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Results for “Local adaptation”

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At least 109 records · Page 6Linked to original sources
PubMed2006

Local adaptation of frankia to different Discaria (Rhamnaceae) host species growing in patagonia.

Frankia BCU110601 (Da) and Frankia BCU110345 (Dc) were isolated from root nodules of Discaria articulata and Discaria chacaye, respectively; Frankia BCU110501 (Dt) was previously isolated from Discaria trinervis. The strains were identical at the 16S sequence and after analysis of RFLP of 16S and 23S rDNA intergenic region. Diversity was revealed at the molecular level after fingerprint analysis by BOX-polymerase chain reaction. The strains were infective and effective on the original host plants. A cross-inoculation assay intra Discaria genus, including D. trinervis, D. articulata, and D. chacaye, with each of these isolated Frankia strains caused effective symbioses with a similar dry weight in each plant species regardless of the inoculated strain. Nevertheless, a differential degree of recognition was revealed: Homologous symbiotic pairs in the case of D. chacaye-Frankia BCU110345 (Dc), D. articulata-Frankia BCU110601 (Da), and D. trinervis-Frankia BCU110501 (Dt) had faster nodulation rates than heterologous pairs. The differences in nodulation rate would suggest the existence of a subspecific level of recognition within a certain cross-inoculation group, pointing to subspecific adaptation occurring in this actinorhizal symbiosis.

Adaptation, Physiological↗
PubMed2006

Automated detection of masses in mammograms by local adaptive thresholding.

In this paper, an algorithm for detection of suspicious masses from mammographic images is presented. The proposed algorithm was tested on a database of 61 mammograms on which masses had previously been marked by experienced radiologists. Results show that the proposed method exhibits for mass detection, a sensitivity of 95.91%. The area under receiver operating characteristic (ROC) Az was 0.946 when enhancement of the original image was performed before detection and 0.938 otherwise. Furthermore in some cases, we could detect some masses that the radiologists were not able to mark out.

Algorithms↗
PubMed2004

Grid powered nonlinear image registration with locally adaptive regularization.

Multi-subject non-rigid registration algorithms using dense deformation fields often encounter cases where the transformation to be estimated has a large spatial variability. In these cases, linear stationary regularization methods are not sufficient. In this paper, we present an algorithm that uses a priori information about the nature of imaged objects in order to adapt the regularization of the deformations. We also present a robustness improvement that gives higher weight to those points in images that contain more information. Finally, a fast parallel implementation using networked personal computers is presented. In order to improve the usability of the parallel software by a clinical user, we have implemented it as a grid service that can be controlled by a graphics workstation embedded in the clinical environment. Results on inter-subject pairs of images show that our method can take into account the large variability of most brain structures. The registration time for images of size 256 x 256 x 124 is 5 min on 15 standard PCs. A comparison of our non-stationary visco-elastic smoothing versus solely elastic or fluid regularizations shows that our algorithm converges faster towards a more optimal solution in terms of accuracy and transformation regularity.

Algorithms↗
PubMed2004

Theoretical derivation of SNR, CNR and spatial resolution for a local adaptive strain estimator for elastography.

Conventional techniques in elastography estimate the axial strain as the gradient of the displacement (time-delay) estimates obtained using cross-correlation of pre- and temporally stretched postcompression radiofrequency (RF) A-line segments. The use of a constant stretch factor for stretching the postcompression A-line is not adequate in the presence of heterogeneous targets that are commonly encountered. This led to the development of several adaptive strain estimation techniques in elastography. Yet, a theoretical framework for the image quality of adaptive strain estimation has not been established. In this work, we develop theoretical expressions for the image quality [measured in terms of the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and spatial resolution] of elastograms obtained using an adaptive strain estimator developed by Alam et al. (1998). We show a linear trade-off between the SNR and axial resolution of the strain elastogram with respect to the window length used for strain estimation. The CNR shows a quadratic tradeoff with the axial resolution with respect to the window length. The SNR, CNR and axial resolution are shown to improve with the ultrasonic bandwidth.

Algorithms↗
PubMed2004

Local adaptation and population differentiation at the interleukin 13 and interleukin 4 loci.

A 25.6 kb region at chromosome 5q31, covering the entire human interleukin 13 (IL-13) and interleukin 4 (IL-4) genes, has been reported to be associated with bronchial asthma. We have examined nucleotide variations at this locus in African, European American, and Japanese populations, using 120 diallelic variants. A block of strong linkage disequilibrium (LD) (mid R:D'mid R:>0.7) spans a 10 kb region containing IL-4 in European American and Japanese populations, and is present but less clear in African samples. Two major haplotypes at IL-4 account for >80% of haplotypes in European Americans and Japanese. These haplotypes are common and quite diverged from each other and the ancestral haplotype, resulting in highly significant deviations from neutrality. F(ST) statistics show that European American and Japanese populations are unusually distinct at the IL-4 locus. The most common haplotype in the European American population is much less common in the Japanese population, and vice versa. This implies that natural selection has acted on IL-4 haplotypes differently in different populations. This selected variation at IL-4 may account for some genetic variance underlying susceptibility to asthma and other allergic diseases. The strong LD observed in the IL-4 region may allow more efficient disease-association studies using this locus.

Adaptation, Physiological↗
PubMed2003

Latitudinal countergradient variation in the common frog (Rana temporaria) development rates--evidence for local adaptation.

Adaptive genetic differentiation along a climatic gradient as a response to natural selection is not necessarily expressed at phenotypic level if environmental effects on population mean phenotypes oppose the genotypic effects. This form of cryptic evolution--called countergradient variation--has seldom been explicitly demonstrated for terrestrial vertebrates. We investigated the patterns of phenotypic and genotypic differentiation in developmental rates of common frogs (Rana temporaria) along a ca. 1600 km latitudinal gradient across Scandinavia. Developmental rates in the field were not latitudinally ordered, but displayed large variation even among different ponds within a given latitudinal area. In contrast, development rates assessed in the laboratory increased strongly and linearly with increasing latitude, suggesting a genetic capacity for faster development in the northern than the southern larvae. Experiments further revealed that environmental effects (temperature and food) could easily override the genetic effects on developmental rates, providing a possible mechanistic explanation as to why the genetic differentiation was not seen in the samples collected from the wild. Our results suggest that the higher developmental rates of the northern larvae are likely to be related to selection stemming from seasonal time constrains, rather than from selection dictated by low ambient temperatures per se. All in all, the results provide a demonstration of environmental effects concealing substantial latitudinally ordered genetic differentiation understandable in terms of adaptation to clinal variation in time constrains.

Adaptation, Physiological↗
PubMed2001

Rapid, local adaptation of zooplankton behavior to changes in predation pressure in the absence of neutral genetic changes.

Organisms producing resting stages provide unique opportunities for reconstructing the genetic history of natural populations. Diapausing seeds and eggs often are preserved in large numbers, representing entire populations captured in an evolutionary inert state for decades and even centuries. Starting from a natural resting egg bank of the waterflea Daphnia, we compare the evolutionary rates of change in an adaptive quantitative trait with those in selectively neutral DNA markers, thus effectively testing whether the observed genetic changes in the quantitative trait are driven by natural selection. The population studied experienced variable and well documented levels of fish predation over the past 30 years and shows correlated genetic changes in phototactic behavior, a predator-avoidance trait that is related to diel vertical migration. The changes mainly involve an increased plasticity response upon exposure to predator kairomone, the direction of the changes being in agreement with the hypothesis of adaptive evolution. Genetic differentiation through time was an order of magnitude higher for the studied behavioral trait than for neutral markers (DNA microsatellites), providing strong evidence that natural selection was the driving force behind the observed, rapid, evolutionary changes.

Adaptation, Physiological↗