89 research outputs found
Self-recognition and Ca2+-dependent carbohydrate–carbohydrate cell adhesion provide clues to the Cambrian explosion
Author Posting. © The Authors, 2009. This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Molecular Biology and Evolution 26 (2009): 2551-2561, doi:10.1093/molbev/msp170.The Cambrian explosion of life was a relatively short period ca. 540 million years ago that
marked a generalized acceleration in the evolution of most animal phyla, but the trigger of this
key biological event remains elusive. Sponges are the oldest extant Precambrian metazoan
phylum and thus a valid model to study factors that could have unleashed the rise of multicellular
animals. One such factor is the advent of self/non-self recognition systems, which would be
evolutionarily beneficial to organisms to prevent germ cell parasitism or the introduction of
deleterious mutations resulting from fusion with genetically different individuals. However, the
molecules responsible for allorecognition probably evolved gradually before the Cambrian
period, and some other (external) factor remains to be identified as the missing triggering event.
Sponge cells associate through calcium-dependent, multivalent carbohydrate-carbohydrate
interactions of the g200 glycan found on extracellular proteoglycans. Single molecule force
spectroscopy analysis of g200-g200 binding indicates that calcium affects the lifetime (+Ca/-Ca:
680 s/3 s) and bond reaction length (+Ca/-Ca: 3.47 Å/2.27 Å). Calculation of mean g200
dissociation times in low and high calcium within the theoretical framework of a cooperative
binding model indicates the non-linear and divergent characteristics leading to either
disaggregated cells or stable multicellular assemblies, respectively. This fundamental
phenomenon can explain a switch from weak to strong adhesion between primitive metazoan
cells caused by the well documented rise in ocean calcium levels at the end of Precambrian time.
We propose that stronger cell adhesion allowed the integrity of genetically uniform animals
composed only of “self” cells, facilitating genetic constitutions to remain within the metazoan
individual and be passed down inheritance lines. The Cambrian explosion might have been
triggered by the coincidence in time of primitive animals endowed with self/non-self recognition,
and of a surge in sea water calcium that increased the binding forces between their calcium-dependent cell adhesion molecules.D.A. and A.K. acknowledge financial support from the Collaborative Research
Center SFB 613 from the Deutsche Forschungsgemeinschaft (DFG), and X.F.-B. acknowledges
financial support from grants BIO2002-00128, BIO2005-01591, and CSD2006-00012 from the
Ministerio de Ciencia y Tecnología, Spain, which included Fondo Europeo de Desarrollo
Regional funds, and from grant 2005SGR-00037 from the Generalitat de Catalunya, Spain
Multiwavelength observations of short time-scale variability in NGC 4151. I. Ultraviolet observations
We present the results of an intensive ultraviolet monitoring campaign on the
Seyfert 1 galaxy NGC 4151, as part of an effort to study its short time-scale
variability over a broad range in wavelength. The nucleus of NGC 4151 was
observed continuously with the {\it International Ultraviolet Explorer} (IUE)
for 9.3 days, yielding a pair of LWP and SWP spectra every 70 minutes,
and during four-hour periods for 4 days prior to and 5 days after the
continuous monitoring period. The sampling frequency of the observations is an
order of magnitude higher than that of any previous UV monitoring campaign on a
Seyfert galaxy.
The continuum fluxes in bands from 1275 \AA\ to 2688 \AA\ went through four
significant and well-defined ``events'' of duration 2 -- 3 days during the
continuous monitoring period. We find that the amplitudes of the continuum
variations decrease with increasing wavelength, which extends a general trend
for this and other Seyfert galaxies to smaller time scales (i.e., a few days).
The continuum variations in all of the UV bands are {\it simultaneous} to
within an accuracy of about 0.15 days, providing a strict constraint on
continuum models. The emission-line light curves show only one major event
during the continuous monitoring (a slow rise followed by a shallow dip), and
do not correlate well with continuum light curves over the (short) duration of
the campaign, because the time scale for continuum variations is apparently
smaller than the response times of the emission lines.Comment: 39 pages, LaTeX, including 7 PostScript figures; To appear in the ApJ
(October 20, 1996) Vol. 47
Multiwavelength observations of short time-scale variability in NGC 4151. IV. Analysis of multiwavelength continuum variability
This paper combines data from the three preceding papers in order to analyze
the multi-waveband variability and spectral energy distribution of the
Seyfert~1 galaxy NGC~4151 during the December 1993 monitoring campaign. The
source, which was near its peak historical brightness, showed strong,
correlated variability at X-ray, ultraviolet, and optical wavelengths. The
strongest variations were seen in medium energy (1.5~keV) X-rays, with a
normalized variability amplitude (NVA) of 24\%. Weaker (NVA = 6\%) variations
(uncorrelated with those at lower energies) were seen at soft -ray
energies of 100~keV. No significant variability was seen in softer
(0.1--1~keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased
from 9\% to 1\% as the wavelength increased from 1275~\AA\ to 6900~\AA. These
data do not probe extreme ultraviolet (1200~\AA\ to 0.1~keV) or hard X-ray
(2--50~keV) variability. The phase differences between variations in different
bands were consistent with zero lag, with upper limits of \ls0.15~day between
1275~\AA\ and the other ultraviolet bands, \ls0.3~day between 1275~\AA\ and
1.5~keV, and \ls1~day between 1275~\AA\ and 5125~\AA. These tight limits
represent more than an order of magnitude improvement over those determined in
previous multi-waveband AGN monitoring campaigns. The ultraviolet fluctuation
power spectra showed no evidence for periodicity, but were instead well-fitted
with a very steep, red power-law ().
If photons emitted at a ``primary" waveband are absorbed by nearby material
and ``reprocessed" to produce emission at a secondary waveband, causality
arguments require that variations in the secondary band follow those in the
primary band. The tight interband correlation and limits on the ultraviolet andComment: 35 pages, LaTeX (including aaspp4), including 7 PostScript figures;
To appear in the ApJ (October 20, 1996) Vol. 47
Habitat selection, facilitation, and biotic settlement cues affect distribution and performance of coral recruits in French Polynesia
Habitat selection can determine the distribution and performance of individuals if the precision with which sites are chosen corresponds with exposure to risks or resources. Contrastingly, facilitation can allow persistence of individuals arriving by chance and potentially maladapted to local abiotic conditions. For marine organisms, selection of a permanent attachment site at the end of their larval stage or the presence of a facilitator can be a critical determinant of recruitment success. In coral reef ecosystems, it is well known that settling planula larvae of reef-building corals use coarse environmental cues (i.e., light) for habitat selection. Although laboratory studies suggest that larvae can also use precise biotic cues produced by crustose coralline algae (CCA) to select attachment sites, the ecological consequences of biotic cues for corals are poorly understood in situ. In a field experiment exploring the relative importance of biotic cues and variability in habitat quality to recruitment of hard corals, pocilloporid and acroporid corals recruited more frequently to one species of CCA, Titanoderma prototypum, and significantly less so to other species of CCA; these results are consistent with laboratory assays from other studies. The provision of the biotic cue accurately predicted coral recruitment rates across habitats of varying quality. At the scale of CCA, corals attached to the “preferred” CCA experienced increased survivorship while recruits attached elsewhere had lower colony growth and survivorship. For reef-building corals, the behavioral selection of habitat using chemical cues both reduces the risk of incidental mortality and indicates the presence of a facilitator
Galaxy bulges and their massive black holes: a review
With references to both key and oft-forgotten pioneering works, this article
starts by presenting a review into how we came to believe in the existence of
massive black holes at the centres of galaxies. It then presents the historical
development of the near-linear (black hole)-(host spheroid) mass relation,
before explaining why this has recently been dramatically revised. Past
disagreement over the slope of the (black hole)-(velocity dispersion) relation
is also explained, and the discovery of sub-structure within the (black
hole)-(velocity dispersion) diagram is discussed. As the search for the
fundamental connection between massive black holes and their host galaxies
continues, the competing array of additional black hole mass scaling relations
for samples of predominantly inactive galaxies are presented.Comment: Invited (15 Feb. 2014) review article (submitted 16 Nov. 2014). 590
references, 9 figures, 25 pages in emulateApJ format. To appear in "Galactic
Bulges", E. Laurikainen, R.F. Peletier, and D.A. Gadotti (eds.), Springer
Publishin
Prediction of peak oxygen uptake in children using submaximal ratings of perceived exertion during treadmill exercise
Purpose: This study assessed the utility of the Children’s Effort Rating Table (CERT) and the Eston-Parfitt (EP) Scale in estimating peak oxygen uptake (V• O2peak) in children, during cardiopulmonary exercise testing (CPET) on a treadmill. Methods: Fifty healthy children (n=21 boys; 9.4 ± 0.9 y) completed a continuous, incremental protocol until the attainment of V• O2peak. Oxygen uptake (V• O2) was measured continuously, and Ratings of Perceived Exertion (RPE) were estimated at the end of each exercise stage using the CERT and the EP Scale. Ratings up to- and including RPE 5 and 7, from both the CERT (CERT 5, CERT 7) and EP Scale (EP 5, EP 7), were linearly regressed against the corresponding V• O2, to both maximal RPE (CERT 10, EP 10) and terminal RPE (CERT 9, EP 9). Results: There were no differences between measured- and predicted V• O2peak from CERT 5, CERT 7, EP 5 and EP 7 when extrapolated to either CERT 9 or EP 9 (P > .05). Pearson’s correlations of r = 0.64-0.86 were observed between measured- and predicted V• O2peak, for all perceptual ranges investigated. However, only EP 7 provided a small difference when considering the Standard Error of Estimate, suggesting that the prediction of V• O2peak from EP 7 would be within 10% of measured V• O2peak. Conclusions: Although robust estimates of V• O2peak may be elicited using both the CERT and EP Scale during a single CPET with children, the most accurate estimates of V• O2peak occur when extrapolating from EP 7
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Multiwavelength Observations of Short-Timescale Variability in NGC 4151. I. Ultraviolet Observations
We present the results of an intensive ultraviolet monitoring campaign on the Seyfert I galaxy NGC 4151, as part of an effort to study its short-timescale variability over a broad range in wavelength. The nucleus of NGC 4151 was observed continuously With the International Ultraviolet Explorer for 9.3 days, yielding a pair of LWP and SWP spectra every ~70 minutes, and during 4 hr periods for 4 days Prior to and 5 days after the continuous-monitoring period. The sampling frequency of the observations is an order of magnitude higher than that of any previous UV monitoring campaign on a Seyfert galaxy. The continuum fluxes in bands from 1275 to 2688 A went through four significant and well-defined events of duration 2-3 days during the continuous-monitoring period. We find that the amplitudes of the continuum variations decrease with increasing wavelength, which extends a general trend for this and other Seyfert galaxies to smaller timescales (i.e., a few days). The continuum variations in all the UV bands are simultaneous to within an accuracy of ~0.15 days, providing a strict constraint on continuum models. The emission-line light curves show only one major event during the continuous monitoring (a slow rise followed by a shallow dip) and do not correlate well with continuum light curves over the short duration of the campaign, because the timescale for continuum variations is apparently smaller than the response times of the emission lines.Astronom
Magna Carta, the Rule of Law and the Limits on Government
This paper surveys the legal tradition that links Magna Carta with the modern concepts of the rule of law and the limits on government. It documents that the original understanding of the rule of law included substantive commitments to individual freedom and limited government. Then, it attempts at explaining how and why such commitments were lost to a formalist interpretation of the rule of law from 1848 to 1939. The paper concludes by arguing how a revival of the substantive commitments of the rule of law is central in a project of reshaping modern states
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