51
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Genomic islands of differentiation in a rapid avian radiation have been driven by recent selective sweeps. Proc Natl Acad Sci U S A 2020; 117:30554-30565. [PMID: 33199636 DOI: 10.1073/pnas.2015987117] [Citation(s) in RCA: 24] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
Numerous studies of emerging species have identified genomic "islands" of elevated differentiation against a background of relative homogeneity. The causes of these islands remain unclear, however, with some signs pointing toward "speciation genes" that locally restrict gene flow and others suggesting selective sweeps that have occurred within nascent species after speciation. Here, we examine this question through the lens of genome sequence data for five species of southern capuchino seedeaters, finch-like birds from South America that have undergone a species radiation during the last ∼50,000 generations. By applying newly developed statistical methods for ancestral recombination graph inference and machine-learning methods for the prediction of selective sweeps, we show that previously identified islands of differentiation in these birds appear to be generally associated with relatively recent, species-specific selective sweeps, most of which are predicted to be soft sweeps acting on standing genetic variation. Many of these sweeps coincide with genes associated with melanin-based variation in plumage, suggesting a prominent role for sexual selection. At the same time, a few loci also exhibit indications of possible selection against gene flow. These observations shed light on the complex manner in which natural selection shapes genome sequences during speciation.
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52
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Bourgeois Y, Ruggiero RP, Hariyani I, Boissinot S. Disentangling the determinants of transposable elements dynamics in vertebrate genomes using empirical evidences and simulations. PLoS Genet 2020; 16:e1009082. [PMID: 33017388 PMCID: PMC7561263 DOI: 10.1371/journal.pgen.1009082] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2020] [Revised: 10/15/2020] [Accepted: 08/25/2020] [Indexed: 12/14/2022] Open
Abstract
The interactions between transposable elements (TEs) and their hosts constitute one of the most profound co-evolutionary processes found in nature. The population dynamics of TEs depends on factors specific to each TE families, such as the rate of transposition and insertional preference, the demographic history of the host and the genomic landscape. How these factors interact has yet to be investigated holistically. Here we are addressing this question in the green anole (Anolis carolinensis) whose genome contains an extraordinary diversity of TEs (including non-LTR retrotransposons, SINEs, LTR-retrotransposons and DNA transposons). We observed a positive correlation between recombination rate and frequency of TEs and densities for LINEs, SINEs and DNA transposons. For these elements, there was a clear impact of demography on TE frequency and abundance, with a loss of polymorphic elements and skewed frequency spectra in recently expanded populations. On the other hand, some LTR-retrotransposons displayed patterns consistent with a very recent phase of intense amplification. To determine how demography, genomic features and intrinsic properties of TEs interact we ran simulations using SLiM3. We determined that i) short TE insertions are not strongly counter-selected, but long ones are, ii) neutral demographic processes, linked selection and preferential insertion may explain positive correlations between average TE frequency and recombination, iii) TE insertions are unlikely to have been massively recruited in recent adaptation. We demonstrate that deterministic and stochastic processes have different effects on categories of TEs and that a combination of empirical analyses and simulations can disentangle these mechanisms.
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Affiliation(s)
- Yann Bourgeois
- School of Biological Sciences, University of Portsmouth, Portsmouth, United Kingdom
- New York University Abu Dhabi, Saadiyat Island Campus, Abu Dhabi, United Arab Emirates
- * E-mail: (YB); (SB)
| | - Robert P. Ruggiero
- New York University Abu Dhabi, Saadiyat Island Campus, Abu Dhabi, United Arab Emirates
- Department of Biology, Southeast Missouri State University, Cape Girardeau, MO, United States of America
| | - Imtiyaz Hariyani
- New York University Abu Dhabi, Saadiyat Island Campus, Abu Dhabi, United Arab Emirates
| | - Stéphane Boissinot
- New York University Abu Dhabi, Saadiyat Island Campus, Abu Dhabi, United Arab Emirates
- * E-mail: (YB); (SB)
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53
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Schrider DR. Background Selection Does Not Mimic the Patterns of Genetic Diversity Produced by Selective Sweeps. Genetics 2020; 216:499-519. [PMID: 32847814 PMCID: PMC7536861 DOI: 10.1534/genetics.120.303469] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2019] [Accepted: 08/04/2020] [Indexed: 12/28/2022] Open
Abstract
It is increasingly evident that natural selection plays a prominent role in shaping patterns of diversity across the genome. The most commonly studied modes of natural selection are positive selection and negative selection, which refer to directional selection for and against derived mutations, respectively. Positive selection can result in hitchhiking events, in which a beneficial allele rapidly replaces all others in the population, creating a valley of diversity around the selected site along with characteristic skews in allele frequencies and linkage disequilibrium among linked neutral polymorphisms. Similarly, negative selection reduces variation not only at selected sites but also at linked sites, a phenomenon called background selection (BGS). Thus, discriminating between these two forces may be difficult, and one might expect efforts to detect hitchhiking to produce an excess of false positives in regions affected by BGS. Here, we examine the similarity between BGS and hitchhiking models via simulation. First, we show that BGS may somewhat resemble hitchhiking in simplistic scenarios in which a region constrained by negative selection is flanked by large stretches of unconstrained sites, echoing previous results. However, this scenario does not mirror the actual spatial arrangement of selected sites across the genome. By performing forward simulations under more realistic scenarios of BGS, modeling the locations of protein-coding and conserved noncoding DNA in real genomes, we show that the spatial patterns of variation produced by BGS rarely mimic those of hitchhiking events. Indeed, BGS is not substantially more likely than neutrality to produce false signatures of hitchhiking. This holds for simulations modeled after both humans and Drosophila, and for several different demographic histories. These results demonstrate that appropriately designed scans for hitchhiking need not consider BGS's impact on false-positive rates. However, we do find evidence that BGS increases the false-negative rate for hitchhiking, an observation that demands further investigation.
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Affiliation(s)
- Daniel R Schrider
- Department of Genetics, University of North Carolina, Chapel Hill, North Carolina 27514
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54
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Mughal MR, Koch H, Huang J, Chiaromonte F, DeGiorgio M. Learning the properties of adaptive regions with functional data analysis. PLoS Genet 2020; 16:e1008896. [PMID: 32853200 PMCID: PMC7480868 DOI: 10.1371/journal.pgen.1008896] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2019] [Revised: 09/09/2020] [Accepted: 05/29/2020] [Indexed: 12/12/2022] Open
Abstract
Identifying regions of positive selection in genomic data remains a challenge in population genetics. Most current approaches rely on comparing values of summary statistics calculated in windows. We present an approach termed SURFDAWave, which translates measures of genetic diversity calculated in genomic windows to functional data. By transforming our discrete data points to be outputs of continuous functions defined over genomic space, we are able to learn the features of these functions that signify selection. This enables us to confidently identify complex modes of natural selection, including adaptive introgression. We are also able to predict important selection parameters that are responsible for shaping the inferred selection events. By applying our model to human population-genomic data, we recapitulate previously identified regions of selective sweeps, such as OCA2 in Europeans, and predict that its beneficial mutation reached a frequency of 0.02 before it swept 1,802 generations ago, a time when humans were relatively new to Europe. In addition, we identify BNC2 in Europeans as a target of adaptive introgression, and predict that it harbors a beneficial mutation that arose in an archaic human population that split from modern humans within the hypothesized modern human-Neanderthal divergence range.
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Affiliation(s)
- Mehreen R. Mughal
- Bioinformatics and Genomics at the Huck Institutes of the Life Sciences, Pennsylvania State University, University Park, Pennsylvania, United States of America
| | - Hillary Koch
- Department of Statistics, Pennsylvania State University, University Park, Pennsylvania, United States of America
| | - Jinguo Huang
- Bioinformatics and Genomics at the Huck Institutes of the Life Sciences, Pennsylvania State University, University Park, Pennsylvania, United States of America
| | - Francesca Chiaromonte
- Department of Statistics, Pennsylvania State University, University Park, Pennsylvania, United States of America
| | - Michael DeGiorgio
- Department of Computer and Electrical Engineering and Computer Science, Florida Atlantic University, Boca Raton, Florida, United States of America
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55
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Cayuela H, Rougemont Q, Laporte M, Mérot C, Normandeau E, Dorant Y, Tørresen OK, Hoff SNK, Jentoft S, Sirois P, Castonguay M, Jansen T, Praebel K, Clément M, Bernatchez L. Shared ancestral polymorphisms and chromosomal rearrangements as potential drivers of local adaptation in a marine fish. Mol Ecol 2020; 29:2379-2398. [DOI: 10.1111/mec.15499] [Citation(s) in RCA: 35] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2019] [Revised: 05/19/2020] [Accepted: 05/26/2020] [Indexed: 12/18/2022]
Affiliation(s)
- Hugo Cayuela
- Institut de Biologie Intégrative et des Systèmes (IBIS) Université Laval Quebec City QC Canada
| | - Quentin Rougemont
- Institut de Biologie Intégrative et des Systèmes (IBIS) Université Laval Quebec City QC Canada
| | - Martin Laporte
- Institut de Biologie Intégrative et des Systèmes (IBIS) Université Laval Quebec City QC Canada
| | - Claire Mérot
- Institut de Biologie Intégrative et des Systèmes (IBIS) Université Laval Quebec City QC Canada
| | - Eric Normandeau
- Institut de Biologie Intégrative et des Systèmes (IBIS) Université Laval Quebec City QC Canada
| | - Yann Dorant
- Institut de Biologie Intégrative et des Systèmes (IBIS) Université Laval Quebec City QC Canada
| | - Ole K. Tørresen
- Centre for Ecological and Evolutionary Synthesis (CEES) Department of Biosciences University of Oslo Oslo Norway
| | - Siv Nam Khang Hoff
- Centre for Ecological and Evolutionary Synthesis (CEES) Department of Biosciences University of Oslo Oslo Norway
| | - Sissel Jentoft
- Centre for Ecological and Evolutionary Synthesis (CEES) Department of Biosciences University of Oslo Oslo Norway
| | - Pascal Sirois
- Département des sciences fondamentales Université du Québec à Chicoutimi Chicoutimi QC Canada
| | - Martin Castonguay
- Fisheries and Oceans Canada Institut Maurice‐Lamontagne Mont‐Joli QC Canada
| | - Teunis Jansen
- GINR‐Greenland Institute of Natural Resources Nuuk Greenland
- DTU Aqua‐National Institute of Aquatic Resources Technical University of Denmark Charlottenlund Castle, Charlottenlund Denmark
| | - Kim Praebel
- Norwegian College of Fishery Science Faculty of Biosciences, Fisheries and Economics UiT The Arctic University of Norway Tromsø Norway
| | - Marie Clément
- Center for Fisheries Ecosystems Research Fisheries and Marine Institute of Memorial University of Newfoundland St. John's NL Canada
- Labrador Institute of Memorial University of Newfoundland Happy Valley‐Goose Bay NL Canada
| | - Louis Bernatchez
- Institut de Biologie Intégrative et des Systèmes (IBIS) Université Laval Quebec City QC Canada
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56
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Adrion JR, Cole CB, Dukler N, Galloway JG, Gladstein AL, Gower G, Kyriazis CC, Ragsdale AP, Tsambos G, Baumdicker F, Carlson J, Cartwright RA, Durvasula A, Gronau I, Kim BY, McKenzie P, Messer PW, Noskova E, Ortega-Del Vecchyo D, Racimo F, Struck TJ, Gravel S, Gutenkunst RN, Lohmueller KE, Ralph PL, Schrider DR, Siepel A, Kelleher J, Kern AD. A community-maintained standard library of population genetic models. eLife 2020; 9:e54967. [PMID: 32573438 PMCID: PMC7438115 DOI: 10.7554/elife.54967] [Citation(s) in RCA: 81] [Impact Index Per Article: 20.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2020] [Accepted: 06/15/2020] [Indexed: 12/18/2022] Open
Abstract
The explosion in population genomic data demands ever more complex modes of analysis, and increasingly, these analyses depend on sophisticated simulations. Recent advances in population genetic simulation have made it possible to simulate large and complex models, but specifying such models for a particular simulation engine remains a difficult and error-prone task. Computational genetics researchers currently re-implement simulation models independently, leading to inconsistency and duplication of effort. This situation presents a major barrier to empirical researchers seeking to use simulations for power analyses of upcoming studies or sanity checks on existing genomic data. Population genetics, as a field, also lacks standard benchmarks by which new tools for inference might be measured. Here, we describe a new resource, stdpopsim, that attempts to rectify this situation. Stdpopsim is a community-driven open source project, which provides easy access to a growing catalog of published simulation models from a range of organisms and supports multiple simulation engine backends. This resource is available as a well-documented python library with a simple command-line interface. We share some examples demonstrating how stdpopsim can be used to systematically compare demographic inference methods, and we encourage a broader community of developers to contribute to this growing resource.
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Affiliation(s)
- Jeffrey R Adrion
- Department of Biology and Institute of Ecology and Evolution, University of OregonEugeneUnited States
| | - Christopher B Cole
- Weatherall Institute of Molecular Medicine, University of OxfordOxfordUnited Kingdom
| | - Noah Dukler
- Simons Center for Quantitative Biology, Cold Spring Harbor LaboratoryCold Spring HarborUnited States
| | - Jared G Galloway
- Department of Biology and Institute of Ecology and Evolution, University of OregonEugeneUnited States
| | - Ariella L Gladstein
- Department of Genetics, University of North Carolina at Chapel HillChapel HillUnited States
| | - Graham Gower
- Lundbeck GeoGenetics Centre, Globe Institute, University of CopenhagenCopenhagenDenmark
| | - Christopher C Kyriazis
- Department of Ecology and Evolutionary Biology, University of California, Los AngelesLos AngelesUnited States
| | | | - Georgia Tsambos
- Melbourne Integrative Genomics, School of Mathematics and Statistics, University of MelbourneMelbourneAustralia
| | - Franz Baumdicker
- Department of Mathematical Stochastics, University of FreiburgFreiburgGermany
| | - Jedidiah Carlson
- Department of Genome Sciences, University of WashingtonSeattleUnited States
| | - Reed A Cartwright
- The Biodesign Institute and The School of Life Sciences, Arizona State UniversityTempeUnited States
| | - Arun Durvasula
- Department of Human Genetics, David Geffen School of Medicine, University of California, Los AngelesLos AngelesUnited States
| | - Ilan Gronau
- The Efi Arazi School of Computer Science, Herzliya Interdisciplinary CenterHerzliyaIsrael
| | - Bernard Y Kim
- Department of Biology, Stanford UniversityStanfordUnited States
| | - Patrick McKenzie
- Department of Ecology, Evolution, and Environmental Biology, Columbia UniversityNew YorkUnited States
| | - Philipp W Messer
- Department of Computational BiologyCornell UniversityIthacaUnited States
| | - Ekaterina Noskova
- Computer Technologies Laboratory, ITMO UniversitySaint PetersburgRussian Federation
| | - Diego Ortega-Del Vecchyo
- International Laboratory for Human Genome Research, National Autonomous University of MexicoJuriquillaMexico
| | - Fernando Racimo
- Lundbeck GeoGenetics Centre, Globe Institute, University of CopenhagenCopenhagenDenmark
| | - Travis J Struck
- Departmentof Molecular and Cellular Biology, University of ArizonaTucsonUnited States
| | - Simon Gravel
- Department of Human Genetics, McGill UniversityMontrealCanada
| | - Ryan N Gutenkunst
- Departmentof Molecular and Cellular Biology, University of ArizonaTucsonUnited States
| | - Kirk E Lohmueller
- Department of Ecology and Evolutionary Biology, University of California, Los AngelesLos AngelesUnited States
- Department of Human Genetics, David Geffen School of Medicine, University of California, Los AngelesLos AngelesUnited States
| | - Peter L Ralph
- Department of Biology and Institute of Ecology and Evolution, University of OregonEugeneUnited States
- Department of Mathematics, University of OregonEugeneUnited States
| | - Daniel R Schrider
- Department of Genetics, University of North Carolina at Chapel HillChapel HillUnited States
| | - Adam Siepel
- Simons Center for Quantitative Biology, Cold Spring Harbor LaboratoryCold Spring HarborUnited States
| | - Jerome Kelleher
- Big Data Institute, Li Ka Shing Centre for Health Information and Discovery, University of OxfordOxfordUnited Kingdom
| | - Andrew D Kern
- Department of Biology and Institute of Ecology and Evolution, University of OregonEugeneUnited States
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57
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Battey CJ, Ralph PL, Kern AD. Predicting geographic location from genetic variation with deep neural networks. eLife 2020; 9:e54507. [PMID: 32511092 PMCID: PMC7324158 DOI: 10.7554/elife.54507] [Citation(s) in RCA: 45] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2019] [Accepted: 06/03/2020] [Indexed: 12/12/2022] Open
Abstract
Most organisms are more closely related to nearby than distant members of their species, creating spatial autocorrelations in genetic data. This allows us to predict the location of origin of a genetic sample by comparing it to a set of samples of known geographic origin. Here, we describe a deep learning method, which we call Locator, to accomplish this task faster and more accurately than existing approaches. In simulations, Locator infers sample location to within 4.1 generations of dispersal and runs at least an order of magnitude faster than a recent model-based approach. We leverage Locator's computational efficiency to predict locations separately in windows across the genome, which allows us to both quantify uncertainty and describe the mosaic ancestry and patterns of geographic mixing that characterize many populations. Applied to whole-genome sequence data from Plasmodium parasites, Anopheles mosquitoes, and global human populations, this approach yields median test errors of 16.9km, 5.7km, and 85km, respectively.
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Affiliation(s)
- CJ Battey
- University of Oregon, Institute of Ecology and EvolutionEugeneUnited States
| | - Peter L Ralph
- University of Oregon, Institute of Ecology and EvolutionEugeneUnited States
| | - Andrew D Kern
- University of Oregon, Institute of Ecology and EvolutionEugeneUnited States
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58
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Mueller JC, Carrete M, Boerno S, Kuhl H, Tella JL, Kempenaers B. Genes acting in synapses and neuron projections are early targets of selection during urban colonization. Mol Ecol 2020; 29:3403-3412. [DOI: 10.1111/mec.15451] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2019] [Accepted: 04/08/2020] [Indexed: 02/06/2023]
Affiliation(s)
- Jakob C. Mueller
- Department of Behavioural Ecology & Evolutionary Genetics Max Planck Institute for Ornithology Seewiesen Germany
| | - Martina Carrete
- Department of Conservation Biology Estación Biológica de Doñana – CSIC Sevilla Spain
- Department of Physical, Chemical and Natural Systems University Pablo de Olavide Sevilla Spain
| | - Stefan Boerno
- Sequencing Core Facility Max Planck Institute for Molecular Genetics Berlin Germany
| | - Heiner Kuhl
- Sequencing Core Facility Max Planck Institute for Molecular Genetics Berlin Germany
- Department of Ecophysiology and Aquaculture Leibniz‐Institute of Freshwater Ecology and Inland Fisheries Berlin Germany
| | - José L. Tella
- Department of Conservation Biology Estación Biológica de Doñana – CSIC Sevilla Spain
| | - Bart Kempenaers
- Department of Behavioural Ecology & Evolutionary Genetics Max Planck Institute for Ornithology Seewiesen Germany
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59
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Abstract
Accurately inferring the genome-wide landscape of recombination rates in natural populations is a central aim in genomics, as patterns of linkage influence everything from genetic mapping to understanding evolutionary history. Here, we describe recombination landscape estimation using recurrent neural networks (ReLERNN), a deep learning method for estimating a genome-wide recombination map that is accurate even with small numbers of pooled or individually sequenced genomes. Rather than use summaries of linkage disequilibrium as its input, ReLERNN takes columns from a genotype alignment, which are then modeled as a sequence across the genome using a recurrent neural network. We demonstrate that ReLERNN improves accuracy and reduces bias relative to existing methods and maintains high accuracy in the face of demographic model misspecification, missing genotype calls, and genome inaccessibility. We apply ReLERNN to natural populations of African Drosophila melanogaster and show that genome-wide recombination landscapes, although largely correlated among populations, exhibit important population-specific differences. Lastly, we connect the inferred patterns of recombination with the frequencies of major inversions segregating in natural Drosophila populations.
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Affiliation(s)
- Jeffrey R Adrion
- Institute of Ecology and Evolution, University of Oregon, Eugene, OR
| | - Jared G Galloway
- Institute of Ecology and Evolution, University of Oregon, Eugene, OR
| | - Andrew D Kern
- Institute of Ecology and Evolution, University of Oregon, Eugene, OR
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60
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Barghi N, Schlötterer C. Distinct Patterns of Selective Sweep and Polygenic Adaptation in Evolve and Resequence Studies. Genome Biol Evol 2020; 12:890-904. [PMID: 32282913 PMCID: PMC7313669 DOI: 10.1093/gbe/evaa073] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/07/2020] [Indexed: 12/15/2022] Open
Abstract
In molecular population genetics, adaptation is typically thought to occur via selective sweeps, where targets of selection have independent effects on the phenotype and rise to fixation, whereas in quantitative genetics, many loci contribute to the phenotype and subtle frequency changes occur at many loci during polygenic adaptation. The sweep model makes specific predictions about frequency changes of beneficial alleles and many test statistics have been developed to detect such selection signatures. Despite polygenic adaptation is probably the prevalent mode of adaptation, because of the traditional focus on the phenotype, we are lacking a solid understanding of the similarities and differences of selection signatures under the two models. Recent theoretical and empirical studies have shown that both selective sweep and polygenic adaptation models could result in a sweep-like genomic signature; therefore, additional criteria are needed to distinguish the two models. With replicated populations and time series data, experimental evolution studies have the potential to identify the underlying model of adaptation. Using the framework of experimental evolution, we performed computer simulations to study the pattern of selected alleles for two models: 1) adaptation of a trait via independent beneficial mutations that are conditioned for fixation, that is, selective sweep model and 2) trait optimum model (polygenic adaptation), that is adaptation of a quantitative trait under stabilizing selection after a sudden shift in trait optimum. We identify several distinct patterns of selective sweep and trait optimum models in populations of different sizes. These features could provide the foundation for development of quantitative approaches to differentiate the two models.
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Affiliation(s)
- Neda Barghi
- Institut für Populationsgenetik, Vetmeduni, Vienna, Austria
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61
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Zou Z, Zhang H, Guan Y, Zhang J. Deep Residual Neural Networks Resolve Quartet Molecular Phylogenies. Mol Biol Evol 2020; 37:1495-1507. [PMID: 31868908 PMCID: PMC8453599 DOI: 10.1093/molbev/msz307] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Phylogenetic inference is of fundamental importance to evolutionary as well as other fields of biology, and molecular sequences have emerged as the primary data for this task. Although many phylogenetic methods have been developed to explicitly take into account substitution models of sequence evolution, such methods could fail due to model misspecification or insufficiency, especially in the face of heterogeneities in substitution processes across sites and among lineages. In this study, we propose to infer topologies of four-taxon trees using deep residual neural networks, a machine learning approach needing no explicit modeling of the subject system and having a record of success in solving complex nonlinear inference problems. We train residual networks on simulated protein sequence data with extensive amino acid substitution heterogeneities. We show that the well-trained residual network predictors can outperform existing state-of-the-art inference methods such as the maximum likelihood method on diverse simulated test data, especially under extensive substitution heterogeneities. Reassuringly, residual network predictors generally agree with existing methods in the trees inferred from real phylogenetic data with known or widely believed topologies. Furthermore, when combined with the quartet puzzling algorithm, residual network predictors can be used to reconstruct trees with more than four taxa. We conclude that deep learning represents a powerful new approach to phylogenetic reconstruction, especially when sequences evolve via heterogeneous substitution processes. We present our best trained predictor in a freely available program named Phylogenetics by Deep Learning (PhyDL, https://gitlab.com/ztzou/phydl; last accessed January 3, 2020).
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Affiliation(s)
- Zhengting Zou
- Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI
| | - Hongjiu Zhang
- Department of Computational Medicine and Bioinformatics, University of Michigan, Ann Arbor, MI
| | - Yuanfang Guan
- Department of Computational Medicine and Bioinformatics, University of Michigan, Ann Arbor, MI
- Department of Internal Medicine, University of Michigan, Ann Arbor, MI
| | - Jianzhi Zhang
- Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI
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62
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Hejase HA, Dukler N, Siepel A. From Summary Statistics to Gene Trees: Methods for Inferring Positive Selection. Trends Genet 2020; 36:243-258. [PMID: 31954511 PMCID: PMC7177178 DOI: 10.1016/j.tig.2019.12.008] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2019] [Revised: 11/15/2019] [Accepted: 12/11/2019] [Indexed: 01/01/2023]
Abstract
Methods to detect signals of natural selection from genomic data have traditionally emphasized the use of simple summary statistics. Here, we review a new generation of methods that consider combinations of conventional summary statistics and/or richer features derived from inferred gene trees and ancestral recombination graphs (ARGs). We also review recent advances in methods for population genetic simulation and ARG reconstruction. Finally, we describe opportunities for future work on a variety of related topics, including the genetics of speciation, estimation of selection coefficients, and inference of selection on polygenic traits. Together, these emerging methods offer promising new directions in the study of natural selection.
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Affiliation(s)
- Hussein A Hejase
- Simons Center for Quantitative Biology, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA.
| | - Noah Dukler
- Simons Center for Quantitative Biology, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA
| | - Adam Siepel
- Simons Center for Quantitative Biology, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA
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63
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Hartfield M, Bataillon T. Selective Sweeps Under Dominance and Inbreeding. G3 (BETHESDA, MD.) 2020; 10:1063-1075. [PMID: 31974096 PMCID: PMC7056974 DOI: 10.1534/g3.119.400919] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/18/2019] [Accepted: 01/18/2020] [Indexed: 12/26/2022]
Abstract
A major research goal in evolutionary genetics is to uncover loci experiencing positive selection. One approach involves finding 'selective sweeps' patterns, which can either be 'hard sweeps' formed by de novo mutation, or 'soft sweeps' arising from recurrent mutation or existing standing variation. Existing theory generally assumes outcrossing populations, and it is unclear how dominance affects soft sweeps. We consider how arbitrary dominance and inbreeding via self-fertilization affect hard and soft sweep signatures. With increased self-fertilization, they are maintained over longer map distances due to reduced effective recombination and faster beneficial allele fixation times. Dominance can affect sweep patterns in outcrossers if the derived variant originates from either a single novel allele, or from recurrent mutation. These models highlight the challenges in distinguishing hard and soft sweeps, and propose methods to differentiate between scenarios.
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Affiliation(s)
- Matthew Hartfield
- Department of Ecology and Evolutionary Biology, University of Toronto, Ontario M5S 3B2, Canada,
- Bioinformatics Research Centre, Aarhus University, Aarhus 8000, Denmark, and
- Institute of Evolutionary Biology, The University of Edinburgh, Edinburgh EH9 3FL, United Kingdom
| | - Thomas Bataillon
- Bioinformatics Research Centre, Aarhus University, Aarhus 8000, Denmark, and
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64
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Abstract
Cognitive abilities can vary dramatically among species. The relative importance of social and ecological challenges in shaping cognitive evolution has been the subject of a long-running and recently renewed debate, but little work has sought to understand the selective dynamics underlying the evolution of cognitive abilities. Here, we investigate recent selection related to cognition in the paper wasp Polistes fuscatus-a wasp that has uniquely evolved visual individual recognition abilities. We generate high quality de novo genome assemblies and population genomic resources for multiple species of paper wasps and use a population genomic framework to interrogate the probable mode and tempo of cognitive evolution. Recent, strong, hard selective sweeps in P. fuscatus contain loci annotated with functions in long-term memory formation, mushroom body development, and visual processing, traits which have recently evolved in association with individual recognition. The homologous pathways are not under selection in closely related wasps that lack individual recognition. Indeed, the prevalence of candidate cognition loci within the strongest selective sweeps suggests that the evolution of cognitive abilities has been among the strongest selection pressures in P. fuscatus' recent evolutionary history. Detailed analyses of selective sweeps containing candidate cognition loci reveal multiple cases of hard selective sweeps within the last few thousand years on de novo mutations, mainly in noncoding regions. These data provide unprecedented insight into some of the processes by which cognition evolves.
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65
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Koropoulis A, Alachiotis N, Pavlidis P. Detecting Positive Selection in Populations Using Genetic Data. Methods Mol Biol 2020; 2090:87-123. [PMID: 31975165 DOI: 10.1007/978-1-0716-0199-0_5] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
High-throughput genomic sequencing allows to disentangle the evolutionary forces acting in populations. Among evolutionary forces, positive selection has received a lot of attention because it is related to the adaptation of populations in their environments, both biotic and abiotic. Positive selection, also known as Darwinian selection, occurs when an allele is favored by natural selection. The frequency of the favored allele increases in the population and, due to genetic hitchhiking, neighboring linked variation diminishes, creating so-called selective sweeps. Such a process leaves traces in genomes that can be detected in a future time point. Detecting traces of positive selection in genomes is achieved by searching for signatures introduced by selective sweeps, such as regions of reduced variation, a specific shift of the site frequency spectrum, and particular linkage disequilibrium (LD) patterns in the region. A variety of approaches can be used for detecting selective sweeps, ranging from simple implementations that compute summary statistics to more advanced statistical approaches, e.g., Bayesian approaches, maximum-likelihood-based methods, and machine learning methods. In this chapter, we discuss selective sweep detection methodologies on the basis of their capacity to analyze whole genomes or just subgenomic regions, and on the specific polymorphism patterns they exploit as selective sweep signatures. We also summarize the results of comparisons among five open-source software releases (SweeD, SweepFinder, SweepFinder2, OmegaPlus, and RAiSD) regarding sensitivity, specificity, and execution times. Furthermore, we test and discuss machine learning methods and present a thorough performance analysis. In equilibrium neutral models or mild bottlenecks, most methods are able to detect selective sweeps accurately. Methods and tools that rely on linkage disequilibrium (LD) rather than single SNPs exhibit higher true positive rates than the site frequency spectrum (SFS)-based methods under the model of a single sweep or recurrent hitchhiking. However, their false positive rate is elevated when a misspecified demographic model is used to build the distribution of the statistic under the null hypothesis. Both LD and SFS-based approaches suffer from decreased accuracy on localizing the true target of selection in bottleneck scenarios. Furthermore, we present an extensive analysis of the effects of gene flow on selective sweep detection, a problem that has been understudied in selective sweep literature.
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Affiliation(s)
- Angelos Koropoulis
- Institute of Computer Science, Foundation for Research and Technology Hellas, Heraklion, Greece
- Computer Science Department, University of Crete, Crete, Heraklion, Greece
| | - Nikolaos Alachiotis
- Institute of Computer Science, Foundation for Research and Technology Hellas, Heraklion, Greece
| | - Pavlos Pavlidis
- Institute of Computer Science, Foundation for Research and Technology Hellas, Heraklion, Greece.
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66
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Torada L, Lorenzon L, Beddis A, Isildak U, Pattini L, Mathieson S, Fumagalli M. ImaGene: a convolutional neural network to quantify natural selection from genomic data. BMC Bioinformatics 2019; 20:337. [PMID: 31757205 PMCID: PMC6873651 DOI: 10.1186/s12859-019-2927-x] [Citation(s) in RCA: 38] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/21/2019] [Accepted: 05/31/2019] [Indexed: 12/25/2022] Open
Abstract
BACKGROUND The genetic bases of many complex phenotypes are still largely unknown, mostly due to the polygenic nature of the traits and the small effect of each associated mutation. An alternative approach to classic association studies to determining such genetic bases is an evolutionary framework. As sites targeted by natural selection are likely to harbor important functionalities for the carrier, the identification of selection signatures in the genome has the potential to unveil the genetic mechanisms underpinning human phenotypes. Popular methods of detecting such signals rely on compressing genomic information into summary statistics, resulting in the loss of information. Furthermore, few methods are able to quantify the strength of selection. Here we explored the use of deep learning in evolutionary biology and implemented a program, called ImaGene, to apply convolutional neural networks on population genomic data for the detection and quantification of natural selection. RESULTS ImaGene enables genomic information from multiple individuals to be represented as abstract images. Each image is created by stacking aligned genomic data and encoding distinct alleles into separate colors. To detect and quantify signatures of positive selection, ImaGene implements a convolutional neural network which is trained using simulations. We show how the method implemented in ImaGene can be affected by data manipulation and learning strategies. In particular, we show how sorting images by row and column leads to accurate predictions. We also demonstrate how the misspecification of the correct demographic model for producing training data can influence the quantification of positive selection. We finally illustrate an approach to estimate the selection coefficient, a continuous variable, using multiclass classification techniques. CONCLUSIONS While the use of deep learning in evolutionary genomics is in its infancy, here we demonstrated its potential to detect informative patterns from large-scale genomic data. We implemented methods to process genomic data for deep learning in a user-friendly program called ImaGene. The joint inference of the evolutionary history of mutations and their functional impact will facilitate mapping studies and provide novel insights into the molecular mechanisms associated with human phenotypes.
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Affiliation(s)
- Luis Torada
- Department of Life Sciences, Silwood Park campus, Imperial College London, Buckhurst Road, Ascot, SL5 7PY UK
| | - Lucrezia Lorenzon
- Department of Life Sciences, Silwood Park campus, Imperial College London, Buckhurst Road, Ascot, SL5 7PY UK
- Department of Electronics, Information and Bioengineering, Politecnico di Milano, piazza Leonardo da Vinci 32, Milan, 20133 Italy
| | - Alice Beddis
- Department of Life Sciences, Silwood Park campus, Imperial College London, Buckhurst Road, Ascot, SL5 7PY UK
| | - Ulas Isildak
- Department of Biological Sciences, Middle East Technical University, METU Üniversiteler Mah. Dumlupınar Blv. No:1, Ankara, 06800 Çankaya Turkey
| | - Linda Pattini
- Department of Electronics, Information and Bioengineering, Politecnico di Milano, piazza Leonardo da Vinci 32, Milan, 20133 Italy
| | - Sara Mathieson
- Department of Computer Science, Swarthmore College, 500 College Ave, Swarthmore, 19081 PA USA
| | - Matteo Fumagalli
- Department of Life Sciences, Silwood Park campus, Imperial College London, Buckhurst Road, Ascot, SL5 7PY UK
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67
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A Deep Learning Approach for Detecting Copy Number Variation in Next-Generation Sequencing Data. G3-GENES GENOMES GENETICS 2019; 9:3575-3582. [PMID: 31455677 PMCID: PMC6829143 DOI: 10.1534/g3.119.400596] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Copy number variants (CNV) are associated with phenotypic variation in several species. However, properly detecting changes in copy numbers of sequences remains a difficult problem, especially in lower quality or lower coverage next-generation sequencing data. Here, inspired by recent applications of machine learning in genomics, we describe a method to detect duplications and deletions in short-read sequencing data. In low coverage data, machine learning appears to be more powerful in the detection of CNVs than the gold-standard methods of coverage estimation alone, and of equal power in high coverage data. We also demonstrate how replicating training sets allows a more precise detection of CNVs, even identifying novel CNVs in two genomes previously surveyed thoroughly for CNVs using long read data.
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68
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Refoyo-Martínez A, da Fonseca RR, Halldórsdóttir K, Árnason E, Mailund T, Racimo F. Identifying loci under positive selection in complex population histories. Genome Res 2019; 29:1506-1520. [PMID: 31362936 PMCID: PMC6724678 DOI: 10.1101/gr.246777.118] [Citation(s) in RCA: 28] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2018] [Accepted: 07/23/2019] [Indexed: 12/24/2022]
Abstract
Detailed modeling of a species' history is of prime importance for understanding how natural selection operates over time. Most methods designed to detect positive selection along sequenced genomes, however, use simplified representations of past histories as null models of genetic drift. Here, we present the first method that can detect signatures of strong local adaptation across the genome using arbitrarily complex admixture graphs, which are typically used to describe the history of past divergence and admixture events among any number of populations. The method-called graph-aware retrieval of selective sweeps (GRoSS)-has good power to detect loci in the genome with strong evidence for past selective sweeps and can also identify which branch of the graph was most affected by the sweep. As evidence of its utility, we apply the method to bovine, codfish, and human population genomic data containing panels of multiple populations related in complex ways. We find new candidate genes for important adaptive functions, including immunity and metabolism in understudied human populations, as well as muscle mass, milk production, and tameness in specific bovine breeds. We are also able to pinpoint the emergence of large regions of differentiation owing to inversions in the history of Atlantic codfish.
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Affiliation(s)
- Alba Refoyo-Martínez
- Lundbeck GeoGenetics Centre, The Globe Institute, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen 1350, Denmark
| | - Rute R da Fonseca
- Centre for Macroecology, Evolution and Climate, The Globe Institute, Faculty of Health and Medical Sciences, University of Copenhagen, Copehnagen 2100, Denmark
| | - Katrín Halldórsdóttir
- Faculty of Life and Environmental Sciences, University of Iceland, Reykjavík 107, Iceland
| | - Einar Árnason
- Faculty of Life and Environmental Sciences, University of Iceland, Reykjavík 107, Iceland
- Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, USA
| | - Thomas Mailund
- Bioinformatics Research Centre, Aarhus University, Aarhus 8000, Denmark
| | - Fernando Racimo
- Lundbeck GeoGenetics Centre, The Globe Institute, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen 1350, Denmark
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69
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Bourgeois Y, Boissinot S. Selection at behavioural, developmental and metabolic genes is associated with the northward expansion of a successful tropical colonizer. Mol Ecol 2019; 28:3523-3543. [PMID: 31233650 DOI: 10.1111/mec.15162] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2018] [Accepted: 05/28/2019] [Indexed: 02/06/2023]
Abstract
What makes a species able to colonize novel environments? This question is key to understand the dynamics of adaptive radiations and ecological niche shifts, but the mechanisms that underlie expansion into novel habitats remain poorly understood at a genomic scale. Lizards from the genus Anolis are typically tropical, and the green anole (Anolis carolinensis) constitutes an exception since it expanded into temperate North America from subtropical Florida. Thus, we used the green anole as a model to investigate signatures of selection associated with colonization of a new environment, namely temperate North America. To this end, we analysed 29 whole-genome sequences, covering the entire native range of the species. We used a combination of recent methods to quantify both positive and balancing selection in northern populations, including FST outlier methods, machine learning and ancestral recombination graphs. We naively scanned for genes of interest and assessed the overlap between multiple tests. Strikingly, we identified many genes involved in behaviour, suggesting that the recent successful colonization of northern environments may have been linked to behavioural shifts as well as physiological adaptation. Using a candidate genes strategy, we determined that genes involved in response to cold or behaviour displayed more frequently signals of selection, while controlling for local recombination rate, gene clustering and gene length. In addition, we found signatures of balancing selection at immune genes in all investigated genetic groups, but also at genes involved in neuronal and anatomical development.
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Affiliation(s)
- Yann Bourgeois
- New York University Abu Dhabi, Abu Dhabi, United Arab Emirates
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70
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Richards EJ, Servedio MR, Martin CH. Searching for Sympatric Speciation in the Genomic Era. Bioessays 2019; 41:e1900047. [PMID: 31245871 PMCID: PMC8175013 DOI: 10.1002/bies.201900047] [Citation(s) in RCA: 40] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2019] [Revised: 04/22/2019] [Indexed: 12/25/2022]
Abstract
Sympatric speciation illustrates how natural and sexual selection may create new species in isolation without geographic barriers. However, recent genomic reanalyses of classic examples of sympatric speciation reveal complex histories of secondary gene flow from outgroups into the radiation. In contrast, the rich theoretical literature on this process distinguishes among a diverse range of models based on simple genetic histories and different types of reproductive isolating barriers. Thus, there is a need to revisit how to connect theoretical models of sympatric speciation and their predictions to empirical case studies in the face of widespread gene flow. Here, theoretical differences among different types of sympatric speciation and speciation-with-gene-flow models are reviewed and summarized, and genomic analyses are proposed for distinguishing which models apply to case studies based on the timing and function of adaptive introgression. Investigating whether secondary gene flow contributed to reproductive isolation is necessary to test whether predictions of theory are ultimately borne out in nature.
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Affiliation(s)
- Emilie J. Richards
- Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill NC
| | - Maria R. Servedio
- Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill NC
| | - Christopher H. Martin
- Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill NC
- Integrative Biology and Museum of Vertebrate Zoology, University of California, Berkeley, CA
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71
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Bourgeois Y, Boissinot S. On the Population Dynamics of Junk: A Review on the Population Genomics of Transposable Elements. Genes (Basel) 2019; 10:genes10060419. [PMID: 31151307 PMCID: PMC6627506 DOI: 10.3390/genes10060419] [Citation(s) in RCA: 67] [Impact Index Per Article: 13.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2019] [Revised: 05/05/2019] [Accepted: 05/21/2019] [Indexed: 01/18/2023] Open
Abstract
Transposable elements (TEs) play an important role in shaping genomic organization and structure, and may cause dramatic changes in phenotypes. Despite the genetic load they may impose on their host and their importance in microevolutionary processes such as adaptation and speciation, the number of population genetics studies focused on TEs has been rather limited so far compared to single nucleotide polymorphisms (SNPs). Here, we review the current knowledge about the dynamics of transposable elements at recent evolutionary time scales, and discuss the mechanisms that condition their abundance and frequency. We first discuss non-adaptive mechanisms such as purifying selection and the variable rates of transposition and elimination, and then focus on positive and balancing selection, to finally conclude on the potential role of TEs in causing genomic incompatibilities and eventually speciation. We also suggest possible ways to better model TEs dynamics in a population genomics context by incorporating recent advances in TEs into the rich information provided by SNPs about the demography, selection, and intrinsic properties of genomes.
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Affiliation(s)
- Yann Bourgeois
- New York University Abu Dhabi, P.O. 129188, Saadiyat Island, Abu Dhabi, United Arab Emirates.
| | - Stéphane Boissinot
- New York University Abu Dhabi, P.O. 129188, Saadiyat Island, Abu Dhabi, United Arab Emirates.
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72
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Abstract
Identifying genomic locations of natural selection from sequence data is an ongoing challenge in population genetics. Current methods utilizing information combined from several summary statistics typically assume no correlation of summary statistics regardless of the genomic location from which they are calculated. However, due to linkage disequilibrium, summary statistics calculated at nearby genomic positions are highly correlated. We introduce an approach termed Trendsetter that accounts for the similarity of statistics calculated from adjacent genomic regions through trend filtering, while reducing the effects of multicollinearity through regularization. Our penalized regression framework has high power to detect sweeps, is capable of classifying sweep regions as either hard or soft, and can be applied to other selection scenarios as well. We find that Trendsetter is robust to both extensive missing data and strong background selection, and has comparable power to similar current approaches. Moreover, the model learned by Trendsetter can be viewed as a set of curves modeling the spatial distribution of summary statistics in the genome. Application to human genomic data revealed positively selected regions previously discovered such as LCT in Europeans and EDAR in East Asians. We also identified a number of novel candidates and show that populations with greater relatedness share more sweep signals.
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Affiliation(s)
- Mehreen R Mughal
- Bioinformatics and Genomics at the Huck Institutes of the Life Sciences, Pennsylvania State University, University Park, PA
| | - Michael DeGiorgio
- Departments of Biology and Statistics, Pennsylvania State University,University Park, PA
- Institute for CyberScience, Pennsylvania State University, University Park, PA
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73
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Detection and Classification of Hard and Soft Sweeps from Unphased Genotypes by Multilocus Genotype Identity. Genetics 2018; 210:1429-1452. [PMID: 30315068 DOI: 10.1534/genetics.118.301502] [Citation(s) in RCA: 40] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2018] [Accepted: 10/08/2018] [Indexed: 11/18/2022] Open
Abstract
Positive natural selection can lead to a decrease in genomic diversity at the selected site and at linked sites, producing a characteristic signature of elevated expected haplotype homozygosity. These selective sweeps can be hard or soft. In the case of a hard selective sweep, a single adaptive haplotype rises to high population frequency, whereas multiple adaptive haplotypes sweep through the population simultaneously in a soft sweep, producing distinct patterns of genetic variation in the vicinity of the selected site. Measures of expected haplotype homozygosity have previously been used to detect sweeps in multiple study systems. However, these methods are formulated for phased haplotype data, typically unavailable for nonmodel organisms, and some may have reduced power to detect soft sweeps due to their increased genetic diversity relative to hard sweeps. To address these limitations, we applied the H12 and H2/H1 statistics proposed in 2015 by Garud et al., which have power to detect both hard and soft sweeps, to unphased multilocus genotypes, denoting them as G12 and G2/G1. G12 (and the more direct expected homozygosity analog to H12, denoted G123) has comparable power to H12 for detecting both hard and soft sweeps. G2/G1 can be used to classify hard and soft sweeps analogously to H2/H1, conditional on a genomic region having high G12 or G123 values. The reason for this power is that, under random mating, the most frequent haplotypes will yield the most frequent multilocus genotypes. Simulations based on parameters compatible with our recent understanding of human demographic history suggest that expected homozygosity methods are best suited for detecting recent sweeps, and increase in power under recent population expansions. Finally, we find candidates for selective sweeps within the 1000 Genomes CEU, YRI, GIH, and CHB populations, which corroborate and complement existing studies.
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74
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Bourgeois Y, Stritt C, Walser JC, Gordon SP, Vogel JP, Roulin AC. Genome-wide scans of selection highlight the impact of biotic and abiotic constraints in natural populations of the model grass Brachypodium distachyon. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2018; 96:438-451. [PMID: 30044522 DOI: 10.1111/tpj.14042] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/16/2018] [Revised: 06/20/2018] [Accepted: 07/17/2018] [Indexed: 06/08/2023]
Abstract
Grasses are essential plants for ecosystem functioning. Quantifying the selective pressures that act on natural variation in grass species is therefore essential regarding biodiversity maintenance. In this study, we investigate the selection pressures that act on two distinct populations of the grass model Brachypodium distachyon without prior knowledge about the traits under selection. We took advantage of whole-genome sequencing data produced for 44 natural accessions of B. distachyon and used complementary genome-wide selection scans (GWSS) methods to detect genomic regions under balancing and positive selection. We show that selection is shaping genetic diversity at multiple temporal and spatial scales in this species, and affects different genomic regions across the two populations. Gene ontology annotation of candidate genes reveals that pathogens may constitute important factors of positive and balancing selection in B. distachyon. We eventually cross-validated our results with quantitative trait locus data available for leaf-rust resistance in this species and demonstrate that, when paired with classical trait mapping, GWSS can help pinpointing candidate genes for further molecular validation. Thanks to a near base-perfect reference genome and the large collection of freely available natural accessions collected across its natural range, B. distachyon appears as a prime system for studies in ecology, population genomics and evolutionary biology.
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Affiliation(s)
- Yann Bourgeois
- New York University Abu Dhabi, PO Box 129188, Saadiyat Island, Abu Dhabi, United Arab Emirates
| | - Christoph Stritt
- Institute of Plant and Microbial Biology, University of Zürich, Zollikerstrasse 107, 8008, Zürich, Switzerland
| | - Jean-Claude Walser
- Genetic Diversity Centre, ETH Zürich, Universitätstrasse 16, Zurich, Switzerland
| | - Sean P Gordon
- DOE Joint Genome Institute, Walnut Creek, CA, 94598, USA
| | - John P Vogel
- DOE Joint Genome Institute, Walnut Creek, CA, 94598, USA
| | - Anne C Roulin
- Institute of Plant and Microbial Biology, University of Zürich, Zollikerstrasse 107, 8008, Zürich, Switzerland
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