This Week in Science

Science  17 Apr 2009:
Vol. 324, Issue 5925, pp. 309
  1. Past as Prelude


    The Sahel region of West Africa suffered terribly from a severe drought during the 1970s to the 1990s. Because of the disastrous social, economic, and environmental impacts of the drought, it is important to predict how common such droughts may be in the future, which raises the question of how often they may have occurred in the past. Shanahan et al. (p. 377) present a 3000-year-long record from two West African lakes, whose volumes and geo-chemistries are sensitive to regional rainfall. Severe, protracted droughts, lasting from decades to centuries, have occurred repeatedly in West Africa over the past few millennia. These results provide a strong incentive to develop better plans for adapting to and mitigating the consequences of the future dry periods expected to occur in a warming climate.

  2. How Does Deep Brain Stimulation Work?

    Deep brain stimulation is used for the treatment of Parkinson's disease when traditional neurotransmitter replacement with L-DOPA fails. However, we do not understand how deep brain stimulation works. Gradinaru et al. (p. 354, published online 19 March) used light-sensitive neuronal modulators driven by cell type-specific promoters together with an implantable optical stimulator to drive or inhibit distinct circuit elements in the brains of Parkinsonian rodents. Surprisingly, neither increasing nor decreasing activity in excitatory neurons of the subthalamic nucleus was sufficient by itself to affect motor symptoms. Instead, direct selective high-frequency stimulation of afferent axons projecting to the subthalamic nucleus yielded robust, consistent, and reversible therapeutic results.

  3. Four-Legged and Fishy

    The emergence of four-legged creatures and the colonization of land by vertebrates were key events in evolution. Fossils of the Devonian tetrapods Ichthyostega and Acanthostega have revealed insights into the locomotory patterns of these early land animals. Callier et al. (p. 364; see the Perspective by Friedman) now compare two series of humeri (upper arm bones) from these genera, assumed to belong to individuals at different growth stages. While the growth of the humerus of Acanthostega shows no indication of a major functional change during life, that of Ichthyostega shows marked differences between juvenile and adult individuals. This suggests that Ichthyostega had a more pronounced aquatic lifestyle when young than when adult. This result is at odds with character analyses for early tetrapods, which show Acanthostega as the most aquatic tetrapod known from complete skeletons. Thus, Ichthyostega may occupy a deeper position in the evolutionary tree than the more aquatic and seemingly more fishlike Acanthostega.

  4. Controllable Domain Patterning


    Most semiconductor devices are built on a platform of silicon coated with a thin layer of silicon oxide. In fact, it is difficult to avoid the oxide overlayer because silicon readily oxidizes and it can also steal oxygen from any oxide layers that are deposited on top of bare silicon. Using molecular-beam epitaxy, Warusawithana et al. (p. 367) managed to grow strontium titanate (SrTiO3) onto a silicon substrate. As the film thickness increased, the SrTiO3 showed a transition from being mostly commensurate with the underlying silicon to being a fully relaxed structure. Thinner, strained films showed ferroelectric behavior and could be patterned with domains of either polarity.

  5. The Clock Strikes Itself

    Fermionic particles such as electrons and certain atoms cannot occupy the same space if they are in the same quantum-mechanical state. This property has inspired atomic clock designs based around fermion ensembles, which should boost signal strength through the presence of multiple absorbing atoms without suffering from collisional broadening. Campbell et al. (p. 360) show, however, that the act of probing an ultra-cold sample of fermionic strontium atoms gives rise to density-dependent frequency shifts consistent with collisions. This phenomenon may be due to a small degree of inhomogeneity in the interaction of the laser pulse with the atom ensemble, which confers slight distinctions on the states of neighboring atoms and so allows them to interact.

  6. Assessing Asteroid Anomalies

    Measurements of isotopic abundances of primitive solar system materials reveal deviations in the stable isotope compositions of some elements compared to those of Earth. These isotope anomalies have been interpreted as signs of imperfect mixing of presolar materials following the collapse of the molecular cloud that formed the solar system. Trinquier et al. (p. 374) report high-precision measurements of titanium isotopic compositions of a variety of meteorites and objects within them, showing that inner solar system planets and asteroids exhibit correlated anomalies in two titanium isotopes that are produced by different stellar processes. Thus, the protosolar molecular cloud was, in fact, homogenized by the time the oldest dated solar system's solids formed, and other isotope anomalies were created within the solar system by a secondary process.

  7. Immune Cell Origins

    In the immune system, dendritic cells, monocytes, and macrophages are critical for the inflammatory response to pathogens. Although these cell types arise from a common hematopoietic precursor, they are functionally diverse, and the developmental relationships between these subsets and where they develop have not been fully delineated in vivo. Liu et al. (p. 392, published online 12 March; see the cover) elucidate the precursor-progeny relationship between mouse conventional dendritic cells, plasmacytoid dendritic cells, and monocytes. Conventional dendritic cells arise in the bone marrow and traffic to the peripheral lymphoid organs, where further differentiation and expansion occurs.

  8. Suffocating Worms


    Lack of oxygen rapidly leads to the demise of most animals. Menuz et al. (p. 381; see the Perspective by Crowder) conducted a screen for mutants of the worm Caenorhabditis elegans that are particularly sensitive to the effects of anoxia. The gene Hyl-2, which encodes a ceramide synthase, is a member of a family of genes that influence life span in yeast. Worms have another gene, hyl-1, that also encodes a related ceramide synthase, whose mutants are instead more resistant to anoxia than normal animals. The two ceramide synthases have different specificities for fatty acyl chains, with HYL-2 preferentially producing ceramide molecules with shorter fatty acyl chains. Thus, the abundance of particular ceramides can influence cell survival mechanisms.

  9. Rusty Glacier

    Blood Falls at the snout of an Antarctic glacier marks the discharge of an iron-rich subglacial water pocket. Mikucki et al. (p. 397) now show that the water pocket is ancient seawater that was trapped some 1.5 to 4 million years ago under the advancing Taylor Glacier. The water is anoxic, cryogenically concentrated, loaded with Fe-II, and has no evidence of sulfide. The isotopic composition of the oxygen in the sulfate indicates that reduction of sulfate to sulfite is occurring by microbially coupled sulfur and iron cycles driven by adenosine 5′-phosphosulfate reductase and probably originating in an organic source of sulfur. This system offers a clear example of how a microbial system can survive for an extended period without photosynthesis or nutrients from an external source.

  10. Think Positive

    Positive feedback cycles can generate a self-perpetuating pattern of behavior. When the outcome is detrimental, however, these cycles can make it difficult for subjects to escape being drawn further into repetitively negative thinking. Cohen et al. (p. 400) performed a multiyear field experiment in which three cohorts of 7th-grade students were given seemingly gentle interventions—a brief writing assignment on personal values—several times throughout their 7th- and 8th-grade school years. Poorly performing African American students who had been assigned to write about self-affirmation displayed significantly smaller declines in their grades than those who had written about someone else's values; the intervention had no effect on the grade trends of highly performing African American or European American students. The intervention appeared to help to prevent the poorly performing group from falling into a cycle of negativity.

  11. Understanding Within Reach

    Mirror neurons are thought to enable the understanding of the actions of others, possibly by internally simulating observed actions. Now Caggiano et al. (p. 403) report that, in monkeys, mirror neurons might have a much broader cognitive role. The visual responses of mirror neurons were recorded while the experimenter was executing motor acts either within the monkey's reaching distance (peripersonal space) or out of reach (extrapersonal space). About half of the recorded mirror neurons exhibited spatial selectivity and discharged only when the observed action was executed in the monkey's peripersonal space or only when it was executed in the monkey's extrapersonal space. Although completely unimportant for “understanding” the observed action, the relative distance between observer and observed action plays a fundamental role in selecting possible subsequent behavioral responses. Thus, the role of mirror neurons is not limited to understanding actions, but encodes observed motor acts in terms of the monkey's own potential behavior.

  12. Strength and Ductility

    Many routes to strengthening metals and alloys involve the creation of internal defects and boundaries but can also lead to a loss of ductility, which is the plastic deformation of a material prior to its fracturing. Lu et al. (p. 349) review the progress that has been made in creating twin boundaries on the nanoscale, which have been shown to strengthen materials while preserving ductility. Twin boundaries show coherence with the surrounding matrix material, allowing for defect generation under an applied stress, and also show good thermodynamic stability at the nanoscale, two additional features needed for good strength while preserving ductility.

  13. Nuts and Bolts of Chemotaxis

    During chemotaxis, the small GTPase Rac is locally activated to extend membrane protrusions in the direction of migration. This localized Rac activation is achieved, at least in part, by focusing Rac GEFs (guanine nucleotide exchange factors) at the leading edge of the cell, yet the mechanism controlling subcellular localization of Rac GEFs is poorly understood. DOCK2 is a Rac GEF that regulates motility and polarity during neutrophil chemotaxis. Nishikimi et al. (p. 384, published online 26 March; see the Perspective by Côté and Vuori) provide evidence that intracellular DOCK2 dynamics are sequentially regulated by two distinct phospholipids. In response to chemoattractants, DOCK2 rapidly translocates to the plasma membrane of chemotaxing neutrophils. This initial translocation is mediated by phosphatidylinositol 3,4,5-trisphosphate (PIP3), whereas subsequent accumulation of DOCK2 at the leading edge requires de novo synthesis of phosphatidic acid (PA). PA focuses DOCK2 localization through direct binding to DOCK2's C-terminal polybasic clusters, resulting in local induction of actin polymerization. When this interaction is blocked, neutrophils fail to form leading edges properly and exhibit defects in chemotaxis. Thus, PA is critical for leading edge formation and stabilization during chemotaxis.

  14. Old Sediment Settlement

    Sediments that are more than 2.4 billion years old tend to show highly anomalous sulfur isotopes that seem to have been fractionated in a non-mass-dependent manner. This phenomenon has been thought to be produced solely by photolysis reactions involving ultraviolet light, which suggests that oxygen levels were low before this time with no significant ozone layer in Earth's atmosphere. Watanabe et al. (p. 370; see the news story by Kerr) now show that reactions between organic matter and sulfate at moderate temperatures can also produce anomalous sulfur isotope fractionations. These experiments may complicate interpretation of the Precambrian signal, though it is not clear why such a reaction would produce an abrupt change in the signal 2.4 billion years ago.

  15. Resequence and You Shall Find

    Genome-wide association studies (GWAS) can identify small chromosomal regions that contribute to individual susceptibility to common diseases, but these studies often stop short of pinpointing the specific genes and sequence variants that play a causal role in the diseases. Now, using DNA from 480 patients with type 1 diabetes and 480 controls, Nejentsev et al. (p. 387, published online 5 March) have sequenced 10 candidate genes for the disease, a subset of which maps to chromosomal regions previously linked to the disease by GWAS. This approach led to the discovery of four specific sequence variants that lower the risk of type 1 diabetes. These variants, which are rare in the population, fall within IFIH1, a gene involved in the host response to infection with RNA viruses.

  16. An Extra Dimension of DNA Evolution

    Conserved sequence of noncoding regions of the genome suggests that these regions should be important functionally, although some seem to have no function, and known functional elements often reside in apparently nonconserved regions. Parker et al. (p. 389, published online 12 March) explain these observations with the finding that similarities in DNA structure, as determined by the hydroxyl radical cleavage patterns, rather than nucleotide sequence, can predict evolutionary conservation of function. Statistical and functional assays suggest that their algorithm, Chai, can identify noncoding regions that have functional roles and show why many noncoding regions are not under primary (sequence) constraint, but rather are under secondary (structural) evolutionary constraint.

  17. Noncoding DNA Rules

    Genes are turned on and off by regulatory factors that bind to specific DNA sequences. Typically, how strongly these factors activate a gene is thought to be governed by their affinity for DNA. Now Meijsing et al. (p. 407) show that variations in these DNA sequences modulate the structure and activity of one particular regulatory factor, the glucocorticoid receptor, indicating that DNA sequence is not simply a docking site, but a signal that influences gene expression.