scholarly journals Human demographic history has amplified the effects of background selection across the genome

2017 ◽  
Author(s):  
Raul Torres ◽  
Zachary A. Szpiech ◽  
Ryan D. Hernandez
Keyword(s):  
Genetic Diversity ◽  
Natural Selection ◽  
Demographic History ◽  
Background Selection ◽  
Dynamic Changes ◽  
Genome Wide ◽  
Neutral Genetic Diversity ◽  
African Populations ◽  
Demographic Processes ◽  
Non Equilibrium

AbstractNatural populations often grow, shrink, and migrate over time. Demographic processes such as these can impact genome-wide levels of genetic diversity. In addition, genetic variation in functional regions of the genome can be altered by natural selection, which drives adaptive mutations to higher frequencies or purges deleterious ones. Such selective processes impact not only the sites directly under selection but also nearby neutral variation through genetic linkage through processes referred to as genetic hitch-hiking in the context of positive selection and background selection (BGS) in the context of purifying selection. While there is extensive literature examining the impact of selection at linked sites at demographic equilibrium, less is known about how non-equilibrium demographic processes impact the effects of hitchhiking and BGS. Utilizing a global sample of human whole-genome sequences from the Thousand Genomes Project and extensive simulations, we investigate how non-equilibrium demographic processes magnify and dampen the consequences of selection at linked sites across the human genome. When binning the genome by inferred strength of BGS, we observe that, compared to Africans, non-African populations have experienced larger proportional decreases in neutral genetic diversity in such regions. We replicate these findings in admixed populations by showing that non-African ancestral components of the genome have also been impacted more severely in these regions. We attribute these differences to the strong, sustained/recurrent population bottlenecks that non-Africans experienced as they migrated out of Africa and throughout the globe. Furthermore, we observe a strong correlation between FST and inferred strength of BGS, suggesting a stronger rate of genetic drift. Forward simulations of human demographic history with a model of BGS support these observations. Our results show that non-equilibrium demography significantly alters the consequences selection at linked sites and support the need for more work investigating the dynamic process of multiple evolutionary forces operating in concert.Author summaryPatterns of genetic diversity within a species are affected at broad and fine scales by population size changes (“demography”) and natural selection. From both population genetics theory and observation of genomic sequence data, it is known that demography can alter genome-wide average neutral genetic diversity. Additionally, natural selection can affect neutral genetic diversity regionally across the genome via selection at linked sites. During this process, natural selection acting on adaptive or deleterious variants in the genome will also impact diversity at nearby neutral sites due to genetic linkage. However, less is well known about the dynamic changes to diversity that occur in regions impacted by selection at linked sites when a population undergoes a size change. We characterize these dynamic changes using thousands of human genomes and find that the population size changes experienced by humans have shaped the consequences of linked selection across the genome. In particular, population contractions, such as those experienced by non-Africans, have disproportionately decreased neutral diversity in regions of the genome inferred to be under strong background selection (i.e., selection at linked sites that is caused by natural selection acting on deleterious variants), resulting in large differences between African and non-African populations.

scholarly journals Demography and natural selection have shaped genome-wide variation in the widely distributed conifer Norway Spruce (Picea abies)

2019 ◽  
Author(s):  
Xi Wang ◽  
Carolina Bernhardsson ◽  
Pär K. Ingvarsson
Keyword(s):  
Genetic Diversity ◽  
Natural Selection ◽  
Picea Abies ◽  
Population Size ◽  
Effective Population Size ◽  
Norway Spruce ◽  
Demographic History ◽  
Sequencing Data ◽  
Effective Population ◽  
Genome Wide

AbstractUnder the neutral theory, species with larger effective population sizes are expected to harbour higher genetic diversity. However, across a wide variety of organisms, the range of genetic diversity is orders of magnitude more narrow than the range of effective population size. This observation has become known as Lewontin’s paradox and although aspects of this phenomenon have been extensively studied, the underlying causes for the paradox remain unclear. Norway spruce (Picea abies) is a widely distributed conifer species across the northern hemisphere and it consequently plays a major role in European forestry. Here, we use whole-genome re-sequencing data from 35 individuals to perform population genomic analyses in P. abies in an effort to understand what drives genome-wide patterns of variation in this species. Despite having a very wide geographic distribution and an enormous current population size, our analyses find that genetic diversity of P.abies is low across a number of populations (p=0.005-0.006). To assess the reasons for the low levels of genetic diversity, we infer the demographic history of the species and find that it is characterised by several re-occurring bottlenecks with concomitant decreases in effective population size can, at least partly, provide an explanation for low polymorphism we observe in P. abies. Further analyses suggest that recurrent natural selection, both purifying and positive selection, can also contribute to the loss of genetic diversity in Norway spruce by reducing genetic diversity at linked sites. Finally, the overall low mutation rates seen in conifers can also help explain the low genetic diversity maintained in Norway spruce.

scholarly journals The temporal dynamics of background selection in non-equilibrium populations

2019 ◽  
Author(s):  
Raul Torres ◽  
Markus G Stetter ◽  
Ryan D Hernandez ◽  
Jeffrey Ross-Ibarra
Keyword(s):  
Natural Selection ◽  
Temporal Dynamics ◽  
Direct Action ◽  
Demographic History ◽  
Empirical Studies ◽  
Natural Populations ◽  
Background Selection ◽  
Size Change ◽  
Demographic Models ◽  
Non Equilibrium

ABSTRACTNeutral genetic diversity across the genome is determined by the complex interplay of mutation, demographic history, and natural selection. While the direct action of natural selection is limited to functional loci across the genome, its impact can have effects on nearby neutral loci due to genetic linkage. These effects of selection at linked sites, referred to as genetic hitchhiking and background selection (BGS), are pervasive across natural populations. However, only recently has there been a focus on the joint consequences of demography and selection at linked sites, and empirical studies have sometimes come to apparently contradictory conclusions as to their combined effects. In order to understand the relationship between demography and selection at linked sites, we conducted an extensive forward simulation study of BGS under a range of demographic models. We found that the relative levels of diversity in BGS and neutral regions vary over time and that the initial dynamics after a population size change are often in the opposite direction of the long-term expected trajectory. Our detailed observations of the temporal dynamics of neutral diversity in the context of selection at linked sites in non-equilibrium populations provides new intuition about why patterns of diversity under BGS vary through time in natural populations and help reconcile previously contradictory observations. Most notably, our results highlight that classical models of BGS are poorly suited for predicting diversity in non-equilibrium populations.

scholarly journals The Temporal Dynamics of Background Selection in Nonequilibrium Populations

Genetics ◽  
2020 ◽  
Vol 214 (4) ◽  
pp. 1019-1030 ◽  
Author(s):  
Raul Torres ◽  
Markus G. Stetter ◽  
Ryan D. Hernandez ◽  
Jeffrey Ross-Ibarra
Keyword(s):  
Natural Selection ◽  
Temporal Dynamics ◽  
Direct Action ◽  
Demographic History ◽  
Empirical Studies ◽  
Natural Populations ◽  
Background Selection ◽  
Size Change ◽  
Demographic Models ◽  
Classical Models

Neutral genetic diversity across the genome is determined by the complex interplay of mutation, demographic history, and natural selection. While the direct action of natural selection is limited to functional loci across the genome, its impact can have effects on nearby neutral loci due to genetic linkage. These effects of selection at linked sites, referred to as genetic hitchhiking and background selection (BGS), are pervasive across natural populations. However, only recently has there been a focus on the joint consequences of demography and selection at linked sites, and some empirical studies have come to apparently contradictory conclusions as to their combined effects. To understand the relationship between demography and selection at linked sites, we conducted an extensive forward simulation study of BGS under a range of demographic models. We found that the relative levels of diversity in BGS and neutral regions vary over time and that the initial dynamics after a population size change are often in the opposite direction of the long-term expected trajectory. Our detailed observations of the temporal dynamics of neutral diversity in the context of selection at linked sites in nonequilibrium populations provide new intuition about why patterns of diversity under BGS vary through time in natural populations and help reconcile previously contradictory observations. Most notably, our results highlight that classical models of BGS are poorly suited for predicting diversity in nonequilibrium populations.

scholarly journals Genome-wide scans provide evidence for positive selection of genes implicated in Lassa fever

2012 ◽  
Vol 367 (1590) ◽  
pp. 868-877 ◽  
Author(s):  
Kristian G. Andersen ◽  
Ilya Shylakhter ◽  
Shervin Tabrizi ◽  
Sharon R. Grossman ◽  
Christian T. Happi ◽  
...  
Keyword(s):  
Natural Selection ◽  
Positive Selection ◽  
Lassa Fever ◽  
West African ◽  
Lassa Virus ◽  
Genome Wide ◽  
African Populations ◽  
Genome Wide Data ◽  
Differential Gene ◽  
Selection Of

Rapidly evolving viruses and other pathogens can have an immense impact on human evolution as natural selection acts to increase the prevalence of genetic variants providing resistance to disease. With the emergence of large datasets of human genetic variation, we can search for signatures of natural selection in the human genome driven by such disease-causing microorganisms. Based on this approach, we have previously hypothesized that Lassa virus (LASV) may have been a driver of natural selection in West African populations where Lassa haemorrhagic fever is endemic. In this study, we provide further evidence for this notion. By applying tests for selection to genome-wide data from the International Haplotype Map Consortium and the 1000 Genomes Consortium, we demonstrate evidence for positive selection in LARGE and interleukin 21 ( IL21 ), two genes implicated in LASV infectivity and immunity. We further localized the signals of selection, using the recently developed composite of multiple signals method, to introns and putative regulatory regions of those genes. Our results suggest that natural selection may have targeted variants giving rise to alternative splicing or differential gene expression of LARGE and IL21 . Overall, our study supports the hypothesis that selective pressures imposed by LASV may have led to the emergence of particular alleles conferring resistance to Lassa fever, and opens up new avenues of research pursuit.

scholarly journals The Effect of Mating System Differences on Nucleotide Diversity at the Phosphoglucose Isomerase Locus in the Plant Genus Leavenworthia

Genetics ◽  
1999 ◽  
Vol 151 (1) ◽  
pp. 343-357 ◽  
Author(s):  
F Liu ◽  
D Charlesworth ◽  
M Kreitman
Keyword(s):  
Genetic Diversity ◽  
Nucleotide Diversity ◽  
Phosphoglucose Isomerase ◽  
Self Incompatibility ◽  
Background Selection ◽  
Selective Sweeps ◽  
Balanced Polymorphism ◽  
Neutral Genetic Diversity ◽  
Low Diversity ◽  
High Diversity

AbstractTo test the theoretical prediction that highly inbreeding populations should have low neutral genetic diversity relative to closely related outcrossing populations, we sequenced portions of the cytosolic phosphoglucose isomerase (PgiC) gene in the plant genus Leavenworthia, which includes both self-incompatible and inbreeding taxa. On the basis of sequences of intron 12 of this gene, the expected low diversity was seen in both populations of the selfers Leavenworthia uniflora and L. torulosa and in three highly inbreeding populations of L. crassa, while high diversity was found in self-incompatible L. stylosa, and moderate diversity in L. crassa populations with partial or complete self-incompatibility. In L. stylosa, the nucleotide diversity was strongly structured into three haplotypic classes, differing by several insertion/deletion sequences, with linkage disequilibrium between sequences of the three types in intron 12, but not in the adjacent regions. Differences between the three kinds of haplotypes are larger than between sequences of this gene region from different species. The haplotype divergence suggests the presence of a balanced polymorphism at this locus, possibly predating the split between L. stylosa and its two inbreeding sister taxa, L. uniflora and L. torulosa. It is therefore difficult to distinguish between different potential causes of the much lower sequence diversity at this locus in inbreeding than outcrossing populations. Selective sweeps during the evolution of these populations are possible, or background selection, or merely loss of a balanced polymorphism maintained by overdominance in the populations that evolved high selfing rates.

scholarly journals Transition from background selection to associative overdominance promotes diversity in regions of low recombination

2019 ◽  
Author(s):  
Kimberly J. Gilbert ◽  
Fanny Pouyet ◽  
Laurent Excoffier ◽  
Stephan Peischl
Keyword(s):  
Genetic Diversity ◽  
Balancing Selection ◽  
Purifying Selection ◽  
Heterozygote Advantage ◽  
Background Selection ◽  
Deleterious Mutations ◽  
Recombination Rates ◽  
Locus Model ◽  
Genome Wide ◽  
Linked Selection

SummaryLinked selection is a major driver of genetic diversity. Selection against deleterious mutations removes linked neutral diversity (background selection, BGS, Charlesworth et al. 1993), creating a positive correlation between recombination rates and genetic diversity. Purifying selection against recessive variants, however, can also lead to associative overdominance (AOD, Ohta 1971, Zhao & Charlesworth, 2016), due to an apparent heterozygote advantage at linked neutral loci that opposes the loss of neutral diversity by BGS. Zhao & Charlesworth (2016) identified the conditions when AOD should dominate over BGS in a single-locus model and suggested that the effect of AOD could become stronger if multiple linked deleterious variants co-segregate. We present a model describing how and under which conditions multi-locus dynamics can amplify the effects of AOD. We derive the conditions for a transition from BGS to AOD due to pseudo-overdominance (Ohta & Kimura 1970), i.e. a form of balancing selection that maintains complementary deleterious haplotypes that mask the effect of recessive deleterious mutations. Simulations confirm these findings and show that multi-locus AOD can increase diversity in low recombination regions much more strongly than previously appreciated. While BGS is known to drive genome-wide diversity in humans (Pouyet et al. 2018), the observation of a resurgence of genetic diversity in regions of very low recombination is indicative of AOD. We identify 21 such regions in the human genome showing clear signals of multi-locus AOD. Our results demonstrate that AOD may play an important role in the evolution of low recombination regions of many species.

scholarly journals Population genetics reveals cryptic lineages and ongoing hybridization in a declining migratory fish species complex

2021 ◽  
Author(s):  
Quentin Rougemont ◽  
Charles Perrier ◽  
Anne-Laure Besnard ◽  
Isabelle Lebel ◽  
Yann Abdallah ◽  
...  
Keyword(s):  
Genetic Diversity ◽  
Life History ◽  
Demographic History ◽  
Life History Strategies ◽  
Secondary Contact ◽  
Allis Shad ◽  
Twaite Shad ◽  
Genetic Diversity And Structure ◽  
Demographic Processes ◽  
Hybridization And Introgression

AbstractDeciphering the effects of historical and recent demographic processes responsible for the spatial patterns of genetic diversity and structure is a key objective in evolutionary and conservation biology. Using genetic analyses, we investigated the demographic history, the contemporary genetic diversity and structure, and the occurrence of hybridization and introgression, of two species of anadromous fish with contrasted life history strategies and which have undergone recent demographic declines, the allis shad (Alosa alosa) and the twaite shad (Alosa fallax). We genotyped 706 individuals from 20 rivers and 5 sites at sea in Southern Europe at microsatellite markers. Genetic structure between populations was lower for the nearly semelparous species allis shad that disperse stronger distance compared to the iteroparous species, twaite shad. Individuals caught at sea were assigned at the river level for twaite shad and at the region level for allis shad. Using an approximate Bayesian computation framework, we inferred that the most likely long term historical divergence scenario between both species implicated historical separation followed by secondary contact accompanied by strong population size decline. Accordingly, we found evidence of contemporary hybridization and introgression between both species. Besides, our results support the existence of cryptic species in the Mediterranean sea. Overall, our results shed light on the interplay between historical and recent demographic processes and life history strategies in shaping population genetic diversity and structure of closely related species. The recent demographic decline of these species’ populations and their hybridization should be carefully considered while implementing conservation programs.

scholarly journals Genomic analysis reveals the genetic diversity, population structure, evolutionary history and relationships of Chinese pepper

2020 ◽  
Vol 7 (1) ◽  
Author(s):  
Shijing Feng ◽  
Zhenshan Liu ◽  
Yang Hu ◽  
Jieyun Tian ◽  
Tuxi Yang ◽  
...  
Keyword(s):  
Genetic Diversity ◽  
Population Structure ◽  
Demographic History ◽  
Genomic Analysis ◽  
Genotyping By Sequencing ◽  
Gansu Province ◽  
Climatic Oscillations ◽  
Genome Wide ◽  
A Genome ◽  
Wild Accessions

Abstract Chinese pepper, mainly including Zanthoxylum bungeanum and Zanthoxylum armatum, is an economically important crop popular in Asian countries due to its unique taste characteristics and potential medical uses. Numerous cultivars of Chinese pepper have been developed in China through long-term domestication. To better understand the population structure, demographic history, and speciation of Chinese pepper, we performed a comprehensive analysis at a genome-wide level by analyzing 38,395 genomic SNPs that were identified in 112 cultivated and wild accessions using a high-throughput genome-wide genotyping-by-sequencing (GBS) approach. Our analysis provides genetic evidence of multiple splitting events occurring between and within species, resulting in at least four clades in Z. bungeanum and two clades in Z. armatum. Despite no evidence of recent admixture between species, we detected substantial gene flow within species. Estimates of demographic dynamics and species distribution modeling suggest that climatic oscillations during the Pleistocene (including the Penultimate Glaciation and the Last Glacial Maximum) and recent domestication events together shaped the demography and evolution of Chinese pepper. Our analyses also suggest that southeastern Gansu province is the most likely origin of Z. bungeanum in China. These findings provide comprehensive insights into genetic diversity, population structure, demography, and adaptation in Zanthoxylum.

scholarly journals Selective signatures and high genome-wide diversity in traditional Brazilian manioc (Manihot esculenta Crantz) varieties

2021 ◽  
Author(s):  
Alessandro Alves-Pereira ◽  
Maria Imaculada Zucchi ◽  
Charles R. Clement ◽  
João Paulo Gomes Viana ◽  
José Baldin Pinheiro ◽  
...  
Keyword(s):  
Genetic Diversity ◽  
Genetic Resources ◽  
Manihot Esculenta ◽  
Evolutionary Dynamics ◽  
Raw Material ◽  
Nucleotide Polymorphisms ◽  
Manihot Esculenta Crantz ◽  
Genome Wide ◽  
Neutral Genetic Diversity ◽  
Effective Use

Knowledge about crops' genetic diversity is essential to promote effective use and conservation of their genetic resources, because genetic diversity enables farmers to adapt their crops to specific needs and is the raw material for breeding efforts. Currently, manioc (Manihot esculenta ssp. esculenta) is one of the most important food crops in the world and has the potential to help achieve food security in the context of on-going climate changes. In this study we assessed the patterns of genome-wide diversity of traditional Brazilian manioc varieties conserved in the gene bank of the Luiz de Queiroz College of Agriculture, University of São Paulo. We used single nucleotide polymorphisms to evaluate the organization of genetic diversity and to identify selective signatures contrasting varieties from different biomes with samples of manioc's wild relative M. esculenta ssp. flabellifolia. We identified signatures of selection putatively associated with resistance genes, plant development and response to abiotic stresses. This presumed adaptive variation might have been important for the initial domestication and for the crop's diversification in response to cultivation in different environments. The neutral variation revealed high levels of genetic diversity within groups of varieties from different biomes and low to moderate genetic divergence among biomes. These results reflect the complexity of manioc's biology and its evolutionary dynamics under traditional cultivation. Our results exemplify how the smallholder practices contribute to the conservation of manioc's genetic resources, maintaining variation of potential adaptive significance and high levels of neutral genetic diversity.

scholarly journals Genetic Diversity and Demographic History of Cajanus spp. Illustrated from Genome-Wide SNPs

PLoS ONE ◽  
2014 ◽  
Vol 9 (2) ◽  
pp. e88568 ◽  
Author(s):  
Rachit K. Saxena ◽  
Eric von Wettberg ◽  
Hari D. Upadhyaya ◽  
Vanessa Sanchez ◽  
Serah Songok ◽  
...  
Keyword(s):  
Genetic Diversity ◽  
Demographic History ◽  
Genome Wide ◽  
History Of
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