Is urbanization scrambling the genetic structure of human populations? A case study

Recent population expansion and increased migration linked to urbanization are assumed to be eroding the genetic structure of human populations. We investigated change in population structure over three generations by analysing both demographic and mitochondrial DNA (mtDNA) data from a random sample of 2351 men from 22 Iranian populations. Potential changes in genetic diversity (θ) and genetic distance (FST) over the last three generations were analysed by assigning mtDNA sequences to populations based on the individual's place of birth or that of their mother or grandmother. Despite the fact that several areas included cities of over one million inhabitants, we detected no change in genetic diversity, and only a small decrease in population structure, except in the capital city (Tehran), which was characterized by massive immigration, increased θ and a large decrease in FST over time. Our results suggest that recent erosion of human population structure might not be as important as previously thought, except in some large conurbations, and this clearly has important implications for future sampling strategies. © 2007 Nature Publishing Group All rights reserved

Dolomitization and burial history of lower triassic carbonate reservoir-rocks in the Persian Gulf (Salman offshore field)

International audienceShallow marine carbonate sedimentation prevailed during the Early Triassic in the northern part of the Persian Gulf (Kangan Formation; offshore Iran). Associated near surface and (to a lesser extent) burial diagenesis had significant impacts on the evolution of reservoir properties of the Kangan Formation. In some Iranian offshore fields such as Salman, pervasive, early dolomitization of the Kangan carbonate rocks is believed to have played an important role in improving the reservoir characteristics of this unit. Detailed petrographic and geochemical analyses indicate that such dolomitization could be explained by means of the sabkha/seepage-reflux model. Furthermore, three types of dolomites, found in Kangan rocks in the Salman Field, have been described: (1) fabric-retentive, (2) fabric-destructive, and (3) cement. Fabric-retentive dolomites are the most frequent commonly found in the studied field, with respect to the other types. Oxygen and carbon-stable isotopic analyses confirm that the fabric-retentive dolomites and associated anhydrites formed at surface and near-surface conditions, whereas the fabric-destructive dolomite formed at temperatures above 50 °C. The burial history of the Salman Field is divided into three major phases (tectonic quiescence and low subsidence rates, and two active phases with high subsidence rates). The fabric-retentive dolomites are mostly related to the first phase, while the fabric-destructive and dolomite cements are related to the second and third phases including shallow and deep burial, respectively

Dual-Tracer PET Using Generalized Factor Analysis of Dynamic Sequences

Purpose: With single-photon emission computed tomography, simultaneous imaging of two physiological processes relies on discrimination of the energy of the emitted gamma rays, whereas the application of dual-tracer imaging to positron emission tomography (PET) imaging has been limited by the characteristic 511-keV emissions.Procedures: To address this limitation, we developed a novel approach based on generalized factor analysis of dynamic sequences (GFADS) that exploits spatio-temporal differences between radiotracers and applied it to near-simultaneous imaging of 2-deoxy-2-[18F]fluoro-D-glucose (FDG) (brain metabolism) and 11C-raclopride (D2) with simulated human data and experimental rhesus monkey data. We show theoretically and verify by simulation and measurement that GFADS can separate FDG and raclopride measurements that are made nearly simultaneously. Results: The theoretical development shows that GFADS can decompose the studies at several levels: (1) It decomposes the FDG and raclopride study so that they can be analyzed as though they were obtained separately. (2) If additional physiologic/anatomic constraints can be imposed, further decomposition is possible. (3) For the example of raclopride, specific and nonspecific binding can be determined on a pixel-by-pixel basis. We found good agreement between the estimated GFADS factors and the simulated ground truth time activity curves (TACs), and between the GFADS factor images and the corresponding ground truth activity distributions with errors less than 7.3±1.3 %. Biases in estimation of specific D2 binding and relative metabolism activity were within 5.9±3.6 % compared to the ground truth values. We also evaluated our approach in simultaneous dual-isotope brain PET studies in a rhesus monkey and obtained accuracy of better than 6 % in a mid-striatal volume, for striatal activity estimation. Conclusions: Dynamic image sequences acquired following near-simultaneous injection of two PET radiopharmaceuticals can be separated into components based on the differences in the kinetics, provided their kinetic behaviors are distinct