160 research outputs found
Witnessing the gradual slow-down of powerful extragalactic jets: The X-ray -- optical -- radio connection
A puzzling feature of the {\it Chandra}--detected quasar jets is that their
X-ray emission decreases faster along the jet than their radio emission,
resulting to an outward increasing radio to X-ray ratio. In some sources this
behavior is so extreme that the radio emission peak is located clearly
downstream of that of the X-rays. This is a rather unanticipated behavior given
that the inverse
Compton nature of the X-rays and the synchrotron radio emission are
attributed to roughly the same electrons of the jet's non-thermal electron
distribution. In this note we show that this morphological behavior can result
from the gradual deceleration of a relativistic flow and that the offsets in
peak emission at different wavelengths carry the imprint of this deceleration.
This notion is consistent with another recent finding, namely that the jets
feeding the terminal hot spots of powerful radio galaxies and quasars are still
relativistic with Lorentz factors . The picture of the
kinematics of powerful jets emerging from these considerations is that they
remain relativistic as they gradually decelerate from Kpc scales to the hot
spots, where, in a final collision with the intergalactic medium, they
slow-down rapidly to the subrelativistic velocities of the hot spot advance
speed.Comment: Submitted in ApJ Letters on Jan. 14, 200
Molecular patterns of sex determination in the animal kingdom: a comparative study of the biology of reproduction
Determining sexual fate is an integral part of reproduction, used as a means to enrich the genome. A variety of such regulatory mechanisms have been described so far and some of the more extensively studied ones are being discussed. For the insect order of Hymenoptera, the choice lies between uniparental haploid males and biparental diploid females, originating from unfertilized and fertilized eggs accordingly. This mechanism is also known as single-locus complementary sex determination (slCSD). On the other hand, for Dipterans and Drosophila melanogaster, sex is determined by the ratio of X chromosomes to autosomes and the sex switching gene, sxl. Another model organism whose sex depends on the X:A ratio, Caenorhabditis elegans, has furthermore to provide for the brief period of spermatogenesis in hermaphrodites (XX) without the benefit of the "male" genes of the sex determination pathway. Many reptiles have no discernible sex determining genes. Their sexual fate is determined by the temperature of the environment during the thermosensitive period (TSP) of incubation, which regulates aromatase activity. Variable patterns of sex determination apply in fish and amphibians. In birds, while sex chromosomes do exist, females are the heterogametic (ZW) and males the homogametic sex (ZZ). However, we have yet to decipher which of the two (Z or W) is responsible for the choice between males and females. In mammals, sex determination is based on the presence of two identical (XX) or distinct (XY) gonosomes. This is believed to be the result of a lengthy evolutionary process, emerging from a common ancestral autosomal pair. Indeed, X and Y present different levels of homology in various mammals, supporting the argument of a gradual structural differentiation starting around the SRY region. The latter initiates a gene cascade that results in the formation of a male. Regulation of sex steroid production is also a major result of these genetic interactions. Similar observations have been described not only in mammals, but also in other vertebrates, emphasizing the need for further study of both normal hormonal regulators of sexual phenotype and patterns of epigenetic/environmental disruption
Sex determination strategies in 2012: towards a common regulatory model?
Sex determination is a complicated process involving large-scale modifications in gene expression affecting virtually every tissue in the body. Although the evolutionary origin of sex remains controversial, there is little doubt that it has developed as a process of optimizing metabolic control, as well as developmental and reproductive functions within a given setting of limited resources and environmental pressure. Evidence from various model organisms supports the view that sex determination may occur as a result of direct environmental induction or genetic regulation. The first process has been well documented in reptiles and fish, while the second is the classic case for avian species and mammals. Both of the latter have developed a variety of sex-specific/sex-related genes, which ultimately form a complete chromosome pair (sex chromosomes/gonosomes). Interestingly, combinations of environmental and genetic mechanisms have been described among different classes of animals, thus rendering the possibility of a unidirectional continuous evolutionary process from the one type of mechanism to the other unlikely. On the other hand, common elements appear throughout the animal kingdom, with regard to a) conserved key genes and b) a central role of sex steroid control as a prerequisite for ultimately normal sex differentiation. Studies in invertebrates also indicate a role of epigenetic chromatin modification, particularly with regard to alternative splicing options. This review summarizes current evidence from research in this hot field and signifies the need for further study of both normal hormonal regulators of sexual phenotype and patterns of environmental disruption
Sex Steroids: Beyond Conventional Dimorphism
Sexual dimorphism is a characteristic of a large number of species, ranging from lower invertebrates to mammals and, last but not least, humans. Recognition of the various factors regulating sexual dimorphism initial establishment (i.e. sex determination and differentiation) and subsequent life-long adaptation to distinct functional and behavioural patterns has remained a hot topic for several decades. As our understanding of the various molecular pathways involved in this process increases, the significant role of sex steroids becomes more evident. At the same time, the recognition of new sites of steroid production (e.g. parts of the brain) and aromatization, as well as new target cells (owing to the pro- posed presence of additional receptors to those classically considered as primary steroid receptors) has lead to the need to revisit their spectrum of actions within a novel, multifactorial context. Thus, anthropology and medicine are presented with the challenge to unravel a major mystery, i.e. that of sexual orientation and differentiation and its potential contri- bution in human evolution and civilization development, taking advantage of the high-tech research tools provided by modern biotechnology. This short review summarizes the basic principles of sex determination and sex steroid function as they have been classically described in the literature and then proceeds to present examples of how modern research methods have started to offer a new insight on the more subtle details of this process, stressing that it is extending to virtually every single part and system of the body
Discriminating Between the Roles of Androgens and Estrogens in Cardiovascular Disease
Cardiovascular disease shows a distinct difference in incidence rates between men and women, a fact that has been known for many years. While initial theories supported that this could be attributed to the protective effect of estrogens in women, attempts to correlate endogenous estrogen levels with cardiovascular risk factors and the progression of atherosclerosis-related indexes indicate otherwise. Similarly, endogenous androgen levels seem to correlate with opposite effects in males and females, whereas exogenous treatment with either androgens or estrogens fails to correspond to scientific expectations entirely. A brief discussion of the merits and pitfalls of placing either estrogens or androgens alone at the root of the problem shows that current understanding is inadequate concerning this major anthropological issue, as it refers to the primary global mortality and morbidity cause
Modelling the spectral evolution of classical double radio sources
The spectral evolution of powerful double radio galaxies (FR II's) is thought
to be determined by the acceleration of electrons at the termination shock of
the jet, their transport through the bright head region into the lobes and the
production of the radio emission by synchrotron radiation in the lobes. Models
presented to date incorporate some of these processes in prescribing the
electron distribution which enters the lobes. We have extended these models to
include a description of electron acceleration at the relativistic termination
shock and a selection of transport models for the head region. These are
coupled to the evolution of the electron spectrum in the lobes under the
influence of losses due to adiabatic expansion, by inverse Compton scattering
on the cosmic background radiation and by synchrotron radiation. The
evolutionary tracks predicted by this model are compared to observation using
the power/source-size (P-D) diagram. We find that the simplest scenario, in
which accelerated particles suffer adiabatic losses in the head region which
become more severe as the source expands produces P-D-tracks which conflict
with observation, because the power is predicted to decline too steeply with
increasing size. Agreement with observation can be found by assuming that
adiabatic losses are compensated during transport between the termination shock
and the lobe by a re-acceleration process distributed throughout the head
region.Comment: 14 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Acceleration of energetic particles by large-scale compressible magnetohydrodynamic turbulence
Fast particles diffusing along magnetic field lines in a turbulent plasma can
diffuse through and then return to the same eddy many times before the eddy is
randomized in the turbulent flow. This leads to an enhancement of particle
acceleration by large-scale compressible turbulence relative to previous
estimates in which isotropic particle diffusion is assumed.Comment: 13 pages, 3 figures, accepted for publication in Ap
Sex Determinants in the Genome – Lessons from the Animal Kingdom
The immense value of sex differentiation as a means of enriching and evolving the genome has been proven by the vast
variety of sex determining mechanisms to which organisms of all kinds resort. From single gene switching pathways
found in lower level organisms to haplodiploid reproduction in hymenoptera, temperature-determined sex in reptiles
and sex chromosomes in mammals and avians, nature and evolution have designated an impressive amount of effort to
ensure that sex-specific variations remain under well-regulated control. Therefore enhancing our efforts to study some of
the strategies recruited for the above may also lead to a better understanding of the inherent complexity of sexual dimorphism
in general
The new sample of giant radio sources III. Statistical trends and correlations
In this paper we analyse whether `giant' radio galaxies (GRGs) differ from
`normal'-size galaxies (NSGs) except for the linear extent of their radio
structure. We compare a number of properties of GRGs with the corresponding
properties of NSGs, and analyse the statistical trends and correlations of
physical parameters, homogeneously determined for the sources, with their
`fundamental' parameters. Using the Pearson partial-correlation test on the
correlation between two variables in the presence of one or two other
variables, we examine which correlation is the strongest. The analysis clearly
shows that GRGs do not form a separate class of radio sources. They most likely
evolve with time from smaller sources, however under specific circumstances.
Analysing properties of GRGs and NSGs together, we find that (i) the core
prominence does not correlate with the total radio luminosity (as does the core
power), but it anti-correlates with the surface brightness of the lobes of
sources, (ii) the energy density (and possibly the internal pressure) in the
lobes is independent of redshift for constant radio luminosity and size of the
sources, (iii) the equipartition magnetic-field strength, transformed into
constant source luminosity and redshift, strongly correlates with the source
size. We argue that this B_{eq} - D correlation reflects a more fundamental
correlation between B_{eq} and the source age, (iv) both the rotation and
depolarisation measures suggest Faraday screens local to the lobes of sources,
however their geometry and the composition of intervening material cannot be
determined from the global polarisation characteristics.Comment: 10 pages, 8 figures, 6 tables. Accepted for publication in A&
Establishing Sexual Dimorphism in Human
Sexual dimorphism, i.e. the distinct recognition of only two sexes per species, is the phenotypic expression of a multistage
procedure at chromosomal, gonadal, hormonal and behavioral level. Chromosomal – genetic sexual dimorphism
refers to the presence of two identical (XX) or two different (XY) gonosomes in females and males, respectively. This is due
to the distinct content of the X and Y-chromosomes in both genes and regulatory sequences, SRY being the key regulator.
Hormones (AMH, testosterone, Insl3) secreted by the foetal testis (gonadal sexual dimorphism), impede Müller duct development,
masculinize Wolff duct derivatives and are involved in testicular descent (hormonal sexual dimorphism).
Steroid hormone receptors detected in the nervous system, link androgens with behavioral sexual dimorphism. Furthermore,
sex chromosome genes directly affect brain sexual dimorphism and this may precede gonadal differentiation
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