This Week in Science

Science  09 Dec 2011:
Vol. 334, Issue 6061, pp. 1321
  1. A Niche Is Not an Island


    Stem cells in the intestine are responsible for the regular replacement of intestinal epithelium throughout life, as well as for recuperation after disease or injury. Two populations of intestinal stem cells have been identified, one in the base and one on the side of the intestinal crypt. Studying mice, Takeda et al. (p. 1420, published online 10 November) found that the relatively quiescent stem cells in the side of the crypt could give rise to the rapidly cycling stem cells in the base of the crypt, and the active cells could also give rise to the quiescent cells. Despite this ability to cross-populate one another, each group of stem cells, however, functions independently and has distinct characteristics.

  2. Worst… Date… Ever

    The Permian-Triassic boundary is marked by the biggest mass extinction in Earth's history, including the extinction of roughly 90% of marine life and 70% of terrestrial species. The exact causes of this catastrophic loss of global biodiversity remain a matter of debate, but precisely dating the peak and duration of the extinction may provide a context from which to base correlations with other geochemical and paleontological records. Shen et al. (p. 1367, published online 17 November) used high-resolution U-Pb geochronology on several sedimentary rock sections from South China to determine robustly the timing of the end-Permian mass extinction to 252.28 ± 0.08 million years ago. This age coincides with decreased oxygen levels in shallow marine environments and expansive wildfires on land, suggesting that dramatic environmental and climatic disturbances—possibly from a massive release of carbon dioxide and methane—lead to the extinction.

  3. Malaria Liver Block

    Outside Africa, Plasmodium vivax is the dominant species of malaria parasite causing between 20 and 50% of the annual ∼515 million malaria cases globally. P. vivax differs from the deadly P. falciparum in that it develops “sleeping stages,” called hypnozoites, in the liver that can give rise to reappearances of parasites in the blood, months or even years after an infected mosquito has made the inoculating bite. The chronic, long-lasting nature of P. vivax infections has a substantial impact on an infected individual's health and economic well-being. Meister et al. (p. 1372, published online 17 November; see the cover) describe a systematic approach using an automated microscopy assay for identifying antimalarial drugs capable of killing liver-stage parasites. A series of orally available imidazolopiperazine compounds were able to prevent malaria parasites from developing within liver cells with a single 15-milligrams-per-kilogram oral dose.

  4. Nanoparticle Remodeling


    When synthesizing nanoparticles, the final product can be heavily dependent on the ingredients used and the preparation conditions. Obtaining nanoparticles with complex shapes can be difficult, especially after the synthesis has been completed. Gonzälez et al. (p. 1377; see the Perspective by Parak) demonstrate how two separate hollowing mechanisms can be used in sequence to produce multicomponent nanostructures. Molecular reducing agents and coordinating complexes were used to facilitate the production of alloy nanostructures of high complexity at room temperature.

  5. Galactic Runaways

    OB runaway stars are young, massive stars that have unusually high velocities with respect to the mean galactic rotation. They are thought to be produced by ejection from a binary system after the companion star explodes as a supernova or by ejection from a young stellar cluster through dynamical interactions between stars. Combining state-of-the-art numerical simulations with observations of OB runaway stars, Fujii and Portegies Zwart (p. 1380, published online 17 November) show that dynamical interactions between single stars and binary systems in young, massive stellar clusters dominates the formation of runaway OB stars in our galaxy.

  6. Guided Catalyst Discovery

    Industrial catalyst development is often a largely empirical process, comprising a screen of many different compounds to gauge improvements in activity directly. Suntivich et al. (p. 1383, published online 27 October; see the Perspective by Vojvodic and Nørskov) present a more predictive, guided path to produce a metal oxide catalyst for water oxidation for use in prospective energy storage applications. A correlation between the catalytic activity of a range of perovskite-type compounds and the electronic structure of the surface transition metal ions within them was observed, which allowed formulation of an optimal catalyst composition of barium, strontium, iron, and cobalt that proved highly effective in alkaline media.

  7. Welcome Insensitivity

    Predicting how much climate will warm in response to anthropogenic greenhouse gas inputs to the atmosphere requires the knowledge of Earth's climate sensitivity, that is, how much the global average surface air temperature would rise if the concentration of atmospheric CO2 were to be doubled from its preindustrial value of around 280 parts per million. Schmittner et al. (p. 1385, published online 24 November; see the Perspective by Hegerl and Russon) combined temperature reconstructions of the Last Glacial Maximum with global climate models to estimate climate sensitivity and conclude that its most likely value is slightly less than that of most current estimates, which suggests that catastrophic climate change from expected near-future atmospheric CO2 levels is unlikely.

  8. Time for Bed

    An important aspect of human behavior and culture is the generation of a safe and comfortable sleeping environment. Wadley et al. (p. 1388) provide a detailed record of the use of bedding by Middle Stone Age humans from the Sibudu rock shelter, South Africa. From ∼77,000 to 38,000 years ago, sleeping mats were made from plants and leaves and sometimes included plants that would have provided some disease protection. Mats were apparently repeatedly burned after ∼73,000 years ago. The abundance of mats increased markedly after ∼58,000 years ago, perhaps reflecting a growth in the local population.

  9. Fear Factor

    Killing and consumption by predators have a very direct impact on a prey population. However, even the fear of predation can have powerful influences on behavior—for example, increasing the amount of time individuals spend being vigilant or reducing their time out in the open. By excluding all possible predators from a small population of song sparrows, Zanette et al. (p. 1398; see the Perspective by Martin) were able to show that the perceived risk of predation decreased the number of offspring successfully reared by 40%. When predator sounds were played to nesting pairs, the birds exposed to high rates of perceived predator presence chose more secluded nest sites and made fewer foraging trips, both of which put their offspring at a disadvantage.

  10. Plant Defenses


    In plants, the R (resistance) proteins represent one arm of the innate defense system against pathogens. Once activated by detection of an invading pathogen's effector protein, the R proteins promote a cascade of defensive responses that can include cellular suicide (see the Perspective by McDowell). Using the plant Arabidopsis and pathogenic Pseudomonas bacteria, Heidrich et al. (p. 1401) found that the plant protein EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1) was important for the nucleo-cytoplasmic communications required to mediate the transition from threat-detection to defense-implementation. Bhattacharjee et al. (p. 1405) identified interactions between the plant EDS1 and SRFR1 (SUPPRESSOR OF rps4-RLD1) proteins in the microsomal fraction of the cytoplasm, where SRFR1 may serve to tether R proteins and temper their responses to initial pathogen detection.

  11. Adult Brain Plasticity

    The sensory cortex is very plastic during the so-called critical period early in life. It is, however, unclear if it retains its plasticity in the adult. Shibata et al. (p. 1413) used brain-imaging data to classify activation patterns in early visual cortices. Adult participants were shown a visual feature and their brain activity was measured. Next, participants were instructed to make the size of a disc on display as large as possible, but were not instructed how to do so. The size of the disc increased depending on how closely activity in the visual cortex corresponded to the pattern that had been measured earlier. After repetitive inductions of the patterns, the subjects' performance improved, which suggests that the early visual cortex is sufficiently plastic to improve perceptual learning.

  12. Shedding Light on Measles

    Seasonal epidemics of measles in West Africa are a major vaccine-preventable cause of childhood mortality, but it has been difficult to measure key parameters needed for epidemiological assessment, including population density changes among the affected human communities. Furthermore, the mechanisms underlying the substantial seasonal fluctuations in measles incidence are poorly understood. Bharti et al. (p. 1424) were able to quantify seasonal fluctuations in human population density by using satellite imagery to measure changes in the brightness of anthropogenic sources of nighttime light. Changes in population density appeared to drive epidemic dynamics. This type of approach for measuring fine-scale changes in human population density has relevance for public health, crisis management, and economic development.

  13. Familiarity Breeds Cooperation

    Many behavioral experiments that are carried out under laboratory conditions and rely on undergraduate subjects, have established the link between self/other (or in/out-group) attitudes and elicited social behaviors. Observing these influences and effects in real-world environments is less common and sometimes less consistent. Alexander and Christia (p. 1392; see the Perspective by Goette and Meier) took advantage of a natural experiment by comparing the behaviors of students in four Mostar high schools: two in the Croat-majority western part of Mostar and two in the Bosniac-majority eastern part of the city. In 2004, a merger between one Croat school and one Bosniac school yielded three high schools with relatively homogeneous or heterogeneous ethnic compositions. Students in the integrated school were more likely to behave cooperatively in a public-goods game and they were also more likely to levy sanctions against noncooperating players in the game.

  14. Cooperative Benefits

    Devising and implementing interventions aimed at improving the lives of pastoral communities requires a knowledge of and sensitivity to local conditions and cultural histories. Evaluating the impact of these interventions and the likely causal pathways requires a battery of measures, both of outcomes and of participant perceptions and judgments. Coppock et al. (p. 1394) provide a case study analysis of a program in Ethiopia spanning almost a decade and describe the higher living standards and aspirations generated by exposure to income diversification and collective action.

  15. Better Is Not Always Best

    Once a cell has evolved a high-affinity transporter for nutrients like phosphate or zinc, why would it need to continue to make low-affinity transmitters as well? Levy et al. (p. 1408) address this question in yeast and show that the lower-affinity form does come in handy. When nutrients are plentiful, the low-affinity transporter is expressed and works fine. As nutrient concentrations are depleted, the low-affinity transporter begins to lose efficiency and the nutrient influx dips. This signals the cell to begin preparation for starvation—at a time earlier than it would have, if it had waited until nutrient concentrations got so low that function of the high-affinity transporter became compromised. Sensing a decrease in nutrient supply before concentrations get low enough to inhibit growth thus appears to be a key reason for expressing transporters of both low and high affinity.

  16. Encoding Memory Identity

    Episodic memory formation requires two distinct types of timing signals. One type of signal gives the memory its unique and temporally ordered fingerprint. The second type of signal controls the retention and integration of temporally discontinuous elements, a process independent of sequence. The ability to bind these signals over time helps to define the identity of the memory episode. Suh et al. (p. 1415, published online 3 November) wanted to delineate a specific neural circuit for transmission of the second timing signal for memory identity and investigate its underlying physiological mechanisms. Conditional transgenic mice were produced in which the direct input from the medial entorhinal cortical layer III to area CA1 of the hippocampus via a monosynaptic pathway was selectively inhibited. These mutant animals were impaired in specific temporal association memory tasks. In contrast, the other major pathway, the trisynaptic pathway from entorhinal cortex to area CA1 via the dentate gyrus and area CA3, was not necessary for temporal association memory. Furthermore, persistent activity appeared to represent the critical requirement for the memory signal from the entorhinal cortical layer III. Memory formation thus requires a persistent timing signal transmitted by the monosynaptic pathway from the entorhinal cortex to the hippocampus to define memory identity.

  17. Helping a Cagemate in Need

    Empathy, a well-known characteristic in humans, occurs when an individual is motivated to help another, while maintaining emotional separation. Thus, it is distinct from emotional contagion where an individual begins to experience the emotions of other individuals, and act similarly. Emotional contagion is known to occur in many mammalian species, but empathy has often been considered unique to primates. Through a controlled experiment in captive rats, Ben-Ami Bartal et al. (p. 1427; see the Perspective by Panksepp) show that the biological roots of empathy could be much deeper than recognized. Rats were highly motivated to release a restrained cagemate, even when they were not permitted any immediate contact with it after release. Furthermore, when presented with chocolate, a highly preferred food, the rats were still motivated to release their cagemate and even shared the food with them. Thus, empathically motivated prosocial behavior is not limited to primates, and—like many other behaviors previously thought to be limited to this group—may serve similarly important functions across species.

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