Biology subjects

Rosenberg, N. A.

Publications and source records attributed to Rosenberg, N. A..

30 records · Page 2Linked to original sources

bioRxiv2019

On the heterozygosity of an admixed population

A population is termed admixed if its members possess recent ancestry from two or more separate sources. As a result of the fusion of source populations with different genetic variants, admixed populations can exhibit high levels of genetic variation, reflecting contributions of their multiple ancestral groups. For a model of an admixed population derived from K source groups, we obtain a relationship between its level of genetic variation, as measured by heterozygosity, and its proportions of admixture from the various source populations. We show that the heterozygosity of the admixed population is at least as great as that of the least heterozygous source population, and that it potentially exceeds the heterozygosities of all of the source populations. The admixture proportions that maximize the heterozygosity possible for an admixed population formed from a specified set of source populations are also obtained under specific conditions. We examine the special case of K = 2 source populations in detail, characterizing the maximal admixture in terms of the heterozygosities of the two source populations and the value of FST between them. In this case, the heterozygosity of the admixed population exceeds the maximal heterozygosity of the source groups if the divergence between them, measured by FST, is large enough, namely above a certain bound that is a function of the heterozygosities of the source groups. We present applications to simulated data as well as to data from human admixture scenarios, providing results useful for interpreting the properties of genetic variability in admixed populations.

genetics↗

bioRxiv2019

Assortative mating and the dynamical decoupling of genetic admixture levels from phenotypes that differ between source populations

Source populations for an admixed population can possess distinct patterns of genotype and pheno-type at the beginning of the admixture process. Such differences are sometimes taken to serve as markers of ancestry--that is, phenotypes that are initially associated with the ancestral background in one source population are taken to reflect ancestry in that population. Examples exist, however, in which genotypes or phenotypes initially associated with ancestry in one source population have decoupled from overall admixture levels, so that they no longer serve as proxies for genetic ancestry. We develop a mechanistic model for describing the joint dynamics of admixture levels and phenotype distributions in an admixed population. The approach includes a quantitative-genetic model that relates a phenotype to underlying loci that affect its trait value. We consider three forms of mating. First, individuals might assort in a manner that is independent of the overall genetic admixture level. Second, individuals might assort by a quantitative phenotype that is initially correlated with the genetic admixture level. Third, individuals might assort by the genetic admixture level itself. Under the model, we explore the relationship between genetic admixture level and phenotype over time, studying the effect on this relationship of the genetic architecture of the phenotype. We find that the decoupling of genetic ancestry and phenotype can occur surprisingly quickly, especially if the phenotype is driven by a small number of loci. We also find that positive assortative mating attenuates the process of dissociation in relation to a scenario in which mating is random with respect to genetic admixture and with respect to phenotype. The mechanistic framework suggests that in an admixed population, a trait that initially differed between source populations might be a reliable proxy for ancestry for only a short time, especially if the trait is determined by relatively few loci. The results are potentially relevant in admixed human populations, in which phenotypes that have a perceived correlation with ancestry might have social significance as ancestry markers, despite declining correlations with ancestry over time.\n\nAuthor SummaryAdmixed populations are populations that descend from two or more populations that had been separated for a long time at the beginning of the admixture process. The source populations typically possess distinct patterns of genotype and phenotype. Hence, early in the admixture process, phenotypes of admixed individuals can provide information about the extent to which these individuals possess ancestry in a specific source population. To study correlations between admixture levels and phenotypes that differ between source populations, we construct a genetic and phenotypic model of the dynamical process of admixture. Under the model, we show that correlations between admixture levels and these phenotypes dissipate over time--especially if the genetic architecture of the phenotypes involves only a small number of loci, or if mating in the admixed population is random with respect to both the admixture levels and the phenotypes. The result has the implication that a trait that once reflected ancestry in a specific source population might lose this ancestry correlation. As a consequence, in human populations, after a sufficient length of time, salient phenotypes that can have social meaning as ancestry markers might no longer bear any relationship to genome-wide genetic ancestry.

evolutionary biology↗

bioRxiv2019

Assortative mating by population of origin in a mechanistic model of admixture

Populations whose mating pairs have levels of similarity in phenotypes or genotypes that differ systematically from the level expected under random mating are described as experiencing assortative mating. Excess similarity in mating pairs is termed positive assortative mating, and excess dissimilarity is negative assortative mating. In humans, empirical studies suggest that mating pairs from various admixed populations--whose ancestry derives from two or more source populations--possess correlated ancestry components that indicate the occurrence of positive assortative mating on the basis of ancestry. Generalizing a two-sex mechanistic admixture model, we devise a model of one form of ancestry-assortative mating that occurs through preferential mating based on source population. Under the model, we study the moments of the admixture fraction distribution for different assumptions about mating preferences, including both positive and negative assortative mating by population. We consider the special cases of assortative mating by population that involve a single admixture event and that consider a model of constant contributions to the admixed population over time. We demonstrate that whereas the mean admixture under assortative mating is equivalent to that of a corresponding randomly mating population, the variance of admixture depends on the level and direction of assortative mating. In contrast to standard settings in which positive assortment increases variation within a population, certain assortative mating scenarios allow the variance of admixture to decrease relative to a corresponding randomly mating population: with the three populations we consider, the variance-increasing effect of positive assortative mating within a population might be overwhelmed by a variance-decreasing effect emerging from mating preferences involving other pairs of populations. The effect of assortative mating is smaller on the X chromosome than the autosomes because inheritance of the X in males depends only on the mothers ancestry, not on the mating pair. Because the variance of admixture is informative about the timing of admixture and possibly about sex-biased admixture contributions, the effects of assortative mating are important to consider in inferring features of population history from distributions of admixture values. Our model provides a framework to quantitatively study assortative mating under flexible scenarios of admixture over time.

evolutionary biology↗

bioRxiv2019

Designing gene drives to limit spillover to non-target populations

The prospect of utilizing CRISPR-based gene-drive technology for controlling populations, such as invasive and disease-vector species, has generated much excitement. However, the potential for spillovers of gene drive alleles from the target population to non-target populations -- events that may be ecologically catastrophic -- has raised concerns. Here, using two-population mathematical models, we investigate the possibility of limiting spillovers and impact on non-target populations by designing differential-targeting gene drives, in which the expected equilibrium gene drive allele frequencies are high in the target population but low in the non-target population. We find that achieving differential targeting is possible with certain configurations of gene drive parameters. Most of these configurations ensure differential targeting only under relatively low migration rates between target and non-target populations. Under high migration, differential targeting is possible only in a narrow region of the parameter space. When migration is increased, differential-targeting states can sharply transition to states of global fixation or global loss of the gene drive. Because fixation of the gene drive in the non-target population could severely disrupt ecosystems, we outline possible ways to avoid this outcome. Our results emphasize that, although gene drive technology is promising, understanding the potential consequences for populations other than the targets requires detailed analysis of gene-drive spillovers, and that ways to limit the unintended effects of gene drives to non-target populations should be explored prior to the application of gene drives in natural settings.

evolutionary biology↗

bioRxiv2019

FST and the triangle inequality for biallelic markers

The population differentiation statistic FST, introduced by Sewall Wright, is often treated as a pairwise distance measure between populations. As was known to Wright, however, FST is not a true metric because allele frequencies exist for which it does not satisfy the triangle inequality. We prove that a stronger result holds: for biallelic markers whose allele frequencies differ across three populations, FST never satisfies the triangle inequality. We study the deviation from the triangle inequality as a function of the allele frequencies of three populations, identifying frequency vectors at which the deviation is maximal. We also examine the implications of the failure of the triangle inequality for the four-point condition for groups of four populations. Next, we examine the extent to which FST fails to satisfy the triangle inequality in genome-wide data from human populations, finding that some loci have frequencies that produce deviations near the maximum. We discuss the consequences of the theoretical results for various types of data analysis, including multidimensional scaling and inference of neighbor-joining trees from pairwise FST matrices.

evolutionary biology↗

bioRxiv2019

Network-based hierarchical population structure analysis for large genomic datasets

Analysis of population structure in natural populations using genetic data is a common practice in ecological and evolutionary studies. With large genomic datasets of populations now appearing more frequently across the taxonomic spectrum, it is becoming increasingly possible to reveal many hierarchical levels of structure, including fine-scale genetic clusters. To analyze these datasets, methods need to be appropriately suited to the challenges of extracting multilevel structure from whole-genome data. Here, we present a network-based approach for constructing population structure representations from genetic data. The use of community detection algorithms from network theory generates a natural hierarchical perspective on the representation that the method produces. The method is computationally efficient, and it requires relatively few assumptions regarding the biological processes that underlie the data. We demonstrate the approach by analyzing population structure in the model plant species Arabidopsis thaliana and in human populations. These examples illustrates how network-based approaches for population structure analysis are well-suited to extracting valuable ecological and evolutionary information in the era of large genomic datasets.

genetics↗

bioRxiv2018

Disease and introgression explain the long-lasting contact zone of Modern Humans and Neanderthals and its eventual destabilization

Neanderthals and modern humans both occupied the Levant for tens of thousands of years prior to modern humans spread into the rest of Eurasia and their replacement of the Neanderthals. That the inter-species boundary remained geographically localized for so long is a puzzle, particularly in light of the rapidity of its subsequent movement. We propose that disease dynamics can explain the localization and persistence of the inter-species boundary; in this view, each species carried pathogens to which it was largely immune and tolerant, but that could spread to the other, vulnerable, species, inducing a significant disease burden. Epidemics and endemic diseases along the interspecies boundary would have mitigated against bands of one species migrating into regions dominated by the other species. Together with decreased population densities and limited inter-group interactions due to disease burden, this mechanism could have resulted in a fixed and narrow contact-zone. We further propose, and support with results from dynamical systems models, that genetic introgression, including transmission of alleles related to the immune system, would have gradually allowed the two species to overcome this barrier to pervasive inter-species interaction, leading to the eventual release of the inter-species boundary from its geographic localization. Asymmetries between the two species in the initial size of their associated \"pathogen package\" could have created feedback loops that influenced the rates at which immunity to and tolerance of the novel pathogens were acquired. These asymmetries could have allowed modern humans to overcome the disease burden earlier than Neanderthals, giving them a significant advantage in their subsequent spread into Eurasia, particularly upon interaction with Neanderthal populations that had previously been far from the original contact zone in the Levant.

evolutionary biology↗

bioRxiv2018

Linkage disequilibrium connects genetic records of relatives typed with disjoint genomic marker sets

In familial searching in forensic genetics, a query DNA profile is tested against a database to determine whether it represents a relative of a database entrant. We examine the potential for using linkage disequilibrium to identify pairs of profiles as belonging to relatives when the query and database rely on nonoverlapping genetic markers. Considering data on individuals genotyped with both microsatellites used in forensic applications and genome-wide SNPs, we find that ~30-32% of parent-offspring pairs and ~35-36% of sib pairs can be identified from the SNPs of one member of the pair and the microsatellites of the other. The method suggests the possibility of performing familial searches of microsatellite databases using query SNP profiles, or vice versa. It also reveals that privacy concerns arising from computations across multiple databases that share no genetic markers in common entail risks not only for database entrants, but for their close relatives as well.

genetics↗

bioRxiv2017

Coalescent theory of migration network motifs

Natural populations display a variety of spatial arrangements, each potentially with a distinctive impact on genetic diversity and genetic differentiation among subpopulations. Although the spatial arrangement of populations can lead to intricate migration networks, theoretical developments have focused mainly on a small subset of such networks, emphasizing the island-migration and stepping-stone models. In this study, we investigate all small network motifs: the set of all possible migration networks among populations subdivided into at most four subpopulations. For each motif, we use coalescent theory to derive expectations for three quantities that describe genetic variation: nucleotide diversity, FST, and half-time to equilibrium diversity. We describe the impact of network properties on these quantities, finding that motifs with a large mean node degree have the largest nucleotide diversity and the longest time to equilibrium, whereas motifs with small density have the largest FST. In addition, we show that the motifs whose pattern of variation is most strongly influenced by loss of a connection or a subpopulation are those that can be split easily into several disconnected components. We illustrate our results using two example datasets--sky island birds of genus Brachypteryx and Indian tigers--identifying disturbance scenarios that produce the greatest reduction in genetic diversity; for tigers, we also compare the benefits of two assisted gene flow scenarios. Our results have consequences for understanding the effect of geography on genetic diversity and for designing strategies to alter population migration networks to maximize genetic variation in the context of conservation of endangered species.

genetics↗

bioRxiv2017

A Genome Scan for Genes Underlying Adult Body Size Differences between Central African Pygmies and their Non-Pygmy Neighbors

BackgroundCentral African hunter-gatherer Pygmy populations have reduced body size compared with their often much larger agricultural non-Pygmy neighbors, potentially reflecting adaptation to the anatomical and physiological constraints of their lifestyle in tropical rainforests. Earlier studies investigating the genetics of the pygmy phenotype have focused on standing height, one aspect of this complex phenotype that is itself a composite of skeletal components with different growth patterns. Here, we extend the investigations of standing height to the variability and genetic architecture of sitting height and subischial leg length as well as body mass index (BMI) in a sample of 406 unrelated West Central African Pygmies and non-Pygmies.\n\nResultsIn addition to their significantly reduced standing height compared with non-Pygmies, we find Pygmies to have significantly shorter sitting heights and subischial leg lengths as well as higher sitting/standing height ratios than non-Pygmies. However, while male Pygmies had significantly lower BMI compared with male non-Pygmies, the BMI of females were instead similar. Consistent with prior observations with standing height, sitting height and subischial leg length were strongly correlated with inferred levels of non-Pygmy genetic admixture while BMI was instead weakly correlated, likely reflecting the greater contribution of non-genetic factors to the determination of body weight compared with height. Using 196,725 SNPs on the Illumina Cardio-MetaboChip with genotypes on 358 Pygmy and 169 non-Pygmy individuals together with single-and multi-marker association approaches, we identified a single genomic region and seven genes associated with Pygmy/non-Pygmy categorization as well as 9, 10, 9, and 10 genes associated with standing and sitting height, sitting/standing height ratio, and subischial leg length, respectively. Many of the genes identified have putative functions consistent with a role in determining their associated trait as well as the complex Central African pygmy phenotype.\n\nConclusionsThese findings highlight the potential of modestly sized datasets of Pygmies and non-Pygmies to detect biologically meaningful associations with traits contributing to the Central African pygmy phenotype. Moreover, they provide new insights into the phenotypic and genetic bases of the complex pygmy phenotype and offer new opportunities to facilitate our understanding of its complex evolutionary origins.

genetics↗

bioRxiv2017

Reply To Lazaridis And Reich: Robust Model-Based Inference Of Male-Biased Admixture During Bronze Age Migration From The Pontic-Caspian Steppe

Comparing the sex-specifically inherited X chromosome to the autosomes in ancient genetic samples, we (1) studied sex-specific admixture for two prehistoric migrations. For each migration, we used several admixture estimation procedures--including ADMIXTURE model-based clustering (2)--comparing X-chromosomal and autosomal ancestry in contemporaneous Central Europeans, and interpreting greater admixture from the migrating population on the autosomes as male-biased migration. For migration into late Neolithic/Bronze Age Central Europeans (\"BA\") from the Pontic-Caspian steppe (\"SP\"), we inferred male-biased admixture at 5-14 males per migrating female.\n\nLazaridis & Reich (3) contest this male-biased migration claim. For simulated individuals, they claim that ADMIXTURE provides biased X-chromosomal ancestry estimates. They argue that if the bias is taken into account, the ...

genetics↗

bioRxiv2016

Mathematical constraints on FST: biallelic markers in arbitrarily many populations

FST is one of the most widely used statistics in population genetics. Recent mathematical studies have identified constraints on FST that challenge interpretations of FST as a measure with potential to range from 0 for genetically similar populations to 1 for divergent populations. We generalize results obtained for population pairs to arbitrarily many populations, characterizing the mathematical relationship between FST, the frequency M of the more frequent allele at a polymorphic biallelic marker, and the number of subpopulations K. We show that for fixed K, FST has a peculiar constraint as a function of M, with a maximum of 1 only if M = i/K for integers i with {lceil}K/2{rciel} [≤] i [≤] K - 1. For fixed M, as K grows large, the range of FST becomes the full closed or half-open unit interval. For fixed K, however, some M < (K - 1)/K always exists at which the upper bound on FST is constrained to be below [Formula]. In each of three migration models--island, rectangular stepping-stone, and linear stepping-stone--we use coalescent simulations to show that under weak migration, FST depends strongly on the allele frequency M when K is small, but not when K is large. Finally, using data on human genetic variation, we employ our results to explain the generally smaller FST values between pairs of continents relative to global FST values. We discuss implications for the interpretation and use of FST.

genetics↗