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Search Results: 1 - 10 of 14916 matches for " Christian Bickel "
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Developmental Origin and Evolution of Bacteriocytes in the Aphid–Buchnera Symbiosis
Christian Braendle,Toru Miura,Ryan Bickel,Alexander W. Shingleton,Srinivas Kambhampati,David L. Stern
PLOS Biology , 2012, DOI: 10.1371/journal.pbio.0000021
Abstract: Symbiotic relationships between bacteria and insect hosts are common. Although the bacterial endosymbionts have been subjected to intense investigation, little is known of the host cells in which they reside, the bacteriocytes. We have studied the development and evolution of aphid bacteriocytes, the host cells that contain the endosymbiotic bacteria Buchnera aphidicola. We show that bacteriocytes of Acyrthosiphon pisum express several gene products (or their paralogues): Distal-less, Ultrabithorax/Abdominal-A, and Engrailed. Using these markers, we find that a subpopulation of the bacteriocytes is specified prior to the transmission of maternal bacteria to the embryo. In addition, we discovered that a second population of cells is recruited to the bacteriocyte fate later in development. We experimentally demonstrate that bacteriocyte induction and proliferation occur independently of B. aphidicola. Major features of bacteriocyte development, including the two-step recruitment of bacteriocytes, have been conserved in aphids for 80–150 million years. Furthermore, we have investigated two cases of evolutionary loss of bacterial symbionts: in one case, where novel extracellular, eukaryotic symbionts replaced the bacteria, the bacteriocyte is maintained; in another case, where symbionts are absent, the bacteriocytes are initiated but not maintained. The bacteriocyte represents an evolutionarily novel cell fate, which is developmentally determined independently of the bacteria. Three of five transcription factors we examined show novel expression patterns in bacteriocytes, suggesting that bacteriocytes may have evolved to express many additional transcription factors. The evolutionary transition to a symbiosis in which bacteria and an aphid cell form a functional unit, similar to the origin of plastids, has apparently involved extensive molecular adaptations on the part of the host cell.
Exclusion in Very Old Age : The Impact of Three Critical Life Events
Stefano Cavalli,Jean-Fran?ois Bickel,Christian J. Lalive d'Epinay
International Journal of Ageing and Later Life , 2007,
Abstract: This paper focuses on relational exclusion (i.e. isolation and non-participation in social activities) in very old age. Based on a five-year study of an octogenarian cohort, the authors investigate the impact of three critical life events (deterioration of health, death of a close relative, entry into a nursing home) on relational life and social involvement. With advancing age, older people withdraw from some social activities, but their relationships with their family and friends remain stable. Life events have a stimulative effect on the support network (especially of family), and only the deterioration of health curbs social activity. This would seem to confirm the existence of a process of disengagement stemming more from the older people's functional or sensory disabilities than from an individual choice.
Das Internet als linguistisches Korpus
Bickel, Hans
Linguistik Online , 2006,
Abstract: This article discusses whether the Internet can be used as a linguistic corpus. It is based on experiences in connection with the Variantenw rterbuch des Deutschen (Dictionary of Standard German Variants), which was compiled 1997-2004. In order to identify national and regional variants of the German language in Germany, Austria and Switzerland, it was necessary to work with a large linguistic corpus that could also provide data on the frequency of rather rare words. The question was: Is the Internet suitable as a corpus for linguistic frequency analysis? The use of the WWW as corpus can be suitable only1. if reliable and reproducible results can be obtained;2. if the results are closely related to the language as it is actually used.The test showed that the Internet is an extremely useful corpus to get information on word frequency. The enormous size and the large number of different text types makes it an extremely versatile corpus, which has a systematic connection to the written language reality.
Depletion forces near a soft surface
Thomas Bickel
Physics , 2003, DOI: 10.1063/1.1568732
Abstract: We investigate excluded-volume effects in a bidisperse colloidal suspension near a flexible interface. Inspired by a recent experiment by Dinsmore et al. (Phys. Rev, Lett. 80, 409 (1998)), we study the adsorption of a mesoscopic bead on the surface and show that depletion forces could in principle lead to particle encapsulation. We then consider the effect of surface fluctuations on the depletion potential itself and construct the density profile of a polymer solution near a soft interface. Surprisingly we find that the chains accumulate at the wall, whereas the density displays a deficit of particles at distances larger than the surface roughness. This non-monotonic behavior demonstrates that surface fluctuations can have major repercusions on the properties of a colloidal solution. On average, the additional contribution to the Gibbs adsorbance is negative. The amplitude of the depletion potential between a mesoscopic bead and the surface increases accordingly.
A note on confined diffusion
Thomas Bickel
Physics , 2006, DOI: 10.1016/j.physa.2006.11.008
Abstract: The random motion of a Brownian particle confined in some finite domain is considered. Quite generally, the relevant statistical properties involve infinite series, whose coefficients are related to the eigenvalues of the diffusion operator. Unfortunately, the latter depend on space dimensionality and on the particular shape of the domain, and an analytical expression is in most circumstances not available. In this article, it is shown that the series may in some circumstances sum up exactly. Explicit calculations are performed for 2D diffusion restricted to a circular domain and 3D diffusion inside a sphere. In both cases, the short-time behaviour of the mean square displacement is obtained.
Hindered mobility of a particle near a soft interface
Thomas Bickel
Physics , 2006, DOI: 10.1103/PhysRevE.75.041403
Abstract: The translational motion of a solid sphere near a deformable fluid interface is studied in the low Reynolds number regime. In this problem, the fluid flow driven by the sphere is dynamically coupled the instantaneous conformation of the interface. Using a two-dimensional Fourier transform technique, we are able to account for the multiple backflows scattered from the interface. The mobility tensor is then obtained from the matrix elements of the relevant Green function. This analysis allows us to express the explicit position and frequency dependence of the mobility. We recover in the steady limit the result for a sphere near a perfectly flat interface. At intermediate time scales, the mobility exhibits an imaginary part, which is a signature of the elastic response of the interface. In the short time limit, we find the intriguing feature that the perpendicular mobility may, under some circumstances, become lower than the bulk value. All those results can be explained from the definition of the relaxation time of the soft interface.
Brownian motion near a liquid-like membrane
Thomas Bickel
Physics , 2006, DOI: 10.1140/epje/i2006-10026-0
Abstract: The dynamics of a tracer molecule near a fluid membrane is investigated, with particular emphasis given to the interplay between the instantaneous position of the particle and membrane fluctuations. It is found that hydrodynamic interactions creates memory effects in the diffusion process. The random motion of the particle is then shown to cross over from a ``bulk'' to a ``surface'' diffusive mode, in a way that crucially depends on the elastic properties of the interface.
Probing nanoscale deformations of a fluctuating interface
Thomas Bickel
Physics , 2014, DOI: 10.1209/0295-5075/106/16004
Abstract: We consider the contribution of thermal capillary waves to the interaction between a fluid-fluid interface and a nearby nanoparticle. Fluctuations are described thanks to an effective interaction potential which is derived using the renormalization group. The general theory is then applied to a spherical particle interacting with the interface through van der Waals forces. Surprisingly enough, we find that fluctuations contribute significantly to the deformation profile. Our study therefore reveals that thermal fluctuations cannot be ignored when probing nanoscale deformations of a soft interface.
Differentiating Matrix Functions
Kelly Bickel
Mathematics , 2011,
Abstract: Multivariable, real-valued functions induce matrix-valued functions defined on the space of d-tuples of n-times-n pairwise-commuting self-adjoint matrices. We examine the geometry of this space of matrices and conclude that the best notion of differentiation of these matrix-valued functions is differentiation along curves. We prove that a C^1 real-valued function always induces a C^1 matrix function and give an explicit formula for the derivative. We also show that every real-valued C^m function defined on an open rectangle in the plane induces a matrix-valued function that can be m-times continuously differentiated along C^m curves.
Fundamental Agler Decompositions
Kelly Bickel
Mathematics , 2012,
Abstract: We use shift-invariant subspaces of the Hardy space on the bidisk to provide an elementary proof of the Agler Decomposition Theorem. We observe that these shift-invariant subspaces are specific cases of Hilbert spaces that can be defined from Agler decompositions and analyze the properties of such Hilbert spaces. We then restrict attention to rational inner functions and show that the shift-invariant subspaces provide easy proofs of several known results about decompositions of rational inner functions. We use our analysis to obtain a result about stable polynomials on the polydisk.
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