343 research outputs found
Brucellose du lièvre en France Identification bactériologique des deux premiers cas en Picardie
Jacotot Henri, Vallée A., Barrière J. Brucellose du Lièvre en France. Identification bactériologique des deux premiers cas en Picardie. In: Bulletin de l'Académie Vétérinaire de France tome 104 n°5, 1951. pp. 283-285
Pour une amélioration de l’actuel protocole de lutte contre la tuberculose bovine
Barrière G., Desbrosse H., Germain L., Hugonet R., Ollivet Ch. Pour une amélioration de l’actuel protocole de lutte contre la tuberculose bovine. In: Bulletin de l'Académie Vétérinaire de France tome 113 n°2, 1960. pp. 107-124
Gap Formation in the Dust Layer of 3D Protoplanetary Disks
We numerically model the evolution of dust in a protoplanetary disk using a
two-phase (gas+dust) Smoothed Particle Hydrodynamics (SPH) code, which is
non-self-gravitating and locally isothermal. The code follows the three
dimensional distribution of dust in a protoplanetary disk as it interacts with
the gas via aerodynamic drag. In this work, we present the evolution of a disk
comprising 1% dust by mass in the presence of an embedded planet for two
different disk configurations: a small, minimum mass solar nebular (MMSN) disk
and a larger, more massive Classical T Tauri star (CTTS) disk. We then vary the
grain size and planetary mass to see how they effect the resulting disk
structure. We find that gap formation is much more rapid and striking in the
dust layer than in the gaseous disk and that a system with a given stellar,
disk and planetary mass will have a different appearance depending on the grain
size and that such differences will be detectable in the millimetre domain with
ALMA. For low mass planets in our MMSN models, a gap can open in the dust disk
while not in the gas disk. We also note that dust accumulates at the external
edge of the planetary gap and speculate that the presence of a planet in the
disk may facilitate the growth of planetesimals in this high density region.Comment: 5 page, 4 figures. Accepted for publication in Astrophysics & Space
Scienc
R&D progress on second-generation crystals for Laue lens applications
The concept of a gamma-ray telescope based on a Laue lens offers the
possibility to increase the sensitivity by more than an order of magnitude with
respect to existing instruments. Laue lenses have been developed by our
collaboration for several years : the main achievement of this R&D program was
the CLAIRE lens prototype. Since then, the endeavour has been oriented towards
the development of efficient diffracting elements (crystal slabs), the aim
being to step from a technological Laue lens to a scientifically exploitable
lens. The latest mission concept featuring a gamma-ray lens is the European
Gamma-Ray Imager (GRI) which intends to make use of the Laue lens to cover
energies from 200 keV to 1300 keV.
Investigations of two promising materials, low mosaicity copper and gradient
concentration silicon-germanium are presented in this paper. The measurements
have been performed during three runs on beamline ID15A of the European
Synchrotron Radiation Facility, and on the GAMS 4 instrument of the Institute
Laue-Langevin (both in Grenoble, France) using highly monochromatic beam of
energy close to 500 keV. Despite it was not perfectly homogeneous, the
presented copper crystal exhibits peak reflectivity of 25% in accordance with
theoretical predictions, and a mosaicity around 26 arcsec, the ideal range for
the realization of a Laue lens such as GRI. Silicon-germanium featuring a
constant gradient have been measured for the very first time at 500 keV. Two
samples showed a quite homogeneous reflectivity reaching 26%, which is far from
the 48% already observed in experimental crystals but a very encouraging
beginning. This results have been used to estimate the performance of the GRI
Laue lens design
Reimagining pheromone signalling in the model nematode Caenorhabditis elegans
Caenorhabditis elegans is an important, widely used developmental and genetic model. A pheromone has long been known to cause juvenile developmental arrest in C. elegans, a phenomenon that is common among nematodes more widely. Many novel effects of this pheromone are now being discovered—most recently, that exogenous supply of this pheromone controls adult worms reproduction. Here, we suggest that to properly understand and interpret these phenomena, C. elegans natural ecology must be considered, about which rather little is known. With this perspective, we suggest that C. elegans pheromone signalling evolves very locally, such that there are different dialects of pheromone signalling among ecological communities and among kin groups, and we also argue that pheromone signals may also evolve to be manipulative and dishonest. New approaches must be undertaken to study these phenomena in C. elegans. While model systems have been tremendously important tools in modern biological research, taking account of their natural history is necessary, and key, to properly understand and interpret laboratory-based discoveries.understand and interpret laboratory-based discoveries
Horizontal DNA transfer mechanisms of bacteria as weapons of intragenomic conflict
Horizontal DNA transfer (HDT) is a pervasive mechanism of diversification in many microbial species, but its primary evolutionary role remains controversial. Much recent research has emphasised the adaptive benefit of acquiring novel DNA, but here we argue instead that intragenomic conflict provides a coherent framework for understanding the evolutionary origins of HDT. To test this hypothesis, we developed a mathematical model of a clonally descended bacterial population undergoing HDT through transmission of mobile genetic elements (MGEs) and genetic transformation. Including the known bias of transformation toward the acquisition of shorter alleles into the model suggested it could be an effective means of counteracting the spread of MGEs. Both constitutive and transient competence for transformation were found to provide an effective defence against parasitic MGEs; transient competence could also be effective at permitting the selective spread of MGEs conferring a benefit on their host bacterium. The coordination of transient competence with cell-cell killing, observed in multiple species, was found to result in synergistic blocking of MGE transmission through releasing genomic DNA for homologous recombination while simultaneously reducing horizontal MGE spread by lowering the local cell density. To evaluate the feasibility of the functions suggested by the modelling analysis, we analysed genomic data from longitudinal sampling of individuals carrying Streptococcus pneumoniae. This revealed the frequent within-host coexistence of clonally descended cells that differed in their MGE infection status, a necessary condition for the proposed mechanism to operate. Additionally, we found multiple examples of MGEs inhibiting transformation through integrative disruption of genes encoding the competence machinery across many species, providing evidence of an ongoing "arms race." Reduced rates of transformation have also been observed in cells infected by MGEs that reduce the concentration of extracellular DNA through secretion of DNases. Simulations predicted that either mechanism of limiting transformation would benefit individual MGEs, but also that this tactic's effectiveness was limited by competition with other MGEs coinfecting the same cell. A further observed behaviour we hypothesised to reduce elimination by transformation was MGE activation when cells become competent. Our model predicted that this response was effective at counteracting transformation independently of competing MGEs. Therefore, this framework is able to explain both common properties of MGEs, and the seemingly paradoxical bacterial behaviours of transformation and cell-cell killing within clonally related populations, as the consequences of intragenomic conflict between self-replicating chromosomes and parasitic MGEs. The antagonistic nature of the different mechanisms of HDT over short timescales means their contribution to bacterial evolution is likely to be substantially greater than previously appreciated
Networks become navigable as nodes move and forget
We propose a dynamical process for network evolution, aiming at explaining
the emergence of the small world phenomenon, i.e., the statistical observation
that any pair of individuals are linked by a short chain of acquaintances
computable by a simple decentralized routing algorithm, known as greedy
routing. Previously proposed dynamical processes enabled to demonstrate
experimentally (by simulations) that the small world phenomenon can emerge from
local dynamics. However, the analysis of greedy routing using the probability
distributions arising from these dynamics is quite complex because of mutual
dependencies. In contrast, our process enables complete formal analysis. It is
based on the combination of two simple processes: a random walk process, and an
harmonic forgetting process. Both processes reflect natural behaviors of the
individuals, viewed as nodes in the network of inter-individual acquaintances.
We prove that, in k-dimensional lattices, the combination of these two
processes generates long-range links mutually independently distributed as a
k-harmonic distribution. We analyze the performances of greedy routing at the
stationary regime of our process, and prove that the expected number of steps
for routing from any source to any target in any multidimensional lattice is a
polylogarithmic function of the distance between the two nodes in the lattice.
Up to our knowledge, these results are the first formal proof that navigability
in small worlds can emerge from a dynamical process for network evolution. Our
dynamical process can find practical applications to the design of spatial
gossip and resource location protocols.Comment: 21 pages, 1 figur
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