446 research outputs found
Experimentally increased group diversity improves disease resistance in an ant species.
A leading hypothesis linking parasites to social evolution is that more genetically diverse social groups better resist parasites. Moreover, group diversity can encompass factors other than genetic variation that may also influence disease resistance. Here, we tested whether group diversity improved disease resistance in an ant species with natural variation in colony queen number. We formed experimental groups of workers and challenged them with the fungal parasite Metarhizium anisopliae. Workers originating from monogynous colonies (headed by a single queen and with low genetic diversity) had higher survival than workers originating from polygynous ones, both in uninfected groups and in groups challenged with M. anisopliae. However, an experimental increase of group diversity by mixing workers originating from monogynous colonies strongly increased the survival of workers challenged with M. anisopliae, whereas it tended to decrease their survival in absence of infection. This experiment suggests that group diversity, be it genetic or environmental, improves the mean resistance of group members to the fungal infection, probably through the sharing of physiological or behavioural defences
Cotton cultivation in Laos
Le Laos est un pays de tradition cotonnière, dont les systèmes de culture sont adaptés à de multiples situations écologiques. Ils comprennent en particulier le riz, le cotonnier, l'arachide. Afin qu'une véritable filière cotonnière, source de devises pour le pays, puisse être développée, les recherches du projet de coopération bilatérale franco-lao de recherche-développement sur les plantes à fibre ont porté sur des systèmes de culture améliorés, comprenant engrais et pesticides, techniques de culture plus performantes et variétés productives. Dans les villages, des structures artisanales d'égrenage et de pressage de la fibre s'organisent, en plus de l'usine de filature installée à Vientiane. Dans certaines régions proches de la Thaïlande, la culture cotonnière d'exportation se développe avec des variétés améliorée
Transport and conservation laws
We study the lowest order conservation laws in one-dimensional (1D)
integrable quantum many-body models (IQM) as the Heisenberg spin 1/2 chain, the
Hubbard and t-J model. We show that the energy current is closely related to
the first conservation law in these models and therefore the thermal transport
coefficients are anomalous. Using an inequality on the time decay of current
correlations we show how the existence of conserved quantities implies a finite
charge stiffness (weight of the zero frequency component of the conductivity)
and so ideal conductivity at finite temperatures.Comment: 6 pages, Late
Time evolution of a quantum many-body system: transition from integrability to ergodicity in thermodynamic limit
Numerical evidence is given for non-ergodic (non-mixing) behavior, exhibiting
ideal transport, of a simple non-integrable many-body quantum system in the
thermodynamic limit, namely kicked model of spinless fermions on a ring.
However, for sufficiently large kick parameters and we recover quantum
ergodicity, and normal transport, which can be described by random matrix
theory.Comment: 4 pages in RevTex (6 figures in PostScript included
Transport in the XX chain at zero temperature: Emergence of flat magnetization profiles
We study the connection between magnetization transport and magnetization
profiles in zero-temperature XX chains. The time evolution of the transverse
magnetization, m(x,t), is calculated using an inhomogeneous initial state that
is the ground state at fixed magnetization but with m reversed from -m_0 for
x0. In the long-time limit, the magnetization evolves into a
scaling form m(x,t)=P(x/t) and the profile develops a flat part (m=P=0) in the
|x/t|1/2 while it
expands with the maximum velocity, c_0=1, for m_0->0. The states emerging in
the scaling limit are compared to those of a homogeneous system where the same
magnetization current is driven by a bulk field, and we find that the
expectation values of various quantities (energy, occupation number in the
fermionic representation) agree in the two systems.Comment: RevTex, 8 pages, 3 ps figure
Integrability and coherence of hopping between 1D correlated electrons systems
We present numerical evidence that the hopping of electrons between chains
described by the model is coherent in the integrable cases ( and
) and essentially incoherent otherwise. This effect is {\it not} related
to the value of the exponent , (which is restricted to the interval
[0,1/8] when ), and we propose that enhanced coherence is
characteristic of integrable systems.Comment: 9 pages, LateX, 4 figures in uuencoded format, submitted to Phys.
Rev. Let
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