This Week in Science

Science  17 Jan 2003:
Vol. 299, Issue 5605, pp. 305
  1. Poynting Out the Aurora's Power Source

    The source of energy to power Earth' auroras is poorly understood. Based on observations by the Polar spacecraft, Keiling et al. (p. 383) suggest that the Poynting flux due to electromagnetic waves, also called Alfvén waves, at high altitudes above the ionosphere contribute energy to power the auroras near the surface.

    KEILING ET AL.
  2. A Lighter Light Source

    It has been suggested that ultraluminous x-ray sources are powered by black holes that have masses of one hundred to several thousand times that of the Sun. Kaaret et al. (p. 365) used x-ray, optical, and radio wavelength observations to show that an ultraluminous source in the dwarf irregular galaxy NGC 5408 is a beamed, relativistic jet. Because the luminosity is beamed, the source could be a stellar-mass black hole rather than something more massive. Nevertheless, these results help explain relativistic processes as well as the distribution and amount of black hole mass contributing to the universe.

  3. A Wet Electrical Switch

    Changes in the properties of a surface, such as hydrophobicity, have been achieved with photochemical reactions or by rearrangements in the material induced by changes in pH or temperature. Lahann et al. (p. 371, see the news story by Service) have switched surface properties electrically without a change in the surface chemistry. They fabricated a self-assembled monolayer that initially had a low surface coverage; once electrically stimulated, the surface molecules underwent a conformational change that exposed different functional groups and that altered the wettability of the surface.

  4. Radiating Photonic Crystals

    One way to detect the very energetic particles emitted in subatomic processes is to follow their Cerenkov radiation, which is emitted when the particle moves through a medium faster than light itself can move because of slowing caused by the material' refractive index. Luo et al. (p. 368) show that photonic crystals, structures of periodic refractive index contrast, should exhibit quite different behavior. Their calculations reveal that radiation should be emitted even when the particles travel below the threshold velocity and that, for certain velocity regimes, the radiation can be emitted backward, rather than in the typical forward direction.

    CREDIT: LUO ET AL.
  5. Getting Around on the Ground

    Two reports focus on the mechanics of walking and running in mammals and birds, respectively. The functions of the epipubic bones and associated abdominal muscles of primitive mammals have offered anatomists a conundrum: while hypothesized to support the pouch of female marsupials, their presence in males and other, nonmarsupial primitive mammals remained unexplained. Reilly and White (p. 400) present evidence that the epipubic bones in marsupials transmit forces between the limb and the body wall during the stance phase. These results establish a functional continuity between primitive mammals and other amniotes, and suggest that the epipubic bones will be a useful tool in reconstructing gaits in extinct vertebrates. Dial (p. 402, see the news story by Pennisi) reports data on wing-assisted incline running, exhibited by many ground birds. Experiments using different textured running surfaces showed the importance of hindlimb traction for birds as they flapped their wings (whether fully feathered, partially feathered, or featherless) in order to ascend inclined obstacles. The acceleration vector imparted by the flapping wings to the body and legs while running uphill acts like the spoiler on a race car to improve traction, rather than to propel the animal skyward. This behavior may provide insight into the possible role of incipient wings in feathered theropod dinosaurs and the evolution of avian flight.

  6. Ancient Ant Farm Pests

    The attine ant-microbe symbiosis involves at least four diverse groups: ants, their cultivated fungi, specialized pathogens of the fungal cultivars, and antibiotic-producing filamentous bacteria. Two of these symbiont groups, the ants and the fungal cultivars, have coevolved for more than 50 million years. Currie et al. (p. 386, see the news story by Pennisi) show that the pathogen interaction is similarly ancient and likely was introduced into the symbiosis when the ants first domesticated the fungal cultivars. There appears to be an ongoing tripartite “arms-race,” with this specialized parasite on one side and the three mutualists on the other.

  7. A Stone's Throw

    The origin of stone circles and streaks that form in alpine and polar regions has been problematic. The sorting is clearly related to repeated freezing of the soil, and previous models have mimicked specific occurrences but not the range of forms observed. Kessler and Werner (p. 380, see the cover and the Perspective by Mann) present a numerical model that captures many aspects of their formation, including how the effects of slopes, stone size, and stone concentration, among other factors, result in different patterns. Their model is based on the interplay of two physical processes that sort stones into rows and then move them along rows and patterns.

  8. Cat and Mouse

    Major changes in the complex life cycles of parasites can evolve rapidly and can dramatically affect host range. Su et al. (p. 414, see the Perspective by Volkman and Hartl) resolve a long-standing question about the unusual population structure of the ubiquitous parasite Toxoplasma gondii. The current dominance of the three predominant clonal lineages of T. gondii is associated with their relatively recent acquisition of oral infectivity, which has allowed rapid spread through many hosts, including humans.

  9. Sharks Attacked

    Sharp reductions in the populations of large predatory species, especially sharks, have taken place over the past two decades in the Northwest Atlantic. Using data from longline fishery records, Baum et al. (p. 389) show that populations of several shark species have been reduced to less than a quarter of their former size, a decline that may herald further major changes in the oceanic food web. Current marine reserves are insufficient to halt this trend, and only the establishment of further reserves in concert with further controls on fishing will prevent extinction.

  10. Blurring of the Pathways

    Apparent motion, such as when flickering lights create the illusion of a moving object, has traditionally been considered to be processed in the “where” pathway of the visual system. A functional magnetic resonance imaging study by Zhuo et al. (p. 417) shows instead that long-range apparent motion primarily activates the anterior temporal lobe, a component of the “what” pathway. This result suggests that long-range apparent motion is associated with brain systems for form perception and argues against the widespread assumption that figural properties are of little importance in determining apparent motion.

    CREDIT: ZHUO ET AL.
  11. Tannin Synthesis

    A long-standing puzzle has been which biochemical pathways and enzymes synthesize condensed tannins, which contribute to food flavor, human health, and cattle digestion. Xie et al. (p. 396, see the Perspective by Bartel and Matsuda) use a heterologous metabolic profiling approach to determine that the BANYULS enzyme of Arabidopsis and Medicago plants functions as an anthocyanidin reductase, which converts anthocyanidins to flavan-3-ols that then serve as building blocks in the polymerization process leading to condensed tannins.

  12. Forming a Maternal Attachment

    The process of embryo implantation during mammalian reproduction involves signaling between the embryo and the uterus. Genbacev et al. (p. 405, see the Perspective by Fazleabas and Kim) report that selectin adhesion systems, which operate in leukocyte capture from bloodstream, function during embryo implantation, too. On the maternal side, selectin oligosaccharide ligands are expressed in the uterine epithelium, and on the fetal side, trophoblast cells express L-selectin receptors. In both mouse and human models, this receptor-ligand pair is shown to mediate successful uterine attachment.

  13. Biting in the Middle

    The proteasome is a cylindrical molecular machine that promotes the proteolysis of a wide variety of cellular substrates. Its architecture is well understood at the molecular level, and it has been assumed that substrates are threaded into the cylinder from their termini for cleavage. Liu et al. (p. 408) have found that in vitro the proteasome can cleave internal sites within a protein, and can even cleave circular substrates.

  14. Biased Defect Diffusion

    Many of the remarkable properties of transition-metal oxides such as TiO2 arise from oxygen vacancies at their surfaces. Schaub et al. (p. 377, see the Perspective by Campbell) examined the diffusion of such defects on the (110) surface of this material and found that defect diffusion is mediated by adsorbed O2. One oxygen atom fills the defect while the other grabs an oxygen atom from the lattice. The net result is that defects move across rather than down the rows.

  15. Power Steering

    In nonlinear optical materials such as lithium tantalate, the application of a sudden laser pulse at the surface of the crystal can launch surface waves (phonon-polariton waves) that travel at an appreciable fraction of the speed of light. Feurer et al. (p. 374) report an approach, similar to phased arrays in radar technology, that operates at lower frequencies for the steering and focusing of terahertz waves. Several points of femtosecond excitations are focused onto the sample through a pulse shaper, and interference effects are used to create complex wavefronts. Such methods could prove useful in the area of terahertz signal processing.

  16. Traffic Between Plant Cells

    Plant cells exchange cytoplasmic contents through specialized channels, plasmodesmata, that connect from one cell to the other. Traffic through these plasmodesmata can be regulated according to the size and identity of the transported components. Lee et al. (p. 392) have identified a protein in tobacco plants, NCAPP1, that is discriminating in its interaction with the proteins traveling through the plasmodesmata. Deletion mutations of NCAPP1 disrupted plasmodesmatal trafficking, with consequences for normal leaf and floral development. NCAPP1 is localized to the endoplasmic reticulum, which connects to the plasmodesmata, and may serve to deliver cargo.

  17. A Double Knockout of Reactive Epitopes

    The scarcity of appropriate human organs for transplantation has driven interest in pig-to-human xenotransplantation. However, the presence of α-1,3-galactosyltransferase in pig cells synthesizes α-1,3Gal epitopes that often lead to rejection. Although there have already been reports of knockouts of one of the two copies of the gene, a relatively fast and easy way to generate pigs whose cells would not evoke this immune response is needed. Phelps et al. (p. 411) started with heterozygous α-1,3Gal fibroblasts and selected for cells totally lacking the gene by means of a bacterial toxin (toxin A from Clostridium difficile) that kills cells with the epitope. Three rounds of cloning based on cells selected in this way resulted in four healthy double-knockout female piglets that lacked the reactive epitopes on cell surfaces. The inactivation was the result of a point mutation.

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