4,641 research outputs found
Australian Organic Market Report 2010
This is the second report the Biological Farmers of Australia has commissioned to help industry bench mark the growth and health of its sectors. This report - another significant milestone in the two decade plus history of the rapidly developing Australian certified organic sector - builds the information base for industry to benchmark production and market value against past and current claims and estimates and will enable monitoring of future growth of the certified organic market in Australia and its farming and production base. In an industry characterised by operational diversity, this report allows for performance assessment by sector. The next publication in this series is planned in 2012 (biennial since the inaugural report in 2008) as a means of providing the wider industry with invaluable and realistic market information
Complex pattern formation in reaction diffusion systems with spatially-varying parameters
Spontaneous pattern formation in reactionâdiffusion systems on a spatially homogeneous domain has been well studied. However, in embryonic development and elsewhere, pattern formation often takes place on a spatially heterogeneous background. We explore the effects of spatially varying parameters on pattern formation in one and two dimensions using the GiererâMeinhardt reactionâdiffusion model. We investigate the effect of the wavelength of a pre-pattern and demonstrate a novel form of moving pattern. We find that spatially heterogeneous parameters can both increase the range and complexity of possible patterns and enhance the robustness of pattern selection
Speed of reaction diffusion in embryogenesis
Reaction diffusion systems have been proposed as mechanisms for patterning during many stages of embryonic development. While much attention has been focused on the study of the steady state patterns formed and the robustness of pattern selection, much less is known about the time scales required for pattern formation. Studies of gradient formation by the diffusion of a single morphogen from a localized source have shown that patterning can occur on realistic time scales over distances of a millimeter or less. Reaction diffusion has the potential to give rise to patterns on a faster time scale, since all points in the domain can act as sources of morphogen. However, the speed at which patterning can occur has hitherto not been explored in depth. In this paper, we investigate this issue in specific reaction diffusion models and address the question of whether patterning via reaction diffusion is fast enough to be applicable to morphogenesis
Pattern formation by lateral inhibition with feedback: a mathematical model of Delta-Notch intercellular signalling
In many developing tissues, adjacent cells diverge in character so as to create a fine-grained pattern of cells in contrasting states of differentiation. It has been proposed that such patterns can be generated through lateral inhibitionâa type cellsâcell interaction whereby a cell that adopts a particular fate inhibits its immediate neighbours from doing likewise. Lateral inhibition is well documented in flies, worms and vertebrates. In all of these organisms, the transmembrane proteins Notch and Delta (or their homologues) have been identified as mediators of the interactionâNotch as receptor, Delta as its ligand on adjacent cells. However, it is not clear under precisely what conditions the Delta-Notch mechanism of lateral inhibition can generate the observed types of pattern, or indeed whether this mechanism is capable of generating such patterns by itself. Here we construct and analyse a simple and general mathematical model of such contact-mediated lateral inhibition. In accordance with experimental data, the model postulates that receipt of inhibition (i.e. activation of Notch) diminishes the ability to deliver inhibition (i.e. to produce active Delta). This gives rise to a feedback loop that can amplify differences between adjacent cells. We investigate the pattern-forming potential and temporal behavior of this model both analytically and through numerical simulation. Inhomogeneities are self-amplifying and develop without need of any other machinery, provided the feedback is sufficiently strong. For a wide range of initial and boundary conditions, the model generates fine-grained patterns similar to those observed in living systems
Tetraspanins are involved in Burkholderia pseudomallei-induced cell-to-cell fusion of phagocytic and non-phagocytic cells
Tetraspanins are four-span transmembrane proteins of host cells that facilitate infections by many pathogens. Burkholderia pseudomallei is an intracellular bacterium and the causative agent of melioidosis, a severe disease in tropical regions. This study investigated the role of tetraspanins in B. pseudomallei infection. We used flow cytometry to determine tetraspanins CD9, CD63, and CD81 expression on A549 and J774A.1 cells. Their roles in B. pseudomallei infection were investigated in vitro using monoclonal antibodies (MAbs) and recombinant large extracellular loop (EC2) proteins to pretreat cells before infection. Knockout of CD9 and CD81 in cells was performed using CRISPR Cas9 to confirm the role of tetraspanins. Pretreatment of A549 cells with MAb against CD9 and CD9-EC2 significantly enhanced B. pseudomallei internalization, but MAb against CD81 and CD81-EC2 inhibited MNGC formation. Reduction of MNGC formation was consistently observed in J774.A1 cells pretreated with MAbs specific to CD9 and CD81 and with CD9-EC2 and CD81-EC2. Data from knockout experiments confirmed that CD9 enhanced bacterial internalization and that CD81 inhibited MNGC formation. Our data indicate that tetraspanins are host cellular factors that mediated internalization and membrane fusion during B. pseudomallei infection. Tetraspanins may be the potential therapeutic targets for melioidosis
A model of primitive streak initiation in the chick embryo
Initiation of the primitive streak in avian embryos provides a well-studied example of a pattern-forming event that displays a striking capacity for regulation. The mechanisms underlying the regulative properties are, however, poorly understood and are not easily accounted for by traditional models of pattern formation, such as reactionâdiffusion models. In this paper, we propose a new activatorâinhibitor model for streak initiation. We show that the model is consistent with experimental observations, both in its pattern-forming properties and in its ability to form these patterns on the correct time-scales for biologically realistic parameter values. A key component of the model is a travelling wave of inhibition. We present a mathematical analysis of the speed of such waves in both diffusive and juxtacrine relay systems. We use the streak initiation model to make testable predictions. By varying parameters of the model, two very different types of patterning can be obtained, suggesting that our model may be applicable to other processes in addition to streak initiation
Large Magellanic Cloud Planetary Nebula Morphology: Probing Stellar Populations and Evolution
Planetary Nebulae (PNe) in the Large Magellanic Cloud (LMC) offer the unique
opportunity to study both the Population and evolution of low- and
intermediate-mass stars, by means of the morphological type of the nebula.
Using observations from our LMC PN morphological survey, and including images
available in the HST Data Archive, and published chemical abundances, we find
that asymmetry in PNe is strongly correlated with a younger stellar Population,
as indicated by the abundance of elements that are unaltered by stellar
evolution (Ne, Ar, S). While similar results have been obtained for Galactic
PNe, this is the first demonstration of the relationship for extra-galactic
PNe. We also examine the relation between morphology and abundance of the
products of stellar evolution. We found that asymmetric PNe have higher
nitrogen and lower carbon abundances than symmetric PNe. Our two main results
are broadly consistent with the predictions of stellar evolution if the
progenitors of asymmetric PNe have on average larger masses than the
progenitors of symmetric PNe. The results bear on the question of formation
mechanisms for asymmetric PNe, specifically, that the genesis of PNe structure
should relate strongly to the Population type, and by inference the mass, of
the progenitor star, and less strongly on whether the central star is a member
of a close binary system.Comment: The Astrophysical Journal Letters, in press 4 figure
Neon and Sulfur Abundances of Planetary Nebulae in the Magellanic Clouds
The chemical abundances of neon and sulfur for 25 planetary nebulae (PNe) in
the Magellanic Clouds are presented. These abundances have been derived using
mainly infrared data from the Spitzer Space Telescope. The implications for the
chemical evolution of these elements are discussed. A comparison with similarly
obtained abundances of Galactic PNe and HII regions and Magellanic Clouds HII
regions is also given. The average neon abundances are 6.0x10(-5) and
2.7x10(-5) for the PNe in the Large and Small Magellanic Clouds respectively.
These are ~1/3 and 1/6 of the average abundances of Galactic planetary nebulae
to which we compare. The average sulfur abundances for the LMC and SMC are
respectively 2.7x10(-6) and 1.0x10(-6). The Ne/S ratio (23.5) is on average
higher than the ratio found in Galactic PNe (16) but the range of values in
both data sets is similar for most of the objects. The neon abundances found in
PNe and HII regions agree with each other. It is possible that a few (3-4) of
the PNe in the sample have experienced some neon enrichment, but for two of
these objects the high Ne/S ratio can be explained by their very low sulfur
abundances. The neon and sulfur abundances derived in this paper are also
compared to previously published abundances using optical data and
photo-ionization models.Comment: 13 pages, 4 tables, 5 figures. Accepted for publication in Ap
Are Optically-Selected Quasars Universally X-Ray Luminous? X-Ray/UV Relations in Sloan Digital Sky Survey Quasars
We analyze archived Chandra and XMM-Newton X-ray observations of 536 Sloan
Digital Sky Survey (SDSS) Data Release 5 (DR5) quasars (QSOs) at 1.7 <= z <=
2.7 in order to characterize the relative UV and X-ray spectral properties of
QSOs that do not have broad UV absorption lines (BALs). We constrain the
fraction of X-ray weak, non-BAL QSOs and find that such objects are rare; for
example, sources underluminous by a factor of 10 comprise \la2% of
optically-selected SDSS QSOs. X-ray luminosities vary with respect to UV
emission by a factor of \la2 over several years for most sources. UV
continuum reddening and the presence of narrow-line absorbing systems are not
strongly associated with X-ray weakness in our sample. X-ray brightness is
significantly correlated with UV emission line properties, so that relatively
X-ray weak, non-BAL QSOs generally have weaker, blueshifted CIV1549
emission and broader CIII]1909 lines. The CIV emission line strength
depends on both UV and X-ray luminosity, suggesting that the physical mechanism
driving the global Baldwin effect is also associated with X-ray emission.Comment: Accepted to Ap
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