Showing papers by "University of Montpellier published in 1996"

Microsatellites, from molecules to populations and back

Abstract: Population genetics studies using microsatellites, and data on their molecular dynamics, are on the increase. But, so far, no consensus has emerged on which mutation model should be used, though this is of paramount importance for analysis of population genetic structure. However, this is not surprising given the variety of microsatellite molecular motifs. Null alleles may be disturbing for population studies, even though their presence can be detected through careful population analyses, while homoplasy seems of little concern, at least over short evolutionary scales. Interspecific studies show that microsatellites are poor markers for phylogenetic inference. However, these studies are fuelling discussions on directional mutation and the role of selection and recombination in their evolution. Nonetheless, it remains true that microsatellites may be considered as good, neutral mendelian markers.

Testing differentiation in diploid populations

Abstract: We examine the power of different exact tests of differentiation for diploid populations. Since there is not necessarily random mating within populations, the appropriate hypothesis to construct exact tests is that of independent sampling of genotypes. There are two categories of tests, Fsrestimator tests and goodness of fit tests. In this latter category, we distinguish “allelic statistics”, which account for the nature of alleles within genotypes, from “genotypic statistics” that do not. We show that the power of Fs+stimator tests and of allelic goodness of fit tests are similar when sampling is balanced, and higher than the power of genotypic goodness of fit tests. When sampling is unbalanced, the most powerful tests are shown to belong to the allelic goodness of fit group.

Deglacial sea-level record from Tahiti corals and the timing of global meltwater discharge

Abstract: THE timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the Earth's surface2,3. Moreover, the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans, which probably disturbed the ocean's thermohaline circulation and, hence, global climate4–7. Sea-level increases bear witness to the melting of continental ice sheets, but only two such records—from Barbados8,9 and New Guinea10,11 corals—have been accurately dated. But these corals overlie active subduction zones, where tectonic movements are large and often discontinuous (especially in New Guinea), so the apparent sea-level records may be contaminated by a complex tectonic component. Here we date fossil corals from Tahiti, which is far from plate boundaries (and thus is likely to be tectonically relatively stable) and remote from the locations of large former ice sheets. The resulting record indicates a large sea-level jump shortly before 13,800 calendar years BP, which corresponds to meltwater pulse 1A in the Barbados coral records8,9. The timing of this event is more accurately constrained in the Tahiti record, revealing that the meltwater pulse coincides with a short and intense climate cooling event12–15 that followed the initiation of the Bolling–Allerod warm period12–16, but preceded the Younger Dryas cold event by about 1,000 years.

Force Distributions in Dense Two-Dimensional Granular Systems

Abstract: Relying on contact dynamics simulations, we study the statistical distribution of contact forces inside a confined packing of circular rigid disks with solid friction. We find the following: (1) The number of normal and tangential forces lower than their respective mean value decays as a power law. (2) The number of normal and tangential forces higher than their respective mean value decays exponentially. (3) The ratio of friction to normal force is uniformly distributed and is uncorrelated with normal force. (4) When normalized with respect to their mean values, these distributions are independent of sample size and particle size distribution.

Local Adaptation and Gene-For-Gene Coevolution in a Metapopulation Model

Abstract: In several reciprocal cross-infection experiments parasites were found to be significantly more adapted to their local host populations than to hosts from distant populations. We developed a metapopulation model, taking explicit account of both population densities and gene frequencies, to determine the influence of ecological and genetical parameters on the local adaptation of the parasites and on the spatial distribution of resistance and virulence genes. Our results point to the predominant effect of ecological parameters such as parasite growth rate and host and parasite migration rates on coevolutionary outcomes. In particular, the parasites are more likely to be adapted to their local host population than to allopatric hosts when the parasite migration rate is larger than the host migration rate. The opposite should be observed whenever hosts migrate more than parasites.

Alternate interpretation of the Messinian salinity crisis: Controversy resolved?

Abstract: A high-resolution chronology of the Mediterranean Messinian salinity crisis is proposed. Two types of evaporite deposition may be distinguished: those in marginal areas vs. those in basinal ones. Their diachroneity is deduced from the stratigraphic relationships linking these evaporites to a major Messinian erosional surface, A two-step model is proposed for the evolution of the salinity crisis through time. During the first phase (from 5.75 to 5.60 Ma), the deposition of marginal evaporites took place in response to a modest sea-level fall; in the second interval (from 5.60 to 5.32 Ma), the Mediterranean basin became isolated. During this later period, the deposition of basinal evaporites and the cutting of the Mediterranean canyons took place.

Recent developments of molecular chemistry for sol-gel processes

Abstract: This review focuses on some aspects of the chemistry involved in the preparation of oxides by sol–gel methods. This field is opening interesting possibilities for molecular chemistry and among those presented herein are: (1) the use of precursors containing functional groups for the preparation of solids with specific chemical reactivity; (2) the non-hydrolytic gelation (NHG) route, a new general method for the synthesis of oxides that allows the use of oxygen donors (ethers, alcohols, alkoxides) other than water; the mechanism is based on oxygen–carbon bond cleavage; (3) organic–inorganic hybrid solids containing covalent bonds for which the chemical reactivity is used to demonstrate the presence of substructure in the globally amorphous solid; the substructure greatly depends on the geometry of the organic moiety.

The African rain forest : main characteristics of changes in vegetation and climate from the Upper Cretaceous to the Quaternary

Abstract: This chapter sets out to give a historical overview of the African rain forest from its origins, towards the end of the Cretaceous period. The areas around the Gulf of Guinea, in particular from Ivory Coast to Nigeria and especially Cameroon, Gabon and Congo, appear to have been already occupied at this time by wet tropical forest formations mainly composed of Angiosperms which were then becoming established. In the course of the Tertiary period the combined effect of the equator being situated further north than now and the development of the Antarctic ice cap favoured the development of wet tropical conditions over a large part of North Africa which in turn led to the extension of tropical forest to various sites on the shores of the Tethys Sea. There were probably at this time common taxa and similar vegetation patterns stretching from the Gulf of Guinea to the Tethys Sea.Towards the end of the Tertiary, the equator reached its present position and the northern hemisphere ice caps appeared, and these phenomena resulted in the disappearance of the forest formations spread across the north of Africa, and the concentration of these formations near the equatorial zone around the Gulf of Guinea and in the Congo–Zaire basin. From 800 000 years ago onwards the marked glacial variations at middle and high latitudes in both hemispheres, with a periodicity of about 100 000 years determined by the orbital variations of the earth around the sun, lowered temperatures in equatorial areas and brought arid climates at times of maximum glacial extension. The most arid periods resulted in the fragmentation of the forest cover, and the forest biotopes and their biodiversity were preserved in a series of refugia. The lowering of temperatures also resulted in the extension of montane flora to low altitudes, with migration of montane flora and fauna between main mountain ranges. These compounded phenomena of isolation and migration, probably involving genie exchange, must have resulted in numerous speciation phenomena. Subsequently, such montane flora or fauna became isolated on mountain areas during periods of maximum warming, in the last instance in the course of the Holocene, when a vast forest cover became re-established from Guinea westwards, and to the East as far as the Lake Victoria area. The phases of maximum fragmentation, which appear to have been connected with only the coldest periods – in the last instance during the second part of isotopic stages 6 (from c. 160 to 130 000 years) and 2 (from c. 24 to 12000 years BP) – relate to less than 10% of the last 800 000 years, and the phases of maximum forest extension would likewise appear to be less than 10% of the period. The remaining 80–90% of the time relates to ‘intermediate situations’ which varied from period to period, and these intermediate extension situations seem to have been the norm over the larger part of the Quaternary, rather than the present situation which is closer to a situation of maximum extension.

Weak antilocalization and spin precession in quantum wells.

Abstract: The results of magnetoconductivity measurements in ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells are presented. The observed magnetoconductivity appears due to the quantum interference, which lead to the weak localization effect. It is established that the details of the weak localization are controlled by the spin splitting of electron spectra. A theory is developed that takes into account both linear and cubic in electron wave-vector terms in spin splitting, which arise due to the lack of inversion center in the crystal, as well as the linear terms that appear when the well itself is asymmetric. It is established that, unlike spin-relaxation rate, contributions of different terms into magnetoconductivity are not additive. It is demonstrated that in the interval of electron densities under investigation [(0.98-1.85)\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ ] all three contributions are comparable and have to be taken into account to achieve a good agreement between the theory and experiment. The results obtained from comparison of the experiment and the theory have allowed us to determine what mechanisms dominate the spin-relaxation in quantum wells and to improve the accuracy of determination of spin-splitting parameters in ${\mathit{A}}_{3}$${\mathit{B}}_{5}$ crystals and two-dimensional structures. \textcopyright{} 1996 The American Physical Society.

Origin and radiation of the house mouse: mitochondrial DNA phylogeny

Abstract: On the basis of patterns of allele frequency variation in nuclear genes (Din et al., in press) it has been proposed that the house mouse M. musculus originated in the northern Indian subcontinent, from where it radiated in several directions to form the well-described peripheral subspecies (M. m. domesticus, M. m. musculus and M. m. castaneus). Here we use a mitochondrial DNA (mtDNA) phylogeny to test this hypothesis and to analyse the historical and demographic events that have accompanied this differentiation. This marker also provides a powerful means to check for genetic continuity between the central and peripheral populations. We studied restriction site polymorphism of samples from India and the Middle East as well as samples from the rest of Eurasia and northern Africa. M. m. domesticus and M. m. musculus are both monophyletic for mtDNA and belong to the subspecies-specific mtDNA lineages that have been described previously. Average nucleotide diversity is low in M. m. musculus (0.2-5%). It is not only higher in M. m. domesticus (0.7-0.9%) but the distribution of pairwise divergence is wider, and the rate of evolution in this branch appears to be higher than in M. m. musculus. The nucleotide diversity found in M. m. castaneus (0.4%) is due to the existence of two rather divergent linages with little intralineage variation. These two lineages are part of a diversified bush of the phylogenetic tree that also comprises several previously undescribed branches and includes all samples from the northern Indian subcontinent and Iran. The degree of diversity found in each of the samples from this region is high (1.2-2.4%) although they come from small geographic areas. This agrees well with the idea that the origin of the radiation was in the northern Indian subcontinent. However, as neither haplotypes on the M. m. domesticus nor on the M. m. musculus branches were found in this region, there appear to be important phylogeographic discontinuities between this central region and these peripherial subspecies. On the basis of the present result and the nuclear data (Din et al., in press), we propose that M. musculus originated in the north of the indian subcontinent. Our calibration of the evolutionary rate of mtDNA in mice suggests that the mouse settlement in this region could be as old as 900 000 years. Possibly from there, a first radiation could have reach the Middle East and the Caspian Sea, where the M. m. domesticus and M. m. musculus lineages, respectively, would have started to differentiate a few hundred thousand years ago, and from where they could have colonised the peripheral part of their ranges only recently.M. m. castaneus appears from its mtDNA to be recent offshoot of the northern Indian population. This multiple and gradual radiation ultimately led to recent peripheral secondary contacts, such as the well-known European hybrid zone.

Gifa V. 4: A complete package for NMR data set processing

Abstract: The Gifa program is designed for processing, displaying and analysing 1D, 2D and 3D NMR data sets. It has been constructed in a modular fashion, based on three independent modules: a set of commands that perform all the basic processing operations such as apodisation functions, a complete set of Fourier Transforms, phasing and baseline correction, peak-picking and line fitting, linear prediction and maximum entropy processing; a set of command language primitives that permit the execution of complex macro commands; and a set of graphic commands that permit to build a complete graphic user interface, allowing the user to interact easily with the program. We have tried to create a versatile program that can be easily extended according to the user's requirements and that is adapted to a novice as well as an experienced user. The program runs on any UNIX computer, with or without graphic display, in interactive or batch mode.

A member of the arthropod defensin family from edible Mediterranean mussels (Mytilus galloprovincialis)

Abstract: Plasma from the mussel Mytilus galloprovincialis previously immunized by injecting them with bacteria contains several bactericidal proteins. One protein, MGD-1, was purified by reverse-phase HPLC of supernatant from acidified cell-free hemolymph. Its biological activity is directed against both gram-positive and gram-negative bacteria but it is not cytotoxic towards human erythrocytes nor protozoa. As determined by mass spectrometry, the molecular mass of MGD-1 is 4418 Da. Primary-structure analysis revealed 38 amino acids including 8 cysteines and a modified amino acid residue in position 28. Computer searches unambiguously recognized the signature of an arthropod defensin, but the presence of two extra cysteines and of one modified amino acid suggest that it is a previously unknown member of that family.

Cyclical behavior of thrust wedges: Insights from high basal friction sandbox experiments

Abstract: Scaled sandbox experiments with high basal friction, simulating the growth of accretionary wedges, display cycles alternating between frontal imbricate thrusting and underthrusting of long, undeformed sheets. By contrast, low basal friction experiments with otherwise similar and constant, initial conditions produce a classic frontal imbricate fan through repeated failure along frontal thrusts. The cyclical behavior observed in high basal friction experiments is expressed by three quantities: (1) the average spacing between frontal thrusts, (2) the advance and retreat of the deformation front, and (3) the frontal slope (Alpha) of the actively deforming wedge. As a long sheet is underthrust, the front is steepened through slumping until the maximum critical angle is reached. Then frontal thrusting resumes and the accretion of imbricate slices builds the wedge forward, thereby lowering the taper to the minimum critical angle. At shallow tapers, a long unit is underthrust and subsequently uplifts, shortens, and steepens the overlying wedge through backthrust deformation, thus completing the cycle. Underthrusting of long units offers a simple mechanism for underplating overlying units. It also provides a possible explanation for temporally and spatially varying wedge geometries in nature, when basal frictions attain 80%–90% of the internal friction.

Cure of Short- and Long-Term Experimental Chagas' Disease Using D0870

Abstract: Chagas9 disease, a protozoan infection by the kinetoplastid Trypanosoma cruzi, constitutes a major public health problem in Latin America. With the use of mouse models of both short- and long-term forms of the disease, the efficacy of D0870, a bis-triazole derivative, was tested. D0870 was able to prevent death and induced parasitological cure in 70 to 90 percent of animals, in both the short- and long-term disease. In contrast, currently used drugs such as nifurtimox or ketoconazole prolonged survival but did not induce significant curing effects. D0870 may be useful in the treatment of human long-term Chagas9 disease, a condition that is currently incurable.

Personal and situational factors influencing intrinsic interest of adolescent girls in school physical education: A structural equation modelling analysis.

Abstract: This study assessed the influence of individual and contextual factors on adolescent girls' interest in school physical education (PE). Specifically, girls (N = 700) were assessed on: (1) perceptions of their PE class climate, using the Perceived Motivational Climate Scale (Cury et al, 1994); (2) dispositional achievement goal orientations, using the French version of the Perception of Success Questionnaire (Durand et al, in press); and (3) the interest and competence subscales from the French version of the Intrinsic Motivation Inventory (Cury et al, 1994). Structural equation modelling analysis, using LISREL VII, showed an excellent fit with the hypothesized model. Specifically, situational class climate was found to be more important than individual goals in influencing pupil interest in PE. However, perceived competence also influenced interest, as did a mastery goal orientation. Results are discussed in the context of theoretical propositions of goal perspectives theory and practical issues of enhancing adolescent girls' interest in physical education.

Origin of the recrystallisation front in the Ronda peridotite by km-scale pervasive porous melt flow

Abstract: It is well established that porous melt flow in the upper mantle may significantly affect partial mantle melt compositions. Less well established are the length-scale of porous flow and whether porous melt flow can be a volumetrically important magmatic process. The only source for observations concerning the length-scale and nature of pervasive porous melt flow are peridotite massifs. Here we present such observations in the form of structural, and major and trace element data from peridotites of the Ronda massif, southern Spain. Trace element concentrations were obtained with high analytical precision (ICP-MS) and include trace elements rarely analysed in peridotites, such as Rb, Th, Nb and Ta. The western portion of the Ronda massif can be divided into two structural facies. The first and oldest is composed of deformed, porphyroclastic spinel peridotites, the second of virtually undeformed granular spinel peridotites. They are separated by a recrystallisation front across which grain growth of all phases occurred. The granular domain can be further subdivided into three subfacies: coarse-granular, fine-granular, and layered-granular peridotites. According to structural facies, km-scale spatial variations unrelated to Ca and Al abundances have been recognised for mg-numbers [atomic Mg/(Mg±Fe)] and incompatible elements such as rare earth elements (REE), Th and high-field-strength elements (HFSE; including Ti). Such variations are reminiscent of those commonly ascribed to mantle metasomatism, but have never been documented on the km-scale. The origin of the recrystallisation front is related to km-scale pervasive melt percolation. Feed-back processes between grain growth and melt fraction could have led to important accumulation of melt at the recrystallisation front, accomplished mainly by melting/dissolution. Variation in melt fraction across the front explains the spatial variation in the degree of recrystallisation, mg-numbers, REE fractionation, and HFSE abundances, and could account for many of the classical differences between basalts from convergent and extensional tectonic settings. Whereas the coarse-granular peridotites reflect a stage of steady-state pervasive porous melt flow, the fine- and layered-granular facies probably reflect the terminate stages of porous melt flow. Processes associated with both domains are pyroxene-forming freezing reactions at decreasing melt volumes, and progressive channelling of melt flow associated with olivine-producing reactions. Both processes show complex overprinting relationships in both time and space.

The late Miocene Panama isthmian strait

Abstract: Miocene sediments of the Caribbean Gatun and Chagres formations, Panama Canal basin, were deposited within an archipelagic strait that connected Caribbean and Pacific waters. Shallow-water (∼ 25 m) benthic foraminifera of the Gatun Formation have a strong Caribbean affinity, indicating that a significant interoceanic, biogeographic barrier had formed at ∼ 8 Ma. However, benthic foraminifera of the overlying Chagres Formation are bathyal and markedly Pacific in affinity, indicating that at ∼ 6 Ma, waters of the Panama isthmian strait deepened to ∼ 200–500 m and Pacific bathyal waters flowed into the Caribbean. The Chagres Formation crops out at the Caribbean entrance to the Panama Canal in a large wedge of cross-laminated sandstone and coquina. The cross-laminations and coarse grain size indicate high-energy currents atypical of bathyal settings. We infer that a jet of the Pacific North Equatorial Countercurrent–Equatorial Undercurrent passed through the Panama isthmian strait to deposit these sediments on the Caribbean side. This later entry of Pacific taxa into the Caribbean had no apparent effect on the subsequent composition of Caribbean faunas.