The Control of Spore Germination in Anemia Phyllitidis (L.) Swartz and Other Ferns

Many fern spores require light for germination. In one family of ferns, the Schizaeaceae, the gibberellins overcome this light requirement and induce dark-germination. Fern gametophytes produce antheridium-inducing substances. The Anemia substance, antheridogen-B, is demonstrated to induce germination in two genera of the Schizaeaceae, Anemia and Mohria. The Pteridium substance, antheridogen-A, stimulates germination of at least ten species in two families of ferns, but is not active on species of the Schizaeaceae. In almost all tested cases the antheridogens are active in inducing germination in the same species on which they also induce antheridia. The hormone-induced dark-germination in Anemia phyllitidis (L.) Swartz is pH-dependent while the light-induced germination is insensitive to changes in pH from 3.5 to 7.0. Light most probably initiates germination in A. phyllitidis by inducing the synthesis of gibberellin-like substances within the spore. The inhibitor of gibberellin biosynthesis, AM0-1618, greatly depresses light-induced germination without affecting gibberellin-induced dark-germination. Gibberellin-like substances, resembling GA 1, 3 and GA 9, are shown to be produced during light-induced germination in quantities of 6 x –10 -9 to 10-7 gram equivalents GA 3 per gram of spores. Synergism between GA 3 and light is observed only under certain conditions and may be explained by a light stimulation of spore permeability. The two antheridogens and the gibberellin-like material produced by the germinating spore of A. phyllitidis are different substances. The antheridogens also are not identical to any of the known gibberellins. Cycloheximide inhibits the gibberellin-induced germination and also inhibits incorporation of C 14-amino acids into TCA-insoluble material. Actinomycin D has no effect on germination, but it is not shown whether the inhibitor enters the dormant spore. Rupture of the spore wall in A. phyllitidis only occurs after a germination stimulus is applied. However, the opening of the spore wall can occur without growth of the spore cell in cycloheximide, 5-fluoro-uracil, or mannitol are applied under certain conditions. The following sequence of events is hypothesized in the course of the natural germination of A. phyllitidis spores: a) light induces synthesis of gibberellin-like substances, b) gibberellin-like factors initiate the synthesis of specific hydrolases, c) the spore wall is weakened, d) the spore cell extends out from the spore coat

Settling the Competition Complexity of Additive Buyers over Independent Items

The competition complexity of an auction setting is the number of additional bidders needed such that the simple mechanism of selling items separately (with additional bidders) achieves greater revenue than the optimal but complex (randomized, prior-dependent, Bayesian-truthful) optimal mechanism without the additional bidders. Our main result settles the competition complexity of $n$ bidders with additive values over $m < n$ independent items at $\Theta(\sqrt{nm})$. The $O(\sqrt{nm})$ upper bound is due to [BW19], and our main result improves the prior lower bound of $\Omega(\ln n)$ to $\Omega(\sqrt{nm})$. Our main result follows from an explicit construction of a Bayesian IC auction for $n$ bidders with additive values over $m<n$ independent items drawn from the Equal Revenue curve truncated at $\sqrt{nm}$ ($\mathcal{ER}_{\le \sqrt{nm}}$), which achieves revenue that exceeds $\text{SRev}_{n+\sqrt{nm}}(\mathcal{ER}_{\le \sqrt{nm}}^m)$. Along the way, we show that the competition complexity of $n$ bidders with additive values over $m$ independent items is exactly equal to the minimum $c$ such that $\text{SRev}_{n+c}(\mathcal{ER}_{\le p}^m) \geq \text{Rev}_n(\mathcal{ER}_{\le p}^m)$ for all $p$ (that is, some truncated Equal Revenue witnesses the worst-case competition complexity). Interestingly, we also show that the untruncated Equal Revenue curve does not witness the worst-case competition complexity when $n > m$: $\text{SRev}_n(\mathcal{ER}^m) = nm+O_m(\ln (n)) \leq \text{SRev}_{n+O_m(\ln (n))}(\mathcal{ER}^m)$, and therefore our result can only follow by considering all possible truncations.Comment: 50 page

Non-equilibrium phase transitions in biomolecular signal transduction

We study a mechanism for reliable switching in biomolecular signal-transduction cascades. Steady bistable states are created by system-size cooperative effects in populations of proteins, in spite of the fact that the phosphorylation-state transitions of any molecule, by means of which the switch is implemented, are highly stochastic. The emergence of switching is a nonequilibrium phase transition in an energetically driven, dissipative system described by a master equation. We use operator and functional integral methods from reaction-diffusion theory to solve for the phase structure, noise spectrum, and escape trajectories and first-passage times of a class of minimal models of switches, showing how all critical properties for switch behavior can be computed within a unified framework

Limitations of the Standard Gravitational Perfect Fluid Paradigm

We show that the standard perfect fluid paradigm is not necessarily a valid description of a curved space steady state gravitational source. Simply by virtue of not being flat, curved space geometries have to possess intrinsic length scales, and such length scales can affect the fluid structure. For modes of wavelength of order or greater than such scales eikonalized geometrical optics cannot apply and rays are not geodesic. Covariantizing thus entails not only the replacing of flat space functions by covariant ones, but also the introduction of intrinsic scales that were absent in flat space. In principle it is thus unreliable to construct the curved space energy-momentum tensor as the covariant generalization of a geodesic-based flat spacetime energy-momentum tensor. By constructing the partition function as an incoherent average over a complete set of modes of a scalar field propagating in a curved space background, we show that for the specific case of a static, spherically symmetric geometry, the steady state energy-momentum tensor that ensues will in general be of the form $T_{\mu\nu}=(\rho+p)U_{\mu}U_{\nu}+pg_{\mu\nu}+\pi_{\mu\nu}$ where the anisotropic $\pi_{\mu\nu}$ is a symmetric, traceless rank two tensor which obeys $U^{\mu}\pi_{\mu\nu}=0$. Such a $\pi_{\mu\nu}$ type term is absent for an incoherently averaged steady state fluid in a spacetime where there are no intrinsic length scales, and in principle would thus be missed in a covariantizing of a flat spacetime $T_{\mu\nu}$. While the significance of such $\pi_{\mu\nu}$ type terms would need to be evaluated on a case by case basis, through the use of kinetic theory we reassuringly find that the effect of such $\pi_{\mu\nu}$ type terms is small for weak gravity stars where perfect fluid sources are commonly used.Comment: Final version to appear in General Relativity and Gravitation (the final publication is available at http://www.springerlink.com). 29 pages, 1 figur

Glycated AAV Vectors: Chemical Redirection of Viral Tissue Tropism

A chemical approach for selective masking of arginine residues on viral capsids, featuring an exogenous glycation reaction has been developed. Reaction of adeno-associated viral (AAV) capsids with the α-dicarbonyl compound, methylglyoxal resulted in formation of arginine adducts. Specifically, surface exposed guanidinium side chains were modified into charge neutral hydroimidazolones, thereby disrupting a continuous cluster of basic amino acid residues implicated in heparan sulfate binding. Consequent loss in heparin binding ability and decrease in infectivity were observed. Strikingly, glycated AAV retained ability to infect neurons in the mouse brain and were redirected from liver to skeletal and cardiac muscle following systemic administration in mice. Further, glycated AAV displayed altered antigenicity demonstrating potential for evading antibody neutralization. Generation of unnatural amino acid side chains through capsid glycation might serve as an orthogonal strategy to engineer AAV vectors displaying novel tissue tropisms for gene therapy applications

Observational Bounds on Cosmic Doomsday

Recently it was found, in a broad class of models, that the dark energy density may change its sign during the evolution of the universe. This may lead to a global collapse of the universe within the time t_c ~ 10^{10}-10^{11} years. Our goal is to find what bounds on the future lifetime of the universe can be placed by the next generation of cosmological observations. As an example, we investigate the simplest model of dark energy with a linear potential V(\phi) =V_0(1+\alpha\phi). This model can describe the present stage of acceleration of the universe if \alpha is small enough. However, eventually the field \phi rolls down, V(\phi) becomes negative, and the universe collapses. The existing observational data indicate that the universe described by this model will collapse not earlier than t_c > 10 billion years from the present moment. We show that the data from SNAP and Planck satellites may extend the bound on the "doomsday" time to t_c > 40 billion years at the 95% confidence level.Comment: 11 pages, 6 figures, revtex

Criticality and Bifurcation in the Gravitational Collapse of a Self-Coupled Scalar Field

We examine the gravitational collapse of a non-linear sigma model in spherical symmetry. There exists a family of continuously self-similar solutions parameterized by the coupling constant of the theory. These solutions are calculated together with the critical exponents for black hole formation of these collapse models. We also find that the sequence of solutions exhibits a Hopf-type bifurcation as the continuously self-similar solutions become unstable to perturbations away from self-similarity.Comment: 18 pages; one figure, uuencoded postscript; figure is also available at http://www.physics.ucsb.edu/people/eric_hirschman