This Week in Science

Science  04 Jul 2008:
Vol. 321, Issue 5885, pp. 12
  1. The Human Honeypot


    The possibility that international investment in protected areas would turn parks into magnets for human immigration (the “honeypot” hypothesis) and thereby reduce conservation effectiveness has been a concern of conservationists, economists, and the development community for some years. Wittemyer et al. (p. 123) now confirm that rates of human population growth around 306 protected areas in 45 countries across Africa and Latin America are nearly twice the country averages. The high population growth around protected areas is correlated with international donor funding to parks and the consequent creation of park-related jobs and services and, disappointingly, is associated with accelerated rates of deforestation.

  2. Reassessing Past Diversity

    Assessing past diversity of life and how it changed over time requires assembly of a database of the many individual and diverse studies of fossils. An early effort was by Jack Sepkoski, and his data on first and last occurrences of marine invertebrates showed an increase in diversity following the Cambrian explosion and particularly since about 100 million years ago. Alroy et al. (p. 97; see the news story by Kerr) have now analyzed a new compilation of more than 3 million specimens resolved to the genus and species level. In contrast to older analyses, the data support a Jurassic increase and imply that the increase in diversity in the Cenozoic was not particularly high relative to earlier times.

  3. Transistor Nanofilms


    For single-walled carbon nanotubes to find use in electronics, a method is needed to extract the semiconducting ones from the metallic ones and to deposit only the semiconductors into dense, aligned patterns on a substrate. While these steps have been accomplished individually or on a small scale, the methods are not amenable to large-scale fabrication. LeMieux et al. (p. 101; see the news story by Service) show that by treating silicon substrates with silane monolayers, which selectively absorb semiconducting carbon nanotubes, they can spin-coat solutions of carbon nanotubes to produce films where the nanotubes are aligned and densely packed. The films show excellent transistor behavior including on/off ratios above 100,000.

  4. Test of the Double Pulsar

    Under general relativity and strong gravity, when massive objects orbit each other closely, their spin and orbital angular momentum should couple. This coupling leads to precession of the spin of each body. This and other related effects have been difficult to test because it requires close observations of the orientation and spin of two massive objects. Breton et al. (p. 104) show that the recently discovered double pulsar provides such a test. The geometry of the system is such that one pulsar eclipses the other when viewed from Earth, blocking some of the radio emissions from the companion, which provides information on its orientation and rotation. Four years of data confirm the relativistic coupling and provide a new test of general relativity and strong gravity theories.

  5. Steps in Organic Film Growth

    An effect often observed in the growth of inorganic thin films is that atoms are more likely to climb step edges between layers than descend. When this effect is seen, the activated nature of descending the step edge is described with a term called the Ehrlich-Schwoebel barrier. Hlawacek et al. (p. 108) explored whether such barriers are at work in the more complex growth of molecules used in organic electronics and light-emitting diodes, in this case, the rodlike molecule para-sexiphenyl. An analysis of atomic force microscopy images for different film thicknesses revealed the presence of a 0.67 electron volt barrier, and that this barrier relaxes when the layers first start to grow. Thus, the film growth changes from layer-by-layer growth to the formation of terraced mounds. Calculations of the transition state suggest the molecule undergoes bending at the step edge, an effect distinct from those seen in atomic growth.

  6. Water and Ice

    Water derived from melting on the surface can be transferred quickly to the base of an ice sheet, lubricating the ice-ground interface and facilitating movement of the ice sheet. How much will global warming accelerate the decay of the polar ice sheets and, consequently, the rate of sea-level rise? Van de Wal et al. (p. 111) present 16 years of data from the Western margin of the Greenland Ice Sheet and show a correlation between meltwater production and ice velocity on a weekly time scale but observe no sign of a positive feedback over an annual time scale. This suggests that the internal drainage system of the ice sheet adjusts to increases in meltwater inputs, and that annual velocities are mainly functions of ice thickness and surface slope.

  7. Toward the Tree of Life

    By quantifying the distribution of phylogenetically informative data across the entire eukaryotes, Sanderson (p. 121) has tackled the problem of reconstructing the complete tree of life. The available data are distributed across virtually all groups of eukaryotes, with about 10% of known species represented, but the distribution of the most informative collections of sequences is patchy. Not surprisingly, most of the information-dense clades are among the charismatic megabiota, such as mammals and other vertebrates and flowering plants, with other rich pockets of data centered on experimental model organisms. A coordinated sampling effort designed to bridge the gaps in phylogenetic information in eukaryotes is now needed, particularly in diverse but poorly studied groups.

  8. A Mechanism for Sudden Infant Death Syndrome?

    Deficits in serotonin neurotransmission have been hypothesized to be involved in sudden infant death syndrome (SIDS), the leading cause of death during the first year of life. Audero et al. (p. 130) describe a sporadic death phenotype in mice with increased serotonin autoinhibition as a result of overexpression of the serotonin 1A autoreceptor (Htr1a). Deficient serotonergic feedback regulation is sufficient to precipitate autonomic crisis and death. Until now, most SIDS research has focused on respiratory or cardiovascular deficits. These new findings, however, suggest that SIDS is associated with a widespread loss of sympathetic tone, including both bradycardia (slow heart rate) and hypothermia.

  9. Spreading the Word

    Individual dendritic spines, the receiving ends of synapses, compartmentalize small diffusible molecules. In particular, Ca2+ signals in spines are synapse-specific. However, synapses interact in subtle ways through diffusible postsynaptic factors, which suggests the existence of molecular signals that are activated at individual synapses but that can spread to other synapses. Harvey et al. (p. 136, published online 12 June) used two-photon glutamate uncaging to induce long-term potentiation (LTP) —the electrophysiological correlate of memory—at single spines while imaging Ras activity using two-photon fluorescence lifetime imaging. Ca2+-dependent Ras activation spread over ∼10 micrometers of dendritic length and invaded nearby spines by diffusion. Neighboring synapses along a short stretch of dendrite may thus be co-regulated due to this spread of signals downstream.

  10. Autophagy from Egg to Embryo


    Autophagy is an intracellular bulk degradation system that is critical as a self-nourishment system in the neonatal starvation period. Atg5 is a gene critical for autophagy, and knockout mice appear almost normal at birth but die immediately thereafter. It has thus been assumed that autophagy might not be important during embryogenesis in mammals. Now Tsukamoto et al. (p. 117) demonstrate that autophagy is essential for the preimplantation development of mammalian embryos. After fertilization, maternal proteins are rapidly degraded and new proteins encoded by the zygotic genome are synthesized. In the absence of maternally- or paternally-derived Atg5, autophagy is indispensable during this egg-to-embryo transition period.

  11. Rat Run

    There is a long-standing view that the functions of the dorsal and ventral hippocampus can be dissociated with respect to spatial information processing, with the dorsal hippocampus specialized for processing spatial information and the ventral hippocampus specialized for fear conditioning or defensive responses. Now, however, Kjelstrup et al. (p. 140; see the Perspective by Hasselmo) show that place cells exist across the entire longitudinal axis of the hippocampus, including the ventral parts that receive little or no input from relevant sensory areas of the cortex. In an 18-meter-long recording track, place cells in the dorsal one-third of the hippocampus scaled up from field sizes of 0 to 1 meter and at the ventral tip of the hippocampus scaled sizes of 5 to 15 meters. The increase in spatial scale across the hippocampus is approximately linear. The typical home range of Rattus norvegicus is only about 30 to 50 meters, and so the gradual increase in spatial scale is probably sufficient for simultaneous high- and low-resolution representation of the rat's entire spatial environment.

  12. Myosins Under Tension

    Myosin I is a single-headed myosin molecule that plays a role in regulating membrane dynamics and structure in eukaryotic cells. Its best-characterized function is to provide tension to sensitize mechano-sensitive ion channels responsible for hearing. Myosin I is thought to function by sensing tension and changing its motile properties in response to changes in loads. Laakso et al. (p. 133) used single-molecule measurements to characterize the motor activity of myosin I. Small, resisting loads (< 2 piconewtons) resulted in a 75 times lower rate of myosin I detachment from actin, dramatically changing its motor properties. This acute sensitivity supports models in which myosin I functions as a molecular force sensor.

  13. Inhibition, Timing, and Cortical Computation

    In the CA1 region of the hippocampus, relatively uniform pyramidal cells coexist with a very heterogeneous group of GABAergic inhibitory interneurons. It remains unclear why there is such a highly structured neuronal machinery and why these interneurons, rather than the excitatory pyramidal neurons, form such a diverse class of cells. Klausberger and Somogyi (p. 53) review the cellular basis of hippocampal network organization. The diversity of the GABAergic interneuron system provides spatial, and also temporal, specificity of neuronal activity. The interneuron diversity and the consequential dynamic timing of synaptic action between different types of interneurons and principal cells may represent a cellular requirement for the cortical computations underlying complex behavior.

  14. Cation Close-Up

    Because layered, double-hydroxide materials incorporate mobile anions in a cationic lattice framework (usually of bound Mg2+ and Al3+), they are of interest for catalytic and geochemical processes that rely on anion exchange. Precise structural studies of these materials have been hampered by the challenge of distinguishing the similarly sized magnesium and aluminum cations in diffraction patterns and by band broadening in nuclear magnetic resonance (NMR) spectra due to occluded water. Sideris et al. (p. 113) surmount this problem by applying highly rapid sample spinning during solid-state 1H NMR spectroscopy. With supporting data from 25Mg and 27Al NMR techniques, they resolve a well-ordered distribution of Mg2+ and Al3+, and further highlight the general utility of the rapid spinning technique for structural characterization of hydrous minerals.

  15. Generating a Biological Oscillator

    The components of many of the biochemical circuits that regulate biological functions have been described, and investigators are now able to explore how the components work together and why regulatory networks are connected in a particular way. Tsai et al. (p. 126) note that, although a simple negative feedback loop can create an oscillator, biological oscillators like the pacemaker of the heart or the cell cycle include both negative and positive feedback loops. Modeling of the properties of a range of circuits of these types confirms that the “negative plus positive feedback” option is better for biological applications. This circuit design enables a broad range of control of the frequency without changes in the amplitude of the output of the oscillator, whereas negative feedback alone made an oscillator, with a more restricted range of function. Even when the frequency of oscillation does not need to be adjusted, there appear to be advantages to the more complicated circuit—it allows oscillating behavior over a much larger range of enzyme concentrations and kinetic constant values, which may contribute to adaptability during evolution.