5,175 research outputs found
Hartree-Fock symmetry breaking around conical intersections
We study the behavior of Hartree-Fock (HF) solutions in the vicinity of
conical intersections. These are here understood as regions of a molecular
potential energy surface characterized by degenerate or nearly-degenerate
eigenfunctions with identical quantum numbers (point group, spin, and electron
number). Accidental degeneracies between states with different quantum numbers
are known to induce symmetry breaking in HF. The most common closed-shell
restricted HF instability is related to singlet-triplet spin degeneracies that
lead to collinear unrestricted HF (UHF) solutions. Adding geometric frustration
to the mix usually results in noncollinear generalized HF (GHF) solutions,
identified by orbitals that are linear combinations of up and down spins. Near
conical intersections, we observe the appearance of coplanar GHF solutions that
break all symmetries, including complex conjugation and time-reversal, which do
not carry good quantum numbers. We discuss several prototypical examples taken
from the conical intersection literature. Additionally, we utilize a recently
introduced a magnetization diagnostic to characterize these solutions, as well
as a solution of a Jahn-Teller active geometry of H.Comment: accepted to JCP December 2017, published online January 201
Enhanced propagation of motile bacteria on surfaces due to forward scattering
How motile bacteria move near a surface is a problem of fundamental
biophysical interest and is key to the emergence of several phenomena of
biological, ecological and medical relevance, including biofilm formation.
Solid boundaries can strongly influence a cell's propulsion mechanism, thus
leading many flagellated bacteria to describe long circular trajectories stably
entrapped by the surface. Experimental studies on near-surface bacterial
motility have, however, neglected the fact that real environments have typical
microstructures varying on the scale of the cells' motion. Here, we show that
micro-obstacles influence the propagation of peritrichously flagellated
bacteria on a flat surface in a non-monotonic way. Instead of hindering it, an
optimal, relatively low obstacle density can significantly enhance cells'
propagation on surfaces due to individual forward-scattering events. This
finding provides insight on the emerging dynamics of chiral active matter in
complex environments and inspires possible routes to control microbial ecology
in natural habitats
Parallel evolution of TCP and B-class genes in Commelinaceae flower bilateral symmetry
<p>Abstract</p> <p>Background</p> <p>Flower bilateral symmetry (zygomorphy) has evolved multiple times independently across angiosperms and is correlated with increased pollinator specialization and speciation rates. Functional and expression analyses in distantly related core eudicots and monocots implicate independent recruitment of class II TCP genes in the evolution of flower bilateral symmetry. Furthermore, available evidence suggests that monocot flower bilateral symmetry might also have evolved through changes in B-class homeotic MADS-box gene function.</p> <p>Methods</p> <p>In order to test the non-exclusive hypotheses that changes in TCP and B-class gene developmental function underlie flower symmetry evolution in the monocot family Commelinaceae, we compared expression patterns of <it>teosinte branched1 </it>(<it>TB1</it>)-like, <it>DEFICIENS </it>(<it>DEF</it>)-like, and <it>GLOBOSA </it>(<it>GLO</it>)-like genes in morphologically distinct bilaterally symmetrical flowers of <it>Commelina communis </it>and <it>Commelina dianthifolia</it>, and radially symmetrical flowers of <it>Tradescantia pallida</it>.</p> <p>Results</p> <p>Expression data demonstrate that <it>TB1</it>-like genes are asymmetrically expressed in tepals of bilaterally symmetrical <it>Commelina</it>, but not radially symmetrical <it>Tradescantia</it>, flowers. Furthermore, <it>DEF</it>-like genes are expressed in showy inner tepals, staminodes and stamens of all three species, but not in the distinct outer tepal-like ventral inner tepals of <it>C. communis</it>.</p> <p>Conclusions</p> <p>Together with other studies, these data suggest parallel recruitment of <it>TB1</it>-like genes in the independent evolution of flower bilateral symmetry at early stages of <it>Commelina </it>flower development, and the later stage homeotic transformation of <it>C. communis </it>inner tepals into outer tepals through the loss of <it>DEF</it>-like gene expression.</p
An expanded evolutionary role for flower symmetry genes
CYCLOIDEA (CYC)-like TCP genes are critical for flower developmental patterning. Exciting recent breakthroughs, including a study by Song et al. published in BMC Evolutionary Biology, demonstrate that CYC-like genes have also had an important role in the evolution of flower form.
See research article http://www.biomedcentral.com/1471-2148/9/244 webcite
Recommended from our members
Insurance impacts survival for children, adolescents, and young adults with bone and soft tissue sarcomas.
BackgroundWhile racial/ethnic survival disparities have been described in pediatric oncology, the impact of income has not been extensively explored. We analyzed how public insurance influences 5-year overall survival (OS) in young patients with sarcomas.MethodsThe University of California San Francisco Cancer Registry was used to identify patients aged 0-39 diagnosed with bone or soft tissue sarcomas between 2000 and 2015. Low-income patients were defined as those with no insurance or Medicaid, a means-tested form of public insurance. Survival curves were computed using the Kaplan-Meier method and compared using log-rank tests and Cox models. Causal mediation was used to assess whether the association between public insurance and mortality is mediated by metastatic disease.ResultsOf 1106 patients, 39% patients were classified as low-income. Low-income patients were more likely to be racial/ethnic minorities and to present with metastatic disease (OR 1.96, 95% CI 1.35-2.86). Low-income patients had significantly worse OS (61% vs 71%). Age at diagnosis and extent of disease at diagnosis were also independent predictors of OS. When stratified by extent of disease, low-income patients consistently had significantly worse OS (localized: 78% vs 84%, regional: 64% vs 73%, metastatic: 23% vs 30%, respectively). Mediation analysis indicated that metastatic disease at diagnosis mediated 15% of the effect of public insurance on OS.ConclusionsLow-income patients with bone and soft tissue sarcomas had decreased OS regardless of disease stage at presentation. The mechanism by which insurance status impacts survival requires additional investigation, but may be through reduced access to care
A CYCâRADâDIVâDRIF interaction likely pre-dates the origin of floral monosymmetry in Lamiales
Background
An outstanding question in evolutionary biology is how genetic interactions defining novel traits evolve. They may evolve either by de novo assembly of previously non-interacting genes or by en bloc co-option of interactions from other functions. We tested these hypotheses in the context of a novel phenotypeâLamiales flower monosymmetryâdefined by a developmental program that relies on regulatory interaction among CYCLOIDEA, RADIALIS, DIVARICATA, and DRIF gene products. In Antirrhinum majus (snapdragon), representing Lamiales, we tested whether components of this program likely function beyond their previously known role in petal and stamen development. In Solanum lycopersicum (tomato), representing Solanales which diverged from Lamiales before the origin of Lamiales floral monosymmetry, we additionally tested for regulatory interactions in this program.
Results
We found that RADIALIS, DIVARICATA, and DRIF are expressed in snapdragon ovaries and developing fruit, similar to their homologs during tomato fruit development. In addition, we found that a tomato CYCLOIDEA ortholog positively regulates a tomato RADIALIS ortholog.
Conclusion
Our results provide preliminary support to the hypothesis that the developmental program defining floral monosymmetry in Lamiales was co-opted en bloc from a function in carpel development. This expands our understanding of novel trait evolution facilitated by co-option of existing regulatory interactions
A Bose gas in a single-beam optical dipole trap
We study an ultracold Bose gas in an optical dipole trap consisting of one
single focused laser beam. An analytical expression for the corresponding
density of states beyond the usual harmonic approximation is obtained. We are
thus able to discuss the existence of a critical temperature for Bose-Einstein
condensation and find that the phase transition must be enabled by a cutoff
near the threshold. Moreover, we study the dynamics of evaporative cooling and
observe significant deviations from the findings for the well-established
harmonic approximation. Furthermore, we investigate Bose-Einstein condensates
in such a trap in Thomas-Fermi approximation and determine analytical
expressions for chemical potential, internal energy and Thomas-Fermi radii
beyond the usual harmonic approximation
- âŠ