Coexistence in an inhomogeneous environment

We examine the two-dimensional extension of the model of Kessler and Sander of competition between two species identical except for dispersion rates. In this class of models, the spatial inhomogeneity of reproduction rates gives rise to an implicit cost of dispersal, due to the tendency to leave favorable locations. Then, as in the Hamilton-May model with its explicit dispersal cost, the tradeoff between dispersal case and the beneficial role of dispersal in limiting fluctuations, leads to an advantage of one dispersal rate over another, and the eventual extinction of the disadvantaged species. In two dimensions we find that while the competition leads to the elimination of one species at high and low population density, at intermediate densities the two species can coexist essentially indefinitely. This is a new phenomenon not present in either the one-dimensional form of the Kessler-Sander model nor in the totally connected Hamilton-May model, and points to the importance of geometry in the question of dispersal

Point mutation of an RGD sequence in the human P2Y2 receptor to a QGD sequence conserves Go-mediated signal transduction

Abstract only availableThe P2Y2 nucleotide receptor is a Go/q coupled receptor that is activated equipotently by extracellular nucleotides such as ATP or UTP and is upregulated in a variety of tissues in response to injury or stress. The biological effects of extracellular nucleotides are mediated through activation of P1 and P2 purinergic receptors. P1 receptors are responsive to adenine and P2 receptors are activated by a variety of nucleotides including ATP and UTP. The P2 receptors are subdivided into two distinct categories, the ionotropic ligand-gated channel (P2X) receptors and G-protein coupled P2Y-receptors, with seven transmembrane domains. Previous studies have shown that the human P2Y2 nucleotide receptor contains an arginine-glycine-aspartic acid (RGD), integrin-binding domain. This domain is located in the first extracellular loop of the receptor and binds specifically to the v3/5 group of integrins. The P2Y2 receptor interacts with the v integrins by the RGD domain to activate Go and induce cell migration. The human and murine P2Y2R's have the RGD integrin-binding domain, whereas the rat homologue has a QGD domain. However, this change in the arginine position in the RGD integrin-binding domain is considered to be a conservative substitution that maintains integrin binding. In order to confirm this assumption we changed the RGD domain of the human P2Y2 receptor into the QGD domain by in vitro mutagenesis. The wild-type and the QGD mutant P2Y2 receptors were transiently transfected into P2 receptor null, 1321 N1 astrocytoma cell line. Preliminary data suggest that the QGD mutant can stimulate PLC dependent intracellular Ca2+ mobilization and also activate cofillin and extracellular signal-regulated kinases (Erk) with equal efficacy and agonist potency as the wild type receptor. Further tests need to be done to verify that integrin bindin and signaling by the Rac and Rho pathways remain unaffected in the QGD mutant to induce integrin dependent cell migration in response to UTP and ATP.NSF-REU Program in Biological Sciences & Biochemistr

Theory and it ab initio calculation of radiative lifetime of excitons in semiconducting carbon nanotubes

We present theoretical analysis and first-principles calculation of the radiative lifetime of excitons in semiconducting carbon nanotubes. An intrinsic lifetime of the order of 10 ps is computed for the lowest optically active bright excitons. The intrinsic lifetime is however a rapid increasing function of the exciton momentum. Moreover, the electronic structure of the nanotubes dictates the existence of dark excitons nearby in energy to each bright exciton. Both effects strongly influence measured lifetime. Assuming a thermal occupation of bright and dark exciton bands, we find an effective lifetime of the order of 10 ns at room temperature, in good accord with recent experiments.Comment: 12 pages, 3 figure