13,987 research outputs found
Developmental constraints on vertebrate genome evolution
Constraints in embryonic development are thought to bias the direction of
evolution by making some changes less likely, and others more likely, depending
on their consequences on ontogeny. Here, we characterize the constraints acting
on genome evolution in vertebrates. We used gene expression data from two
vertebrates: zebrafish, using a microarray experiment spanning 14 stages of
development, and mouse, using EST counts for 26 stages of development. We show
that, in both species, genes expressed early in development (1) have a more
dramatic effect of knock-out or mutation and (2) are more likely to revert to
single copy after whole genome duplication, relative to genes expressed late.
This supports high constraints on early stages of vertebrate development,
making them less open to innovations (gene gain or gene loss). Results are
robust to different sources of data-gene expression from microarrays, ESTs, or
in situ hybridizations; and mutants from directed KO, transgenic insertions,
point mutations, or morpholinos. We determine the pattern of these constraints,
which differs from the model used to describe vertebrate morphological
conservation ("hourglass" model). While morphological constraints reach a
maximum at mid-development (the "phylotypic" stage), genomic constraints appear
to decrease in a monotonous manner over developmental time
Thalamocortical synchronization and cognition: implications for schizophrenia?
Cognitive deficits are a core dysfunction in schizophrenia. In this issue of Neuron, Parnaudeau et al. (2013) investigated synchronization in thalamocortical pathways in an animal model to address the disconnection between brain regions as a mechanism for working memory impairments in the disorder.implicated dysfunctional neural oscillations in the explanation of cognitive deficits and certain clinical symptoms of schizophrenia. Specifically, we will focus on findings that have examined neural oscillations during 1) perceptual processing, 2) working memory and executive processes and 3) spontaneous activity. The importance of the development of paradigms suitable for human and animal models is discussed as well as the search for mechanistic explanation for oscillatory dysfunctions
Turbulent-like fluctuations in quasistatic flow of granular media
We analyze particle velocity fluctuations in a simulated granular system
subjected to homogeneous quasistatic shearing. We show that these fluctuations
share the following scaling characteristics of fluid turbulence in spite of
their different physical origins: 1) Scale-dependent probability distribution
with non-Guassian broadening at small time scales; 2) Power-law spectrum,
reflecting long-range correlations and the self-affine nature of the
fluctuations; 3) Superdiffusion with respect to the mean background flow
Dynamic glass transition: bridging the gap between mode-coupling theory and the replica approach
We clarify the relation between the ergodicity breaking transition predicted
by mode-coupling theory and the so-called dynamic transition predicted by the
static replica approach. Following Franz and Parisi [Phys. Rev. Lett. 79, 2486
(1997)], we consider a system of particles in a metastable state characterized
by non-trivial correlations with a quenched configuration. We show that the
assumption that in a metastable state particle currents vanish leads to an
expression for the replica off-diagonal direct correlation function in terms of
a replica off-diagonal static four-point correlation function. A factorization
approximation for this function results in an approximate closure for the
replica off-diagonal direct correlation function. The replica off-diagonal
Ornstein-Zernicke equation combined with this closure coincides with the
equation for the non-ergodicity parameter derived using the mode-coupling
theory.Comment: revised version; to be published in EP
Strain versus stress in a model granular material: a Devil's staircase
The series of equilibrium states reached by disordered packings of rigid,
frictionless discs in two dimensions, under gradually varying stress, are
studied by numerical simulations. Statistical properties of trajectories in
configuration space are found to be independent of specific assumptions ruling
granular dynamics, and determined by geometry only. A monotonic increase in
some macroscopic loading parameter causes a discrete sequence of
rearrangements. For a biaxial compression, we show that, due to the statistical
importance of such events of large magnitudes, the dependence of the resulting
strain on stress direction is a Levy flight in the thermodynamic limit.Comment: REVTeX, 4 pages, 5 included PostScript figures. New version altered
throughout text, very close to published pape
Internal avalanches in models of granular media
We study the phenomenon of internal avalanching within the context of
recently introduced lattice models of granular media. The avalanche is produced
by pulling out a grain at the base of the packing and studying how many grains
have to rearrange before the packing is once more stable. We find that the
avalanches are long-ranged, decaying as a power-law. We study the distriution
of avalanches as a function of the density of the packing and find that the
avalanche distribution is a very sensitive structural probe of the system.Comment: 12 pages including 9 eps figures, LaTeX. To appear in Fractal
Two-photon optics of Bessel-Gaussian modes
In this paper we consider geometrical two-photon optics of Bessel-Gaussian
modes generated in spontaneous parametric down-conversion of a Gaussian pump
beam. We provide a general theoretical expression for the orbital angular
momentum (OAM) spectrum and Schmidt number in this basis and show how this may
be varied by control over the radial degree of freedom, a continuous parameter
in Bessel-Gaussian modes. As a test we first implement a back-projection
technique to classically predict, by experiment, the quantum correlations for
Bessel-Gaussian modes produced by three holographic masks, a blazed axicon,
binary axicon and a binary Bessel function. We then proceed to test the theory
on the down-converted photons using the binary Bessel mask. We experimentally
quantify the number of usable OAM modes and confirm the theoretical prediction
of a flattening in the OAM spectrum and a concomitant increase in the OAM
bandwidth. The results have implications for the control of dimensionality in
quantum states.Comment: 8 pages, 10 figure
Vonnisoplegging by roof met verswarende omstandighede
Uit teks: In ’n saak wat op 6 September 2002 voor die Kaapse Provinsiale Afdeling gedien het, is die hof, by monde van Griesel R en Oosthuizen Wnr, met die interessante vraag gekonfronteer of daar verswarende omstandighede aanwesig is waar die klaer tydens ’n rooftog met ’n speelgoed vuurwapen gedreig is. Die klaer was nie ten tye van die rooftog bewus van die feit dat die beskuldigde nie met ’n ware vuurwapen gewapen was nie
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