos com base em simulações apontarem as vantagens e desvantagens de cada método e os fatores que associados aos resultados, é necessário que se façam testes com base em dados reais para que sejam mais eficazes na detecção de barreiras genéticas.
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Fraïsse, Christelle. "Génétique de l’adaptation et de la spéciation : théorie et analyse de données de séquençage haut-débit dans le complexe d’espèces Mytilus edulis." Thesis, Montpellier 2, 2014. http://www.theses.fr/2014MON20107/document.
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Les génomes sont affectés par des régimes de sélection conflictuels. Ceci est particulièrement bien illustré par le concept de barrière semi-perméable au flux génique, issu de la littérature des zones hybrides. Certains gènes contribuent à empêcher le mélange entre lignées génétiques différenciées, soit parce qu'ils participent à l'adaptation aux conditions environnementales locales, soit parce qu'ils sont incompatibles avec les gènes d'autres lignées. D'autres parties du génome sont soit neutres, soit soumises à une sélection qui tend à homogénéiser les différentes lignées entre elles. Dans la première partie de cette thèse, des modèles d'évolution de l'isolement reproductif sont présentés pour expliquer les patrons d'isolements observés dans les expériences d'hybridation au laboratoire. Par modélisation classique d'incompatibilités génétiques de type Dobzhansky-Muller, il est montré que l'asymétrie et la complexité des incompatibilités sont imparfaitement expliquées par un filtre évolutif, c.a.d. une vitesse d'accumulation différente entre types d'incompatibilité. Une approche complémentaire de modélisation quantitative à l'aide d'une extension du modèle géométrique de Fisher a permis de préciser quelles conditions de divergence entre lignées isolées conduisaient à un effet fortement délétère des mutations dans les génotypes hybrides. L'importance relative du niveau d'épistasie moyen, de la distribution des effets des mutations et des modalités de l'adaptation de chaque lignée est discutée. La seconde partie de cette thèse profite des avancées techniques de la génomique pour étudier l'histoire de la spéciation et de l'adaptation dans un complexe d'espèces non-modèles, les moules du genre Mytilus. Une méthode statistique d'inférence de scénarios de spéciation est présentée. Les résultats montrent que les moules Européennes ont connu une histoire complexe de divergence stricte suivie d'une période de connectivité périodique. En accord avec le concept de barrière semi-perméable au flux génique, il est montré que les taux d'introgression sont hétérogènes le long du génome. Ensuite, des scans génomiques de la différenciation ont été menés entre paires de populations du complexe d'espèces. L'analyse de la variation génétique et des généalogies d'allèles sur une échelle chromosomique localisée a permis de reconstituer l'histoire évolutive de plus de 1000 régions du génome des moules. Cette analyse a révélé qu'une cause majeure, mais insoupçonnée, de la différenciation génétique intraspécifique est l'introgression différentielle d'allèles étrangers. Globalement, cette thèse montre non seulement le rôle majeur de la biogéographie de la spéciation, c.a.d. des patrons temporels et spatiaux du flux de gènes, dans notre compréhension de la biodiversité actuelle, mais aussi sa surprenante complexité et l'étendue de ses conséquences sur l'évolution des génomesGenomes are affected by conflicting selective regimes. This is particularly well illustrated by the concept of semi-permeable barriers to gene flow, as found in the hybrid zones literature. Some genes contribute to the prevention of mixing between differentiated genetic lineages, either because they are involved in adaptation to local environmental conditions, or because they are incompatible with alleles from other genetic lineages. Other parts of the genome are either neutral, or subjected to selection which tends to homogenize the genetic lineages. In the first part of this thesis, models of the evolution of reproductive isolation are presented to explain the isolation patterns observed in experimental hybridizing crosses between incipient species. Using standard models of Dobzhansky-Muller genetic incompatibilities, it is shown that the asymmetry and complexity of incompatibilities are not well explained by there being an “evolutionary sieve”, i.e. a different rate of accumulation between incompatibilities. A complementary approach to quantitative modeling (an extension of Fisher's Geometric Model) then clarifies which conditions of divergence between allopatric lines led to highly deleterious effects in hybrid genotypes. The relative importance of mean levels of fitness epistasis, the distribution of mutation sizes, and the way lineages adapt to new environmental conditions is discussed. The second part of this thesis takes advantage of technical advances in genomics to study the history of speciation and adaptation in a non-model species complex, Mytilus mussels. A statistical method of inferring speciation scenarios is presented. Results show that European mussels experienced a complex history of strict divergence followed by a period of periodic connectivity. In agreement with the concept of semi-permeable barriers to gene flow, it is shown that introgression rates are heterogeneous along the genome. Next, genome scans of differentiation were conducted between pairs of populations of the species complex. The analysis of genetic variation and allele genealogies on a small chromosomal scale allowed to reconstruct the evolutionary history of more than 1000 genomic regions. This analysis reveals that a major cause of intraspecific differentiation is the differential introgression of foreign alleles. Overall, this thesis shows not only that biogeography of speciation, i.e. the temporal and spatial patterns of gene flow, play a major role in our understanding of existing biodiversity, but also its amazing complexity and extent of its impact on genome evolution
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Bryant, Litticia M. "Cryptic diversity and evolutionary relationships among Australian closed-forest Melomys (Rodentia: Muridae) and related Australo-Papuan mosaic-tailed rats." Thesis, Queensland University of Technology, 2013. https://eprints.qut.edu.au/60846/1/Litticia_Bryant_Thesis.pdf.
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This thesis investigates patterns of evolution in a group of native Australo-Papuan rodents. Past climatic change and associated sea level fluctuations, and fragmentation of wet forests in eastern Australia has facilitated rapid radiation, diversification and speciation in this group. This study adds to our understanding of the evolution of Australia’s rainforest fauna and describes the evolutionary relationships of a new genus of Australian rodent.
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Fitzpatrick, Benjamin Minault. "Speciation and barriers to gene flow /." For electronic version search Digital dissertations database. Restricted to UC campuses. Access is free to UC campus dissertations, 2004. http://uclibs.org/PID/11984.
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El, Ayari Tahani. "Barrières au flux génique en Méditerranée Occidentale : étude de la différenciation génétique chez deux mollusques marins, Mytilus galloprovincialis & Stramonita haemastoma." Thesis, Montpellier, 2015. http://www.theses.fr/2015MONTS226/document.
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La génétique des populations a révélé que la diversité génétique des espèces marines était très souvent distribuée de façon discrète dans l’espace, en mosaïque de patchs populationnels génétiquement homogènes délimités par des discontinuités appelées barrière au flux génique. L’objectif de cette thèse était de contribuer à mieux comprendre les processus expliquant l’origine, le maintien et la position des barrières génétiques au niveau de la zone de transition entre l’Atlantique et la Méditerranée. Dans un premier temps a été étudiée la structure génétique de la moule Mytilus galloprovincialis. Contrairement au cline abrupt et étroit reporté en Espagne, nous avons découvert en Algérie une vaste zone hybride mosaïque sur 600 km de côtes à l'Est du front océanique Almeria-Oran. Dans un deuxième temps a été menée une étude de la structure génétique du gastéropode marin Stramonita haemastoma. Nous avons découvert deux lignées cryptiques différentiellement fixées pour des haplogroupes mitochondriaux, et différenciées sur 3 marqueurs microsatellites développés dans cette thèse. La distribution spatiale en mosaïque est étonnante avec un patch de la lignée atlantique enclavé au nord de la Méditerranée occidentale et bordé par une zone hybride au sud dans la région de Valence. Ces deux études mettent en avant l’importance de l’isolement reproductif intrinsèque dans l’explication de la distribution mosaïque de la diversité génétique marine. Bien que les frontières entre patchs correspondent à des barrières physiques à la dispersion ou à des écotones, l’hydrographie et l’environnement n’expliquent sans doute que la position des discontinuités génétiques mais ni leur origine ni leur maintienPopulation genetics has revealed the genetic diversity of marine species is often subdivided into a mosaic of discrete patches, within which populations are genetically homogeneous, delineated by discontinuities called barriers to gene flow. The aim of this thesis was to contribute to better understand the processes explaining the origin, maintenance and location of genetic barriers at the Atlantic/Mediterranean transition zone. First, we studied the genetic structure of the mussel Mytilus galloprovincialis. In contrast to the abrupt narrow cline reported in Spain, we discovered along the Algerian coastline a 600 km wide mosaic hybrid zone eastward of the Almeria-Oran oceanic front. Second, we studied the genetic structure of a marine gastropod Stramonita haemastoma. We discovered two cryptic lineages differentially fixed for alternative mitochondrial haplogroups, and differentiated at three microsatellite markers developed in this PhD work. Surprisingly, the spatial distribution proved to be an unusual mosaic with a patch of the Atlantic lineage enclaved in the north of the Western Mediterranean Sea, bordered in the South by a hybrid zone in eastern Spain around Valencia. These two studies highlight the importance of intrinsic reproductive isolation in explaining the mosaic distribution of the marine genetic diversity. Although boundaries between patches coincide with physical barriers to dispersal or ecotones, hydrography and environment mainly explain the position of the genetic discontinuities but neither their origin nor their maintenance
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Burban, Ewen. "Approche génomique de la détection des barrières au flux de gènes." Electronic Thesis or Diss., Université de Rennes (2023-....), 2024. http://www.theses.fr/2024URENB007.
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La caractérisation des mécanismes qui sous-tendent l'isolement reproductif entre des lignées divergentes est essentielle pour comprendre le processus de spéciation. Au cours de leur évolution, les populations développent progressivement un isolement reproductif (IR) en passant par des étapes intermédiaires, souvent appelées "zone grise de la spéciation". L'établissement de l'IR se manifeste par l'apparition de régions génomiques qui agissent comme des barrières réduisant le flux de gènes local par rapport au reste du génome. Les approches de génomique des populations impliquent donc l'identification de locus avec des signatures spécifiques, différentes du reste du génome. Cependant, d'autres processus peuvent créer des signatures similaires, ce qui fait de la détection des barrières une tâche difficile. Dans ma thèse, j'ai développé un nouvel outil, RIDGE - Reproductive Isolation Detection using Genomic Polymorphisms – un nouvel outil libre et portable adapté en particulier aux approches comparatives. RIDGE utilise une approche ABC (Approximate Bayesian Computation) et de “model averaging” basée sur des “random forest” pour prendre en compte divers scénarios de divergence entre lignées. Il prend en compte l'hétérogénéité du taux de migration, de la sélection en liaison et de la recombinaison le long du génome, estimant la proportion de barrières et effectuant des tests par locus pour détecter les barrières au flux génique. Des simulations et des analyses de jeux de données publiés sur des paires d'espèces de corbeaux indiquent que RIDGE est efficace pour détecter la migration en cours et identifier les locus barrières, même pour des temps de divergence récents. De plus, la contribution des statistiques résumées varie en fonction du jeux de données, ce qui met en évidence la complexité des signaux génomiques des barrières et l’intérêt de combiner plusieurs statistiques résumées. Par la suite, j'ai appliqué RIDGE à des paires de populations sauvages/domestiques : le maïs (allogame) et le millet (autogame), les deux ayant été domestiquées il y a environ 9 000 ans. Des flux de gènes entre les formes ont été documentés dans ces deux systèmes. Les modèles avec migration continue au cours du temps et hétérogène le long du génome sont clairement ressortis comme dominants. RIDGE a également démontré sa capacité à distinguer les locus barrière des locus de domestication (qui ont subi des balayages sélectifs au sein des formes domestiques). Les perspectives de ce travail comprennent l'application de RIDGE à de multiples paires population/espèce englobant un large spectre de divergence afin de déterminer les bases génomiques de l’IR au cours de la spéciation, de tester la théorie de «l’effet boule de neige” formulée par Orr en 1995 ou de déterminer la nature des gènes de spéciationCharacterizing the mechanisms that underlie reproductive isolation between diverging lineages is central in understanding the speciation process. As populations evolve, they gradually develop reproductive isolation (RI) by passing through intermediate steps, often referred to as the "gray zone of speciation". This isolation is marked by the emergence of genomic regions acting as barriers to local gene flow, distinct from the rest of the genome. Detecting these barrier loci involves identifying outlier loci with specific signatures. However, other processes can create similar patterns, which challenges barrier loci detection. In my thesis, I developed a new tool, RIDGE - Reproductive Isolation Detection using Genomic Polymorphisms, a novel free and portable tool tailored for this purpose in a comparative framework. RIDGE utilizes an Approximate Bayesian Computation model-averaging approach based on a random forest to accommodate diverse scenarios of lineage divergence. It considers heterogeneity in migration rate, linked selection, and recombination, estimates barrier proportion and conducts locus-scale tests for gene flow barriers. Simulations and analyses of published datasets in crow species pairs demonstrate RIDGE's efficacy in detecting ongoing migration and identifying barrier loci, even for recent divergence times. Furthermore, the contribution of summary statistics varies depending on the dataset, highlighting the complexity of gene flow barrier genomic signals and the interest of combining several statistics. Subsequently, I applied RIDGE to wild/domestic pairs in maize (an outcrosser), and foxtail millet (a selfer), both domesticated around 9,000 years ago. Gene flow between forms has been reported in these two systems. Consistently, models with ongoing migration and heterogeneity in migration rate were clearly dominant over other models. RIDGE also demonstrated its ability to distinguish between barrier loci and domestication loci (that experienced selective sweeps within the domestic forms). The perspectives of this work include applying RIDGE to multiple population/species pairs encompassing a large spectrum of divergence to determine the genomic pattern of RI during speciation, to test the snowball theory formulated by Orr in 1995 or to determine the nature of speciation genes
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Kruuk, Loeske E. B. "Barriers to gene flow : a Bombina (fire-bellied toad) hybrid zone and multilocus cline theory." Thesis, University of Edinburgh, 1997. http://hdl.handle.net/1842/11015.
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1. In a stable hybrid zone between two taxa, natural selection creates a barrier to gene flow which counterbalances the homogenising effects of dispersal and interbreeding. Studies of hybrid zones can contribute to our understanding of the nature of reproductive isolation between two populations, and hence of speciation mechanisms. For example, reproductive isolation may be generated by natural selection acting against alleles in the wrong environment or against alleles in the wrong genetic background. In this thesis, I consider these issues with (i) a study of a hybrid zone between the fire-bellied toads Bombina bombina and B. variegata and (ii) development of analytical and simulation models of geographical variation maintained by dines at multiple loci. 2. In a transect across the Bombina hybrid zone in Croatia, the pattern of change in three phenotypic traits (leg length, belly pattern and egg size) corresponds to the stepped dines observed in previous studies of diagnostic allozyme loci. There is close concordance between the mean values of the traits and between estimates of linkage disequilibrium calculated from associations between alternative phenotypic and genetic variables. Clines in allozyme frequency and spot score are wider in males than in females, suggesting differential dispersal patterns. 3. There is direct evidence for hybrid dysfunction in the field: in samples collected from Bombina breeding sites, embryonic mortality, larval mortality and frequency of larval developmental abnormalities all increase significantly towards the centre of the hybrid zone. However, a cohort analysis of adult toads within a central region shows no evidence of differential mortality with respect to genotype. 4. Bombina bombina and B. variegata typically use different breeding habitats, with B. variegata showing strong avoidance of the semi-permanent ponds favoured by B. bombina. The abundance of aquatic predators is shown to differ between the alternative breeding habitats preferred by either species. Behavioural experiments show that the feeding strategy of B. bombina larvae reduces their risk of predation, relative to that of B. variegata larvae, thus demonstrating an adaptive advantage to the adults habitat preference. As the availability of either habitat type changes across the zone, these adaptations imply that differential adaptation across an environmental gradient is also creating a barrier to gene flow between the taxa. 5. An analytical model of multilocus dines maintained by differential adaptation to alternative environments is developed, considering in particular the effect of selection on neutral markers. The dynamics are qualitatively similar to those of previous models of heterozygote disadvantage. Computer simulations are used to test the analytical predictions, and the restrictions imposed by assumptions of weak selection. These show that while dine shape can accurately estimate parameters such as fitness in hybrid populations, others, such as the number of genes under selection, may be less robust. 6. The effect of a habitat preference in a hybrid zone such as Bombina is explored with a simulation model. Pooling across habitats, overall statistical associations increase with preference strength, illustrating the effect of the habitat preference in maintaining the integrity of the parental genomes. However, the resulting magnitude of linkage disequilibrium and deviations from Hardy-Weinberg proportion within habitats vary nonmonotonically with the strength of the preference. The resulting shape of the zone will be largely dependent on the underlying distribution of habitat availability. 7. The Bombina hybrid zone is therefore maintained by both endogenous and exogenous selection. Although theoretical models show that the dynamics of either regime are similar, the results imply that divergence during allopatry has been driven by adaptation to alternative environments, rather than solely non-adaptive factors. The resulting reproductive isolation has therefore been, at least partially, determined by ecological factors, and environmentally-mediated factors such as a habitat preference will generate further barriers to gene flow between two populations.
Book chapters on the topic "Gene flow barrier"
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Liu, Yong-Bo, and Xin-Yu Wang. "Gene flow mitigation by ecological approaches." In Gene flow: monitoring, modeling and mitigation, 125–36. Wallingford: CABI, 2021. http://dx.doi.org/10.1079/9781789247480.0009.
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Abstract With an increased area of cultivating genetically modified (GM) plants worldwide, the ecological risks of transgenic plants released into the environment have caused concern. One of the risks is the occurrence of gene flow between GM plants and non-GM plants, including their wild relatives. Gene flow data from oilseed rape (Brassica napus), cotton (Gossypium hirsutum), maize (Zea mays), soybean (Glycine max), rice (Oryza sativa), and wheat (Triticum aestivum) indicate that the frequency of pollen-mediated gene flow is negatively related with distance between donor and recipient plants, and the frequency is relatively high in closely related species. We discuss five main ecological approaches to mitigate gene flow from GM plants to non-GM plants, including distance isolation, border or trap crops, barrier crops, agricultural practices, and through biological means. The required isolation distance has been adopted in managing GM crops in some countries, and cultivating tall crops, or border or trap crops, can decrease the requisite isolation distance to mitigate gene flow. Combining several approaches is more effective than a single approach in mitigating gene flow, because the frequency of pollen-mediated gene flow depends on plant genotype, flowering time, wind speed and direction, and other factors. Thus, in the framework of biosafety assessment of GM plants, mitigating the occurrence of gene flow between GM and non-GM plants is a key step to decrease the ecological risk of post- commercial cultivation of GM plants.
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Richard, A. J. "Hybridisation - Reproductive Barriers to Gene Flow." In Gene Flow from GM Plants, 78–112. Oxford, UK: Blackwell Publishing Ltd, 2007. http://dx.doi.org/10.1002/9780470988497.ch4.
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Melo, Martim, Luis M. P. Ceríaco, and Rayna C. Bell. "Biogeography and Evolution in the Oceanic Islands of the Gulf of Guinea." In Biodiversity of the Gulf of Guinea Oceanic Islands, 141–70. Cham: Springer International Publishing, 2022. http://dx.doi.org/10.1007/978-3-031-06153-0_6.
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AbstractAs with most archipelagos, geography played a central role in the assembly and evolution of the endemic-rich biological communities of the Gulf of Guinea oceanic islands. The islands are located at moderate distances from the species-rich African continent that surrounds them to the east and north. This proximity facilitated colonization by many branches of the tree of life, but gene flow between the islands and continent was low enough that many lineages evolved in isolation once they reached the archipelago, resulting in many endemic species. Furthermore, several of the island taxa belong to groups typically considered to be “poor dispersers” across sea barriers, which strongly supports a role for natural rafts in seeding the islands. Oceanic currents, including the freshwater pathways that extend from large river drainages into the Gulf of Guinea during the rainy season, also support this hypothesis. The distances between the islands are equivalent to those between the islands and the continent such that inter-island dispersal events appear to be relatively rare and thus few taxa are shared between them. Still, the islands present multiple cases of secondary contact leading to hybridization and genetic introgression between closely related lineages—providing several models to study the role and consequences of gene flow in evolution. Most taxa for which molecular estimates of divergence time have been derived are much younger than the ages of the islands. This pattern is consistent with high species turnover, likely resulting from a combination of small island sizes, proximity to the African continent and a long history of intense volcanic activity. The Gulf of Guinea oceanic islands provide multiple examples of classical adaptations to island life (the “island syndrome”), including giants and dwarves, ornament and color loss, among others. In addition, emerging studies of birds are highlighting the importance of competition regimes in driving phenotypic change—with examples of both character release (low inter-specific competition) and character displacement (inter-specific competition upon secondary contact). Collectively, the Gulf of Guinea oceanic islands offer unique opportunities to study adaptation and speciation in a range of taxa and contexts.
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Mayr, Ernst, and Jared Diamond. "The Establishment of Geographic Isolates." In The Birds of Northern Melanesia, 216–17. Oxford University PressNew York, NY, 2001. http://dx.doi.org/10.1093/oso/9780195141702.003.0028.
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Abstract Genetic differentiation within an initially conspecific population can proceed in three different ways. Two of those ways were already mentioned in chapter 16 in connection with peripatric and dichopatric speciation. The first is through primary isolation: colonizing individuals from a source population may succeed in crossing a major barrier to reach an area or island previously unoccupied by their species. There, they found a new population, which thus is immediately isolated from the source population by the barrier. The barrier may be a water gap between islands, or an ecological barrier within a single land mass (such as a mountain range separating lowland areas, a low-lying area separating mountains, or a dry savanna or desert separating forested areas). Already at its moment of founding, the new population may differ from its parent source population as a result of the founder effect; the gene frequencies of the founding individuals may happen to differ from the average gene frequencies in the source population. Once founded, the new population may or may not survive long enough for genetic differentiation to proceed further, depending on the colonized island’s area and the population density of the species. In addition, the continued flow of conspecific individuals from the source population across the barrier may or may not be high enough to inhibit differentiation, depending especially on the width of the barrier and the vagility of the species.
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Ingram, Neil, Sylvia Hixson Andrews, and Jane Still. "The Birth and Death of Species." In Evolution. Oxford University Press, 2021. http://dx.doi.org/10.1093/hesc/9780198862574.003.0002.
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This chapter focuses on the birth and death of species, which has been referred to be the nature of evolution. It also notes the modern classification system based on Linnaeus' Systema Naturae, a framework featuring genus and kingdoms of species. The chapter also notes how reproduction isolation is required for speciation. Genetic isolation is the result of a physical or geographical barrier to gene flow during the process of allopatric speciation. Sympatric speciation is involved in the emergence of hybrids, especially among plant species. Extinction, the chapter explains, is an essential part of evolution as new niches and set of speciation take over.
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Hill, Geoffrey E. "Mitonuclear speciation." In Mitonuclear Ecology, 143–78. Oxford University Press, 2019. http://dx.doi.org/10.1093/oso/9780198818250.003.0007.
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Current models of speciation assume that species arise when nuclear genotypes diverge following the disruption of gene flow between populations. This chapter explores the idea that speciation is specifically the result of divergence in coadapted mitonuclear gene complexes with divergence of most nuclear genes playing little or no role in speciation. To maintain mitonuclear coadaptation, nuclear genes must coevolve with rapidly changing mitochondrial genes. According to the mitonuclear compatibility concept of species, mitonuclear coevolution in isolated populations leads to speciation because population-specific mitonuclear coadaptations create between-population mitonuclear incompatibilities and hence barriers to gene flow between populations. In addition, selection for adaptive divergence of products of mitochondrial genes can lead to rapid fixation of novel mitochondrial genotypes between populations and consequently to disruption in gene flow between populations as the initiating step in animal speciation. The chapter considers the evidence for the involvement of mitonuclear compatibility in the process of speciation and the implications for this new concept of speciation and species.
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Arnold, Michael L. "Barriers to gene flow." In Evolution through Genetic Exchange, 62–81. Oxford University Press, 2007. http://dx.doi.org/10.1093/acprof:oso/9780199229031.003.0004.
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Patricia Rendón-Huerta, Erika, Carlos Abraham García-García, and Luis Felipe Montaño Estrada. "Effect of Helicobacter pylori on Tight Junctions in Gastric Epithelia." In Helicobacter pylori - From First Isolation to 2020 [Working Title]. IntechOpen, 2021. http://dx.doi.org/10.5772/intechopen.96607.
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Molecular complexes grouped under the names of tight, adherent or gap junction regulate the flow of water, ions and macromolecules through epithelium paracellular spaces. The main constituents of tight junctions are claudins, a family of 26 different proteins whose expression and distribution are tissue specific but varies in tumors. A change in claudin 1, 3, 4, 5, 6, 7, 9 and 18 expression, that contributes to lose epithelial cohesion, has been associated to enhanced cell proliferation, migration, and invasiveness in gastric neoplastic tissue. Chronic inflammation process induced by H. pylori infection, a major risk factor for gastric cancer development, disrupts tight junctions via CagA gene, Cag pathogenicity island, and VacA, but the effect upon the epithelial barrier of H. pylori lipopolysaccharides or H. pylori-induced up-regulation of mTOR and ERK signaling pathways by microRNA-100 establishes new concepts of proof.
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Cornelissen, Germaine, and Kuniaki Otsuka. "Integumentary System." In Chronobiology and Chronomedicine, 374–96. Royal Society of Chemistry, 2023. http://dx.doi.org/10.1039/bk9781839167553-00374.
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Skin is a complex organ, consisting of several layers, each containing different kinds of cells performing different functions, including defense mechanisms against pathogens, chemicals, UV light, and mechanical injury, hydration, temperature regulation, sebum production, skin blood flow and barrier function. Skin is regulated by the central clock, while skin cells also harbor their own molecular circuitry, with each cell containing a biochemical oscillator consisting of interlocked transcription–translation feedback loops. After a brief overview of skin structure and organization, illustrative examples of 24-hour variations in some functions of skin are presented. The involvement of the circadian clock in several cells playing key roles in skin and affecting the expression of distinct gene sets and physiological functions is then reviewed. The implication of the circadian clock machinery present in different skin cells is further discussed in relation to various skin diseases, ranging from skin cancer to skin infections, inflammatory skin diseases, and wound healing. To conclude, some thoughts on strategies to maintain skin health and counteract disease conditions in the skin are offered, opening a vast field for future investigation.
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West-Eberhard, Mary Jane. "Speciation." In Developmental Plasticity and Evolution. Oxford University Press, 2003. http://dx.doi.org/10.1093/oso/9780195122343.003.0035.
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In sexually reproducing organisms, speciation is lineage branching—the origin of reproductive isolation between sister populations descended from a single interbreeding parent population. Obviously, speciation is a process of fundamental importance in evolution. In sexually reproducing organisms, every persistent branching point of a phylogenetic tree, whether between very similar species or higher taxa, reflects a speciation event. Because complete reproductive isolation means the end of gene flow between populations, there is no doubt that it can facilitate genetic and phenotypic divergence. So speciation is a major cause of the diversification of living things. In nonsexual or uniparental populations, isolation between divergent populations may also be called speciation, but reduced gene flow can play no role. Such populations may become genetically distinctive and divergent due to differences in mutation, selection, and drift and thereby qualify as species under some definitions (see M. B. Williams, 1992, for a discussion of the species concept in asexual organisms). This chapter deals only with speciation in sexually reproducing organisms. By the usual view of speciation, some barrier to interbreeding comes first, followed or accompanied by genetic and phenotypic divergence. Reproductive isolation leads to divergence. Here I argue that the reverse may sometimes occur—that divergence, mediated by developmental plasticity and selection, may sometimes originate first and contribute to the evolution of reproductive isolation. As discussed in part III, evolution by disruptive and frequencydependent selection can produce a developmental switch between alternative phenotypes rather than loss of intermediate genotypes. This is particularly well documented in insects, often leading to misidentification of intraspecific morphs as species. Since polymorphic insects may have host-associated morphs, host shifts accompanied by distinctive morphology cannot be assumed to represent sympatric speciation or host-race formation, and sympatric speciation hypotheses need to decisively eliminate the possibility of a role for sympatric divergence in the form of polymorphism or behavioral and physiological plasticity. Such intraspecific host shifts may contribute to speciation, whether sympatric or allopatric, as discussed further below.
Conference papers on the topic "Gene flow barrier"
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Vazquez, Louis C., Erik Hagel, Bradley J. Willenberg, Christopher D. Batich, and Malisa Sarntinoranont. "Effect of Polymer Coated Needles on Infusate Backflow During Convection-Enhanced Delivery." In ASME 2010 Summer Bioengineering Conference. American Society of Mechanical Engineers, 2010. http://dx.doi.org/10.1115/sbc2010-19557.
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Currently, many central nervous system disorders cannot be treated effectively using conventional drug delivery methods such as oral and intravenous drug administration. Therapeutic agents for such disorders often contain polar proteins with high molecular weight compounds (i.e. enzymes, antibodies and gene vectors) that are too large to diffuse through the tight junctions of the blood brain barrier (BBB) [1]. Moreover, it has been shown that low molecular weight compounds, though highly diffusive within brain tissue and tumors, have a limited distribution of just a few millimeters from the site of delivery due to loss via capillaries [1]. Direct infusion into the brain using convection-enhanced delivery (CED) as a supplement to diffusion is a technique that can circumvent these limitations by allowing one to utilize bulk flow to achieve much greater drug concentrations throughout the targeted area [1].
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Noh, D. H., S. Kim, J. Eun, and Y. R. Kim. "A New Laboratory Testing Platform to Study Saturation, Swelling/contraction, Desiccation Cracking, and Permeability of Gas/liquid of Engineered Barrier Materials at High Pressure-Temperature Conditions." In 57th U.S. Rock Mechanics/Geomechanics Symposium. ARMA, 2023. http://dx.doi.org/10.56952/arma-2023-0753.
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ABSTRACT This study introduces a new testing platform, which allows an application of high pressure (up to 5 MPa), high temperature (up to 200°C), and simultaneous measurements of vertical deformation, stress, and the permeability of gas/liquid. The test cell is designed for the radial fluid flow to minimize the risk of a preferential leak along the gap between the test specimen and the internal wall of the test cell, which can be prominent during the thermally-induced contraction of bentonite. The testing platform is calibrated by examining the swelling pressure during the saturation process and water permeability of the fully saturated bentonite specimen. After that, a temperature rise, up to 80°C, is imposed on the test cell to monitor the change in the vertical stress. With the partially saturated bentonite specimen, the temperature is raised up to ∼150°C, beyond the boiling point, to induce further desiccation cracks. After that, change in the vertical stress is continuously monitored during the temperature increase and ensuing nitrogen injection to measure the gas permeability. Once the heating period is completed, the test specimen is taken out of the cell and investigated further using the micro-CT machine to capture the distribution of internal desiccation cracks. This study also examines the effect of bentonite reinforcement using inorganic microfibers on the development of desiccation cracks. INTRODUCTION Bentonite has been widely used for many engineering applications, such as for the barrier of pollutant transfer, for the stability of engineered construction, etc., due to its characteristics of high swelling pressure, low hydraulic permeability, and adsorption capacity (Smith et al. 1980; Yong et al., 1986; Cho et al. 2000). These characteristics improve even further as the dry density of bentonite increases. Bentonite has also been considered the best candidate as an engineered barrier material for a geological repository of high-level nuclear wastes, including spent nuclear fuels (SNF). Most of the past geological repository concepts suggested the temperature at the canister of SNF should not exceed 100°C to better control the sealing performance of bentonite (Martin et al. 2014; Abootalebi and Siemens 2018). On the other hand, however, several current research efforts are examining the impact of higher temperatures, up to 200°C, on the safety functions of buffer materials, which in turn, can help to achieve shorter cooling times on the ground, more efficient packaging, reduced need for disposal containers and transportation, and smaller facility footprints (Pusch 1980; Gens and Garcia-Fontanet 1997; Gens et al. 2022; Chang et al. 2022).
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Noh, D. H., S. Kim, J. Eun, and Y. R. Kim. "High-Temperature Performance of Compacted Bentonite Blocks for High-Level Nuclear Waste Repository: A Study of Desiccation Crack Control and Gas Permeability with Glass Microfiber Reinforcement." In 58th U.S. Rock Mechanics/Geomechanics Symposium. ARMA, 2024. http://dx.doi.org/10.56952/arma-2024-0513.
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ABSTRACT: Geological repositories for high-level nuclear waste require barriers with low permeability to prevent radionuclide migration. Compacted bentonite, which exhibits low permeability and high swelling capacity, is considered an ideal candidate for such applications. However, the thermal conditions within repositories, potentially reaching temperatures of 200°C, pose a risk of moisture loss and desiccation cracking, which can compromise the sealing performance of bentonite. This study investigates the use of glass microfiber reinforcement to enhance the structural stability of bentonite blocks against thermal challenges in geological repositories. We investigated the gas permeability and crack formation in bentonite blocks reinforced with varying concentrations of glass microfiber. The results indicated that the fiber content of C = 1% effectively reduces crack size without altering the gas permeability, maintaining the sealing performance close to that of pure bentonite. However, a higher value of the gas permeability was found in the bentonite block with the fiber content at C = 2%. X-ray CT imaging provided insights into the microstructural changes, confirming the role of the glass fiber in mitigating crack formation. This study underscores the importance of optimizing fiber content in bentonite-based barriers to ensure the good sealing performance of waste containment systems under high thermal conditions. 1. INTRODUCTION Deep geological repositories use highly compacted bentonite blocks to seal radioactive waste, thereby preventing leachate from the radioactive waste into the surrounding environment. As such, thermo-hydro-mechanically coupled behaviors of bentonite blocks have been intensely studied to enhance their performance for the deep geological repository of high-level nuclear wastes (Smith et al., 1980; Yong et al., 1986; Cho et al., 1999; Cho et al., 2000; Lloret et al., 2003). In the geological repository setting, bentonite would surround the radioactive waste canister in the form of a compacted block with a high dry density. This bentonite block may encounter high temperatures as high as 200°C, based on factors such as waste heat emission, spacing of waste packages, and the thermal conductivity of the bentonite blocks and host rock (Martin et al., 2014; Abootalebi and Siemens, 2018). When the temperature of the repository exceeds 100°C, the bentonite buffer may lose moisture in its pores. As a result, not only does the swelling pressure reduce, but desiccation cracks also form due to the shrinkage in the volume (Villar and Lloret, 2004; Gebrenegus et al., 2011; Ye et al., 2013; Sánchez et al., 2013; Kim et al., 2021). The reduction in the swelling pressure and the presence of such desiccation cracks could compromise the desired sealing performance of the bentonite block (Pusch, 1980; Gens and Garcia-Fontanet, 1997; Martin et al., 2006; Åkesson et al., 2009; Chang et al., 2022; Gens et al., 2022;). In this regard, the addition of additives such as graphite aggregates, sands, copper wires or powder, and inorganic microfibers have been examined by several research groups to improve the sealing performance of the bentonite buffer, mainly by increasing the thermal conductivity or imparting the tensile stress between bentonite particles (Liu et al., 2019; Wang and Hadgu, 2020; Peng et al., 2021; Feng et al., 2024). On the other hand, adding those additives brought up a new concern that the inclusion at high percentages could potentially compromise the hydraulic sealing capacity of bentonite blocks because it may lead to predominant fluid flow pathways. Therefore, further in-depth study is needed to examine the optimal contents of such additives that can help maintain the sealing integrity while improving the mechanical performance of bentonite blocks.
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Rassouli, F. S., E. Haghighat, R. Juanes, and M. D. Zoback. "A Thermo-Visco-Plastic Study of the Wolfcamp Shale: Experiments and Modeling." In 58th U.S. Rock Mechanics/Geomechanics Symposium. ARMA, 2024. http://dx.doi.org/10.56952/arma-2024-0063.
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ABSTRACT: Here, we present a preliminary laboratory study of visco-plastic deformation at representative reservoir temperatures. We performed three loading and unloading stages on each sample, measuring time-dependent deformation after each stage. For two samples, we started at room temperature (24 °C), then increased temperature to 50 °C and then from 50°C to 80°C. We reported that the elevated temperatures soften the samples and increase the creep deformation rate. However, the creep behavior of the sample under a cyclic loading pattern and at a constantly elevated reservoir temperature is similar to what we observed at room temperature in our previous studies, following creep-hardening processes. To better isolate the visco-plastic deformation at elevated temperatures, we performed one experiment at 80 °C and then applied the loading and unloading cycles to the sample. This sample showed creep hardening after applying three cycles of creep loading at 80°C. We extended a visco-plastic creep model that accurately reproduced room temperature experiments to account for the model's temperature dependence on visco-plastic parameters. Here, our modeling is focused on describing the mechanism, however, we realize the need for anisotropic considerations, which we postpone to a later study. 1. INTRODUCTION Propagation of hydraulic fractures requires the pressure inside the fractures to exceed the magnitude of the least principal stress. In this context, vertical propagation of hydraulic fractures in unconventional shale formations is controlled by variations of the magnitude of the least principal stress with depth. In previous papers, we showed that the magnitude of stress variation is a function of the relative degree of viscoplastic stress relaxation (Sone and Zoback, (2014), Rassouli and Zoback (2016), Ma and Zoback (2017), and Xu et al. (2017)). This time-dependent viscoplastic behavior is affected by the mechanical and mineralogical properties of the rocks, as well as the reservoir stress and thermal conditions. The degree of anisotropy and the direction of the applied differential stress with respect to the bedding plane of the rock formations are correlated with their time-dependent deformation. In general, a higher viscoplastic deformation is expected in isostress conditions, where the direction of the applied differential stress is orthogonal to the orientation of bedding planes. (Sone and Zoback (2013), and Rassouli and Zoback (2016)). Shale rocks with higher porosity and preexisting micro-cracks show higher creep compliance (Sone and Zoback (2014), and Rassouli and Zoback (2018)). Variations in the lithology of the formation change the viscoplastic response of the reservoirs (Alalli and Zoback (2018)). Higher clay, kerogen, and soft mineral contents in addition to the presence of porous carbonate minerals enhance the viscoplastic deformation of the rocks (Sone and Zoback (2014), Rassouli and Zoback (2016), Ma and Zoback (2017), and Rassouli and Zoback (2018)). Water content increases the time-dependent deformation of shales by reducing the frictional coefficient between the grains (Rassouli and Zoback (2018)). As the ductile flow of minerals is known as a thermally activated process, the higher temperature at reservoir depths increases the time-dependent deformation of shales (Rassouli (2017), and Rybacki et al. (2017)). The stress regime and the level of the reservoir anisotropy have a direct effect on the viscoplastic deformation of shales. Higher confining pressure increases the shear strength and reduces the ductile deformation of rocks (Sone and Zoback (2014), and Geng et al. (2017)). The viscoplastic deformation in normal/strike-slip faulting systems results in stress relaxation, meaning that the three principal stresses become more isotropic such that the least principal stress increases (Sone and Zoback (2014)). This would make some formations more difficult to fracture hydraulically or it could create stress regime barriers to the vertical propagation of hydraulic fractures from other intervals (Xu et al. (2017)).
Reports on the topic "Gene flow barrier"
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Kirk, James. The Columbia River as a Barrier to Gene Flow in the Vagrant Shrew, Sorex vagrans vagrans Baird. Portland State University Library, January 2000. http://dx.doi.org/10.15760/etd.2550.
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