This Week in Science

Science  30 Sep 2005:
Vol. 309, Issue 5744, pp. 2133
  1. Jam Session


    The demixing of a binary fluid mixture in the presence of colloidal particles was studied by Stratford et al. (p. 2198; see the Perspective by Poulin) through computer simulations. The particles were chosen so that they exhibited neutral wetting with the two liquids and would remained trapped at the interface between the two liquid phases. As coarsening between the fluids proceeded, the interface becomes shorter and the particles became more concentrated and reached a jammed state. This phenomenon can arrest the phase separation and lead to a metastable bicontinuous gel.

  2. Dynamic Spin Control in Double Quantum Dots

    The coupling of electron spins between adjacent quantum dots can form the basis of a quantum logic gate. However, each electron on a dot couples to the large and random background field of about 1 million nuclear spins in the substrate, and these interactions lead to spin-state memory loss and mixing between spin-singlet and spin-triplet states. Recent work has looked at mitigating the spin-state mixing statically by controlling the coupling strength between quantum dots or by polarizing the background nuclear magnetic field. Using fast voltage pulses to control the exchange interaction between the electrons on adjacent dots, Petta et al. (p. 2180, published online 1 September 2005; see the Perspective by DiVincenzo) now show that dynamical coherent control of the spin states can also be achieved, which leads to a substantially increased lifetime of the prepared coupled spin states.

  3. Imaging Spin Transport

    “Spintronics” technology will use the spin state of electrons, rather than charge, to represent information, and will require a number of transport properties to be brought together. For example, it would be useful to be able to inject a spin-polarized current electrically with a ferromagnetic source contact, modulate the polarization of the propagating spin current with an electric field, and then detect the spin current with a ferromagnetic drain contact. Crooker et al. (p. 2191) report magneto-optical Kerr effect images of spin-polarized electrons in a lateral Fe-GaAs-Fe heterostructure, and provide a detailed account of the length scales governing the injection of spin-polarized electrons into the GaAs semiconductor layer.

  4. Instant Gratification


    The molecular chaperone Hsp90 allows various organisms to exploit existing genetic variation depending upon the prevailing environmental conditions. Cowen and Lindquist (p. 2185; see the Perspective by Heitman) establish a new role for Hsp90 in the evolution of adaptive traits. In fungal species separated by ˜1 billion years of evolution, Hsp90 potentiates the evolution of drug resistance by enabling immediate phenotypic consequences from new mutations. Increased temperature can abolish fungal drug resistance, which provides an explicit mechanism by which fever might be beneficial to the host. In fungal pathogens that are already recalcitrant to antifungal therapy, inhibiting Hsp90 improves response to treatment and, if given in the initial stages of therapy, may impede the de novo evolution of drug resistance.

  5. Winding Down

    Low-mass stars like the Sun form with their surfaces rotating rapidly, but the rotation slows over time because of magnetic braking and momentum exchange that creates internal velocity gradients. Models of these velocity patterns are in conflict with helioseismology as well as with observations of the element lithium at the stellar surface. Charbonnel and Talon (p. 2189) report a model that correctly accounts for both the rotation patterns and lithium abundance in Sun-like stars. The best model incorporates internal gravity waves, much like those responsible for Earth's alternating easterly and westerly zonal winds called the quasibiennial oscillation.

  6. Mammals, Oxygen, and Oceans

    The atmospheric concentration of O2 has varied considerably during the past 205 million years, rising irregularly from around 10% at the beginning of the Jurassic to 21% today, with a maximum of more than 23% during the Tertiary. How might these changes have affected the evolution of animals? Falkowski et al. (p. 2202) used their carbon isotopic measurements of carbonates and organic matter, along with published records of sulfur isotopes, to produce a high-resolution reconstruction of atmospheric O2 concentration since the early Jurassic. They find that O2 levels approximately doubled over the course of their record, in association with enhanced burial of organic matter on continental shelves resulting from the formation of passive continental margins during the opening of the Atlantic Ocean. There were relatively fast changes in the Jurassic and since the start of the Eocene. The authors suggest that the rise of O2 levels was a key factor in the evolution, radiation, and the increase in average size of placental mammals since the mid-Cretaceous.

  7. Keeping Up Appearances

    Despite the hundreds of studies of mating systems in socially monogamous vertebrates, little is known about the decision rules that drive females' allocation of paternity to their social, versus extra-pair, mates. These decision rules underlie the control and function of the variable reproductive strategies that are prevalent in nature. In a field population of barn swallows (Hirundo rustica), Safran et al. (p. 2210) analyzed genetic measures of paternity before and after a known signal of male quality (plumage coloration) was manipulated. The females shift paternity to more colorful males, which suggests the presence of continual, flexible decision rules for paternity allocation. Thus, it is important for male birds to maintain their signals of quality even after they form a pair bond.

  8. Retinoic Acid Responder


    Retinoic acid causes changes in gene expression that are essential for development of spinal motor neurons in the chick. Rao and Sockanathan (p. 2212) now find that glycerophosphodiester phosphodiesterase 2 (GDE2) shows increased expression in response to retinoic acid. In developing embryos, GDE2 was necessary and sufficient to promote differentiation of motor neurons.

  9. Rev1 Rescues Replication

    To maintain the fidelity of stored DNA codes, DNA polymerases use the complementarity of the nucleotide bases to ensure the correct incorporation of the incoming base against the template base: A with T, G with C, and so forth. Nair et al. (p. 2219) now show that unlike other polymerases, the highly specialized Y family polymerase Rev1 does not use the complementarity of the template G to incorporate the incoming C. Rather, the protein itself specifies the identity of the incoming base: Both the template G and incoming C are bound to the protein, and not to each other. In this way, Rev1 can replicate through damaged G residues that would otherwise stop the processing of replicative polymerases. Thus, Rev1 can rescue the genome from further potentially lethal damage.

  10. Keeping Options Open

    The brain's visual cortex is normally constructed to balance inputs from both eyes. When input is unbalanced during an early critical period, such as when vision from one eye is blocked, the visual cortex adjusts accordingly. However, the critical period is finite. Beyond this time of juvenile flexibility, the cortex cannot readjust to unbalanced visual inputs. McGee et al. (p. 2222; see the news story by Miller) now find that mutations in the Nogo-66 receptor (NgR) can keep the ocular dominance critical period in mice from closing. Closure of the critical period for whisker barrel fields is not affected by NgR mutations, which suggests that there may be more than one mechanism governing the extent of different critical periods.

  11. To Neglect or Not to Neglect…

    Unilateral neglect patients usually ignore events in one-half of the world around them. Thiebaut de Schotten et al. (p. 2226; see the Perspective by Gaffan) used intraoperative direct intracranial stimulation to assess the role of cortical and subcortical areas in attentional neglect. Two patients undergoing surgery for tumor resection were subjected to direct electrical stimulation of areas in the parietal and temporal lobes (lesions of which have been implicated in attentional neglect), as well as in an underlying region of subcortical white matter. Stimulation of the supramarginal gyrus and the caudal superior temporal gyrus produced behavior typical for unilateral neglect. The most profound effect was observed during stimulation of an area of underlying white matter that corresponded to the superior occipitofrontal fasciculus that connects the parietal and the frontal cortex.

  12. Restricted Activities of the Sleeping Brain

    The departure of consciousness as we experience “the death of each day's life …” has puzzled neuroscientists, who have noticed little change in cortical neuron firing rates between quiet wakefulness and non-REM (rapid eye movement) sleep. Massimini et al. (p. 2228) now can assess whether the directional connections between brain areas might weaken with the onset of sleep. They applied transcranial magnetic stimulation (TMS) to the premotor area and monitored neural activity in the whole brain with electroencephalography. TMS-evoked activity, which spread to distant cortical areas when subjects were awake, remained locally confined after they fell asleep.

  13. Looking Out Below

    Transmission electron microscopy can be used to study embedded microstructures, but it is a laborious process that requires very careful specimen preparation. Arslan et al. (p. 2195) show that Z-contrast tomography in a scanning transmission electron microscopy can reveal embedded structures with a resolution approaching one cubic nanometer. They track the formation of tin quantum dots inside a silicon matrix that form during the phase separation of the tin. Silicon atoms diffuse into the tin-containing layer and leave behind vacancies that cluster into voids that have a truncated octahedron shape. Tin atoms then fill the voids. By examining the growth of one of these void-filling quantum dots that lie between the tin-bearing layers, they determine the particular mechanism of growth of the dots.

  14. Acid Test

    Higher surface-ocean temperatures, as well as higher concentrations of dissolved CO2 (which make the ocean more acidic), can be harmful to corals, but at what levels those two effects are dangerous is not well understood. Clues can be found from studies of their past variability, both to see how resilient are corals to change and to understand how natural cycles might aggravate or ameliorate future anthropogenic trends. Pelejero et al. (p. 2204) analyzed the isotopic composition of boron in coral from the southwestern tropical Pacific to reconstruct the natural variability of seawater pH for the past 300 years. Large variations of pH occurred with a strong 55-year periodicity similar to that of the weather pattern called the Interdecadal Pacific Oscillation.

  15. Evolving Methodology

    The study of evolutionary relationships between biological phenomena—organisms, molecules, regulatory systems, and so forth—can provide significant insight, but the methods for unraveling such relations are far from foolproof. Mossel and Vigoda (p. 2207) consider theoretical phylogenetic analyses using Bayesian inference and the popular Markov chain Monte Carlo algorithms. For a mixture of characters from two phylogenetic trees, they find that the Markov chains can rapidly converge on an erroneous tree, and convergence on the correct tree is very slow. Thus, for some mixtures, the data should be decomposed into nonconflicting signals and treated separately.

  16. Understanding Regioselective Halogenation

    The biosynthetic pathways of many natural products involve flavin adenine dinucleotide (FAD)-dependent halogenases, but how these enzymes catalyze regioselective halogenation has been unclear. Dong et al. (p. 2216) provide insight into the mechanism based on the structure of tryptophan 7-halogenase (PrnA), which regioselectively chlorinates tryptophan. Within the enzyme, tryptophan and FAD are separated by a tunnel 10 angstrom in length and bound by distinct enzyme modules. The authors suggest that FAD is involved in forming HOCl, which then moves along the tunnel toward the substrate tryptophan. The HOCl is likely to be spatially constrained and activated by residues in the tunnel to achieve regioselective chlorination.

  17. Similar, But Not the Same

    Inositol 1,4,5-trisphosphate (IP3) is an intracellular second messenger that causes release of calcium from intracellular stores in response to signals sensed by receptors on the cell surface. One of the subtypes of receptors for IP3, IP3R1, is expressed predominantly in the brain and is important for motor functions and learning. The physiological roles of the other two, IP3R2 and IP3R3, which are widely expressed in mammalian tissues, have been unclear. Futatsugi et al. (p. 2232) studied knock-out mice lacking one or both proteins. The animals appeared to be similar at birth, but within 4 weeks, double-knock-out animals lost weight and died. These animals had markedly impaired exocrine secretion in the salivary gland and pancreas, where these receptors have a key role in calcium signaling that regulates secretion.

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