PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Genetic Structures”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2Linked to original sources
PubMed2001

[Geographic distribution of surnames and genetic structure: the county of Ardeche at the beginning of the twentieth century].

The analysis of the geographical distribution of surnames (whose transmission in the European societies is similar to that of the Y chromosome) allows to study in an exhaustive way the spatial and temporal human population structure and the influence of environmental variations on this structure. The genetic structure of the Ardèche county was analysed through a study of the surname characteristics (diversity, frequency, geographical dispersion, etc.) of individuals born between 1891 and 1915. The estimation of the intra-community average consanguinity and the inter-district genetic relationships reveals a clear differentiation between populations of the mountainous part (geographically isolated, highly inbred) and those of the Rhône valley (less isolated, weakly inbred). As predicted by the isolation by distance model, inter-population coefficients of kinship decreases as distance increases, confirming the presence of a spatial structure, characterized by neighborhood exchange and resulting from former migratory flow.

Alleles↗
PubMed2001

Spatial genetic structure of allozyme polymorphisms within populations of Pinus Strobus (Pinaceae).

The population structure of genetic variation for four allozyme loci was investigated for two populations (one old growth, OG, and the other logged, SS) of eastern white pine (Pinus strobus). Both seedlings and reproductive adults were studied for both study populations. Spatial autocorrelation statistics were used to examine the distribution of allozyme polymorphisms. The spatial genetic structure in adults of population OG indicated that individual genotypes were distributed in a structured, isolation-by-distance manner, consistent with observed levels of pollen and seed dispersal. In contrast, adult genotypes in population SS were nearly randomly distributed, probably as a result of logging. Nonetheless, spatial structuring of genotypes of seedlings occurred at both sites, indicating the power of limited seed flow, as well as temporal Wahlund effects, to create structure. None of four loci in both seedling populations showed a significant departure from Hardy-Weinberg proportions, whereas one and two significant deviations were found for loci in the two respective adult populations. These departures may be attributed to episodic reproductive events.

Journal Article↗
PubMed1999

Spatial autocorrelation analysis of individual multiallele and multilocus genetic structure.

Population genetic theory predicts that plant populations will exhibit internal spatial autocorrelation when propagule flow is restricted, but as an empirical reality, spatial structure is rarely consistent across loci or sites, and is generally weak. A lack of sensitivity in the statistical procedures may explain the discrepancy. Most work to date, based on allozymes, has involved pattern analysis for individual alleles, but new PCR-based genetic markers are coming into vogue, with vastly increased numbers of alleles. The field is badly in need of an explicitly multivariate approach to autocorrelation analysis, and our purpose here is to introduce a new approach that is applicable to multiallelic codominant, multilocus arrays. The procedure treats the genetic data set as a whole, strengthening the spatial signal and reducing the stochastic (allele-to-allele, and locus-to-locus) noise. We (i) develop a very general multivariate method, based on genetic distance methods, (ii) illustrate it for multiallelic codominant loci, and (iii) provide nonparametric permutational testing procedures for the full correlogram. We illustrate the new method with an example data set from the orchid Caladenia tentaculata, for which we show (iv) how the multivariate treatment compares with the single-allele treatment, (v) that intermediate frequency alleles from highly polymorphic loci perform well and rare alleles poorly, (vi) that a multilocus treatment provides clearer answers than separate single-locus treatments, and (vii) that weighting alleles differentially improves our resolution minimally. The results, though specific to Caladenia, offer encouragement for wider application.

Alleles↗
PubMedDate not supplied

[Influence of selection forms on genetic structure of domestic animals].

Analysis of genetic structure of pure and crossbred sheep and cattle with the use of genetic-biochemical markers was carried out. Data on breed- and locus-specific genetic traits, stable in the time, were obtained. In sheep, breed-specific peculiarities of genetic structure partly related with the belonging of breeds to breed groups with fine-, semi-fine and coarse wool. The preferable influence of artificial selection at the phenotype complex traits in comparison with natural selection and animal origin on the genetic structure of crossbred sheep was revealed. However, the more profound effects of natural selection on the genetic structure of crossbred cattle were observed.

Alleles↗
PubMed2004

Trypanosoma cruzi: genetic structure of populations and relevance of genetic variability to the pathogenesis of chagas disease.

Chagas disease, caused by the protozoan Trypanosoma cruzi, has a variable clinical course, ranging from symptomless infection to severe chronic disease with cardiovascular or gastrointestinal involvement or, occasionally, overwhelming acute episodes. The factors influencing this clinical variability have not been elucidated, but it is likely that the genetic variability of both the host and the parasite are of importance. In this work we review the the genetic structure of T. cruzi populations and analyze the importance of genetic variation of the parasite in the pathogenesis of the disease under the light of the histotropic-clonal model.

Animals↗
PubMed2000

Differences in genetic structure between two Japanese beech (Fagus crenata Blume) stands.

To examine the effects of forest cutting on within-population genetic structure, the genetic structure and variability of two Japanese beech (Fagus crenata Blume) stands with contrasting histories in relation to cutting were investigated. Six hundred and sixty beech trees, covering two hectares in total, were mapped and genetically analysed using nine isozyme loci encoding eight enzyme systems. The proportion of polymorphic loci, the average number of alleles per locus, the effective number of alleles per locus, the expected heterozygosity and the observed heterozygosity were 78, 3.3, 1.31, 0.200 and 0.189, respectively, in a secondary stand (designated AK) cut during the 1920s. Corresponding figures were 78, 3.3, 1.33, 0.203 and 0.193, respectively, in a primary stand designated KU. The inbreeding coefficient and the grand mean of the number of alleles in common (NAC) were 0.055 and 1.684 in AK, and 0. 042 and 1.649 in KU, respectively. The genetic variability was slightly but significantly lower in AK. The genetic structure of the two stands was strikingly different. The proportions of positively significant Moran's I and SND values found in the shortest distance class were 0.86 and 0.38 for AK, and 0.14 and 0.29 for KU, respectively. Furthermore, significant linkage disequilibrium was observed in AK, but none at all in KU. To examine which, if any, differences in the genetic structure would be likely to influence succeeding generations, we simulated a self-thinning process. The simulation suggested that reduced genetic variability and linkage disequilibrium would have significant influence in the AK stand for several generations.

Alleles↗
PubMed2004

Heterogeneous genetic structure in a Fagus crenata population in an old-growth beech forest revealed by microsatellite markers.

The within-population genetic structure of Fagus crenata in a 4-ha plot (200 x 200 m) of an old-growth beech forest was analysed using microsatellite markers. To assess the genetic structure, Moran's I spatial autocorrelation coefficient was calculated. Correlograms of Moran's I showed significant positive values less than 0.100 for short-distance classes, indicating weak genetic structure. The genetic structure within the population is created by limited seed dispersal, and is probably weakened by overlapping seed shadow, secondary seed dispersal, extensive pollen flow and the thinning process. Genetic structure was detected in a western subplot of 50 x 200 m with immature soils and almost no dwarf bamboos (Sasa spp.), where small and intermediate-sized individuals were distributed in aggregations with high density because of successful regeneration. By contrast, genetic structure was not found in an eastern subplot of the same size with mature soils and Sasa cover, where successful regeneration was prevented, and the density of the small and intermediate-sized individuals was low. Moreover, genetic structure of individuals in a small-size class (diameter at breast height < 12 cm) was more obvious than in a large-size class (diameter at breast height >/= 12 cm). The apparent genetic structure detected in the 4-ha plot was therefore probably the result of the structure in the western portion of the plot and in small and intermediate-sized individuals that successfully regenerated under the favourable environment. The heterogeneity in genetic structure presumably reflects variation in the density that should be affected by differences in regeneration dynamics associated with heterogeneity in environmental conditions.

Demography↗
PubMed2006

Founder events as determinants of within-island and among-island genetic structure of Daphnia metapopulations.

The genetic structure of metapopulations offers insights into the genetic consequences of local extinction and recolonization. We studied allozyme variation in rock pool metapopulations of two species of waterfleas (Daphnia) with the aim to understand how these dynamics influence genetic differentiation. We screened 138 populations of D. magna and 65 populations of D. longispina from an area in the archipelago of southern Finland. The pools from which they were sampled are separated by distances between 1.5 and 4710 m and located on a total of 38 islands. The genetic population structure of the two species was strikingly similar, consistent with their similar metapopulation ecology. The mean F(PT) value (differentiation among pools with respect to the total metapopulation) was 0.55 and a hierarchical analysis showed that genetic differentiation was strong (>0.25) among pools within islands as well as among whole islands. Within islands, pairwise genetic differentiation increased with geographic distance, indicating isolation by distance due to spatially limited dispersal. Previous studies have shown strong founder events occurring during colonization in our metapopulation. We suggest that the genetic population structure in the studied metapopulations is largely explained by three consequences of these founder events: (i) strong drift during colonization, (ii) local inbreeding, which results in hybrid vigour and increased effective migration rates after subsequent immigration, and (iii) effects of selection through hitchhiking of neutral genes with linked loci under selection.

Animals↗
PubMedDate not supplied

The effects of habitat fragmentation on the genetic structure of small mammal populations.

We present five case studies highlighting the effects of habitat fragmentation on the genetic structure of small mammal populations. The studies reflect different spatial scales and components of genetic variation. In marginal and central populations of Sigmodon hispidus we found less allozymic variation within the marginal population, whereas patterns of morphological variability were the converse. In the rice rat (Oryzomys spp.), nucleotide diversity in mtDNA was similar in an island population in the Florida Keys to mainland populations in the Everglades. This observation contrasts with insular vole populations (Microtus spp.), where isolation on islands results in genetic structuring. Temporal changes in abundance in mainland populations had no effects on genetic differentiation (FST values) because subpopulations did not experience bottlenecks. In an experimentally fragmented landscape, fragmentation influenced demographic processes but not genetic structure. We conclude that (1) with extreme fragmentation, small mammal populations become depauperate of genetic variation and differentiate genetically; (2) different components of genetic variation lead to different genetic structuring; (3) spatial and temporal scales should both be considered when examining genetic structure of populations; (4) demographic and ecological processes are more likely influenced by fragmentation than genetic structure; and (5) there is an interaction between demographic processes and genetic structure.

Animals↗
PubMed2004

Comparison of the fine-scale genetic structure of three dipterocarp species.

We investigated the fine-scale genetic structure of three tropical-rainforest trees, Hopea dryobalanoides, Shorea parvifolia and S. acuminata (Dipterocarpaceae), in Peninsular Malaysia, all of which cooccurred within a 6-ha plot in Pasoh Forest Reserve. A significant genetic structure was found in H. dryobalanoides, weaker (but still significant) genetic structure in S. parvifolia and nonsignificant structure in S. acuminata. Seeds of all three species are wind dispersed, and their flowers are thought to be insect pollinated. The most obvious difference among these species is their height: S. parvifolia and S. acuminata are canopy species, whereas H. dryobalanoides is a subcanopy species. Clear differences were also found among these species in their range of seed dispersal, which depends on the height of the release point; so taller trees disperse their seed more extensively. The estimates of seed dispersal area were consistent with the degree of genetic structure found in the three species. Therefore, tree height probably had a strong influence on the fine-scale genetic structure of the three species.

DNA, Plant↗
PubMed2005

Rapidly declining fine-scale spatial genetic structure in female red deer.

A growing literature now documents the presence of fine-scale genetic structure in wild vertebrate populations. Breeding population size, levels of dispersal and polygyny--all hypothesized to affect population genetic structure--are known to be influenced by ecological conditions experienced by populations. However the possibility of temporal or spatial variation in fine-scale genetic structure as a result of ecological change is rarely considered or explored. Here we investigate temporal variation in fine-scale genetic structure in a red deer population on the Isle or Rum, Scotland. We document extremely fine-scale spatial genetic structure (< 100 m) amongst females but not males across a 24-year study period during which resource competition has intensified and the population has reached habitat carrying capacity. Based on census data, adult deer were allocated to one of three subpopulations in each year of the study. Global F(ST) estimates for females generated using these subpopulations decreased over the study period, indicating a rapid decline in fine-scale genetic structure of the population. Global F(ST) estimates for males were not different from zero across the study period. Using census and genetic data, we illustrate that, as a consequence of a release from culling early in the study period, the number of breeding females has increased while levels of polygyny have decreased in this population. We found little evidence for increasing dispersal between subpopulations over time in either sex. We argue that both increasing female population size and decreasing polygyny could explain the decline in female population genetic structure.

Animals↗
PubMed1994

Population genetic structure and ecotoxicology.

Electrophoretic analyses of population genetic structure, both in the laboratory and in the field, have documented significant shifts in allozyme genotype frequencies in a variety of aquatic taxa as a result of environmental impacts. Studies are documented which indicate that contaminants may select for individuals with tolerant allozyme genotypes, causing the potential loss of individuals with sensitive genotypes. This may diminish the genetic variability and fitness of affected populations and make them more susceptible to extinction following a subsequent stress. Future research involving population genetic structure and ecotoxicology should focus on determining the mechanism of sensitivity, documenting multigenerational effects of chronic laboratory exposure on population genetic composition, investigating whether previously stressed and genetically impacted populations are more susceptible to further natural and/or anthropogenic stressors, and establishing the utility of population genetic structure as a sensitive monitor of impacts in aquatic systems and their subsequent remediation.

Animals↗
PubMed1998

Fine-scale spatial genetic structure with nonuniform distribution of individuals.

This paper presents the first theoretical study of spatial genetic structure within nonuniformly distributed continuous plant populations. A novel individual-based model of isolation by distance was constructed to simulate genetic evolution within such populations. We found larger values of spatial genetic autocorrelations in highly clumped populations than in uniformly distributed populations. Most of this difference was caused by differences in mean dispersal distances, but aggregation probably also produced a slight increase in spatial genetic structure. Using an appropriate level of approximation of the continuous distribution of individuals in space, we assessed the potential effects of density, seed and pollen dispersal, generation overlapping, and overdominance selection at an independent locus, on fine-scale genetic structure, by varying them separately in a few particular cases with extreme clumping. When selfing was allowed, all these input variables influenced both aggregation and spatial genetic structure. Most variations in spatial genetic structure were closely linked to variations in clumping and/or local density. When selfing was not allowed, spatial genetic structure was lower in most cases.

Biological Evolution↗
PubMed1999

Life cycle variation and the genetic structure of nematode populations.

Few data are available on population genetic structure in nematode species, and little of the available data allows direct comparison of the genetic structures of species having different life cycles. Here we use mtDNA sequence data to describe the genetic structure of a heterorhabditid nematode, and compare results to published data on other nematode species. Heterorhabditis marelatus is a parasite of soil-dwelling insects. Its life cycle and local ecology should result in small effective population sizes and restricted gene flow. As predicted, H. marelatus shows much lower mtDNA diversity within populations and over the species as a whole, and has a much more strongly subdivided population structure, than parasites of mobile vertebrate hosts. From data such as these we can begin to generalize about the effects of life cycle variation on genetic structure in different nematode species.

Animals↗
PubMedDate not supplied

Spatial and genetic structure within populations of wild American ginseng (Panax quinquefolius L., Araliaceae).

Spatial structure and fine-scale genetic structure were analyzed for the medicinal plant American ginseng (Panax quinquefolius L.) to more fully understand biological processes within wild populations. P. quinquefolius has been harvested for more than 250 years and is now considered threatened or rare throughout its range. Plants within four protected and four unprotected populations were significantly clumped based on Ripley's univariate analysis. Analysis with Ripley's bivariate test determined that juvenile plants were significantly clumped with adult plants at the shortest distance classes in all populations. Although plants were highly clumped, we found that significant fine-scale genetic structure was restricted to the shortest distance classes based on estimates of coancestry (f(ij)). In most cases, estimates of f(ij) were more significant among juveniles than among adults, especially at the shortest distance classes. The spatial structure of ginseng seems to result from the establishment and persistence of plants in favorable microhabitats coupled with limited seed dispersal around maternal individuals. There were no differences in patterns of fine-scale genetic structure between protected and unprotected populations.

Age Factors↗
PubMed2006

Inferring colonization history from analyses of spatial genetic structure within populations of Pinus strobus and Quercus rubra.

Many factors interact to determine genetic structure within populations including adult density, the mating system, colonization history, natural selection, and the mechanism and spatial patterns of gene dispersal. We examined spatial genetic structure within colonizing populations of Quercus rubra seedlings and Pinus strobus juveniles and adults in an aspen-white pine forest in northern Michigan, USA. A 20-year spatially explicit demographic study of the forest enables us to interpret the results in light of recent colonization of the site for both species. We assayed 217 Q. rubra seedlings and 171 P. strobus individuals at 11 polymorphic loci using nine allozyme systems. Plant genotypes and locations were used in an analysis of spatial genetic structure. Q. rubra and P. strobus showed similar observed levels of heterozygosity, but Q. rubra seedlings have less heterozygosity than expected. Q. rubra seedlings show spatial genetic clumping of individuals on a scale to 25 m and levels of genetic relatedness expected from the clumped dispersion of half-siblings. In contrast, P. strobus has low levels of genetic relatedness at the smallest distance class and positive spatial genetic structure at scales < 10 m within the plot. The low density of adult Q. rubra outside the study plot and limited, spatially clumped rodent dispersal of acorns is likely responsible for the observed pattern of spatial genetic structure and the observed heterozygote deficit (i.e. a Wahlund effect). We attribute weaker patterns observed in P. strobus to the longer dispersal distance of seeds and the historical overlap of seed shadows from adults outside of the plot coupled with the overlap of seed shadows from younger, more recently established reproductive adults. The study demonstrates the utility of long-term demographic data in interpreting mechanisms responsible for generating contemporary patterns of genetic structure within populations.

Genetic Variation↗
PubMed2001

Fine-scale spatial genetic structure of eight tropical tree species as analysed by RAPDs.

The fine-scale spatial genetic structure of eight tropical tree species (Chrysophyllum sanguinolentum, Carapa procera, Dicorynia guianensis, Eperua grandiflora, Moronobea coccinea, Symphonia globulifera, Virola michelii, Vouacapoua americana) was studied in populations that were part of a silvicultural trial in French Guiana. The species analysed have different spatial distribution, sexual system, pollen and seed dispersal agents, flowering phenology and environmental demands. The spatial position of trees and a RAPD data set for each species were combined using a multivariate genetic distance method to estimate spatial genetic structure. A significant spatial genetic structure was found for four of the eight species. In contrast to most observations in temperate forests, where spatial structure is not usually detected at distances greater than 50 m, significant genetic structure was found at distances up to 300 m. The relationships between spatial genetic structure and life history characteristics are discussed.

DNA, Plant↗