This Week in Science

Science  19 Mar 1999:
Vol. 283, Issue 5409, pp. 1813
  1. A Sinking Slab of Crust?

    Global tomographic models have revealed velocity anomalies in the mantle that may represent remnants of subducted slabs. Kaneshima and Helffrich (p. 1888) used short-period seismic waves to image a small-scale, low-velocity layer in the lower mantle beneath the Mariana and Izu-Bonin subduction zones in the Western Pacific. The authors suggest that this small, previously undetected dipping layer [about 500 kilometer (km) by 300 km in extent but only 8 km thick] is an undeformed fragment of oceanic crust that was subducted about 170 million years ago along the paleo-Indonesian subduction zone. This interpretation implies that mantle mixing was too weak to destroy this small, isolated slab remnant.

  2. Chaos in the Outer Solar System

    Previous numerical simulations have showed that orbits could be chaotic for bodies in the outer solar system, but the mechanism for producing chaos was not determined and it was also possible that the simulated chaos was a numerical artifact. Murray and Holman (p. 1877) derived an analytical solution to the problem and showed in numerical simulations that chaos in the outer solar system can be caused by interactions between the orbits of Jupiter, Saturn, and Uranus or of Saturn, Uranus, and Neptune. The simulated chaotic zones are very narrow, so there is about a 20% chance that the jovian planets are currently in a chaotic zone.

  3. Molecular Chains

    Dipolar molecules might be expected to align and form chains if dipole-dipole interactions dominate. However, other forces, such as van der Waals forces, would favor the formation of more compact structures. Nauta and Miller (p. 1895) show that when the strongly dipolar hydrogen cyanide molecules self-assemble inside cold, superfluid liquid helium droplets, linear chains of up to eight molecules form. In the gas phase, such clusters are known to form more compact cyclical structures. It is unlikely that the weak interactions with the superfluid heliu. “solvent” change the relative stabilities of the structures significantly. Rather, the formation of structures that do not represent the global energy minimum is attributed to their assembly mechanism, which is strongly influenced by the dipole-dipole interactions.

  4. A Deeper Mantle Boundary?

    Geochemical data have implied that Earth's mantle is layered, with the boundary between an upper and a lower convective system marked by a change in seismic velocities at a depth of 660 kilometers. However, recent geophysical observations seem to show that subducted ocean crust penetrated this boundary and extended to much greater depths (see the news story by Kerr). Van der Hilst and Kárason (p. 1885) show that recent tomographic models imply that the seismic structure of the mantle changes at a much greater depth of 1600 kilometers. Kellogg et al. (p. 1881) present results from a series of models to argue that this depth may mark the convective boundary in the mantle.

  5. An Electron Heart

    A bias voltage is generally used to create electron currents in devices. Switkes et al. (p. 1905; see the Perspective by Altshuler and Glazman) abandon this usual method of bias-driven electron flow and report on a mechanism in which the electrons are pumped between two electron reservoirs by means of a periodic asymmetric deformation of an intervening quantum dot. In this system, the flow of electrons is not quantized but depends on the magnitude and frequency of the deformation.

  6. An Insulator for Gallium Arsenide

    Gallium arsenide (GaAs) has several properties that make it superior for optoelectronics compared to silicon (Si), including higher charge carrier mobilities and a wider, direct band gap that allows high-temperature operation. However, GaAs-based devices are less commonly used than their Si counterparts because of the notorious difficulties in fabricating defect-free insulating oxide barriers on GaAs. Hong et al. (p. 1897) describe the successful growth by molecular beam epitaxy of a thin (∼15 angstroms) gadolinium-based oxide on GaAs that has a low density of interface states, exhibits low leakage currents across the interface, and is stable against surface stress.

  7. Less Assembly Required

    The familiar liquid-crystal displays of laptop computers are often the costliest component because of the numerous steps associated with their fabrication. Vorflusev and Kumar (p. 1903) show that some steps could be eliminated by preparing liquid-crystal films through phase separation in a polymer-liquid-crystal mixture induced by ultraviolet illumination. In particular, the alignment of the liquid crystal can be achieved with a single substrate, whereas two are usually required. In addition, the resulting cells switched their display state faster than conventional cells and exhibited a gradation in their optical response.

  8. Forming Young Minds

    The formation of appropriate connections between neurons in the brain, for example, in the visual cortex, is influenced by a blend of nature (molecular specifications of neighbors) and nurture (incoming visual input after birth), but what of the connections between areas of the brain? Paus et al. (p. 1908) present structural magnetic resonance imaging data from a sample of 111 children, ages 4 to 17, and describe age-related changes in interareal fiber tracts, which were assessed as increases in white matter density. Maturation of the fronto-temporal pathway was significantly greater in the left hemisphere than in the right, a result consistent with the maturation of brain areas involved in language.

  9. Peptides Pattern Flowers

    The pattern of petals in a flower, and even the pattern of flowers in a cluster, is determined by the development of the shoot meristems. In the plant Arabidopsis, the CLAVATA1 gene, which encodes a receptor-like molecule, and the CLAVATA3 gene are both expressed during development of the meristem but in different layers of cells. Fletcher et al. (p. 1911; see the news story by Barinaga) show that the CLAVATA3 gene encodes a small protein that may serve as a messenger between cells, perhaps signaling to the CLAVATA1 protein found in neighboring cells. Thus, a peptide signal may be critical to pattern formation of tissues derived from the meristem.

  10. MHC Mimic

    How similar to a major histocompatibility complex (MHC) class I protein does a molecule have to be in order to look like one? Sánchez et al. (p. 1914) analyzed the crystal structure of ZAG, a protein that stimulates lipolysis in fat cells. Although only 30 to 40% identical in sequence, and unable to bind the class I light chain β2-microglobulin or peptides, ZAG retains the same general tertiary structure, down to a pocket available for ligand binding. This finding illustrates the versatility of the basic structure and raises questions about the function and evolution of the ancestral protein.

  11. Becoming an Animal

    The evolution of animals from protozoans involved several radical changes, including the formation of multicellular structures, the shift from radial to bilateral symmetry, and the development of digestive cavities. Ruiz-Trillo et al. (p. 1919; see the cover and the news story by Pennisi) have used molecular analyses to clarify the position of certain curiously primitive worms in this family tree. Previously thought to be secondarily simplified flatworms of the Platyhelminthes order, worms of the Acoela group are now seen to represent the earliest step in evolution of bilaterally symmetrical organisms.

  12. Switching Prion States

    The human prion protein, implicated in neurodegenerative disorders, is found in a nonpathological soluble conformation rich in α-helix and in a pathological, fibrillar conformation rich in β sheet. Jackson et al. (p. 1935) have identified conditions in vitro that allow soluble human prion protein to switch between an α-helix rich conformation and a β sheet rich-conformation. The retention of th. “pathological” conformation in solution may be important in understanding both the pathology and the biochemistry of this important protein.

  13. Dendrite and Neurite Dynamics

    As a neuron develops, one of its neurites elongates to become the axon, and the others become dendrites. Bradke and Dotti (p. 1931) found that neurites that had growth cones with low levels of assembled actin were most likely to become axons, and that by locally breaking down the actin cytoskeleton in a dendrite, could be induced to form an axon. Subsequent electrical activity can change the morphology and topology of the remaining dendrites. Maletic-Savatic et al. (p. 1923; see the Perspective by Smith) present evidence from two-photon scanning imaging, which shows that, after brief periods of high-frequency synaptic stimulation, the dendrites of postsynaptic neurons start to grow new filopodia. This sprouting requires stimulation at high frequencies and can be antagonized by blocking N-methyl-D-aspartate receptors, which links this response to activity-dependent neuronal development and functional plasticity.

  14. Hold the Holes

    Organic light-emitting devices were initially plagued with very low operating lifetimes. Eventually a number of methods that modified current injection at the transparent indium tin oxide contact (which introduces positive charge carriers, o. “holes”) were found and thus achieved long lifetimes (thousands of hours). Aziz et al. 1900 tested a number of device configurations and show that these approaches work by limiting the rate of hole transport so that the electroluminescent AlQ3 molecule [tris(8-hydroxyquinoline)aluminum] does not form its unstable cation, which degrades rapidly.

  15. Chemistry in Microflasks

    Kinetics studies can benefit from methods that allow small numbers of molecules to be isolated or that can rapidly mix reagents. Chiu et al. 1892 have used biomimetic containers, phopspholipd vesicles between 1 and 5 micrometers in diameter, to do both. Vesicles can be held in optical traps and their contents mixed by electroporation or electrofusion. The resulting reactions can be followed with laser fluorescence microscopy.

  16. Flies, Eyes, and Reliability

    Reliable behavior depends on reliable neuronal output, and the reliability of neurons depends on the variability of their responses to certain sensory inputs. Warzecha and Egelhaaf p. 1927 present recordings from a precisely defined nerve cell in the fly's visual system, the H1 neuron. The variance of its discharges was not proportional to the mean spike count, regardless of the dynamics of the stimuli, and the neuronal response variances were similar for different stimulus dynamics. These results question previous findings, which concluded that the H1 neuron shows a large response variability only after constant stimuli and that dynamic stimuli give more reproducible outputs.

  17. North American Carbon Sink

    S. Fan et al. (Reports, 16 Oct. p. 442) use. “atmospheric and oceanic carbon dioxide data and models” to estimat. “the spatial distribution of terrestrial carbon dioxide uptake.” They concluded that North Americ. “is the best constrained continent,” with a larg. “carbon sink.”

    E. A. Holland and S. Brown comment tha. “direct estimates of forest C uptake” suggest that the report may have overestimated the magnitude of the sink. C. S. Potter and S. A. Klooster also studied the question usin. “newly derived terrestrial C fluxes” predicted fro. “forward modeling” simulations and arrived a. “merely a transient C sink pattern in North America.”

    Fan et al. respond with a. “intercomparison” between recent studies of C uptake. They state that, unlike their report, the comment. “estimate terrestrial C uptake by invoking specific causes. The difference between [the various] estimates is the subject of intense research and may be resolved by a growing network of atmospheric and ecological observations.”

    The full text of these comments can be seen at

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