This Week in Science

Science  01 May 2009:
Vol. 324, Issue 5927, pp. 565
  1. Shedding Light on a Problem


    Human activities—mainly the application of fertilizers to farmland—have increased the availability of nutrients in terrestrial and aquatic ecosystems. In grasslands, nutrient enrichment reduces plant species diversity, but the mechanisms underlying this loss of biodiversity remain unclear. Hautier et al. (p. 636) use an experimental manipulation, addition of supplementary light to the grassland understory, which supports the idea that competition for light is the major mechanism of plant diversity loss.

  2. MESSENGER from Mercury

    The spacecraft MESSENGER passed by Mercury in October 2008, in what was the second of three fly-bys before it settles into the planet's orbit in 2011. Another spacecraft visited Mercury in the mid-1970s, which mapped 45% of the planet's surface. Now, after MESSENGER, only 10% of Mercury's surface remains to be imaged up close. Denevi et al. (p. 613) use this near-global data to look at the mechanisms that shaped Mercury's crust, which likely formed by eruption of magmas of different compositions over a long period of time. Like the Moon, Mercury's surface is dotted with impact craters. Watters et al. (p. 618) describe a well-preserved impact basin, Rembrandt, which is second in size to the largest known basin, Caloris. Unlike Caloris, Rembrandt is not completely filled by material of volcanic origin, preserving clues to its formation and evolution. It displays unique patterns of tectonic deformation, some of which result from Mercury's contraction as its interior cooled over time. Mercury's exosphere and magnetosphere were also observed (see the Perspective by Glassmeier). Magnetic reconnection is a process whereby the interplanetary magnetic field lines join the magnetospheric field lines and transfer energy from the solar wind into the magnetosphere. Slavin et al. (p. 606) report observations of intense magnetic reconnection 10 times as intense as that of Earth. McClintock et al. (p. 610) describe simultaneous, high-resolution measurements of Mg, Ca, and Na in Mercury's exosphere, which may shed light on the processes that create and maintain the exosphere.

  3. Vive La Différence

    How closely do climate changes in the Northern and Southern Hemispheres resemble each other? Much discussion has concentrated on the Holocene, the warm period of the past 11,500 years in which we now live, which represents a baseline to which contemporary climate change can be compared. Schaefer et al. (p. 622; see the Perspective by Balco) present a chronology of glacial movement over the last 7000 years in New Zealand, which they compare to similar records from the Northern Hemisphere. Clear differences are observed between the histories of glaciers in the opposing hemispheres, which may be owing to regional controls. Thus, neither of two popular arguments—that the hemispheres change in-phase or that they change in an anti-phased manner—appear to be correct.

  4. The Birds and the Dinosaurs


    The extent to which primary tissues are preserved in ancient fossils remains controversial. Schweitzer et al. (p. 626; see the news story by Service) describe well-preserved tissues and primary collagen sequences from the femur of an 80-million-year-old hadrosaur. The fossil preserved structures resembling primary bone tissues and vessels. Both extracts and tissue pieces were analyzed in multiple laboratories by mass spectrometry, which revealed ancient collagen sequences that support a close relation between birds and dinosaurs.

  5. Glasslike Supersolid

    Recent experiments with solid helium confined to the ring of a torsional oscillator at extremely low temperatures have been interpreted in terms of an exotic supersolid phase—a crystalline solid that somehow flows like a superfluid. However, behavioral differences between samples have raised many questions (see the Perspective by Saunders). Hunt et al. (p. 632) present a comprehensive study of the relaxation dynamics of the torsional oscillator system as a function of time and temperature. The data provides evidence for a “supersolid glass,” where glassy behavior of crystal dislocations and superfluidity can coexist. In a separate theoretical study, Anderson (p. 631) argues that supersolidity ought to be a ground state for all bose solids, but that defects in the sample may mask the supersolid signature.

  6. Tactical Target

    Intramembrane proteolysis by the γ-secretase complex is important during development and in the pathology of Alzheimer's disease. γ-Secretase has usually been considered as a homogeneous activity. Serneels et al. (p. 639, published online 19 March; see the Perspective by Golde and Kukar) now show that the Aph1B component of the γ-secretase complex is responsible for the generation of long β-amyloid species involved in Alzheimer's disease. In a mouse model of Alzheimer's disease, full knockout of Aph1B improved disease phenotypes, without the sort of toxicity previously observed when targeting γ-secretase more generally.

  7. Single-Cell Brain Switching

    Different behavioral states of an animal are characterized by distinct patterns of global brain activity. For example, slow-wave sleep is associated with membrane potential transitions in cortical neurons and high-amplitude, low-frequency local field potential and electroencephalogram signals. In contrast, during rapid—eye-movement sleep and wakefulness, low-amplitude, high-frequency cortical activity dominates with neuronal membrane potentials fluctuating around a depolarized level. Li et al. (p. 643) asked whether action potentials in a single neuron can change global network activity. In a series of in vivo single-cell stimulation experiments, using intracellular stimulation via a whole-cell patch pipette, a brief episode of intracellular stimulation switched brain states.

  8. Dieter's Dilemma

    The ability to exercise self-control is central to human success and well-being. However, little is known about the neurobiological underpinnings of self-control and how or why these neural mechanisms might differ between successful and unsuccessful decision-makers. Hare et al. (p. 646) used brain imaging in a dieting population undergoing real-life decisions between a healthy or a tempting, yet nutritionally inferior, choice of food. Activity in the ventromedial prefrontal cortex correlated with the value of the stimulus, termed goal value. Importantly, this activity integrated both health and taste values in individuals who were able to exert self-control in their choices, while reflecting only taste in those unable to exert self-control.

  9. Hard Graft


    In plants, it is not clear whether heritable changes can be induced by tissue grafting—for example, when one plant is grafted onto a different root stock. Stegemann and Bock (p. 649) now show that genetic material is transferred between plants across graft junctions. This suggests that eukaryotic cells can indeed exchange genetic information, and in this way, grafting allows genes to cross species barriers. Although the exchange is limited to the site around the graft, the finding blurs the boundary between natural gene transfer and genetic engineering techniques and suggests that natural grafting may serve as a mechanism of horizontal gene transfer during evolution.

  10. Circadian Oscillations

    The 24-hour day-night cycle plays an important role in mammalian physiology and behavior and, as most travelers are well aware, there is an intimate link between our in-built circadian clocks and metabolic rhythms. This link is in part forged by the protein deacetylase SIRT1, which regulates the clock's molecular circuitry. SIRT1 uses as a cofactor the cellular metabolite NAD+, which is synthesized through a salvage pathway that includes the enzyme nicotinamide phosphoribosyltransferase (NAMPT) (see the Perspective by Wijnen). Ramsey et al. (p. 651; published online 19 March) and Nakahata et al. (p. 654, published online 12 March) now show that NAMPT and NAD+ levels oscillate during the daily 24-hour cycle and that this oscillation is regulated by the circadian clock. Furthermore, the oscillations in NAD+ modulate the activity of SIRT1 feeding back into the circadian clock.

  11. Edited Information Flow

    RNA is generally a faithful mirror of the information encoded in DNA. However, posttranscriptional RNA editing is a widespread phenomenon that can result in a translated protein differing from its gene code. A class of deaminase enzymes related to human APOBEC (apolipoprotein B editing complex protein) edit adenosines in eukaryotes. Randau et al. (p. 657) have discovered an addition to this enzyme family in prokaryotes that edits cytosine to uridine at the highly conserved and structurally critical position 8 in the majority of transfer RNAs of the hyperthermophilic archaebacterium Methanopyrus kandleri.

  12. Gene Expression in Hybrids

    A large proportion of genes differ in their expression patterns between closely related species, and this divergence is believed to be an important driver of phenotypic evolution. However, little is known about the genetic basis of this divergence. Tirosh et al. (p. 659) use allele-specific expression profiling of two yeast species and their hybrid to identify which genes diverged in expression owing to changes in their genomic loci (cis) and which genes diverged owing to changes in their regulators (trans). The data can be used to explain the distinct gene expression pattern observed in the hybrid.