2,668 research outputs found
Asexual and sexual replication in sporulating organisms
This paper develops models describing asexual and sexual replication in
sporulating organisms. Replication via sporulation is the replication strategy
for all multicellular life, and may even be observed in unicellular life (such
as with budding yeast). We consider diploid populations replicating via one of
two possible sporulation mechanisms: (1) Asexual sporulation, whereby adult
organisms produce single-celled diploid spores that grow into adults
themselves. (2) Sexual sporulation, whereby adult organisms produce
single-celled diploid spores that divide into haploid gametes. The haploid
gametes enter a haploid "pool", where they may recombine with other haploids to
form a diploid spore that then grows into an adult. We consider a haploid
fusion rate given by second-order reaction kinetics. We work with a simplified
model where the diploid genome consists of only two chromosomes, each of which
may be rendered defective with a single point mutation of the wild-type. We
find that the asexual strategy is favored when the rate of spore production is
high compared to the characteristic growth rate from a spore to a reproducing
adult. Conversely, the sexual strategy is favored when the rate of spore
production is low compared to the characteristic growth rate from a spore to a
reproducing adult. As the characteristic growth time increases, or as the
population density increases, the critical ratio of spore production rate to
organism growth rate at which the asexual strategy overtakes the sexual one is
pushed to higher values. Therefore, the results of this model suggest that, for
complex multicellular organisms, sexual replication is favored at high
population densities, and low growth and sporulation rates.Comment: 8 pages, 5 figures, to be submitted to Journal of Theoretical
Biology, figures not included in this submissio
Proton NMR measurements of the local magnetic field in the paramagnetic metal and antiferromagnetic insulator phases of -(BETS)FeCl
Measurements of the H-NMR spectrum of a small ( 4 g) single
crystal of the organic conductor -(BETS)FeCl are reported
with an applied magnetic field = 9 T parallel to the a-axis in
the -plane over a temperature range 2.0 180 K. They provide the
distribution of the static local magnetic field at the proton sites in the
paramagnetic metal (PM) and antiferromagnetic insulator (AFI) phases, along
with the changes that occur at the PMAFI phase transition. The spectra have
six main peaks that are significantly broadened and shifted at low . The
origin of these features is attributed to the large dipolar field from the 3d
Fe ion moments (spin = 5/2). Their amplitude and
dependence are modeled using a modified Brillouin function that includes a
mean field approximation for the total exchange interaction () between
one Fe ion and its two nearest neighbors. A good fit is obtained using
= 1.7 K. At temperatures below the PMAFI transition temperature
= 3.5 K, an extra peak appears on the high frequency side of the
spectrum and the details of the spectrum become smeared. Also, the rms
linewidth and the frequency shift of the spectral distribution are
discontinuous, consistent with the transition being first-order. These
measurements verify that the dominant local magnetic field contribution is from
the Fe ions and indicate that there is a significant change in the
static local magnetic field distribution at the proton sites on traversing the
PM to AFI phase transition.Comment: 11 pages, 7 figures. Revised version of cond-mat/0605044 resubmitted
to Phys. Rev. B in response to comments of Editor and reviewer
Impairment of left atrial function and cryptogenic stroke : potential insights in the pathophysiology of stroke in the young
Background: Stroke is one of the leading causes of morbidity and mortality with a significant percentage classified as cryptogenic. Left atrial (LA) remodelling, a substrate for atrial fibrillation (AF) and stroke development, may play a role in identification of the aetiology of cryptogenic stroke. We aimed to examine LA function to gain mechanistic insights into the pathophysiology of cryptogenic stroke in young patients otherwise at low risk for cardiovascular disease. Methods: Patients aged <60 years without traditional cardiovascular risk factors and who were diagnosed with ischaemic cryptogenic stroke or TIA were evaluated and compared to healthy controls and patients with paroxysmal AF with a CHA2DS2-VA score of 0. Conventional and novel left ventricular (LV) and LA echocardiographic parameters between the three groups were assessed. Results: Each group consisted of thirty patients. There were no significant differences in LV parameters (LVEF, LV endoGLS) between groups. LA strain in stroke patients was significantly lower compared to the controls (median 33%; interquartile range (IQ) [32/39] vs 31 [27/34]; p = 0.008). LA strain was significantly lower in AF patients compared to stroke patients (median 21% [19/22] vs 31% [27/34]; p < 0.0001). Conclusion: A stepwise reduction in measures of LA function was appreciated between controls, young stroke and paroxysmal AF groups. This may indicate dynamic LA remodelling occurring in the young stroke population and suggest a shared causal mechanism for stroke development in this group. LA strain may further refine the risk for cardioembolic stroke
Dissecting the Cosmic Infrared Background with 3D Instruments
The cosmic infrared background (CIB) consists of emission from distant,
dusty, star-forming galaxies. Energetically, the CIB is very important as it
contains as much energy as the extragalactic optical background. The nature and
evolutionary status of the objects making up the background are, however,
unclear. The CIB peaks at ~150 microns, and as such is most effectively studied
from space. The limited apertures of space-borne telescopes set the angular
resolution that can be attained, and so even Herschel, with its 3.5m diameter,
will be confusion-limited at this wavelengths at ~5mJy. The bulk of the
galaxies contributing to the CIB are fainter than this, so it is difficult to
study them without interferometry. Here we present the results of a preliminary
study of an alternative way of probing fainter than the continuum confusion
limit using far-IR imaging spectroscopy. An instrument capable of such
observations is being planned for SPICA - a proposed Japanese mission with an
aperture equivalent to that of Herschel and more than 2 orders of magnitude
more sensitive. We investigate the potential of imaging spectrometers to break
the continuum confusion limit. We have simulated the capabilities of a
spectrometer with modest field of view (2'x2'), moderate spectral resolution
(R~1-2000) and high sensitivity. We find that such an instrument is capable of
not only detecting line emission from sources with continuum fluxes
substantially below the confusion limit, but also of determining their
redshifts and, where multiple lines are detected, some emission line
diagnostics. We conclude that 3-D imaging spectrometers on cooled far-IR space
telescopes will be powerful new tools for extragalactic far-IR astronomy.Comment: Accepted for publication in Astronomy & Astrophysic
Ionic Liquid-Based Microemulsions in Catalysis
The design and properties of surface-active
ionic liquids that
are able to form stable microemulsions with heptane and water are
presented, and their promise as reaction media for thermomorphic palladium-catalyzed
cross-coupling reactions is demonstrated
First detection of [CII]158um at high redshift: vigorous star formation in the early universe
We report the detection of the 2P_3/2 -> 2P_1/2 fine-structure line of C+ at
157.74 micron in SDSSJ114816.64+525150.3 (hereafter J1148+5251), the most
distant known quasar, at z=6.42, using the IRAM 30-meter telescope. This is the
first detection of the [CII] line at high redshift, and also the first
detection in a Hyperluminous Infrared Galaxy (L_FIR > 10^13 Lsun). The [CII]
line is detected at a significance level of 8 sigma and has a luminosity of 4.4
x 10^9 Lsun. The L_[CII]/L_FIR ratio is 2 x 10^-4, about an order of magnitude
smaller than observed in local normal galaxies and similar to the ratio
observed in local Ultraluminous Infrared Galaxies. The [CII] line luminosity
indicates that the host galaxy of this quasar is undergoing an intense burst of
star formation with an estimated rate of ~3000 Msun/yr. The detection of C+ in
SDSS J1148+5251 suggests a significant enrichment of metals at z ~ 6 (age of
the universe ~870 Myr), although the data are consistent with a reduced carbon
to oxygen ratio as expected from chemical evolutionary models of the early
phases of galaxy formation.Comment: 5 pages, 2 figures, accepted by A&A Letter
Strong [CII] emission at high redshift
We report the detection of the [CII]157.74um fine-structure line in the
lensed galaxy BRI 0952-0115 at z=4.43, using the APEX telescope. This is the
first detection of the [CII] line in a source with L_FIR < 10^13 L_sun at high
redshift. The line is very strong compared to previous [CII] detections at
high-z (a factor of 5-8 higher in flux), partly due to the lensing
amplification. The L_[CII]/L_FIR ratio is 10^-2.9, which is higher than
observed in local galaxies with similar infrared luminosities. Together with
previous observations of [CII] at high redshift, our result suggests that the
[CII] emission in high redshift galaxies is enhanced relative to local galaxies
of the same infrared luminosity. This finding may result from selection effects
of the few current observations of [CII] at high redshift, and in particular
the fact that non detections may have not been published (although the few
published upper limits are still consistent with the [CII] enhancement
scenario). If the trend is confirmed with larger samples, it would indicate
that high-z galaxies are characterized by different physical conditions with
respect to their local counterparts. Regardless of the physical origin of the
trend, this effect would increase the potential of the [CII]158um line to
search and characterize high-z sources.Comment: Accepted for publication in A&A Letters, 5 pages, 2 figure
CO2 Concentration Limits, The Costs and Benefits of Control, and The Potential for International Agreement
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