Levels Of Admixture Research Articles

Geographic variation in hybridization and ecological differentiation between three syntopic, morphologically similar species of montane lizards

To understand factors shaping species boundaries in closely related taxa, a powerful approach is to compare levels of genetic admixture at multiple points of contact and determine how this relates to intrinsic and extrinsic factors, such as genetic, morphological and ecological differentiation. In the Australian Alps, the threatened alpine bog skink Pseudemoia cryodroma co-occurs with two morphologically and ecologically similar congeners, P.entrecasteauxii and P.pagenstecheri, and all three species are suspected to hybridize. We predicted that the frequency of hybridization should be negatively correlated with genetic divergence, morphological differentiation and microhabitat separation. We tested this hypothesis using a mitochondrial locus, 13 microsatellite loci, morphological and microhabitat data and compared results across three geographically isolated sites. Despite strong genetic structure between species, we detected hybridization between all species pairs, including evidence of backcrossed individuals at the two sites where all three species are syntopic. Hybridization frequencies were not consistently associated with genetic, morphological or ecological differentiation. Furthermore, P.entrecasteauxii and P.pagenstecheri only hybridized at the two sites where they are syntopic with P.cryodroma, but not at the largest site where P.cryodroma was not recorded, suggesting that P.cryodroma may serve as a bridging species. This study reveals the complex dynamics within a three species hybrid zone and provides a baseline for assessing the impact of climate change and anthropogenic habitat modification on future hybridization frequencies.

Hybridization and endangered species protection in the molecular era.

After decades of discussion, there is little consensus on the extent to which hybrids between endangered and nonendangered species should be protected by US law. As increasingly larger, genome-scale data sets are developed, we can identify individuals and populations with even trace levels of genetic admixture, making the 'hybrid problem' all the more difficult. We developed a decision-tree framework for evaluating hybrid protection, including both the processes that produced hybrids (human-mediated or natural) and the ecological impact of hybrids on natural ecosystems. We then evaluated our decision tree for four case studies drawn from our own work and briefly discuss several other cases from the literature. Throughout, we highlight the management outcomes that our approach provides and the nuances of hybridization as a conservation problem.

Genetic basis of adult migration timing in anadromous steelhead discovered through multivariate association testing.

Migration traits are presumed to be complex and to involve interaction among multiple genes. We used both univariate analyses and a multivariate random forest (RF) machine learning algorithm to conduct association mapping of 15 239 single nucleotide polymorphisms (SNPs) for adult migration-timing phenotype in steelhead (Oncorhynchus mykiss). Our study focused on a model natural population of steelhead that exhibits two distinct migration-timing life histories with high levels of admixture in nature. Neutral divergence was limited between fish exhibiting summer- and winter-run migration owing to high levels of interbreeding, but a univariate mixed linear model found three SNPs from a major effect gene to be significantly associated with migration timing (p < 0.000005) that explained 46% of trait variation. Alignment to the annotated Salmo salar genome provided evidence that all three SNPs localize within a 46 kb region overlapping GREB1-like (an oestrogen target gene) on chromosome Ssa03. Additionally, multivariate analyses with RF identified that these three SNPs plus 15 additional SNPs explained up to 60% of trait variation. These candidate SNPs may provide the ability to predict adult migration timing of steelhead to facilitate conservation management of this species, and this study demonstrates the benefit of multivariate analyses for association studies.

Geographic distribution of genetic diversity in populations of Rio Grande Chub Gila pandora

In the southwestern United States (US), the Rio Grande chub (Gila pandora) is state-listed as a fish species of greatest conservation need and federally listed as sensitive due to habitat alterations and competition with non-native fishes. Characterizing genetic diversity, genetic population structure, and effective number of breeders will assist with conservation efforts by providing a baseline of genetic metrics. Genetic relatedness within and among G. pandora populations throughout New Mexico was characterized using 11 microsatellite loci among 15 populations in three drainage basins (Rio Grande, Pecos, Canadian). Observed heterozygosity (HO) ranged from 0.71–0.87 and was similar to expected heterozygosity (0.75–0.87). Rio Ojo Caliente (Rio Grande) had the highest allelic richness (AR = 15.09), while Upper Rio Bonito (Pecos) had the lowest allelic richness (AR = 6.75). Genetic differentiation existed among all populations with the lowest genetic variation occurring within the Pecos drainage. STRUCTURE analysis revealed seven genetic clusters. Populations of G. pandora within the upper Rio Grande drainage (Rio Ojo Caliente, Rio Vallecitos, Rio Pueblo de Taos) had high levels of admixture with Q-values ranging from 0.30–0.50. In contrast, populations within the Pecos drainage (Pecos River and Upper Rio Bonito) had low levels of admixture (Q = 0.94 and 0.87, respectively). Estimates of effective number of breeders (N b ) varied from 6.1 (Pecos: Upper Rio Bonito) to 109.7 (Rio Grande: Rio Penasco) indicating that populations in the Pecos drainage are at risk of extirpation. In the event that management actions are deemed necessary to preserve or increase genetic diversity of G. pandora, consideration must be given as to which populations are selected for translocation.

A Genome-Wide Association Study on the Seedless Phenotype in Banana (Musa spp.) Reveals the Potential of a Selected Panel to Detect Candidate Genes in a Vegetatively Propagated Crop.

Banana (Musa sp.) is a vegetatively propagated, low fertility, potentially hybrid and polyploid crop. These qualities make the breeding and targeted genetic improvement of this crop a difficult and long process. The Genome-Wide Association Study (GWAS) approach is becoming widely used in crop plants and has proven efficient to detecting candidate genes for traits of interest, especially in cereals. GWAS has not been applied yet to a vegetatively propagated crop. However, successful GWAS in banana would considerably help unravel the genomic basis of traits of interest and therefore speed up this crop improvement. We present here a dedicated panel of 105 accessions of banana, freely available upon request, and their corresponding GBS data. A set of 5,544 highly reliable markers revealed high levels of admixture in most accessions, except for a subset of 33 individuals from Papua. A GWAS on the seedless phenotype was then successfully applied to the panel. By applying the Mixed Linear Model corrected for both kinship and structure as implemented in TASSEL, we detected 13 candidate genomic regions in which we found a number of genes potentially linked with the seedless phenotype (i.e. parthenocarpy combined with female sterility). An additional GWAS performed on the unstructured Papuan subset composed of 33 accessions confirmed six of these regions as candidate. Out of both sets of analyses, one strong candidate gene for female sterility, a putative orthologous gene to Histidine Kinase CKI1, was identified. The results presented here confirmed the feasibility and potential of GWAS when applied to small sets of banana accessions, at least for traits underpinned by a few loci. As phenotyping in banana is extremely space and time-consuming, this latest finding is of particular importance in the context of banana improvement.

The invasion of Senecio pterophorus across continents: multiple, independent introductions, admixture and hybridization

Senecio pterophorus (Compositae) is a perennial shrub native to eastern South Africa that was introduced into the Western Cape in South Africa and Australia approximately 100 years ago and into Europe (Italy and Spain) more than 25–30 years ago. In this study, the aims were to unravel the putative sources of the introduced populations and identify the changes in genetic diversity after invasion using molecular markers and phylogeographic and population genetic analyses. We sampled the entire area of distribution for S. pterophorus extensively. Based on the results, three lineages were established along a latitudinal and climatic gradient in the native range (south, central, central/north) with high levels of admixture. Multiple, independent introductions occurred in the four invaded ranges. The central/northern lineage (humid climate) was the primary source for all of the invaded regions (with drier climates), although a secondary role was revealed for the southern lineage in the Western Cape and the central/northern lineage in Australia and Spain. The genetic diversity was slightly lower in the Spanish and Australian populations than that in the native populations. A variety of demographic and genetic processes affected the amount and structure of genetic diversity in the invaded areas, including multiple introductions and admixture (Western Cape, Australia and Spain) as well as pre-invasive hybridization (Italy). The patterns of dispersion support a hypothesis of rapid evolution of S. pterophorus after invasion in response to novel climatic conditions.

CMS Maize: A Tool to Reduce the Distance between GM and non‐GM Maize

SummaryTo limit admixture of genetically modified (GM) with non‐GM crops in the early stages of production, codes of good agricultural practice for GM crops have been developed. In addition to general measures, crop‐specific measures including the establishment of ‘isolation distances’ are required. The use of biological means like cytoplasmic male sterility (CMS) in maize provide more flexible isolation distances particularly in cases where GM maize requires higher levels of containment/confinement to ensure coexistence with agricultural production systems that do not cultivate GM plants. This article discusses the role of CMS where plants develop no or no vital pollen but the female fertility is not affected, as a biological means of reducing isolation requirements. Results of the PRICE project and other EU‐funded projects provide evidence that current isolation distances set by EU Member States are disproportionate for achieving admixture levels below the 0.9 per cent GM admixture threshold for labelling and may lead to unnecessary costs for farmers. Distances might be reduced or replaced by the use of GM CMS maize as it provides an efficient biological confinement tool and increases the flexibility for ‘good agricultural practices of GM crop cultivation’.

English

In this study we genotyped 14 microsatellites to assess genetic diversity, population stratification and demographic dynamics using Egyptian local goats (Zaraibi, Baladi, Saidi and Barki) and the Shami (Damascus) goat from the Middle East and West Asia close to the geographic center of goat domestication. We observed high levels of allelic and genetic diversity that was partitioned into six gene pools. Cluster analyses separated Zaraibi and Shami, which were identified with independent gene pools of potential ancestral backgrounds. The analyses provided further evidence for extensive sharing of genetic variation, revealing, varying levels of admixture among the study populations. This finding was supported by AMOVA analysis, which indicated that the proportion of genetic variation due to differences among populations was 7.06%. Our results most likely indicate that multiple waves of introduction of diverse gene pools and recent flock intermixing has created and maintained a unique set of caprine biodiversity in Eastern North Africa emphasizing the importance of the region as one of the hotbeds of African animal biodiversity. Key words: Admixture, Bayesian clustering, bottleneck, expansion, Egypt.

Contemporary patterns of genetic diversity of cedrela fissilis offer insight into the shaping of seasonal forests in eastern South America.

We investigated how genetic diversity is distributed across the range of Cedrela fissilis, a tree species associated with seasonal neotropical forests, to gain insights into competing biogeographic scenarios that explain how disjunct distributions of these forests were shaped. A total of 250 samples were sampled from 18 sites across the species' range in Brazil and eastern Bolivia and genotyped with 10 microsatellite loci. An array of complementary methods-F statistics, analyses of molecular variance (AMOVA), and clustering analyses-assessed genetic diversity, population differentiation, and structure. Most of the genetic diversity (82.5%) was partitioned within populations, but about 12% was due to differences among groups of populations on either side of the Cerrado or located within the Cerrado; mean expected heterozygosity and mean observed heterozygosity were 0.821 and 0.704, respectively. The 250 samples were sorted into two Bayesian groups: one group for each side of the Cerrado. The populations showed varying levels of admixture, with the greatest admixture evident in populations located toward central Brazil. In C. fissilis, genetic diversity is structured according to geography: the Atlantic range and the Chiquitano range each harbor a genealogical lineage. Interfertility and varying levels of admixture between lineages provide strong evidence that the lineages evolved under geographic, but not genetic, isolation. Admixture is of recent origin, owing to population expansion. Cedrela fissilis shares this dual pattern of distribution of genetic diversity with other phylogenetically unrelated taxa that are typically associated with seasonal forests.

Chocó, Colombia: a hotspot of human biodiversity.

Chocó is a state located on the Pacific coast of Colombia that has a majority Afro-Colombian population. The objective of this study was to characterize the genetic ancestry, admixture and diversity of the population of Chocó, Colombia. Genetic variation was characterized for a sample of 101 donors (61 female and 40 male) from the state of Chocó. Genotypes were determined for each individual via the characterization of 610,545 single nucleotide polymorphisms genome-wide. Haplotypes for the uniparental mitochondrial DNA (female) and Y-DNA (male) chromosomes were also determined. These data were used for comparative analyses with a number of worldwide populations, including putative ancestral populations from Africa, the Americas and Europe, along with several admixed American populations. The population of Chocó has predominantly African genetic ancestry (75.8%) with approximately equal parts European (13.4%) and Native American (11.1%) ancestry. Chocó shows relatively high levels of three-way genetic admixture, and far higher levels of Native American ancestry, compared to other New World African populations from the Caribbean and the United States. There is a striking pattern of sex-specific ancestry in Chocó, with Native American admixture along the female lineage and European admixture along the male lineage. The population of Chocó is also characterized by relatively high levels of overall genetic diversity compared to both putative ancestral populations and other admixed American populations. These results suggest a unique genetic heritage for the population of Chocó and underscore the profound human genetic diversity that can be found in the region.

Population Genetic Structure and Reproductive Strategy of the Introduced Grass Centotheca lappacea in Tropical Land-Use Systems in Sumatra.

Intensive transformation of lowland rainforest into oil palm and rubber monocultures is the most common land-use practice in Sumatra (Indonesia), accompanied by invasion of weeds. In the Jambi province, Centotheca lappacea is one of the most abundant alien grass species in plantations and in jungle rubber (an extensively used agroforest), but largely missing in natural rainforests. Here, we investigated putative genetic differentiation and signatures for adaptation in the introduced area. We studied reproductive mode and ploidy level as putative factors for invasiveness of the species. We sampled 19 populations in oil palm and rubber monocultures and in jungle rubber in two regions (Bukit Duabelas and Harapan). Amplified fragment length polymorphisms (AFLP) revealed a high diversity of individual genotypes and only a weak differentiation among populations (FST = 0.173) and between the two regions (FST = 0.065). There was no significant genetic differentiation between the three land-use systems. The metapopulation of C. lappacea consists of five genetic partitions with high levels of admixture; all partitions appeared in both regions, but with different proportions. Within the Bukit Duabelas region we observed significant isolation-by-distance. Nine AFLP loci (5.3% of all loci) were under natural diversifying selection. All studied populations of C. lappacea were diploid, outcrossing and self-incompatible, without any hints of apomixis. The estimated residence time of c. 100 years coincides with the onset of rubber and oil palm planting in Sumatra. In the colonization process, the species is already in a phase of establishment, which may be enhanced by efficient selection acting on a highly diverse gene pool. In the land-use systems, seed dispersal might be enhanced by adhesive spikelets. At present, the abundance of established populations in intensively managed land-use systems might provide opportunities for rapid dispersal of C. lappacea across rural landscapes in Sumatra, while the invasion potential in rainforest ecosystems appears to be moderate as long as they remain undisturbed.

Indigenous Arabs are descendants of the earliest split from ancient Eurasian populations.

An open question in the history of human migration is the identity of the earliest Eurasian populations that have left contemporary descendants. The Arabian Peninsula was the initial site of the out-of-Africa migrations that occurred between 125,000 and 60,000 yr ago, leading to the hypothesis that the first Eurasian populations were established on the Peninsula and that contemporary indigenous Arabs are direct descendants of these ancient peoples. To assess this hypothesis, we sequenced the entire genomes of 104 unrelated natives of the Arabian Peninsula at high coverage, including 56 of indigenous Arab ancestry. The indigenous Arab genomes defined a cluster distinct from other ancestral groups, and these genomes showed clear hallmarks of an ancient out-of-Africa bottleneck. Similar to other Middle Eastern populations, the indigenous Arabs had higher levels of Neanderthal admixture compared to Africans but had lower levels than Europeans and Asians. These levels of Neanderthal admixture are consistent with an early divergence of Arab ancestors after the out-of-Africa bottleneck but before the major Neanderthal admixture events in Europe and other regions of Eurasia. When compared to worldwide populations sampled in the 1000 Genomes Project, although the indigenous Arabs had a signal of admixture with Europeans, they clustered in a basal, outgroup position to all 1000 Genomes non-Africans when considering pairwise similarity across the entire genome. These results place indigenous Arabs as the most distant relatives of all other contemporary non-Africans and identify these people as direct descendants of the first Eurasian populations established by the out-of-Africa migrations.

Editorial: Advances in Farm Animal Genomic Resources.

Editorial: Advances in Farm Animal Genomic Resources.

Three genetic groups of the Eucalyptus stem canker pathogen Teratosphaeria zuluensis introduced into Africa from an unknown source.

The Eucalyptus stem canker pathogen Teratosphaeria zuluensis was discovered in South Africa in 1988 and it has subsequently been found in several other African countries as well as globally. In this study, the population structure, genetic diversity and evolutionary history of T. z uluensis were analysed using microsatellite markers to gain an enhanced understanding of its movement in Africa. Isolates were collected from several sites in Malawi, Mozambique, Uganda and Zambia. Data obtained were compared with those previously published for a South African population. The data obtained from 334 isolates, amplified across eight microsatellite loci, were used for assignment, differentiation and genetic diversity tests. STRUCTURE analyses, θ st and genetic distances revealed the existence of two clusters, one dominated by isolates from South Africa and the other by isolates from the Zambezi basin including Malawi, Mozambique and Zambia. High levels of admixture were found within and among populations, dominated by the Mulanje population in Malawi. Moderate to low genetic diversity of the populations supports the previously held view that the pathogen was introduced into Africa. The clonal nature of the Ugandan population suggests a very recent introduction, most likely from southern Africa.

Genetics against race: Science, politics and affirmative action in Brazil.

This article analyses interrelations between genetic ancestry research, political conflict and social identity. It focuses on the debate on race-based affirmative action policies, which have been implemented in Brazil since the turn of the century. Genetic evidence of high levels of admixture in the Brazilian population has become a key element of arguments that question the validity of the category of race for the development of public policies. In response, members of Brazil’s black movement have dismissed the relevance of genetics by arguing, first, that in Brazil race functions as a social – rather than a biological – category, and, second, that racial classification and discrimination in this country are based on appearance, rather than on genotype. This article highlights the importance of power relations and political interests in shaping public engagements with genetic research and their social consequences.

Genetic Relationship in Cicer Sp. Expose Evidence for Geneflow between the Cultigen and Its Wild Progenitor.

There is a debate concerning mono- or poly-phyletic origins of the Near Eastern crops. In parallel, some authors claim that domestication was not possible within the natural range of the wild progenitors due to wild alleles flow into the nascent crops. Here we address both, the mono- or poly-phyletic origins and the domestications within or without the natural range of the progenitor, debates in order to understand the relationship between domesticated chickpea (Cicer arietinum L.) and its wild progenitor (C. reticulatum Ladizinsky) with special emphasis on its domestication centre in southeastern Turkey. A set of 103 chickpea cultivars and landraces from the major growing regions alongside wild accessions (C. reticulatum, C. echinospermum P.H Davis and C. bijugum K.H. Rech) sampled across the natural distribution range in eastern Turkey were genotyped with 194 SNPs markers. The genetic affinities between and within the studied taxa were assessed. The analysis suggests a mono-phyletic origin of the cultigen, with several wild accession as likely members of the wild stock of the cultigen. Clear separation between the wild and domesticated germplasm was apparent, with negligible level of admixture. A single C. reticulatum accession shows morphological and allelic signatures of admixture, a likely result of introgression. No evidence of geneflow from the wild into domesticated germplasm was found. The traditional farming systems of southeaster Turkey are characterized by occurrence of sympatric wild progenitor—domesticated forms of chickpea (and likewise cereals and other grain legumes). Therefore, both the authentic crop landraces and the wild populations native to the area are a unique genetic resource. Our results grant support to the notion of domestication within the natural distribution range of the wild progenitor, suggesting that the Neolithic domesticators were fully capable of selecting the desired phenotypes even when facing rare wild-domesticated introgression events.

Analysis of admixture in Native American populations from Colombia

The current Colombian population is the result of genetic admixture between Native Americans, Europeans and Africans. Around 82% of the population is considered admixed, 15% are Afro-Colombians and just 3% are Native Americans. There are about 81 native groups in Colombia. In this work, a sample of 121 non-related individuals from three Native American groups were analysed for 46 ancestry informative InDel markers. One of these groups was the Pijaos that, because of their combative nature, was almost exterminated during European conquest and colonial times. The other two studied groups belong to communities that have been less subjected to admixture with non-Natives. The Barí, known as “Motilones” or “Dobukubi”, is a Native group that inhabits the Serranía del Perija, Norte de Santander, since pre-Colombian times. They still keep their original language, the Barí-ara, which belongs to the Chibcha family. We have also studied a sample of Natives from Guainía, formed by different groups that migrated from the Amazonia and Orinoquian regions, including the Desana, Curripaco, Puinave, Cubeo, Guaunano and Tucano, all belonging to Tucano and Arawak linguistic groups. This study allow determining genotypic and allelic frequencies for 46 ancestry informative InDels and to estimate Native American, European and African admixture proportions in three Colombian native communities. The results showed a very low European and African admixture in the Barí and Guainia native groups in contrast to the high levels of admixture presented by the Pijao.

PREDICTION OF GLOBAL AND LOCAL SIMMENTAL AND RED HOLSTEIN FRIESIAN ADMIXTURE LEVELS IN SWISS FLECKVIEH CATTLE

PREDICTION OF GLOBAL AND LOCAL SIMMENTAL AND RED HOLSTEIN FRIESIAN ADMIXTURE LEVELS IN SWISS FLECKVIEH CATTLE

A long-term genetic study reveals complex population dynamics of multiple-source plant reintroductions

A major challenge of using multiple-source populations in reintroductions for restoration and species conservation is to adequately assess the trade-off between the benefits of counteracting inbreeding depression via heterosis, and the risks of maladaptation and reduced fitness through outbreeding depression. In the 1990s, populations of the perennial plant Arenaria grandiflora rapidly declined and were nearing extinction in the Fontainebleau forest, France. This was likely due to inbreeding depression and/or fixation of deleterious alleles. To restore A. grandiflora in the Fontainebleau forest, a reintroduction experiment was conducted in 1999 using transplants from both local and non-local populations. Eight and twelve years later, we carried out a genetic study using microsatellite markers to assess the temporal dynamics of the genetic composition of reintroduced populations. We show that genetic diversity increased significantly in the reintroduced populations compared to the local-source population and that this increased diversity has been maintained for over ten years, highlighting the benefits of mixing individuals of multiple-source populations for restoration of small and inbred populations. Our results also suggest that the level of individual admixture between local and non-local genetic sources might affect individual fitness, which is influenced by the opposing effects of heterosis, inbreeding and outbreeding depressions and local adaptation.

Classification of bentgrass (Agrostis) cultivars and accessions based on microsatellite (SSR) markers

Genetic relationships among Agrostis species used for turf have been difficult to discern. Recent studies have either confirmed or contradicted previously proposed genetic relationships based on chromosome pairing behavior of inter-specific hybrids. The objective of the current study was to assess genetic relationships among Agrostis cultivars and accessions by using newly available A. stolonifera microsatellite (SSR) markers. Nuclear SSR (nuSSR) and chloroplast SSR (cpSSR) markers were used to genotype 16 individuals from each of 74 Agrostis cultivars and accessions. Genetic relationships based on nuSSR markers most closely resembled species relationships proposed by Jones in the 1950s. Contrary to the work of Jones, genetic relationships based on cpSSR markers indicated that A. canina was more closely related to A. stolonifera than to A. capillaris. We hypothesize that chloroplast introgression via interspecific hybridization between A. canina and A. stolonifera resulted in these species sharing common chloroplast genome lineages, while maintaining disparate nuclear genome lineages. Genetic relationships within Agrostis species based on nuSSR markers closely matched known pedigree relationships. Bayesian clustering analysis of nuSSR markers indicated that most modern seeded A. stolonifera cultivars exhibited high levels of admixture. Our study confirms that nuSSR markers distinguish Agrostis species and cultivars, and are valuable for studying genetic diversity and genetic relationships within the genus Agrostis.