This Week in Science

Science  17 Mar 2006:
Vol. 311, Issue 5767, pp. 1517
  1. Starting Statues Sooner


    When Dutch sailors arrived on Easter Island in 1722, they encountered a famished population of Polynesians living on a denuded landscape marked by giant stone statues. It has been generally assumed that colonists arrived on the island between about 400 and 1000 A.D.; only later, around 1200 A.D., did they erect the statues and cleared the once-abundant forests. Hunt and Lipo (p. 1603) present radiocarbon dates from a recent excavation on Easter Island and analyze previous dates from other sites. Their dates and analysis imply that colonization occurred near the time of statue construction. If so, then irreversible deforestation may have started immediately after the Polynesians arrived.

  2. Mobility for Artificial Muscles

    Electrically powered motor or actuators can serve as artificial muscles in robots or prosthetic limbs, but significant “down times” will likely occur if their power needs are met by rechargeable batteries. Ebron et al. (p. 1580; see the Perspective by Madden) demonstrate two alternative approaches that use fuel cells. In one approach, a catalyst containing carbon nanotubes acts as muscle, fuel cell electrode, and supercapacitor electrode in a hydrogen-fueled system. In the other approach that can be fueled by hydrogen, methanol, or formic acid, a shape-memory alloy is used; this artificial muscle achieves actuator stroke and power density comparable to that of natural skeletal muscle and generates stresses that are one hundred times greater.

  3. Sporadic Spokes

    Dark radial streaks or spokes in Saturn's main B-ring were first seen with the Voyager space probes, and later by the Hubble Space Telescope. In 1998, they faded from view from the Earth as Saturn's rings became oriented edge on. Contrary to expectations, the spokes remained absent even when the Cassini spacecraft flew close to the rings in 2004 but then reappeared faintly in September 2005. These latter findings suggested that the spokes are intermittent features whose presence depends on the rings' angle to the Sun. Mitchell et al. (p. 1587) use Cassini data to model the formation of spokes as charged dust particles are lifted into the plasma above the ring plane by electrostatic forces. They find a sharp switch in the spokes' visibility, such that they disappear abruptly when the rings are open to the Sun, and also predict when the spokes are likely to appear clearly.

  4. Deflection Detection

    A promising approach for highly sensitive detection of biomolecules makes use of microfabricated cantilevers decorated with receptors or other molecules that would bind a molecule of interest. Binding creates a surface stress that deflects the cantilever. However, this deflection is small (on the order of tens of nanometers), and the methods used to date (optical, capacitive, and piezoelectric) have various limitations. Shekhawat et al. (p. 1592, published online 2 February) show that they can build a field-effect transistor into the cantilever that responds to surface stresses. Detected deflection changes of ∼5 nanometers can be followed and allows detection of biotin and antibodies.

  5. Construction in Tight Spaces


    Forming high-aspect-ratio metal or semiconducting wires can be difficult because the main fabrication technique, chemical vapor deposition (CVD), does not work well when filling long narrow channels. Sazio et al. (p. 1583) have developed a modified CVD process that allows for the integration of functional materials within an optical waveguide, which can tolerate a much higher pressure CVD process. Specifically, metals and semiconductors with lateral dimensions down to a few nanometers are formed within microstructured optical fibers.

  6. The Ringdown Cycle

    The use of spectroscopy for chemical analysis often requires tradeoffs between bandwidth (how much of the spectral range is being recorded), resolution, and data acquisition speed. For example, in cavity-ringdown spectroscopy (CRDS), adsorption by molecules depletes light that is bouncing back and forth in an optical cavity, and the light adsorption curve can provide extremely high detection limits. However, the range of frequencies that can be followed is limited. Thorpe et al. (p. 1595) created a broadband version of CRDS by coupling an optical frequency comb to a high-finesse optical cavity whose mirror position could be finely adjusted, and followed the simultaneous decay of numerous ringdown modes. They obtained spectral data across a 100-nanometer wavelength range in the visible and near-infrared for species such as water and ammonia.

  7. Observing Proteins One by One

    Detection of single messenger RNA (mRNA) molecules has led to exciting insights into gene expression in live cells. Yu et al. (p. 1600) have developed a method to image single protein molecules in living Escherichia coli cells. They expressed a membrane-targeted version of yellow fluorescent protein (YFP) and, under repressed conditions, detected individual membrane-localized YFP molecules as they were being synthesized. The protein molecules were expressed in bursts, and each burst originated from a stochastically transcribed mRNA molecule. The technique may make it possible to study the dynamics of the many proteins present in low numbers per cell.

  8. A Catalog of Avian Flu

    Large-scale sequence analysis of avian flu isolates based on 4339 virus genes from many wild birds confirms long-known facts of flu biology, such as the variability of hemagglutinin and neuraminidase sequences, the frequency of reassortment, and the restricted compatibility of internal virion subunits. Obenauer et al. (p. 1576, published online 26 January; see the Perspective by Krug) have developed the means to characterize these viruses by a technique they term “proteotyping” and use the method to identify specific combinations of genes and gene products that travel together. They also identified a previously overlooked motif that appears to correlate closely with virulence, at least in strains of avian origin.

  9. Higher Brain Functions in Primary Visual Cortex

    According to the classical textbook view, the early stages of visual cortex operate as a hard-wired, feature-detecting system and are little affected by nonvisual features of external stimuli. However, Shuler and Bear (p. 1606) show that neurons in primary visual cortex (area V1) have very different response patterns during presentation of the same stimuli at early and late stages of visual discrimination training. They found an association of responses of area V1 neurons with the timing of a reward. Animals were trained to receive water after a certain number of licks, on a tube, after stimulation of one eye. Reward time was different for both eyes, and neurons in the primary visual cortex predicted the time of the reward in trained, but not in naïve, animals.

  10. Managing the Neural Production Line


    Neural progenitors in the developing brain interact with neighboring cells through αE-catenin-containing adherens junctions. Lien et al. (p. 1609; see the Perspective by DiCicco-Bloom) found that conditional knock-out of the αE-catenin gene during embryonic brain development resulted in mice whose brains at birth contained twice as many cells as normal. It seems that the area of cell surface occupied by adherens junctions defines the density of cells and regulates cellular proliferation such that enough, but not too many, brain cells are produced.

  11. Eye of Lizard

    The parietal eye of lizards responds to light and dark but does not form images. Su et al. (p. 1617) show that blue light and green light, working through opsins unlike those in visual eyes, send antagonistic signals to a key cyclic guanosine monophosphate (cGMP) phosphodiesterase. Subsequent alterations in cGMP concentrations modulate channel openings to depolarize or hyperpolarize the parietal photoreceptor cells. Comparison of the opsins and signaling molecules involved suggests an evolutionary trajectory by which the parietal eye diverged from the visual eyes.

  12. Promising Therapy for Progeria?

    Progerias are a group of rare genetic disorders characterized by the onset in children of symptoms typically seen in aging individuals, such as osteoporosis, vascular disease, and hair loss. Several progeroid disorders are caused by mutations that alter the function of prelamin A, a protein that helps maintain the structural integrity of the cell nucleus. Cells from patients with progeria display dramatic changes in nuclear architecture because prelamin A remains aberrantly attached to the nuclear membrane by virtue of a farnesyl lipid modification. In a mouse model of progeria, Fong et al. (p. 1621, published online 16 February; see the 17 February news story by Travis) now show that a drug that inhibits protein farnesylation (farnesyltransferase inhibitor, or FTI) and that is already in clinical development for potential anticancer activity can ameliorate symptoms of the disease. FTI-treated mice had greater grip strength were less likely to develop rib fractures and, in a short-term study, appeared to live slightly longer than untreated mice.

  13. Watching Waves Collide

    The wavelike behavior of atoms is among the least intuitive properties imposed by quantum mechanics. Although diatomic molecules like iodine (I2) are often depicted as two balls connected by a spring, their vibration actually involves overlapping nuclear waves. Katsuki et al. (p. 1589) have used two-pulse ultrafast laser excitation to visualize the interference pattern that results from vibrational excitation of I2. The first pulse accesses an electronically excited superposition of vibrational states. The second pulse is then variably tuned in frequency and time to induce fluorescence selectively when the nuclear waves collide and interfere. The spectra resolve peaks and troughs with subangstrom resolution and correspond well to theoretical calculations.

  14. Past Experience Is Key

    Models of animal decision-making often assume that choices are based on the fitness consequences that each choice yields. Fitness gains, in turn, depend on both the intrinsic properties of the options and the state of the subject at the time of the choice. However, recent studies in humans and other vertebrates have shown that preferences may reflect more closely the subject's state at the time of learning than at the time of choice. Pompilio et al. (p. 1613) now describe similar behavior in an invertebrate. In the desert locust, the state-dependent benefit experienced on acquaintance with a source of reward drove later choices. This finding contrasts with normative theories of choice in biology and economics, which rely on present rather than past benefit and psychological models of reinforcement learning that use absolute reward magnitude rather than state-dependent benefit.

  15. Survival of the Fittest?

    An asexual population undergoing selection, such as cancer cells or a virus switching its host-range, may experience clonal interference, in which numerous beneficial mutations create competing lineages. Hegreness et al. (p. 1615) use numerical simulations and analysis of mixed and marked bacterial populations to show that, regardless of the underlying distribution of individual mutant cells, the evolution of a population can be approximated by an equivalent model, in which all beneficial mutations confer the same fitness advantage. This equivalence principle can be used to predict other measures of adaptation in such populations, including the degree of polymorphism and the average fitness of mutant lineages.