Home  About us  Contact
 

Population Frequency (population + frequency)

Distribution by Scientific Domains
Life Sciences70%
Medical Sciences20%

Selected Abstracts

Threatened Peripheral Populations in Context: Geographical Variation in Population Frequency and Size and Sexual Reproduction in a Clonal Woody Shrub

CONSERVATION BIOLOGY, Issue 3 2007
SARAH B. YAKIMOWSKI
especies en riesgo; límites de distribución; poblaciones periféricas; reproducción sexual; Vaccinium stamineum Abstract:,Geographically peripheral populations of widespread species are often the focus of conservation because they are locally rare within political jurisdictions. Yet the ecology and genetics of these populations are rarely evaluated in a broader geographic context. Most expectations concerning the ecology and evolution of peripheral populations derive from the abundant-center model, which predicts that peripheral populations should be less frequent, smaller, less dense, and have a lower reproductive rate than central populations. We tested these predictions and in doing so evaluated the conservation value of peripheral populations for the clonal shrub Vaccinium stamineum L. (Ericaceae, deerberry), which is listed as threatened in Canada. Based on 51 populations sampled from the center to the northern range limits over 2 years, population frequency and size declined toward the range limit, but ramet density increased. Sexual reproductive output varied widely among populations and between years, with many populations producing very few seeds, but did not decline toward range margins. In fact seed mass increased steadily toward range limit, and this was associated with faster germination and seedling growth, which may be adaptive in seasonal northern environments. Our results did not support the prediction that clonal reproduction is more prevalent in peripheral populations or that it contributed antagonistically to the wide variation in seed production. Peripheral populations of V. stamineum are as productive as central populations and may be locally adapted to northern environments. This emphasizes the importance of a broad geographical perspective for evaluating the ecology, evolution, and conservation of peripheral populations. Resumen:,Las poblaciones geográficamente periféricas de una especie de amplia distribución a menudo son el foco de conservación porque son raras localmente dentro de jurisdicciones políticas. Sin embargo, la ecología y genética de estas poblaciones son evaluadas poco frecuentemente en un contexto geográfico más amplio. La mayoría de las expectaciones relacionadas con la ecología y evolución de las poblaciones periféricas se derivan del modelo centro-abundante, que predice que las poblaciones periféricas son menos frecuentes, más pequeñas, menos densas y menor tasa reproductiva que poblaciones centrales. Probamos estas predicciones y al hacerlo evaluamos el valor de conservación de poblaciones periféricas de una especie de arbusto clonal (Vaccinium stamineum L., Ericaceae), que está enlistada como amenazada en Canadá. Con base en 51 poblaciones muestreadas del centro hacia los límites norteños de su distribución durante 2 años, la frecuencia y tamaño poblacional declinó hacia los límites de su distribución, pero la densidad de rametos aumentó. La reproducción sexual varió ampliamente entre las poblaciones y entre años, con muchas poblaciones produciendo muy pocas semillas, pero no declinó hacia los límites de su distribución. De hecho, la masa de semillas incrementó sostenidamente hacia los límites, y esto se asoció a una acelerada germinación y crecimiento de plántulas, lo cual puede ser adaptativo en ambientes norteños estacionales. Nuestros resultados no sustentaron la predicción de que la reproducción clonal es más prevaleciente en poblaciones periféricas o que contribuye antagónicamente a la amplia variación en la producción de semillas. Las poblaciones periféricas de V. stamineum son tan productivas como las poblaciones centrales y pueden estar adaptadas localmente a ambientes norteños. Esto enfatiza la importancia de una perspectiva geográfica amplia cuando se evalúa la ecología, evolución y conservación de poblaciones periféricas. [source]

East meets west: adaptive evolution of an insect introduced for biological control

JOURNAL OF APPLIED ECOLOGY, Issue 3 2008
C. B Phillips
Summary 1A possible explanation for low success rates when introducing natural enemies to new regions for biological control of insect pests is that they fail to adapt to their new conditions. Therefore it has been widely recommended that biological control practitioners increase the probability of local adaptation by maximizing the genetic variation released. An alternative recommendation is to use climate matching to identify native populations that may already possess traits suited to the new region. However, support for these recommendations is weak through lack of empirical evidence that local adaptation is important to biological control. 2This study examined how genetic drift and selection influenced the population frequencies of two asexually reproducing, genetically differentiated parasitoid biotypes that were introduced to New Zealand from South America for biological control. Other than by mutation, the biotypes were genetically fixed due to the absence of recombination both within and between biotypes. This meant that adaptive evolution could occur only if selection acted on any traits that varied between the biotypes introduced from South America. 3The two parasitoid biotypes were released simultaneously at 14 sites and their frequencies were monitored for up to 10 years. Changes in biotype frequency were consistent with strong directional selection favouring one of the South American biotypes, thus generating established parasitoid populations that were better adapted to New Zealand conditions than those that had originally been released. This local adaptation of the control agent contributed to greater mortality of the pest. 4Synthesis and applications. This study provides the first clear demonstration of the importance of releasing natural enemy genetic variation in new regions to foster adaptive evolution and improve success rates in classical biological control. However, the benefit to biological control of maximizing the genetic variation released needs to be balanced against possible risks to non-target species. The results do not support the concept of choosing sampling sites for putative biological control agents based solely on climatic similarities between the source location and the intended region of introduction. [source]

Carbohydrate metabolic pathway genes associated with quantitative trait loci (QTL) for obesity and type 2 diabetes: Identification by data mining

BIOTECHNOLOGY JOURNAL, Issue 9 2010
Dr. Vijayalakshmi Varma
Abstract Increasing consumption of refined carbohydrates is now being recognized as a primary contributor to the development of nutritionally related chronic diseases such as obesity and type 2 diabetes mellitus (T2DM). A data mining approach was used to evaluate the role of carbohydrate metabolic pathway genes in the development of obesity and T2DM. Data from public databases were used to map the position of the carbohydrate metabolic pathway genes to known quantitative trait loci (QTL) for obesity and T2DM and for examining the pathway genes for the presence of sequence and structural genetic variants such as single nucleotide polymorphisms (SNPs) and copy number variants (CNS), respectively. The results demonstrated that a majority of the genes of the carbohydrate metabolic pathways are associated with QTL for obesity and many for T2DM. In addition, some key genes of the pathways also encode non-synonymous SNPs that exhibit significant differences in population frequencies. This study emphasizes the significance of the metabolic pathways genes in the development of disease phenotypes, its differential occurrence across populations and between individuals, and a strategy for interpreting an individuals' risk for disease. [source]

Threatened Peripheral Populations in Context: Geographical Variation in Population Frequency and Size and Sexual Reproduction in a Clonal Woody Shrub

CONSERVATION BIOLOGY, Issue 3 2007
SARAH B. YAKIMOWSKI
especies en riesgo; límites de distribución; poblaciones periféricas; reproducción sexual; Vaccinium stamineum Abstract:,Geographically peripheral populations of widespread species are often the focus of conservation because they are locally rare within political jurisdictions. Yet the ecology and genetics of these populations are rarely evaluated in a broader geographic context. Most expectations concerning the ecology and evolution of peripheral populations derive from the abundant-center model, which predicts that peripheral populations should be less frequent, smaller, less dense, and have a lower reproductive rate than central populations. We tested these predictions and in doing so evaluated the conservation value of peripheral populations for the clonal shrub Vaccinium stamineum L. (Ericaceae, deerberry), which is listed as threatened in Canada. Based on 51 populations sampled from the center to the northern range limits over 2 years, population frequency and size declined toward the range limit, but ramet density increased. Sexual reproductive output varied widely among populations and between years, with many populations producing very few seeds, but did not decline toward range margins. In fact seed mass increased steadily toward range limit, and this was associated with faster germination and seedling growth, which may be adaptive in seasonal northern environments. Our results did not support the prediction that clonal reproduction is more prevalent in peripheral populations or that it contributed antagonistically to the wide variation in seed production. Peripheral populations of V. stamineum are as productive as central populations and may be locally adapted to northern environments. This emphasizes the importance of a broad geographical perspective for evaluating the ecology, evolution, and conservation of peripheral populations. Resumen:,Las poblaciones geográficamente periféricas de una especie de amplia distribución a menudo son el foco de conservación porque son raras localmente dentro de jurisdicciones políticas. Sin embargo, la ecología y genética de estas poblaciones son evaluadas poco frecuentemente en un contexto geográfico más amplio. La mayoría de las expectaciones relacionadas con la ecología y evolución de las poblaciones periféricas se derivan del modelo centro-abundante, que predice que las poblaciones periféricas son menos frecuentes, más pequeñas, menos densas y menor tasa reproductiva que poblaciones centrales. Probamos estas predicciones y al hacerlo evaluamos el valor de conservación de poblaciones periféricas de una especie de arbusto clonal (Vaccinium stamineum L., Ericaceae), que está enlistada como amenazada en Canadá. Con base en 51 poblaciones muestreadas del centro hacia los límites norteños de su distribución durante 2 años, la frecuencia y tamaño poblacional declinó hacia los límites de su distribución, pero la densidad de rametos aumentó. La reproducción sexual varió ampliamente entre las poblaciones y entre años, con muchas poblaciones produciendo muy pocas semillas, pero no declinó hacia los límites de su distribución. De hecho, la masa de semillas incrementó sostenidamente hacia los límites, y esto se asoció a una acelerada germinación y crecimiento de plántulas, lo cual puede ser adaptativo en ambientes norteños estacionales. Nuestros resultados no sustentaron la predicción de que la reproducción clonal es más prevaleciente en poblaciones periféricas o que contribuye antagónicamente a la amplia variación en la producción de semillas. Las poblaciones periféricas de V. stamineum son tan productivas como las poblaciones centrales y pueden estar adaptadas localmente a ambientes norteños. Esto enfatiza la importancia de una perspectiva geográfica amplia cuando se evalúa la ecología, evolución y conservación de poblaciones periféricas. [source]

ASYMMETRY IN STRUCTURAL DEFENSES: INSIGHTS INTO SELECTIVE PREDATION IN THE WILD

EVOLUTION, Issue 9 2003
C. A. Bergstrom
Abstract Assessment of geographical patterns in fluctuating asymmetry (small, random differences between sides of bilateral characters) among populations shows promise as a tool to resolve the relative biomechanical importance of traits, in addition to being a possible indicator of habitat quality. We used 115 endemic freshwater populations of threespine stickleback (Gasterosteus aculeatus) from Haida Gwaii (Queen Charlotte Islands), British Columbia, Canada, to explore the degree of concordance between geographical variation of asymmetry in a predator defense structure (bony lateral plates) and geographical variation in several indirect measures of predation regime as well as several abiotic habitat variables. We found a geographical cline in the population frequency of lateral plate asymmetries, with reduced asymmetry in the southern clear-water regions of the archipelago characterized by long reaction distances and greater chance of capture by predators, and elevated asymmetry in the northern stained-water regions with poor visibility and low chances of capture. Lateral plate asymmetry was strongly correlated with expression of several defensive armor traits, including total plate numbers among populations, mean cross-sectional diameter of stickleback with the dorsal and pelvic spines erect, and mean degree of overlap between the plates and spine supports. There were no significant correlations between frequency of asymmetric fish and any of our abiotic habitat variables. Stickleback with structural plate asymmetries had fewer trout-induced scars than symmetric fish in the significant majority of populations, and there was a decrease in structural plate asymmetry with age in stained-water habitats, suggesting that trout predators may be selectively removing asymmetric fish in some lakes. This study provides evidence that geographical variation in developmental stability of threespine stickleback, as seen in the frequencies of asymmetry, reflects differences among populations in the importance of structural defenses to fitness rather than differences in habitat quality, and that asymmetry may be a target of selection by predators in wild populations. [source]

Closely linked cis -acting modifier of expansion of the CGG repeat in high risk FMR1 haplotypes,

HUMAN MUTATION, Issue 12 2007
S. Ennis
Abstract In its expanded form, the fragile X triplet repeat at Xq27.3 gives rise to the most common form of inherited mental retardation, fragile X syndrome. This high population frequency persists despite strong selective pressure against mutation-bearing chromosomes. Males carrying the full mutation rarely reproduce and females heterozygous for the premutation allele are at risk of premature ovarian failure. Our diagnostic facility and previous research have provided a large databank of X chromosomes that have been tested for the FRAXA allele. Using this resource, we have conducted a detailed genetic association study of the FRAXA region to determine any cis -acting factors that predispose to expansion of the CGG triplet repeat. We have genotyped SNP variants across a 650-kb tract centered on FRAXA in a sample of 877 expanded and normal X chromosomes. These chromosomes were selected to be representative of the haplotypic diversity encountered in our population. We found expansion status to be strongly associated with a ,50-kb region proximal to the fragile site. Subsequent detailed analyses of this region revealed no specific genetic determinants for the whole population. However, stratification of chromosomes by risk subgroups enabled us to identify a common SNP variant which cosegregates with the subset of D group haplotypes at highest risk of expansion (,=17.84, p=0.00002). We have verified that this SNP acts as a marker of repeat expansion in three independent samples. Hum Mutat 28(12), 1216,1224, 2007. © 2007 Wiley-Liss, Inc. [source]

IL23R haplotypes provide a large population attributable risk for Crohn's disease

INFLAMMATORY BOWEL DISEASES, Issue 9 2008
Kent D. Taylor PhD
Abstract Background: The IL-23 pathway plays a pivotal role in the development of chronic mucosal inflammation seen in the inflammatory bowel diseases. Multiple studies have now established the contribution of the interleukin 23 receptor gene (IL23R) to Crohn's disease (CD) risk in general and of the IL23R R381Q variant in particular. The aim of this work was to estimate the total contribution of this gene to CD risk test using a haplotype approach. Methods: In all, 763 CD subjects and 254 controls were genotyped for single nucleotide polymorphisms in the IL23R gene using Illumina and ABI methods. Haplotypes were assigned using PHASEv2 and tested for association with CD by chi-square and permutation. Results: Haplotypes with both increased and decreased risk for CD were observed in 2 of the 4 observed blocks (Block 2 H1: 55.4% control, 64% CD, P = 0.019; H2: 64.5% control, 54.4% CD, P = 0.006; Block 3 H1: 55.8% control, 64.4% CD, P = 0.013; H2: 47.0% control, 36.6% CD, P = 0.001). The population attributable risk for these haplotypes was substantially larger than that estimated for the IL23R R381Q variant (Block 2 H1 and block 3 H1 ,20%, compared with ,4% for Block 3 H6, containing the variant). Conclusions: These observations suggest that IL23R makes a substantial contribution to CD susceptibility, larger than that estimated from the population frequency of the R381Q variant. These observations also support the expectation that finding "hits" from genomewide association studies will be but an important chapter in the story of unraveling the genetic contribution to CD, rather than the final chapter that brings clarity to all the plot twists of a complicated story. (Inflamm Bowel Dis 2008) [source]

Effect of Different Levels of Wheat Straw Soil Surface Coverage on Weed Flora in Vicia faba Crops

JOURNAL OF AGRONOMY AND CROP SCIENCE, Issue 4 2003
D. Bilalis
Abstract The crop system implemented on organic farms requires that crop residues are incorporated into the soil following the end of wheat cultivation. This system leads to a reduction in soil moisture and the creation of favourable conditions for the emergence of weeds. In contrast, covering the soil with 60 % crop residues was found to maintain soil moisture and lead to a reduction in dry weed mass, population density and population frequency as well as a reduction in population diversity, regardless of the initial weed flora before the implementation of different soil tillage systems. Finally, an increase in the prevalence of broad-leafed weeds was observed as the level of soil cover increased. [source]

Psoriasis and the eye: Prevalence of eye disease in Singaporean Asian patients with psoriasis

THE JOURNAL OF DERMATOLOGY, Issue 12 2007
Nisha S. CHANDRAN
ABSTRACT There is little published data on the incidence of eye disease in Asian patients with psoriasis. We determined the frequency of ocular complications in Singaporean Asian patients with chronic plaque psoriasis and related these to extent and severity of psoriasis, family history, treatment and presence of arthritis. A cross-sectional prevalence investigation was carried out in 100 patients who received a comprehensive eye examination. Psoriasis extent and severity was graded by the Lattice System Physician's Global Assessment (LS-PGA). Two patients (four eyes) had uveitis, one of whom had psoriatic arthritis (2% incidence). Presence or absence of uveitis correlated with mean LS-PGA scores. Sixty-three patients had cataract unrelated to previous steroid or phototherapy treatment; in younger (<50 years) patients they were commoner than in those with higher (>5) LS-PGA scores. Three eyes in two patients (2% prevalence) had glaucomatous optic neuropathy unrelated to previous treatment, and comparable with expected population frequency. These findings, although limited by lack of data from a comparable control population, suggest that eye complications are common in Asian patients with psoriasis and eye symptoms should be elicited during history taking. Besides signs and symptoms of eye disease, an LS-PGA score of more than 5 should prompt referral for ophthalmological examination. [source]

Study of Regions of Extended Homozygosity Provides a Powerful Method to Explore Haplotype Structure of Human Populations

ANNALS OF HUMAN GENETICS, Issue 2 2008
D. Curtis
Summary Previous investigations have reported linkage disequilibrium occurring between nearby polymorphisms, a block-like structure for such relationships, some instances where surprisingly few haplotypes are found and regions of extended homozygosity which are especially marked around centromeres and which are especially common on the X chromosome. We investigated the distribution and nature of regions of extended homozygosity in a sample of 1411 subjects included in a genome wide association study. Regions of extended homozygosity over 1Mb are common, with an average of 35.9 occurring per subject, and containing on average 73 homozygous markers. They have a markedly non-random distribution. They are relatively common on the X chromosome and are seen at centromeres but are also concentrated at other chromosomal regions where presumably recombination is rare. They seem to be a consequence of some haplotypes being very common in the population and although sometimes this reflects the effect of a very common haplotype we also note that there are examples of two or three common haplotypes, each very different from each other, underlying this effect. Regions of extended homozygosity are commoner than previously appreciated. They result from the presence of extended haplotypes with high population frequency. Such regions concentrate in particular locations. The haplotypes involved are sometimes markedly disparate from each other. These regions offer a valuable opportunity for further investigation, in particular with regard to their ancestral history. [source]