This Week in Science

Science  25 Jan 2002:
Vol. 295, Issue 5555, pp. 581
  1. In Brevia

    The ice cover and ecology of the lakes on an Antarctic island have changed dramatically over the last 50 years in response to a 1°C rise in air temperature, according to data collected by Quayle et al. (p. 645).

  2. Improving Plants in China

    The use of genetic engineering to improve crop plants has been the subject of intense debate. Although the technology offers potential for improving nutritional quality and growth traits of a variety of crops, public concern about the ecological impact of such engineered crops and about their value as foodstuffs has slowed their acceptance in some countries. Huang et al. (p. 674) have analyzed the state of affairs in China. Surveys from 1997 through 2000 indicate avid exploration of the potential of genetically modified crops, and their analysis indicates reduced costs and usage of pesticides for certain crops.

  3. Organic Adsorption on Clays

    Extensive beds of black, organic-rich shale were deposited at several times in the past, and particularly during the Mesozoic. These deposits have been interpreted as signifying periods when ocean circulation was restricted and reduced carbon was formed and preserved in a low-oxygen environment. Their presence has led to inferences of past ocean and climate conditions; these beds also are the source for much petroleum. Kennedy et al. (p. 657) suggest that their origin may in fact have been controlled by the clay mineralogy present (which may reflect continental weathering) rather than ocean conditions. They show that, like preservation of reduced carbon species in modern environments, organic carbon in a typical Mesozoic black shale is associated with smectite, a clay mineral that allows fixing of carbon species on interlayer sites, facilitating its capture and preservation.

  4. Electrically Controlled Magnetism

    For the developing field of spintronics in which the spin properties of the carriers are encoded with information, magnetic semiconductors need to be developed if spins are to be injected into adjacent semiconductors with high efficiency. Although magnetic semiconductors have been realized, they are not generally compatible with the group IV materials (Si, Ge, or C) that dominate microelectronics. Using molecular beam epitaxy, Park et al. (p. 651) prepared Mn-doped Ge with a ferromagnetic ordering temperature of 116 K. Their design includes a gate structure that allows the carrier density to be modified, thereby providing a mechanism for switching the magnetic behavior on and off.

  5. Bespoke Microchannels

    One obstacle arising from the persistent shrinking of devices is the difficulty in mixing fluid streams, which prefer to adopt laminar flow patterns. External mixing devices do exist, but they require either moving parts or an external power source. Stroock et al. (p. 647) have tailored a microfluidic system with a series of asymmetric herringbones laid onto the floor of the channels. As the fluid passes over these ridges, backflow is initiated, causing two parallel inlet streams to mix chaotically. An additional benefit is that the fluid tends to flow as a plug, which usually is difficult to achieve in a microfluidic system where surface effects dominate and Poiseuille flow is the norm.

  6. Caught Between Love and Hate

    When water is trapped between two hydrophilic surfaces, it wets both surfaces and shows well defined responses to dynamical shearing of the surfaces. When water is confined between two hydrophobic surfaces, if the separation distance becomes critically small, the water will spontaneously eject. So what happens when water is forced into a love-hate relationship? Using a surface force apparatus, Zhang et al. (p. 663) show that the interaction of the water with the hydrophobic surface is complex, with only glancing touches of the surface and with giant fluctuations in the shear moduli.

  7. Tightening the Noose on Carbon Triple Bonds

    The sp hybridization of the carbon atoms in a triple bond tends to force linear geometries on the triple bond and its substituents (C-C≡C-C) and to destabilize small rings containing triple bonds. Suzuki et al. (p. 660) report that a five-membered ring containing a triple bond was formed as part of a zirconium complex. The structure was determined by x-ray crystallography, and the presence of a triple bond was confirmed by nuclear magnetic resonance and reactivity studies.

  8. Diversity at a Distance

    Beta diversity is a measure of the pattern of species diversity at different scales and with distance and is a potentially powerful indicator of ecological processes. However, calculating it requires gathering large quantities of data, a tall challenge in tropical forests. Condit et al. (p. 666; see the Perspective by Duivenvoorden et al.) document patterns of beta diversity in three different parts of the American tropics, using data from more than 70 sites and 1000 species. They show higher species turnover in Central America than in Amazonia, correlating with greater habitat diversity in the isthmus. Limited seed dispersal, often thought to be a factor leading to patchy distributions and variation in community composition, appears not to be a critical determinant of beta diversity.

  9. We Found It, You Can Relax Now

    The hormone relaxin has long been recognized as important to mammalian reproductive processes (see the Perspective by Ivell), and the development of therapeutics to treat preterm labor disorders and delivery problems has been stifled by the elusiveness of its cognate receptor. Hsu et al. (p. 671) have identified a G protein-coupled receptor that mediates the effects of relaxin. This is somewhat surprising because a structurally similar hormone, insulin, signals through a receptor-type tyrosine kinase.

  10. Plasmodium in vitro

    Malaria parasites have complex life cycles consisting of distinct stages, split between vertebrate and mosquito hosts. In contrast to the mosquito stages, techniques for replicating the vertebrate stages in vitro are well established. Al-Olayan et al. (p. 677) have succeeded in culturing mosquito stages of the malaria parasite P. berghei in vitro. This mouse parasite is a commonly used model for human malaria. Ookinetes developed from gametocytes in Schneider's insect medium; a large proportion of these developed into oocysts, from which mouse-infective sporozoites were obtained that were able to complete the cycle in mosquitoes.

  11. Traffic Signaling

    Legionellapneumophila invades host cells and takes up residence in a specialized intracellular vacuole. In order to change the characteristics of the phagocytic vacuole, Legionella have been predicted to inject proteins into the host cell cytoplasm through a secretion apparatus encoded by the dot/icm genes. Nagai et al. (p. 679) have identified a protein termed RalF that appears to be a substrate for secretion. Once in the host cell, RalF acts to recruit and activate a key protein in host membrane traffic, ARF1.

  12. Cells Stabbed in the Gut

    Helicobacter pylori is a prevalent human parasite (infecting half the world's population) and is linked to a variety of gut disorders, including severe gastritis and gastric carcinoma. Higashi et al. (p. 683) have elucidated the steps taken by the bacterial CagA protein that transform host cells. After CagA protein is injected by H. pylori into host cells, it is phosphorylated on tyrosine residues by host kinases. Phosphorylated CagA then binds to a host tyrosine phosphatase, SHP-2, which stimulates SHP-2 translocation to the cell surface where it displays its phosphatase activity. Active membrane-associated SHP-2 then stimulates morphological changes in the host cell that are the prelude to cellular transformation.

  13. Decomposition of the Averages

    Event-related potentials (ERPs) are widely used in electroencephalographic studies. It is usually assumed that ERPs evoked by brief stimuli reflect neural activity within discrete, functionally defined processing regions in the brain. The potentials recorded as the average of a large number of stimulus trials should thus be a simple combination of time-stationary neuronal population activity and otherwise asynchronous noise, which is presumed to average out. Makeig et al. (p. 690) used independent component analysis to decompose the ERP into different fractions arising from different brain areas. An incoming stimulus alters the phase relationship of synchronized activity in multiple brain regions, and these phase imbalances create the peaks in the ERP. Thus, in contrast to the traditional view, the recorded potential is very much affected by the ongoing rhythmic brain activity before the stimulus.

  14. Staying Put

    In the developing nervous system, axon pathfinding from the neuronal cell body to the synaptic target turns out to be only part of the story. Even after pathfinding has been accomplished, mechanical forces produced by movements of the organism or further developmental changes in tissue sizes and relationships can push those axons around. Aurelio et al. (p. 686; see the news story by Vogel) find that in Caenorhabditis elegans, additional cues are delivered to keep axons stabilized within the correct tracts in the ventral nerve cord. The cues derive from the PVT neuron, which is one of the first to establish the ventral nerve tract early in development. The steadying effect of the PVT neuron on later established axons is apparently mediated through the function of secreted zig proteins.

  15. Locally Deformed Crystallization

    Bulk metallic glasses have become more than just scientific curiosities because of their unique properties: They show high strength, localized structure changes when subject to large deformations, and greater hardness than crystalline alloys of similar composition. Previous experiments have shown that under high energy, ball milling, or severe bending, localized nanocrystallites form at the locations of highest deformation. Kim et al. (p. 654) demonstrate that even local-scale, weak indentation can cause the nanocrystallites to form. The crystals are similar to those that form when the glasses are heated, indicating that similar molecular reorganizations are occurring. This has implications both in the application of these materials to engineering structures and in the design of amorphous-nanocrystalline alloys with enhanced properties.

  16. Minding Myosin

    The molecular motor myosin participates in complex structures, both in muscle and in other cells, during the formation of the cytokinesis contractile ring. Proteins like UNC-45 from Caenorhabditis elegans have been implicated in promoting appropriate myosin-based activities. Barral et al. (p. 669) describe how UNC-45 can act as a molecular chaperone for myosin, in association with another chaperone, Hsp90.

  17. RNA Dialectic

    When foreign nucleic acid makes its way into plant or animal cells, a defense mechanism (cosuppression or RNA interference) converts it into small, 20- to 25-nucleotide pieces, which then prevent transcription of any complementary sequences. Tijsterman et al. (p. 694) exposed cells from the worm Caenorhabditis elegans to antisense RNAs. These antisense RNAs do cause silencing but, unlike RNA interference—mediated silencing, this action does not require the rde-1 and rde-4 genes, although it does require the RNA helicases MUT-7 and MUT-14. The authors propose that the helicase activity of MUT-14 permits synthesis of double-stranded RNA from the introduced antisense RNA, which is then cleaved into the 20- to 25-nucleotide pieces by the enzyme DICER.

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