1
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Harris M, Kim BY, Garud N. Enrichment of hard sweeps on the X chromosome compared to autosomes in six Drosophila species. Genetics 2024; 226:iyae019. [PMID: 38366786 PMCID: PMC10990427 DOI: 10.1093/genetics/iyae019] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2023] [Revised: 01/17/2024] [Accepted: 01/18/2024] [Indexed: 02/18/2024] Open
Abstract
The X chromosome, being hemizygous in males, is exposed one-third of the time increasing the visibility of new mutations to natural selection, potentially leading to different evolutionary dynamics than autosomes. Recently, we found an enrichment of hard selective sweeps over soft selective sweeps on the X chromosome relative to the autosomes in a North American population of Drosophila melanogaster. To understand whether this enrichment is a universal feature of evolution on the X chromosome, we analyze diversity patterns across 6 commonly studied Drosophila species. We find an increased proportion of regions with steep reductions in diversity and elevated homozygosity on the X chromosome compared to autosomes. To assess if these signatures are consistent with positive selection, we simulate a wide variety of evolutionary scenarios spanning variations in demography, mutation rate, recombination rate, background selection, hard sweeps, and soft sweeps and find that the diversity patterns observed on the X are most consistent with hard sweeps. Our findings highlight the importance of sex chromosomes in driving evolutionary processes and suggest that hard sweeps have played a significant role in shaping diversity patterns on the X chromosome across multiple Drosophila species.
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Affiliation(s)
- Mariana Harris
- Department of Computational Medicine, University of California Los Angeles, Los Angeles, CA 90095, USA
| | - Bernard Y Kim
- Department of Biology, Stanford University, Stanford, CA 94305, USA
| | - Nandita Garud
- Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, CA 90095, USA
- Department of Human Genetics, University of California Los Angeles, Los Angeles, CA 90095, USA
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2
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Wolff R, Garud NR. Pervasive selective sweeps across human gut microbiomes. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.12.22.573162. [PMID: 38187688 PMCID: PMC10769429 DOI: 10.1101/2023.12.22.573162] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/09/2024]
Abstract
The human gut microbiome is composed of a highly diverse consortia of species which are continually evolving within and across hosts. The ability to identify adaptations common to many host gut microbiomes would not only reveal shared selection pressures across hosts, but also key drivers of functional differentiation of the microbiome that may affect community structure and host traits. However, to date there has not been a systematic scan for adaptations that have spread across host microbiomes. Here, we develop a novel selection scan statistic, named the integrated linkage disequilibrium score (iLDS), that can detect the spread of adaptive haplotypes across host microbiomes via migration and horizontal gene transfer. Specifically, iLDS leverages signals of hitchhiking of deleterious variants with the beneficial variant, a common feature of adaptive evolution. We find that iLDS is capable of detecting simulated and known cases of selection, and moreover is robust to potential confounders that can also elevate LD. Application of the statistic to ~20 common commensal gut species from a large cohort of healthy, Western adults reveals pervasive spread of selected alleles across human microbiomes mediated by horizontal gene transfer. Among the candidate selective sweeps recovered by iLDS is an enrichment for genes involved in the metabolism of maltodextrin, a synthetic starch that has recently become a widespread component of Western diets. In summary, we demonstrate that selective sweeps across host microbiomes are a common feature of the evolution of the human gut microbiome.
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Affiliation(s)
- Richard Wolff
- Department of Ecology and Evolutionary Biology, UCLA
| | - Nandita R. Garud
- Department of Ecology and Evolutionary Biology, UCLA
- Department of Human Genetics, UCLA
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3
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Harris M, Kim B, Garud N. Enrichment of hard sweeps on the X chromosome compared to autosomes in six Drosophila species. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.06.21.545888. [PMID: 38106201 PMCID: PMC10723260 DOI: 10.1101/2023.06.21.545888] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/19/2023]
Abstract
The X chromosome, being hemizygous in males, is exposed one third of the time increasing the visibility of new mutations to natural selection, potentially leading to different evolutionary dynamics than autosomes. Recently, we found an enrichment of hard selective sweeps over soft selective sweeps on the X chromosome relative to the autosomes in a North American population of Drosophila melanogaster. To understand whether this enrichment is a universal feature of evolution on the X chromosome, we analyze diversity patterns across six commonly studied Drosophila species. We find an increased proportion of regions with steep reductions in diversity and elevated homozygosity on the X chromosome compared to autosomes. To assess if these signatures are consistent with positive selection, we simulate a wide variety of evolutionary scenarios spanning variations in demography, mutation rate, recombination rate, background selection, hard sweeps, and soft sweeps, and find that the diversity patterns observed on the X are most consistent with hard sweeps. Our findings highlight the importance of sex chromosomes in driving evolutionary processes and suggest that hard sweeps have played a significant role in shaping diversity patterns on the X chromosome across multiple Drosophila species.
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Affiliation(s)
- Mariana Harris
- Department of Computational Medicine, University of California Los Angeles, Los Angeles California, United States of America
| | - Bernard Kim
- Department of Biology, Stanford University, Stanford, California, United States of America
| | - Nandita Garud
- Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles California, United States of America
- Department of Human Genetics, University of California, Los Angeles, California, United States of America
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4
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Pivirotto AM, Platt A, Patel R, Kumar S, Hey J. Analyses of allele age and fitness impact reveal human beneficial alleles to be older than neutral controls. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.10.09.561569. [PMID: 37873438 PMCID: PMC10592680 DOI: 10.1101/2023.10.09.561569] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/25/2023]
Abstract
A classic population genetic prediction is that alleles experiencing directional selection should swiftly traverse allele frequency space, leaving detectable reductions in genetic variation in linked regions. However, despite this expectation, identifying clear footprints of beneficial allele passage has proven to be surprisingly challenging. We addressed the basic premise underlying this expectation by estimating the ages of large numbers of beneficial and deleterious alleles in a human population genomic data set. Deleterious alleles were found to be young, on average, given their allele frequency. However, beneficial alleles were older on average than non-coding, non-regulatory alleles of the same frequency. This finding is not consistent with directional selection and instead indicates some type of balancing selection. Among derived beneficial alleles, those fixed in the population show higher local recombination rates than those still segregating, consistent with a model in which new beneficial alleles experience an initial period of balancing selection due to linkage disequilibrium with deleterious recessive alleles. Alleles that ultimately fix following a period of balancing selection will leave a modest 'soft' sweep impact on the local variation, consistent with the overall paucity of species-wide 'hard' sweeps in human genomes.
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Affiliation(s)
| | - Alexander Platt
- Temple University, Department of Biology, Philadelphia PA 19122, USA
- University of Pennsylvania, Department of Genetics, Philadelphia PA 19104, USA
| | - Ravi Patel
- Temple University, Department of Biology, Philadelphia PA 19122, USA
- Institute for Genomics and Evolutionary Medicine, Temple University, PA 19122, USA
| | - Sudhir Kumar
- Temple University, Department of Biology, Philadelphia PA 19122, USA
- Institute for Genomics and Evolutionary Medicine, Temple University, PA 19122, USA
| | - Jody Hey
- Temple University, Department of Biology, Philadelphia PA 19122, USA
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5
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Matheson J, Bertram J, Masel J. Human deleterious mutation rate implies high fitness variance, with declining mean fitness compensated by rarer beneficial mutations of larger effect. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.09.01.555871. [PMID: 37732183 PMCID: PMC10508744 DOI: 10.1101/2023.09.01.555871] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/22/2023]
Abstract
Each new human has an expected Ud = 2 - 10 new deleterious mutations. This deluge of deleterious mutations cannot all be purged, and therefore accumulate in a declining fitness ratchet. Using a novel simulation framework designed to efficiently handle genome-wide linkage disequilibria across many segregating sites, we find that rarer, beneficial mutations of larger effect are sufficient to compensate fitness declines due to the fixation of many slightly deleterious mutations. Drift barrier theory posits a similar asymmetric pattern of fixations to explain ratcheting genome size and complexity, but in our theory, the cause is Ud > 1 rather than small population size. In our simulations, Ud ~2 - 10 generates high within-population variance in relative fitness; two individuals will typically differ in fitness by 15-40%. Ud ~2 - 10 also slows net adaptation by ~13%-39%. Surprisingly, fixation rates are more sensitive to changes in the beneficial than the deleterious mutation rate, e.g. a 10% increase in overall mutation rate leads to faster adaptation; this puts to rest dysgenic fears about increasing mutation rates due to rising paternal age.
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Affiliation(s)
- Joseph Matheson
- Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA
- Department of Ecology, Behavior, and Evolution, University of California San Diego, San Diego, CA, 92093, USA
| | - Jason Bertram
- Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA
- Department of Mathematics, University of Western Ontario, London ON, Canada
| | - Joanna Masel
- Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA
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6
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Dwivedi SL, Heslop-Harrison P, Spillane C, McKeown PC, Edwards D, Goldman I, Ortiz R. Evolutionary dynamics and adaptive benefits of deleterious mutations in crop gene pools. TRENDS IN PLANT SCIENCE 2023; 28:685-697. [PMID: 36764870 DOI: 10.1016/j.tplants.2023.01.006] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/11/2022] [Revised: 12/03/2022] [Accepted: 01/18/2023] [Indexed: 05/13/2023]
Abstract
Mutations with deleterious consequences in nature may be conditionally deleterious in crop plants. That is, while some genetic variants may reduce fitness under wild conditions and be subject to purifying selection, they can be under positive selection in domesticates. Such deleterious alleles can be plant breeding targets, particularly for complex traits. The difficulty of distinguishing favorable from unfavorable variants reduces the power of selection, while favorable trait variation and heterosis may be attributable to deleterious alleles. Here, we review the roles of deleterious mutations in crop breeding and discuss how they can be used as a new avenue for crop improvement with emerging genomic tools, including HapMaps and pangenome analysis, aiding the identification, removal, or exploitation of deleterious mutations.
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Affiliation(s)
| | - Pat Heslop-Harrison
- Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, China; Department of Genetics and Genome Biology, University of Leicester, Leicester, LE1 7RH, UK
| | - Charles Spillane
- Agriculture and Bioeconomy Research Centre, Ryan Institute, University of Galway, University Road, Galway, H91 REW4, Ireland
| | - Peter C McKeown
- Agriculture and Bioeconomy Research Centre, Ryan Institute, University of Galway, University Road, Galway, H91 REW4, Ireland
| | - David Edwards
- School of Biological Sciences and Institute of Agriculture, University of Western Australia, Perth, WA 6009, Australia
| | - Irwin Goldman
- Department of Horticulture, College of Agricultural and Life Sciences, University of Wisconsin Madison, WI 53706, USA
| | - Rodomiro Ortiz
- Department of Plant Breeding, Swedish University of Agricultural Sciences, Alnarp, SE 23053, Sweden.
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7
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Moutinho AF, Eyre-Walker A, Dutheil JY. Strong evidence for the adaptive walk model of gene evolution in Drosophila and Arabidopsis. PLoS Biol 2022; 20:e3001775. [PMID: 36099311 PMCID: PMC9470001 DOI: 10.1371/journal.pbio.3001775] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2021] [Accepted: 08/01/2022] [Indexed: 11/19/2022] Open
Abstract
Understanding the dynamics of species adaptation to their environments has long been a central focus of the study of evolution. Theories of adaptation propose that populations evolve by “walking” in a fitness landscape. This “adaptive walk” is characterised by a pattern of diminishing returns, where populations further away from their fitness optimum take larger steps than those closer to their optimal conditions. Hence, we expect young genes to evolve faster and experience mutations with stronger fitness effects than older genes because they are further away from their fitness optimum. Testing this hypothesis, however, constitutes an arduous task. Young genes are small, encode proteins with a higher degree of intrinsic disorder, are expressed at lower levels, and are involved in species-specific adaptations. Since all these factors lead to increased protein evolutionary rates, they could be masking the effect of gene age. While controlling for these factors, we used population genomic data sets of Arabidopsis and Drosophila and estimated the rate of adaptive substitutions across genes from different phylostrata. We found that a gene’s evolutionary age significantly impacts the molecular rate of adaptation. Moreover, we observed that substitutions in young genes tend to have larger physicochemical effects. Our study, therefore, provides strong evidence that molecular evolution follows an adaptive walk model across a large evolutionary timescale. This study uses population genomic datasets from Arabidopsis and Drosophila to show that young genes adapt faster and are subject to mutations of larger fitness effects, providing strong evidence that molecular evolution follows an adaptive walk model across a large evolutionary timescale.
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Affiliation(s)
- Ana Filipa Moutinho
- Department of Evolutionary Genetics, Max Planck Institute for Evolutionary Biology, Plön, Germany
- School of Life Sciences, University of Sussex, Brighton, United Kingdom
- * E-mail:
| | - Adam Eyre-Walker
- School of Life Sciences, University of Sussex, Brighton, United Kingdom
| | - Julien Y. Dutheil
- Department of Evolutionary Genetics, Max Planck Institute for Evolutionary Biology, Plön, Germany
- Unité Mixte de Recherche 5554 Institut des Sciences de l’Evolution, CNRS, IRD, EPHE, Université de Montpellier, Montpellier, France
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8
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Tang J, Huang M, He S, Zeng J, Zhu H. Uncovering the extensive trade-off between adaptive evolution and disease susceptibility. Cell Rep 2022; 40:111351. [PMID: 36103812 DOI: 10.1016/j.celrep.2022.111351] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2022] [Revised: 06/13/2022] [Accepted: 08/23/2022] [Indexed: 11/03/2022] Open
Abstract
Favored mutations in the human genome may make the carriers adapt to changing environments and lifestyles but also susceptible to specific diseases. The scale and details of the trade-off between adaptive evolution and disease susceptibility are unclear because most favored mutations in different populations remain unidentified. As no statistical test can discriminate favored mutations from nearby hitchhiking neutral ones, we report a deep-learning network (DeepFavored) to integrate multiple statistical tests and divide identifying favored mutations into two subtasks. We identify favored mutations in three human populations and analyzed the correlation between favored/hitchhiking mutations and genome-wide association study (GWAS) sites. Both favored and hitchhiking neutral mutations are enriched in GWAS sites with population-specific features, and the enrichment and population specificity are prominent in genes in specific Gene Ontology (GO) terms. These provide evidence for extensive and population-specific trade-offs between adaptive evolution and disease susceptibility. The unveiled scale helps understand and investigate differences and diseases of humans.
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Affiliation(s)
- Ji Tang
- Bioinformatics Section, School of Basic Medical Sciences, Southern Medical University, Guangzhou 510515, China
| | - Maosheng Huang
- Bioinformatics Section, School of Basic Medical Sciences, Southern Medical University, Guangzhou 510515, China; School of Medical Information and Engineering, Guangdong Pharmaceutical University, Guangzhou 510006, China
| | - Sha He
- Bioinformatics Section, School of Basic Medical Sciences, Southern Medical University, Guangzhou 510515, China
| | - Junxiang Zeng
- Bioinformatics Section, School of Basic Medical Sciences, Southern Medical University, Guangzhou 510515, China
| | - Hao Zhu
- Bioinformatics Section, School of Basic Medical Sciences, Southern Medical University, Guangzhou 510515, China; Guangdong-Hong Kong-Macao Greater Bay Area Center for Brain Science and Brain-Inspired Intelligence, Southern Medical University, Guangzhou 510515, China.
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9
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Aggeli D, Marad DA, Liu X, Buskirk SW, Levy SF, Lang GI. Overdominant and partially dominant mutations drive clonal adaptation in diploid Saccharomyces cerevisiae. Genetics 2022; 221:6569837. [PMID: 35435209 PMCID: PMC9157133 DOI: 10.1093/genetics/iyac061] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2021] [Accepted: 04/06/2022] [Indexed: 11/14/2022] Open
Abstract
Identification of adaptive targets in experimental evolution typically relies on extensive replication and genetic reconstruction. An alternative approach is to directly assay all mutations in an evolved clone by generating pools of segregants that contain random combinations of evolved mutations. Here, we apply this method to six Saccharomyces cerevisiae clones isolated from four diploid populations that were clonally evolved for 2,000 generations in rich glucose medium. Each clone contains 17-26 mutations relative to the ancestor. We derived intermediate genotypes between the founder and the evolved clones by bulk mating sporulated cultures of the evolved clones to a barcoded haploid version of the ancestor. We competed the resulting barcoded diploids en masse and quantified fitness in the experimental and alternative environments by barcode sequencing. We estimated average fitness effects of evolved mutations using barcode-based fitness assays and whole genome sequencing for a subset of segregants. In contrast to our previous work with haploid evolved clones, we find that diploids carry fewer beneficial mutations, with modest fitness effects (up to 5.4%) in the environment in which they arose. In agreement with theoretical expectations, reconstruction experiments show that all mutations with a detectable fitness effect manifest some degree of dominance over the ancestral allele, and most are overdominant. Genotypes with lower fitness effects in alternative environments allowed us to identify conditions that drive adaptation in our system.
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Affiliation(s)
- Dimitra Aggeli
- Department of Biological Sciences, Lehigh University, Bethlehem, PA18015, USA
| | - Daniel A Marad
- Department of Biological Sciences, Lehigh University, Bethlehem, PA18015, USA
| | - Xianan Liu
- Joint Initiative for Metrology in Biology, SLAC National Accelerator Laboratory, Stanford University, Stanford, CA94025, USA
| | - Sean W Buskirk
- Department of Biological Sciences, Lehigh University, Bethlehem, PA18015, USA.,Department of Biology, West Chester University, West Chester, PA19383, USA
| | - Sasha F Levy
- Joint Initiative for Metrology in Biology, SLAC National Accelerator Laboratory, Stanford University, Stanford, CA94025, USA
| | - Gregory I Lang
- Department of Biological Sciences, Lehigh University, Bethlehem, PA18015, USA
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10
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Di C, Murga Moreno J, Salazar-Tortosa DF, Lauterbur ME, Enard D. Decreased recent adaptation at human mendelian disease genes as a possible consequence of interference between advantageous and deleterious variants. eLife 2021; 10:69026. [PMID: 34636724 PMCID: PMC8526059 DOI: 10.7554/elife.69026] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2021] [Accepted: 10/02/2021] [Indexed: 11/27/2022] Open
Abstract
Advances in genome sequencing have improved our understanding of the genetic basis of human diseases, and thousands of human genes have been associated with different diseases. Recent genomic adaptation at disease genes has not been well characterized. Here, we compare the rate of strong recent adaptation in the form of selective sweeps between mendelian, non-infectious disease genes and non-disease genes across distinct human populations from the 1000 Genomes Project. We find that mendelian disease genes have experienced far less selective sweeps compared to non-disease genes especially in Africa. Investigating further the possible causes of the sweep deficit at disease genes, we find that this deficit is very strong at disease genes with both low recombination rates and with high numbers of associated disease variants, but is almost non-existent at disease genes with higher recombination rates or lower numbers of associated disease variants. Because segregating recessive deleterious variants have the ability to interfere with adaptive ones, these observations strongly suggest that adaptation has been slowed down by the presence of interfering recessive deleterious variants at disease genes. These results suggest that disease genes suffer from a transient inability to adapt as fast as the rest of the genome.
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Affiliation(s)
- Chenlu Di
- University of Arizona Department of Ecology and Evolutionary Biology, Tucson, United States
| | - Jesus Murga Moreno
- Institut de Biotecnologia i de Biomedicina and Departament de Genètica i de Microbiologia, Universitat Autònoma de Barcelona, Barcelona, Spain
| | | | - M Elise Lauterbur
- University of Arizona Department of Ecology and Evolutionary Biology, Tucson, United States
| | - David Enard
- University of Arizona Department of Ecology and Evolutionary Biology, Tucson, United States
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11
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Jones MR, Mills LS, Jensen JD, Good JM. The Origin and Spread of Locally Adaptive Seasonal Camouflage in Snowshoe Hares. Am Nat 2020; 196:316-332. [DOI: 10.1086/710022] [Citation(s) in RCA: 21] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
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12
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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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13
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Lachance J. Echoes of the neutralist‐selectionist debate in an era of population genomics. Evolution 2019. [DOI: 10.1111/evo.13703] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Affiliation(s)
- Joseph Lachance
- School of Biological Sciences, Georgia Institute of Technology Atlanta Georgia 30332
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14
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Haller BC, Galloway J, Kelleher J, Messer PW, Ralph PL. Tree-sequence recording in SLiM opens new horizons for forward-time simulation of whole genomes. Mol Ecol Resour 2019; 19:552-566. [PMID: 30565882 PMCID: PMC6393187 DOI: 10.1111/1755-0998.12968] [Citation(s) in RCA: 102] [Impact Index Per Article: 20.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2018] [Revised: 11/06/2018] [Accepted: 11/09/2018] [Indexed: 12/12/2022]
Abstract
There is an increasing demand for evolutionary models to incorporate relatively realistic dynamics, ranging from selection at many genomic sites to complex demography, population structure, and ecological interactions. Such models can generally be implemented as individual-based forward simulations, but the large computational overhead of these models often makes simulation of whole chromosome sequences in large populations infeasible. This situation presents an important obstacle to the field that requires conceptual advances to overcome. The recently developed tree-sequence recording method (Kelleher, Thornton, Ashander, & Ralph, 2018), which stores the genealogical history of all genomes in the simulated population, could provide such an advance. This method has several benefits: (1) it allows neutral mutations to be omitted entirely from forward-time simulations and added later, thereby dramatically improving computational efficiency; (2) it allows neutral burn-in to be constructed extremely efficiently after the fact, using "recapitation"; (3) it allows direct examination and analysis of the genealogical trees along the genome; and (4) it provides a compact representation of a population's genealogy that can be analysed in Python using the msprime package. We have implemented the tree-sequence recording method in SLiM 3 (a free, open-source evolutionary simulation software package) and extended it to allow the recording of non-neutral mutations, greatly broadening the utility of this method. To demonstrate the versatility and performance of this approach, we showcase several practical applications that would have been beyond the reach of previously existing methods, opening up new horizons for the modelling and exploration of evolutionary processes.
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Affiliation(s)
- Benjamin C. Haller
- Dept. of Biological Statistics and Computational Biology, Cornell University, Ithaca, NY 14853, USA
| | - Jared Galloway
- Institute of Ecology and Evolution, University of Oregon, Eugene, OR 97403, USA
| | - Jerome Kelleher
- Big Data Institute, Li Ka Shing Centre for Health Information and Discovery, University of Oxford, Oxford, OX3 7FZ, UK
| | - Philipp W. Messer
- Dept. of Biological Statistics and Computational Biology, Cornell University, Ithaca, NY 14853, USA
| | - Peter L. Ralph
- Institute of Ecology and Evolution, University of Oregon, Eugene, OR 97403, USA
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15
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Kim BY, Huber CD, Lohmueller KE. Deleterious variation shapes the genomic landscape of introgression. PLoS Genet 2018; 14:e1007741. [PMID: 30346959 PMCID: PMC6233928 DOI: 10.1371/journal.pgen.1007741] [Citation(s) in RCA: 55] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2018] [Revised: 11/13/2018] [Accepted: 10/05/2018] [Indexed: 11/19/2022] Open
Abstract
While it is appreciated that population size changes can impact patterns of deleterious variation in natural populations, less attention has been paid to how gene flow affects and is affected by the dynamics of deleterious variation. Here we use population genetic simulations to examine how gene flow impacts deleterious variation under a variety of demographic scenarios, mating systems, dominance coefficients, and recombination rates. Our results show that admixture between populations can temporarily reduce the genetic load of smaller populations and cause increases in the frequency of introgressed ancestry, especially if deleterious mutations are recessive. Additionally, when fitness effects of new mutations are recessive, between-population differences in the sites at which deleterious variants exist creates heterosis in hybrid individuals. Together, these factors lead to an increase in introgressed ancestry, particularly when recombination rates are low. Under certain scenarios, introgressed ancestry can increase from an initial frequency of 5% to 30–75% and fix at many loci, even in the absence of beneficial mutations. Further, deleterious variation and admixture can generate correlations between the frequency of introgressed ancestry and recombination rate or exon density, even in the absence of other types of selection. The direction of these correlations is determined by the specific demography and whether mutations are additive or recessive. Therefore, it is essential that null models of admixture include both demography and deleterious variation before invoking other mechanisms to explain unusual patterns of genetic variation. Individuals from distinct populations sometimes will produce fertile offspring and will exchange genetic material in a process called hybridization. Genomes of hybrid individuals often show non-random patterns of hybrid ancestry across the genome, where some regions have a high frequency of ancestry from the second population and other regions have less. Typically, this pattern has been attributed to adaptive introgression, where beneficial genetic variants are passed from one population to the other, or to genomic incompatibilities between these distinct species. However, other mechanisms could lead to these heterogeneous patterns of ancestry in hybrids. Here we use simulations to investigate whether deleterious mutations affect the patterns of introgressed ancestry across genomes. We show that when ancestry from a larger population is added to a smaller population, the ancestry from the larger population dramatically increases in frequency because it carries fewer deleterious mutations. This occurs even in the absence of beneficial mutations in either population. Additionally, we show that differences in sex chromosome evolution relative to autosomes, or differences in mating system, can affect patterns of introgression in similar ways. Our study argues that deleterious mutations should be included in population genetic models used to identify unusual regions of the genome that appear to be under selection in hybrids.
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Affiliation(s)
- Bernard Y. Kim
- Department of Ecology and Evolutionary Biology, University of California, Los Angeles, California, United States of America
| | - Christian D. Huber
- Department of Ecology and Evolutionary Biology, University of California, Los Angeles, California, United States of America
| | - Kirk E. Lohmueller
- Department of Ecology and Evolutionary Biology, University of California, Los Angeles, California, United States of America
- Interdepartmental Program in Bioinformatics, University of California, Los Angeles, California, United States of America
- Department of Human Genetics, David Geffen School of Medicine, University of California, Los Angeles, California, United States of America
- * E-mail:
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Moyers BT, Morrell PL, McKay JK. Genetic Costs of Domestication and Improvement. J Hered 2017; 109:103-116. [DOI: 10.1093/jhered/esx069] [Citation(s) in RCA: 103] [Impact Index Per Article: 14.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2017] [Accepted: 08/02/2017] [Indexed: 12/12/2022] Open
Affiliation(s)
- Brook T Moyers
- Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO
| | - Peter L Morrell
- Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN (Morrell)
| | - John K McKay
- Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO
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17
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Range Expansion Compromises Adaptive Evolution in an Outcrossing Plant. Curr Biol 2017; 27:2544-2551.e4. [DOI: 10.1016/j.cub.2017.07.007] [Citation(s) in RCA: 56] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2017] [Revised: 05/22/2017] [Accepted: 07/04/2017] [Indexed: 01/04/2023]
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18
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Revisiting Adaptive Potential, Population Size, and Conservation. Trends Ecol Evol 2017; 32:506-517. [PMID: 28476215 DOI: 10.1016/j.tree.2017.03.012] [Citation(s) in RCA: 125] [Impact Index Per Article: 17.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2017] [Revised: 03/28/2017] [Accepted: 03/31/2017] [Indexed: 11/24/2022]
Abstract
Additive genetic variance (VA) reflects the potential for evolutionary shifts and can be low for some traits or populations. High VA is critical for the conservation of threatened species under selection to facilitate adaptation. Theory predicts tight associations between population size and VA, but data from some experimental models, and managed and natural populations do not always support this prediction. However, VA comparisons often have low statistical power, are undertaken in highly controlled environments distinct from natural habitats, and focus on traits with limited ecological relevance. Moreover, investigations of VA typically fail to consider rare alleles, genetic load, or linkage disequilibrium, resulting in deleterious effects associated with favored alleles in small populations. Large population size remains essential for ensuring adaptation.
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Lenz TL, Spirin V, Jordan DM, Sunyaev SR. Excess of Deleterious Mutations around HLA Genes Reveals Evolutionary Cost of Balancing Selection. Mol Biol Evol 2016; 33:2555-64. [PMID: 27436009 PMCID: PMC5026253 DOI: 10.1093/molbev/msw127] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Deleterious mutations are expected to evolve under negative selection and are usually purged from the population. However, deleterious alleles segregate in the human population and some disease-associated variants are maintained at considerable frequencies. Here, we test the hypothesis that balancing selection may counteract purifying selection in neighboring regions and thus maintain deleterious variants at higher frequency than expected from their detrimental fitness effect. We first show in realistic simulations that balancing selection reduces the density of polymorphic sites surrounding a locus under balancing selection, but at the same time markedly increases the population frequency of the remaining variants, including even substantially deleterious alleles. To test the predictions of our simulations empirically, we then use whole-exome sequencing data from 6,500 human individuals and focus on the most established example for balancing selection in the human genome, the major histocompatibility complex (MHC). Our analysis shows an elevated frequency of putatively deleterious coding variants in nonhuman leukocyte antigen (non-HLA) genes localized in the MHC region. The mean frequency of these variants declined with physical distance from the classical HLA genes, indicating dependency on genetic linkage. These results reveal an indirect cost of the genetic diversity maintained by balancing selection, which has hitherto been perceived as mostly advantageous, and have implications both for the evolution of recombination and also for the epidemiology of various MHC-associated diseases.
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Affiliation(s)
- Tobias L Lenz
- Division of Genetics, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School Evolutionary Immunogenomics, Department of Evolutionary Ecology, Max Planck Institute for Evolutionary Biology, Plön, Germany
| | - Victor Spirin
- Division of Genetics, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School
| | - Daniel M Jordan
- Division of Genetics, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School
| | - Shamil R Sunyaev
- Division of Genetics, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School Program in Medical and Population Genetics, The Broad Institute, Cambridge, MA
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Heterozygote Advantage Is a Common Outcome of Adaptation in Saccharomyces cerevisiae. Genetics 2016; 203:1401-13. [PMID: 27194750 DOI: 10.1534/genetics.115.185165] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2015] [Accepted: 05/15/2016] [Indexed: 12/31/2022] Open
Abstract
Adaptation in diploids is predicted to proceed via mutations that are at least partially dominant in fitness. Recently, we argued that many adaptive mutations might also be commonly overdominant in fitness. Natural (directional) selection acting on overdominant mutations should drive them into the population but then, instead of bringing them to fixation, should maintain them as balanced polymorphisms via heterozygote advantage. If true, this would make adaptive evolution in sexual diploids differ drastically from that of haploids. The validity of this prediction has not yet been tested experimentally. Here, we performed four replicate evolutionary experiments with diploid yeast populations (Saccharomyces cerevisiae) growing in glucose-limited continuous cultures. We sequenced 24 evolved clones and identified initial adaptive mutations in all four chemostats. The first adaptive mutations in all four chemostats were three copy number variations, all of which proved to be overdominant in fitness. The fact that fitness overdominant mutations were always the first step in independent adaptive walks supports the prediction that heterozygote advantage can arise as a common outcome of directional selection in diploids and demonstrates that overdominance of de novo adaptive mutations in diploids is not rare.
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Abstract
Approximately 2-4% of genetic material in human populations outside Africa is derived from Neanderthals who interbred with anatomically modern humans. Recent studies have shown that this Neanderthal DNA is depleted around functional genomic regions; this has been suggested to be a consequence of harmful epistatic interactions between human and Neanderthal alleles. However, using published estimates of Neanderthal inbreeding and the distribution of mutational fitness effects, we infer that Neanderthals had at least 40% lower fitness than humans on average; this increased load predicts the reduction in Neanderthal introgression around genes without the need to invoke epistasis. We also predict a residual Neanderthal mutational load in non-Africans, leading to a fitness reduction of at least 0.5%. This effect of Neanderthal admixture has been left out of previous debate on mutation load differences between Africans and non-Africans. We also show that if many deleterious mutations are recessive, the Neanderthal admixture fraction could increase over time due to the protective effect of Neanderthal haplotypes against deleterious alleles that arose recently in the human population. This might partially explain why so many organisms retain gene flow from other species and appear to derive adaptive benefits from introgression.
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