This Week in Science

Science  27 Oct 2000:
Vol. 290, Issue 5492, pp. 669
  1. Quantum Mechanics Gets Bigger

    The general impression we have of quantum mechanics is that it manifests itself on the microscopic scale, and that the quantum-mechanical effects of large-scale objects are so small that they can effectively be ignored. Van der Wal et al. (p. 773; see the Perspective by Tesche) report on the quantum superposition of two states of a superconducting loop placed in an applied magnetic field. The superposition consists of clockwise- and anticlockwise-flowing supercurrents established in the ring to counteract the magnetic field. Such large-scale quantum systems have potential applications in quantum computing.

  2. Running Rings in Superfluids

    A particle moving below a critical velocity through a superfluid can do so unhindered, but above the critical velocity a vortex, or a superposition of normal (viscous) and superfluid states, is created. Kivotides et al. (p. 777) report model results on the dynamics of the vortex evolution which suggest that the structure may be more complicated than initially thought. A triple-ring structure develops in which the superfluid vortex ring is accompanied by not one, but two normal-fluid vortex rings. This coupled ring structure is coherent and dissipative.

  3. Seeing Through Quantum Dots

    With their discrete energy levels and tunable occupancy, quantum dots have been shown to serve as ideal systems for investigating interactions between an individual electronic spin within an environment of conduction electrons. Such an interaction, the Kondo effect, manifests itself as an enhancement in the conductance of the dot. Yang et al. (p. 779) used a double-path electron interferometer to measure the phase evolution of the electrons as they traversed the dot to show that the phase provides a sensitive signature of the Kondo regime. The sensitivity of the phase measurement allows them to determine the binding energy of the singlet state.

  4. Tuning In to Energy Transfer

    The transfer of energy between molecules is an important process in many biological and solid-state systems and can occur via a number of different pathways. For example, when the separation of the molecules is relatively large compared to the wavelength of the light, energy transfer occurs via the emission and absorption of a photon. For small separations, however, transfer may be nonradiative. Andrew and Barnes (p. 785) show how this nonradiative, or Förster, transfer can be enhanced by limiting the number of photonic modes available to an emitted photon of the donor molecule. The result provides the potential for enhanced photon-harvesting systems and optical networks.

  5. GDNF to the Rescue

    The gradual loss of dopamine neurons in the nigrostriatal pathway of the brain causes the motor abnormalities characteristic of Parkinson's disease (PD). Although glial cell line-derived growth factor (GDNF) is known to sustain growth of dopamine neurons in culture, attempts to rescue dopamine nerve cell loss with GDNF in animal models of PD have proved disappointing. Kordower et al. (p. 767; see the Perspective by Olson) present encouraging results with GDNF gene therapy in two primate models of PD: aged rhesus monkeys and monkeys treated with the selective nerve toxin MPTP. GDNF protein was expressed in the nigrostriatal pathway for up to 8 months after delivery of the gene in a lentiviral vector to this area by direct injection. Production of GDNF prevented degeneration of dopamine neurons and prompted some regeneration, which correlated with amelioration of motor deficits, as measured by a hand reach test.

  6. Hurricane Aftermath

    Data on the long-term effects of hurricanes on vegetation are being accumulated from a number of sites in the tropics. In a 10-year study, Vandermeer et al. (p. 788) evaluated the effects on tree species diversity of Hurricane Joan (1988) in a Nicaraguan rainforest. They found an accumulation of species well beyond the species diversity of nearby undamaged forest in a manner that conforms to the “intermediate disturbance hypothesis” of diversity maintenance. Because of the large spatial scale of the disturbance, populations of pioneer species are recruitment-limited for a sufficiently long period that other species can gain a foothold and grow into the newly developing canopy.

  7. Tracking Salmon that Have Swum

    Salmon abundance has been declining recently in the Pacific Northwest, but the relative contributions of overfishing, degradation of habitat, and climate change have been uncertain, in part because we have lacked a long-term record of salmon abundance. Finney et al. (p. 795; see the news story by Brown) have now developed such a record based on nitrogen isotopes and diatom records from sediments in several lakes in southern Alaska that helps resolve these competing effects. The nitrogen isotope signature is dominated by input from spawning salmon that pick up most of their nitrogen in the ocean. When they die after spawning, their nitrogen input in turn fertilizes diatom growth. The record, which extends back about 300 years, reveals large decadal variations in salmon abundance that can be related to climate cycles and a more dramatic decline in the past century that reflects overfishing. The lack of nitrogen input from spawning salmon may be directly impacting ecosystems and further inhibiting the salmon's recovery.

  8. Split Cycle

    Caged hamsters usually run vigorously in their wheels once a day for a few hours, always at the same time, but sometimes this period of activity “splits” and they run twice a day, for shorter times, 12 hours apart. The cause of this splitting behavior has now been clearly visualized by de la Iglesia et al. (p. 799), who examined the biological clock located in two symmetrical groups of cells, the suprachiasmatic nuclei, on either side of the midline in the hypothalamus. When their cycles are revealed by staining for messenger RNA of the clock gene per in hamsters showing split running behavior, the two halves of the clock are exactly 12 hours out of phase with one another, each hemiclock likely controlling one bout of running per day.

  9. Signaling Through an "AND" Gate

    Control of cell shape and motility requires integration of multiple signals that ultimately influence polymerization of the actin cytoskeleton. The WASP protein (for Wiskott-Aldrich syndrome protein, defects in which cause thrombocytopenia, eczema, and immunodeficiency in humans) interacts with the small guanosine triphosphatase Cdc42 and phosphatidylinositol 4,5-bisphosphate (PIP2), which are both mediators of signaling pathways that cause alterations in the actin cytoskeleton. WASP also interacts with the actin-related protein 2/3 (Arp2/3) complex, which stimulates actin nucleation. Prehoda et al. (p. 801; see the Perspective by Fawcett and Pawson) examined how the WASP protein processes multiple inputs to coordinate the activity of Arp2/3 and actin polymerization. Their results indicate that N-WASP (neuronal WASP) exists in a “closed” state in which Arp2/3 is bound but inactive and the binding sites for Cdc42 and Arp2/3 are inaccessible. Binding of either Cdc42 or PIP2 appears to promote an active conformation. Activation by Cdc42 and PIP2 is highly cooperative, and thus WASP can function as a “coincidence detector” or “logical ‘AND’ gate” that is highly activated when it receives signals from both Cdc42 and PIP2.

  10. As the Worm Turns

    The availability of the complete genome sequence of the nematode, Caenorhabditis elegans, has made it possible for Hill et al. (p. 809) to follow gene expression in an entire multicellular organism during the course of development. The authors monitored changes in gene expression on microarrays containing 98% of the open reading frames of the nematode in oocytes, six stages of development, and aged worms. Over the course of development, there is a transition from evolutionarily conserved genes to worm-specific genes.

  11. Learning While Sleeping

    The songs sung by birds are learned and can be quite complicated. How do birds learn to produce consistent rhythmic patterns of sounds and to distinguish their own song from those of other birds of the same species and of different species? Dave et al. (p. 812) have succeeded in monitoring neuronal activity in the sensory-motor center (robustus archistriatalis) of zebra finches while the birds were singing and while they were asleep. Single unit recording revealed a remarkably similar pattern of action potentials when the bird was hearing its own song played back (auditory response) and when the bird actually sung (premotor activity). The temporal match suggests that these birds have implemented a mechanism for reinforcement learning with delayed reward and, in fact, may be rehearsing their songs in an “off-line” mode in order to become pitch-perfect.

  12. Giving Form to CTLA-4 Interactions

    Signals delivered by cell surface receptor CTLA-4 are critical for ensuring that the T cells do not overstep their mark when responding to antigen. How this is accomplished is not fully understood, but it involves—at least in part—direct inhibitory signals. The efficiency of these signals likely relies on the assembly of inhibitory signaling complexes at the interface between the antigen-presenting cell and the T cell. Ostrov et al. (p. 816) have solved the crystal structure of CTLA-4 and in so doing revealed how the organization of this receptor might facilitate the arrangement of such complexes. Their data suggest that CTLA-4 forms dimers that are unlike those made by other members of the immunoglobulin supergene family in that they can interact simultaneously with two of its natural B7 ligands. This organization could help to explain the proficiency of CTLA-4 in regulating the immune responses.

  13. Joining the Anvil Chorus

    Diamond anvil cells are routinely used to study materials under high pressure, not only because of diamond's hardness, but because its transparency allows many analytical techniques to be used for in situ studies. Xu and Mao (p. 783) have developed moissanite anvil cells (MACs) that use the synthetic mineral (hexagonal silicon carbide) as the pressure vessel. Although moissanite is softer than diamond, the synthetic crystals have several potential advantages. They are larger (which allows larger sample sizes), less expensive (which allows for more experiments), and provide a clear window over certain spectral regions where diamond peaks interfere with measurements (and thus allow superhard materials to be studied).

  14. Low Temperature Transfer?

    About a dozen martian meteorites have been found on Earth, and models suggest that they were ejected from Mars during large-impact events that heated the ejecta to high temperatures (above 1000°C). Weiss et al. (p. 791) analyzed the magnetic properties of iron oxides and iron sulfides in a sample of martian meteorite, ALH84001, using a new ultrahigh resolution magnetometer. Their results indicate that ALH84001 was not heated above 40°C before the impact event that transferred this material to Earth. Thus, a low temperature transfer of some martian ejecta, which would allow the survival of possible micro-organisms suggested to be present in ALH84001, needs to be considered in impact models.

  15. Divide and Diversify

    Diploid organisms have two copies of each chromosome in their cells and thus, in forming gametes, they must reduce this number to one copy per germ cell. The special division that achieves this reduction, meiosis, is very often accompanied by recombination between pairs of homologous chromosomes, before they are partitioned into the gametes. Borde et al. (p. 806) address the question of whether there is a link between the replication of the chromosomes at the beginning of meiosis and the recombination that happens shortly thereafter. Using yeast as a model system, they show that the time between replication and recombination is always constant (1.5 to 2 hours), even if different parts of a chromosome are induced to start replicating at different times. Thus, replication initiates a process that culminates in the exchange of sequences between sister chromosomes, a potent source of variation in the genome.

  16. Self-Similarity and Clustering in Species Distributions

    Condit et al. (Reports, 26 May, p. 1414), studying six different tropical forest sites, found that individuals of most tree species in the forests tended to be spatially aggregated rather than randomly distributed. Ostling et al., in a comment, observe that the clustering described by Condit et al. “is similar to that expected for a species with a self-similar spatial distribution.” They derive an expression for the principal clustering measurement used by Condit et al., relative neighborhood density, in terms of parameters related to self-similarity, and present quantitative comparisons showing a close fit between the species distributions reported by Condit et al. and those expected for plots with a self-similar distribution. Condit finds the comment by Ostling et al. “elegant and interesting,” and agrees that self-similarity underlies the observed patterns-but argues that self-similarity holds at the community level rather than the species level. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/290/5492/671a

Navigate This Article