This Week in Science

Science  21 Sep 2001:
Vol. 293, Issue 5538, pp. 2165
  1. All-in-One Dot Characterization

    The nanoscopic size and discrete energy levels of quantum dots make them attractive candidates for very high density optical components. However, obtaining information simultaneously on both the spatial location of excitations in a quantum dot and their spectral properties can be challenging. Guest et al. (p 2224; see the Perspective by Koch and Knorr) have overcome this limitation by combining the spatial sensitivity of scanning near-field optical microscopy with the spectral sensitivity of a nonlinear optical pump-probe technique. They can access and excite individual energy levels of the quantum dot and monitor the dynamics of such excitation processes.

  2. Mesowires for Hydrogen Detection

    Submicrometer wires of palladium embedded in a polymer matrix are shown to decrease their resistance at room temperature in the presence of hydrogen, unlike bulk palladium, which becomes more resistive as the hydride forms. Favier et al. (p. 2227) find that hydrogen adsorption lowers the resistance by expanding microcrystallites in the wires, which then heals tiny “break junctions” in the wire. These effects are seen even when gases that normally poison hydrogen sensing were present, including O2, CO, and CH4. The rapid response and low power requirements suggest their use as sensors and switches.

  3. Coupled Kondo Systems

    Quantum dots provide an ideal system for investigating the many-body Kondo effect, in which the interaction of a magnetic impurity with a surrounding sea of electrons enhances conduction through the dot that would otherwise be suppressed by Coulomb blockade. However, when quantum dots are coupled, the Kondo effect must compete with spin-spin interactions. Jeong et al. (p. 2221) present an experimental study of a coupled pair of quantum dots which shows that the transport through the system can be explained through a simple model involving the spin configuration on each dot. They also observed a splitting of the Kondo resonance peak at zero bias which suggests that many-body bonding-antibonding Kondo states form in this artificial molecule.

    CREDIT: JEONG ET AL.
  4. A Primitive Messenger

    The Tagish Lake meteorite is an unusual carbonaceous chondrite that was recovered from a frozen lake in Canada soon after its fall and before significant terrestrial alteration could occur. Two reports now provide important information about the origin of the meteorite and its well-preserved carbonaceous material. Hiroi et al. (p. 2234) compared reflectance spectra of the Tagish Lake meteorite with those of different types of asteroids. The D-type asteroids provided the best spectral fit; these asteroids are located near the Kirkwood gaps in the main asteroid belt. Some of these asteroids may be driven into the gaps as a consequence of gravitational interactions with Jupiter. As a result, their orbits would become chaotic, leading to collisions that could transfer material to the inner solar system and eventually to Earth. Pizzarello et al. (p. 2236) analyzed the organic matter in the Tagish Lake meteorite and found it to be dominated by water-soluble carboxyl and dicarboxyl compounds, to be relatively low in amino acids, and to contain insoluble carbon with aromatic character. This composition is distinct from those of other carbonaceous chondrites and implies an origin on a primitive parent body that had accreted solar nebular and interstellar material. Thus, the organic matter may represent the least altered material from the early solar nebula currently accessible to terrestrial scientists.

  5. Whale Ankles

    Whales, the largest marine mammals, are thought to have evolved from the land to the sea about 40 to 50 million years ago. Gingerich et al. (p. 2239; see the cover and the Perspective by Rose) have found two new species of early whales, Artiocetus clavis and Rodhocetus balochistanensis, in marine deposits from the Tethys Sea in Pakistan. The ankle bones of these species are similar to those found in artiodactyls, suggesting that the whales belong to Artiodactyla rather than Mesonychia. In addition, the fore and hind limbs of Rodhocetus indicate that it was a paddler in the shallow seas of the Tethys.

  6. Climate and Tropical Speciation

    Early models suggested that the high species richness of the tropics was the result of a gradual accumulation of species in a stable climate. Evidence that tropical climates have not remained stable, especially during the last 2 million years of the Pleistocene, led to the idea that tropical diversity might have originated recently and that speciation has been driven by environmental instability. Richardson et al. (p. 2242; see the Perspective by Bermingham and Dick) show that the model of recent speciation is far more accurate for Inga, a species-rich genus of neotropical rain forest trees. Nuclear and plastid DNA sequences and a molecular clock approach suggest recent, rapid diversification in Inga that resemble recent radiations of plant species on oceanic islands.

  7. One Out of Many

    Can many small patches of suitable habitat really act as a large patches if they are connected by corridors and “stepping stone” patches? Hale et al. (p. 2246; see the news story by Kaiser) analyzed DNA microsatellite variation to show the use of stepping stone patches of habitat by the red squirrel in northern Britain and dispersal through a patchy habitat during a 50-year period by a species that evolved in an unfragmented landscape. Samples of red squirrels were obtained before and after patches of a new forest were planted to connect isolated populations from northern England and southern Scotland. The squirrels used the patches of new forest as stepping stones for dispersal from the southeast of Scotland to the northwest of England.

  8. Burning Evenly

    From sloths to squirrels, from weed to hibernating polar bears, the world of bioorganisms seems to present a wide diversity of metabolic rates. Gillooly et al. (p. 2248; see the news story by Brown) have analyzed measurements of metabolism among plants, animals, and microbes to derive a general model that describes the factors determining metabolic rate. Within a range of biologically relevant temperatures, metabolic rates are predominantly determined by temperature and body mass. Even unusual states, such as hibernation, thus need not invoke additional mechanisms for adjustment of metabolic rates.

  9. Phantom Mendelian Inheritance

    Bardet-Biedl syndrome (BBS) is an inherited disorder characterized by a variable phenotype that can include defects in vision, polydactyly, obesity, and learning disabilities. Historically, BBS has been viewed as a classical Mendelian recessive trait in which mutations at a single genetic locus suffice to cause disease. Katsanis et al. (p. 2256; see the Perspective by Burghes et al.) now show that in at least 40% of BBS patients, manifestation of the syndrome requires mutation of three alleles at two different genetic loci. This work blurs the well-defined boundary between Mendelian traits and complex traits such as diabetes, which are caused by the interaction of multiple genes, and will likely stimulate a re-examination of theoretical models of inheritance.

  10. Analyzing the Staph

    Staphylococcus aureus is a common cause of infection for which there are diminishing therapeutic options. By combining two existing technologies in a new way, Ji et al. (p. 2266) developed a library of antisense RNAs that are inducible with tetracycline, which means the respective genes can be turned on and off not just in vitro but also in infected mice. The roles played by specific genes in bacterial growth and behavior can be examined in complex ecologies, such as at the site of infection, and thereby clinically relevant drug or vaccine targets can be identified.

  11. Making Signals Stick

    In T cells, adapter proteins couple the T cell receptor (TCR) to multiple cellular signaling pathways. T cells also depend on cellular adhesion through integrins to perform many of their tasks, and Peterson et al. (p. 2263) and Griffiths et al. (p. 2260) show that the adapter SLAP-130/Fyb is directly involved this process. Using mice genetically deficient SLAP-130/Fyb, both groups observed that this protein was not required for inducing TCR-mediated calcium flux and protein tyrosine kinase activation, but was needed for clustering and activation of the integrin, LFA-1, which led to cellular adhesion. This process occurred in the presence of normal TCR-induced actin polymerization, which suggests that SLAP-130/Fyb couples TCR-dependent motility of the cytoskeleton with integrin activation.

    CREDIT: GRIFFITHS ET AL.
  12. The Independent Dendrite

    The extensive dendritic trees of neurons are by no means passive receivers but are actively involved in processing and transforming synaptic inputs. Wei et al. (p. 2272) examined the passive and active transformations that an individual dendritic terminal segment performs on its excitatory inputs. By combining electrophysiological recordings at the cell bodies of hippocampal pyramidal neurons in organotypic cultures and visually guided local application of caged glutamate over dendritic arbors, they showed that distal arbors may generate an all-or-none regenerative response that spreads in a restricted dendritic region. These results now demonstrate directly that dendritic trees can be partitioned into independent functional subunits and that voltage-dependent calcium channels play a crucial role in this compartmentalization.

  13. Volcanic Pressure Redirects Shear Waves

    Shear-wave anisotropy in the crust normally shows the faster moving component directed parallel to the maximum compressive stress. Miller and Savage (p. 2231) observed the faster wave directed perpendicular to the maximum stress on Mount Ruapehu, New Zealand, during its 1994 eruption. An increase in pore fluid pressure or overpressurization of the magma reservoir explains the change and suggests that localized volcanic stresses can overcome the regional stress field. Changes in anisotropy could therefore be used to predict eruptions.

  14. Dicer in RNAi and Development

    RNA interference (RNAi) is the process by which double-stranded RNA induces sequence-specific posttranscriptional gene silencing. The enzyme Dicer has been implicated in the generation of short interfering RNAs (siRNAs), which are thought to be the effectors of the gene silencing. Knight and Bass (p. 2269) now provide genetic evidence in Caenorhabditis elegans that the ribonuclease III activity of Dicer is important for RNAi in vivo and that mutation of the dicer-1 gene causes sterility. Thus, Dicer seems to have a role in germ line development, or physiology, or both.

  15. Not Following the Clock

    Rodents and flies living in constant darkness are physically active for a period of time each ∼24-hour period, a cycle that is driven by the animal's endogenous molecular clock. Williams et al. (p. 2251) report that fruit flies with a null mutation in neurofibromatosis-1 (Nf1) show defects in rhythmic behavioral outputs and in the mitogen-activated protein kinase (MAPK) pathway, but not in the clock itself. The authors propose that one of the downstream genes of the clock in flies is Nf1, and that Nf1 signals through the Ras/MAPK pathway, possibly triggered by pigment dispersing factor (PDF). These results, which now partly define the output pathway for the circadian clock in flies, may also illuminate the mechanism by which human Nf1 acts as a tumor suppressor.

  16. Roots of Clinical Resistance to STI-571 Cancer Therapy

    Studying 11 patients with chronic myeloid leukemia who had relapsed after initially successful treatment with the kinase inhibitor STI-571, Gorre et al. (Reports, 3 August 2001, p. 876) tied drug resistance to “progressive BCR-ABL gene amplification” in three patients and to a point mutation in the BCR-ABL kinase domain in six patients. In separate comments, Barthe et al. and Hochhaus et al. report the results of two studies that were unable to find the point mutation cited by Gorre et al. in any of a total of 44 relapsed patients. Gambacorti-Passerini et al., in a third comment, argue that the Gorre et al. focus on “cellular” mechanisms gave insufficient weight to “organismic” explanations of STI-571 resistance, such as the binding and inactivation of the drug by α1 acid glycoprotein. Gorre et al., in their response, raise questions about the alternative model suggested by Gambacorti-Passerini et al., and suggest that the alternative results reported by Barthe et al. and Hochhaus et al. may trace to “differences in the patient population studied or the methods used to detect the mutation.” The full text of these comments can be seen at

    www.sciencemag.org/cgi/content/full/293/5538/2163a

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