5,708 research outputs found
Ancient homeobox gene loss and the evolution of chordate brain and pharynx development: deductions from amphioxus gene expression
Homeobox genes encode a large superclass of transcription factors with widespread roles in animal development. Within chordates there are over 100 homeobox genes in the invertebrate cephalochordate amphioxus and over 200 in humans. Set against this general trend of increasing gene number in vertebrate evolution, some ancient homeobox genes that were present in the last common ancestor of chordates have been lost from vertebrates. Here, we describe the embryonic expression of four amphioxus descendants of these genesâAmphiNedxa, AmphiNedxb, AmphiMsxlx and AmphiNKx7. All four genes are expressed with a striking asymmetry about the leftâright axis in the pharyngeal region of neurula embryos, mirroring the pronounced asymmetry of amphioxus embryogenesis. AmphiMsxlx and AmphiNKx7 are also transiently expressed in an anterior neural tube region destined to become the cerebral vesicle. These findings suggest significant rewiring of developmental gene regulatory networks occurred during chordate evolution, coincident with homeobox gene loss. We propose that loss of otherwise widely conserved genes is possible when these genes function in a confined role in development that is subsequently lost or significantly modified during evolution. In the case of these homeobox genes, we propose that this has occurred in relation to the evolution of the chordate pharynx and brain
Undamped nonequilibrium dynamics of a nondegenerate Bose gas in a 3D isotropic trap
We investigate anomalous damping of the monopole mode of a non-degenerate 3D
Bose gas under isotropic harmonic confinement as recently reported by the JILA
TOP trap experiment [D. S. Lob- ser, A. E. S. Barentine, E. A. Cornell, and H.
J. Lewandowski (in preparation)]. Given a realistic confining potential, we
develop a model for studying collective modes that includes the effects of
anharmonic corrections to a harmonic potential. By studying the influence of
these trap anharmonicities throughout a range of temperatures and collisional
regimes, we find that the damping is caused by the joint mechanisms of
dephasing and collisional relaxation. Furthermore, the model is complimented by
Monte Carlo simulations which are in fair agreement with data from the JILA
experiment.Comment: 11 pages, 6 figure
GRB Afterglows from Anisotropic Jets
Some progenitor models of gamma-ray bursts (GRBs) (e.g., collapsars) may
produce anisotropic jets in which the energy per unit solid angle is a
power-law function of the angle (). We calculate light
curves and spectra for GRB afterglows when such jets expand either in the
interstellar medium or in the wind medium. In particular, we take into account
two kinds of wind: one () possibly from a typical red
supergiant star and another () possibly from a Wolf-Rayet
star. We find that in each type of medium, one break appears in the late-time
afterglow light curve for small but becomes weaker and smoother as
increases. When , the break seems to disappear but the afterglow decays
rapidly. Thus, one expects that the emission from expanding, highly anisotropic
jets provides a plausible explanation for some rapidly fading afteglows whose
light curves have no break. We also present good fits to the optical afterglow
light curve of GRB 991208. Finally, we argue that this burst might arise from a
highly anisotropic jet expanding in the wind () from a red
supergiant to interpret the observed radio-to-optical-band afterglow data
(spectrum and light curve).Comment: 12 pages + 10 figures, accepted by Ap
Structure Speaks: User-Centered Design and Professional Development
This reflective essay situates a yearlong professional development endeavor led by a site of the National Writing Project within the language of technical communication. Developing rural writing teachers through four distinct design featuresâneeds assessment, frequent contact, website redesign, collaborative planning through Google Docsâthis work sought to put participants and providers on equal levels, sharing control of programming when possible. Professional development providers and teacher educators ultimately must model practices they desire to impacting students in the classroom
Sympathetic cooling of trapped fermions by bosons in the presence of particle losses
We study the sympathetic cooling of a trapped Fermi gas interacting with an
ideal Bose gas below the critical temperature of the Bose-Einstein
condensation. We derive the quantum master equation, which describes the
dynamics of the fermionic component, and postulating the thermal distribution
for both gases we calculate analytically the rate at which fermions are cooled
by the bosonic atoms. The particle losses constitute an important source of
heating of the degenerate Fermi gas. We evaluate the rate of loss-induced
heating and derive analytical results for the final temperature of fermions,
which is limited in the presence of particle losses.Comment: 7 pages, 2 figures, EPL style; final versio
âWe Were the Teachers, not the Observersâ: Transforming Teacher Preparation through Placements in a Creative, After-School Program
Teacher preparation at one university shifts pre-service observation to hands-on integration of the arts in an after-school program called Razorback Writers
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