1,192 research outputs found
Open-Ended Evolutionary Robotics: an Information Theoretic Approach
This paper is concerned with designing self-driven fitness functions for
Embedded Evolutionary Robotics. The proposed approach considers the entropy of
the sensori-motor stream generated by the robot controller. This entropy is
computed using unsupervised learning; its maximization, achieved by an on-board
evolutionary algorithm, implements a "curiosity instinct", favouring
controllers visiting many diverse sensori-motor states (sms). Further, the set
of sms discovered by an individual can be transmitted to its offspring, making
a cultural evolution mode possible. Cumulative entropy (computed from ancestors
and current individual visits to the sms) defines another self-driven fitness;
its optimization implements a "discovery instinct", as it favours controllers
visiting new or rare sensori-motor states. Empirical results on the benchmark
problems proposed by Lehman and Stanley (2008) comparatively demonstrate the
merits of the approach
Genetic Assimilation and Canalisation in the Baldwin Effect
The Baldwin Effect indicates that individually learned behaviours acquired during an organismâs lifetime can influence the evolutionary path taken by a population, without any direct Lamarckian transfer of traits from phenotype to genotype. Several computational studies modelling this effect have included complications that restrict its applicability. Here we present a simplified model that is used to reveal the essential mechanisms and highlight several conceptual issues that have not been clearly defined in prior literature. In particular, we suggest that canalisation and genetic assimilation, often conflated in previous studies, are separate concepts and the former is actually not required for non-heritable phenotypic variation to guide genetic variation. Additionally, learning, often considered to be essential for the Baldwin Effect, can be replaced with a more general phenotypic plasticity model. These simplifications potentially permit the Baldwin Effect to operate in much more general circumstances
Sensitivity of the g-mode frequencies to pulsation codes and their parameters
From the recent work of the Evolution and Seismic Tools Activity (ESTA,
Lebreton et al. 2006; Monteiro et al. 2008), whose Task 2 is devoted to compare
pulsational frequencies computed using most of the pulsational codes available
in the asteroseismic community, the dependence of the theoretical frequencies
with non-physical choices is now quite well fixed. To ensure that the accuracy
of the computed frequencies is of the same order of magnitude or better than
the observational errors, some requirements in the equilibrium models and the
numerical resolutions of the pulsational equations must be followed. In
particular, we have verified the numerical accuracy obtained with the Saclay
seismic model, which is used to study the solar g-mode region (60 to
140Hz). We have compared the results coming from the Aarhus adiabatic
pulsation code (ADIPLS), with the frequencies computed with the Granada Code
(GraCo) taking into account several possible choices. We have concluded that
the present equilibrium models and the use of the Richardson extrapolation
ensure an accuracy of the order of in the determination of the
frequencies, which is quite enough for our purposes.Comment: 10 pages, 5 figures, accepted in Solar Physic
The role of apoptosis in the development of AGM hematopoietic stem cells revealed by Bcl-2 overexpression
Apoptosis is an essential process in embryonic tissue remodeling and adult
tissue homeostasis. Within the adult hematopoietic system, it allows for
tight regulation of hematopoietic cell subsets. Previously, it was shown
that B-cell leukemia 2 (Bcl-2) overexpression in the adult increases the
viability and activity of hematopoietic cells under normal and/or
stressful conditions. However, a role for apoptosis in the embryonic
hematopoietic system has not yet been established. Since the first
hematopoietic stem cells (HSCs) are generated within the
aortagonad-mesonephros (AGM; an actively remodeling tissue) region
beginning at embryonic day 10.5, we examined this tissue for expression of
apoptosis-related genes and ongoing apoptosis. Here, we show expression of
several proapoptotic and antiapoptotic genes in the AGM. We also generated
transgenic mice overexpressing Bcl-2 under the control of the
transcriptional regulatory elements of the HSC marker stem cell antigen-1
(Sca-1), to test for the role of cell survival in the regulation of AGM
HSCs. We provide evidence for increased numbers and viability of Sca-1(+)
cells in the AGM and subdissected midgestation aortas, the site where HSCs
are localized. Most important, our in vivo transplantation data show that
Bcl-2 overexpression increases AGM and fetal liver HSC activity, strongly
suggesting that apoptosis plays a role in HSC development
Embryonal subregion-derived stromal cell lines from novel temperature-sensitive SV40 T antigen transgenic mice support hematopoiesis
Throughout life, the hematopoietic system requires a supportive
microenvironment that allows for the maintenance and differentiation of
hematopoietic stem cells (HSC). To understand the cellular interactions
and molecules that provide these functions, investigators have previously
established stromal cell lines from the late gestational stage and adult
murine hematopoietic microenvironments. However, the stromal cell
microenvironment that supports the emergence, expansion and maintenance of
HSCs during mid-gestational stages has been largely unexplored. Since
several tissues within the mouse embryo are known to harbor HSCs (i.e.
aortagonads-mesonephros, yolk sac, liver), we generated numerous stromal
cell clones from these mid-gestational sites. Owing to the limited cell
numbers, isolations were performed with tissues from transgenic embryos
containing the ts SV40 Tag gene (tsA58) under the transcriptional control
of constitutive and ubiquitously expressing promoters. We report here that
the growth and cloning efficiency of embryonic cells (with the exception
of the aorta) is increased in the presence of the tsA58 transgene.
Furthermore, our results show that the large panel of stromal clones
isolated from the different embryonal subregions exhibit heterogeneity in
their ability to promote murine and human hematopoietic differentiation.
Despite our findings of heterogeneity in hematopoietic growth factor gene
expression profiles, high-level expression of some factors may influence
hematopoietic differentiation. Interestingly, a few of these stromal
clones express a recently described chordin-like protein, which is an
inhibitor of bone morphogenic proteins and is preferentially expressed in
cells of the mesenchymal lineage
Decomposition and nutrient release of leguminous plants in coffee agroforestry systems.
Leguminous plants used as green manure are an important nutrient source for coffee plantations, especially for soils with low nutrient levels. Field experiments were conducted in the Zona da Mata of Minas Gerais State, Brazil to evaluate the decomposition and nutrient release rates of four leguminous species used as green manures (Arachis pintoi, Calopogonium mucunoides, Stizolobium aterrimum and
Stylosanthes guianensis) in a coffee agroforestry system under two different climate conditions. The initial N contents in plant residues varied from 25.7 to 37.0 g kg-1 and P from 2.4 to 3.0 g kg-1. The lignin/N, lignin/polyphenol and(lignin+polyphenol)/N ratios were low in all residues studied. Mass loss rates were highest in the first 15 days, when 25 % of the residues were decomposed. From 15 to 30 days, the decomposition rate decreased on both farms. On the farm in Pedra Dourada (PD), the decomposition constant k increased in the order C. mucunoides < S. aterrimum < S. guianensis < A. pintoi. On the farm in Araponga (ARA), there was no difference in the decomposition rate among leguminous plants. The N release rates varied from 0.0036 to 0.0096 d-1. Around 32 % of the total N content in the plant material was released in the first 15 days. In ARA, the N concentration in the S. aterrimum residues was always significantly higher than in the other residues. At the end of 360 days, the N released was 78 % in ARA and 89 % in PD of the initial content. Phosphorus was the most rapidly released nutrient (k values from 0.0165 to 0.0394 d-1). Residue decomposition and nutrient release did not correlate with initial residue chemistry and biochemistry, but differences in climatic conditions between the two study sites modified the decomposition rate constants
Triggering an eruptive flare by emerging flux in a solar active-region complex
A flare and fast coronal mass ejection originated between solar active
regions NOAA 11514 and 11515 on July 1, 2012 in response to flux emergence in
front of the leading sunspot of the trailing region 11515. Analyzing the
evolution of the photospheric magnetic flux and the coronal structure, we find
that the flux emergence triggered the eruption by interaction with overlying
flux in a non-standard way. The new flux neither had the opposite orientation
nor a location near the polarity inversion line, which are favorable for strong
reconnection with the arcade flux under which it emerged. Moreover, its flux
content remained significantly smaller than that of the arcade (approximately
40 %). However, a loop system rooted in the trailing active region ran in part
under the arcade between the active regions, passing over the site of flux
emergence. The reconnection with the emerging flux, leading to a series of jet
emissions into the loop system, caused a strong but confined rise of the loop
system. This lifted the arcade between the two active regions, weakening its
downward tension force and thus destabilizing the considerably sheared flux
under the arcade. The complex event was also associated with supporting
precursor activity in an enhanced network near the active regions, acting on
the large-scale overlying flux, and with two simultaneous confined flares
within the active regions.Comment: Accepted for publication in Topical Issue of Solar Physics: Solar and
Stellar Flares. 25 pages, 12 figure
Non-geometric backgrounds, doubled geometry and generalised T-duality
String backgrounds with a local torus fibration such as T-folds are naturally
formulated in a doubled formalism in which the torus fibres are doubled to
include dual coordinates conjugate to winding number. Here we formulate and
explore a generalisation of this construction in which all coordinates are
doubled, so that the doubled space is a twisted torus, i.e. a compact space
constructed from identifying a group manifold under a discrete subgroup. This
incorporates reductions with duality twists, T-folds and a class of flux
compactifications, together with the non-geometric backgrounds expected to
arise from these through T-duality. It also incorporates backgrounds that are
not even locally geometric, and suggests a generalisation of T-duality to a
more general context. We discuss the effective field theory arising from such
an internal sector, give a world-sheet sigma model formulation of string theory
on such backgrounds and illustrate our discussion with detailed examples.Comment: 81 page
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