This Week in Science

Science  17 Aug 2007:
Vol. 317, Issue 5840, pp. 868
  1. Overturning Assumptions


    In the north Atlantic, warm surface waters flow northward and eastward from the Florida Strait, and the northward flows return as southward-flowing deep water. This meridional overturning circulation (MOC) transports huge quantities of heat from low to high northern latitudes. Global climate models have suggested that the flux of water transported might be decreased by global warming, which could have an important effect on climate, particularly that of Europe. However, there has not been a sufficiently long or detailed observational record to evaluate whether significant weakening has occurred (see the Perspective by Church). Cunningham et al. (p. 935) and Kanzow et al. (p. 938) now provide annual records of the strength of the MOC using an array of moored instruments deployed across the Atlantic basin at a latitude of 26.5°N. The strength of the MOC varied by more than a factor of 8 during a 1-year period from a low of 4.0 sverdrups (1 Sv equals 1 million cubic meters per second) to a high of 34.9 Sv, with an average flow of 18.7 ± 5.6 Sv. Fluctuations of the different transport components of the MOC largely compensate each other, which means that robust estimates of the flow can be made over intra-annual periods. Thus, an earlier claim that the MOC has decreased by 8 Sv during the past decade, made on the basis of only a few instantaneous measurements during that period, was premature and reflected short-term natural variability.

  2. Giant Smoking Gun

    Type 1a supernovae are valuable as cosmological distance probes because their intrinsic brightness can be inferred. However, we know little about how they occur or the nature of their progenitor star systems. It is widely thought that type 1a supernovae mark the catastrophic death of a white dwarf after it has cannibalized a companion star within a binary star system. By detecting a whiff of gas blown off during a type 1a supernova outburst, Patat et al. (p. 924, published online 12 July) determined the stellar type of its companion from its absorption line characteristics. The expansion velocities of the gas appear to favor a red-giant companion at the time of the explosion.

  3. Bright Boron Nitride Crystals


    The hexagonal form of boron nitride (hBN) resembles graphite in that it is used as a lubricant and has good thermal conductivity, but unlike graphite, it is an electrical insulator. The large direct band gap of hBN makes it a possible candidate for emission of deep ultraviolet light, which may be of use in the electronics industry or for medical treatments. Kubota et al. (p. 932) have developed a method to synthesize high-purity crystals at atmospheric pressure using nickel-molybdenum as a solvent. The crystals showed intense emission at a wavelength of 215 nanometers at room temperature.

  4. Into Sharper Focus

    In high-end optical systems, zone plates cut off the out-of-phase components of light so that a sharp focus is achieved further down the optic axis. However, these patterned zone plates still create a focus that suffers from classical diffractive wavelength limitations. Merlin (p. 927, published online 12 July) presents theoretical work which shows that it should be possible to design planar structures that force convergence of the near-field component of the optical fields, which would provide a subwave-length focus at some point down the axis. This approach might lead to a general route for achieving subwavelength focusing and imaging that complements the negative refraction route to such superlensing effects.

  5. Atom-Like Quantum Dots

    Quantum dots are often referred to as artificial atoms because they exhibit discrete energy levels that arise through quantum-confinement effects. However, quantum dots are typically formed from hundreds if not thousands of individual atoms, so that many-body, or bulk-like, effects are also present under intense optical excitation. These effects have been used by Xu et al. (p. 929) to tune the absorption and gain of quantum dots driven by two different optical fields. They observed Autler-Townes splitting and the Mollow spectrum in the absorption spectrum of a single quantum dot, and they demonstrated gain without population inversion. These results should enable further well-controlled studies with quantum dots and open the way for applications such as quantum logic gates and optical switches.

  6. One Too Many

    Aneuploid cells, often observed in cancer, have at least one extra or missing chromosome, but little is known about the effects of aneuploidy on cell physiology. Torres et al. (p. 916; see the Perspective by Jallepalli and Pellman) systematically created strains of yeast that contained an extra copy of one or more yeast chromosomes and tested how this influenced cell function. Some effects, like a characteristic pattern of gene expression, seemed to result from the increase in DNA content itself, whereas others, like increased glucose uptake, depended on increased transcription or translation of the extra genes. Some properties were shared regardless of which chromosome was duplicated, including a surprising one—inhibition of cellular proliferation. This result calls into question whether aneuploidy might be a cause or an effect of cancer.

  7. Plant Speciation

    Much theory of speciation has focused on animal systems. Because plants are sessile organisms, they offer some advantages over animals in sampling, tracking, and measuring, as well as extended possibilities for manipulations in experimental gardens and growth chambers. Rieseberg and Willis (p. 910) review the mechanisms of reproductive isolation in plants and their underlying genetics, as well as focus on the role of polyploidy in speciation, which occurs much more in plants than in animals.

  8. Mom's Anticipating Some Help

    An almost universal observation in studies of cooperative vertebrates is that offspring receive more food in the presence of helpers than in their absence. Nevertheless, helper effects on offspring mass and survival have been surprisingly difficult to detect. Russell et al. (p. 941; cover) show that, in the presence of helpers, Australian female fairy-wrens lay smaller eggs, of lower nutritional content, that give rise to lighter chicks. Helper effects on chick provisioning rates are wholly obscured by this maternal reduction in egg investment, which in turn helps the mother survive into the next breeding season.

  9. HIV-1 Goes Genome-Wide

    Elucidating the genetic variability in the response of individuals to human immunodeficiency virus-1 (HIV-1) infection will play an increasing part in developing effective treatments. Fellay et al. (p. 944, published online 19 July) report a whole-genome association study of host response to HIV-1, focusing on viral load during the early stages of infection. Two immune-related polymorphisms were identified that together could account for 15% of the total variation seen in the viral load between patients.

  10. Smell the CO2


    Although invertebrates are known to sense and show behavioral responses to concentrations of CO2 similar to those in the earth's atmosphere, it has been unclear whether the mammalian olfactory system also can sense such amounts of CO2. Hu et al. (p. 953) describe a set of olfactory neurons that appear to allow detection of concentrations of CO2 about 70% greater than those in air. The neurons express carbonic anhydrase II, which catabolizes CO2, and appears to be required as part of the sensing mechanism.

  11. Understanding Selective Synapse Elimination

    Synapse elimination is a hallmark of neural circuit maturation during development. Many synapses are eliminated after an initial phase of synapse formation. However, little is known about the molecular machinery that executes synaptic elimination, nor why certain synapses are selectively eliminated while other synapses persist and grow. Ding et al. (p. 947; see the Perspective by Miller) examined synapse elimination in the nematode worm and found that a ubiquitin E3 ligase complex plays a key role. The activity of the ubiquitin-proteasome system was tightly regulated by a synaptic adhesion molecule, which protected certain synapses from selective elimination.

  12. Fleeting Memories

    How persistent is our memory, how is it maintained, and how can it be disrupted? It has recently been shown that PKMζ, a protein kinase C isoform, is critical for maintaining hippocampus-dependent spatial memory and long-term potentiation. Using a conditioning taste aversion paradigm, Shema et al. (p. 951; see the news story by Miller) found that long-term memory could be erased by infusion of a PKMζ inhibitor, ZIP, into the insular cortex. The activity of PKMζ was specifically involved in the storage of memories but not in their acquisition.

  13. Membrane Crossing

    The mechanisms by which proteins get into or across biological membranes have been difficult to elucidate. Two studies now describe the structures of outer membrane protein (Omp) complexes that mediate protein translocation in bacteria (see the Perspective by Tommassen). Kim et al. (p. 961) solved the structure of the periplasmic portion of YaeT (also called Omp85) from Escherichia coli, which forms the central part of a complex that accepts polypeptides that have crossed the inner membrane and inserts them into the outer membrane (and possibly facilitates their folding). Five polypeptide transport-associated (POTRA) domains were characterized, and the effects of their individual deletion on YaeT complex assembly were examined. Clantin et al. (p. 957) solved the structure of the related transporter FhaC that secretes filamentous hemagglutinin (FHA), the major adhesin of the whooping cough agent Bordetella pertussis, to the cell surface. FhaC functions as a monomer without accessory proteins, and the POTRA domains bind FHA to facilitate its transport through the channel.