This Week in Science

Science  01 Jun 2001:
Vol. 292, Issue 5522, pp. 1605
  1. Climate Change Caught in Film

    Marine cyanobacteria precipitated calcium carbonate throughout most of the Phanerozoic, when the oceans contained sufficiently high concentrations of dissolved calcium to allow supersaturation of CaCO3 to occur in the biofilms of these organisms. Almost no calcified cyanobacteria are found in the Precambrian, however, and after the late Cretaceous they are no longer found. Arp et al. (p. 1701; see the cover) have combined a model of photosynthetic cyanobacterial biofilm calcification and estimates of paleoatmospheric CO2 concentrations to show that Cretaceous oceans must have had calcium concentrations significantly lower than in the earlier Phanerozoic, and that Precambrian oceans must have had high concentrations of dissolved inorganic carbon. Each of these conditions would have prevented the formation of CaCO3 by cyanobacteria.

  2. A Dynamic Hole in the Ice

    Since the time it was first seen in the mid-1970s, a hole in the winter ice (called a “polynya”) of the Weddell Sea, Antarctica, has appeared at irregular intervals, but always in the same general vicinity. This formation of this irregularly shaped opening—which can reach the size of Great Britain—has long been suspected to be related to the presence of the Maud Rise, a nearby massive seamount, but the mechanism by which the polynya is created and sustained has remained uncertain. Holland (p. 1697; see the Perspective by Lemke) develops a dynamic model of the Weddell Polynya, in contrast to the commonly held thermodynamic model, that explains the structure as the result of large, cyclonic oceans eddies produced by prevailing ocean currents being shed from the Maud Rise seamount. This polynya may have an important role in deep-water formation and ocean-atmosphere CO2 exchange.

  3. Shell Games

    For species with an abundant fossil record, the integration of paleontological and molecular data permits the tracking of evolutionary responses to past climatic changes. Hellberg et al. (p. 1707) demonstrate that a northward expansion of the geographic range of a Californian marine gastropod caused by the Late Pleistocene climatic change resulted in rapid morphological evolution. Significant morphological evolution at the population level resulted from these climate-driven range shifts that were concentrated in the recolonized part of the species' geographic range. These findings contradict the traditional expectation that morphological evolution should be concentrated in Pleistocene refugia. As well as its value to paleobiologists and evolutionary biologists, this work has implications for conservation biologists and policy-makers concerned with the biotic consequences of climate change.

    CREDIT: M. E. HELLBERG ET AL.
  4. Forming Planets in Extreme Environments

    Planetesimals are thought to form through the aggregation of particles within an orbiting disk of gas and dust around a central star. Simulations have had difficulty forming planets in these disks because the dust and gas escape before a large enough planetesimal can form. Throop et al. (p. 1686) model the effects of ultraviolet radiation from young stars in the Trapezium cluster on a nearby planetary disk, Orion 114–426, in the Orion nebula. Relatively large silicate grains (maximum diameter of 1 meter) can form within the inner edge of the disk and can avoid destruction by ultraviolet radiation from other stars. The presence of large grains is also consistent with the authors' recent observations in Orion 114–426. These grains may become the starting material for the formation of Earth-size planets. However, the formation of Jupiter-size planets under these radiative conditions remains elusive.

  5. Computing with Ions

    The physical implementation of quantum computers will likely require the ability to perform a series of accurately controllable quantum operations on a set of two-level systems (the qubits). One such architecture is based on a linear series of ions held in a trap, in which computation involves manipulating the states of the individual, but entangled, ions. Duan et al. (p. 1695; see the Perspective by Lloyd) propose a geometric approach to the manipulation and storage of information for quantum computation of such an array of ions. A set of quantum gates would be obtained by constructing looped paths in the parameter space available to the ions. As the ions are manipulated in this parameter space, they obtain a geometrical, or Berry phase, that is then used to keep tabs on their evolution and that can then be mapped to the computational space.

  6. Feeding the Kids

    Offspring demand, and parents supply. The tensions in this relationship, well known to many human parents, are present in various forms across the entire animal kingdom, and the behavioral ecology of parent-offspring conflict has received much attention. Agrawal et al. (p. 1710) present an experimental study with burrower bugs (Sehirus cinctus) that begins to address parent-offspring conflict from a genetic as well as behavioral perspective. The authors use cross-fostering coupled with manipulations of clutch size or food levels to show that female burrower bugs provision their offspring in response to their need (clutch size and hunger level). Members of the same family resemble each other in elicitation. Offspring levels of elicitation and levels of maternal provisioning were negatively related (providing sufficient food may require less signaling for food).

  7. Lost and Found

    One of the best collections of dinosaurs from northern Africa, found by Ernst Stromer in Egypt and housed in Munich, was lost along with most of his records during a bombing raid in 1944. The lack of many African dinosaur specimens has hampered understanding the effects of the breakup of Pangea in the Late Mesozoic on dinosaur faunas. Smith et al. (p. 1704; see the news story by Stokstad) have now apparently rediscovered Stromer's site in the Bahariya Formation of Egypt. They found a huge sauropod that may have topped out at just under 90 metric tons and 30 meters in length, making it the largest known sauropod and largest dinosaur from Africa. This sauropod and other dinosaurs may have lived in a mangrove environment.

  8. Shared Recipe for a Synapse

    Clustering of membrane proteins is essential for signaling by T lymphocytes and neurons and, in both cases, depends on the transit of membrane microdomains across the plane of the cell surface. Khan et al. (p. 1681; see the Perspective by Trautmann and Vivier) show that agrin, a protein required for organization of the neuromuscular synapse, is also expressed in T cells and influences the formation of the immune synapse. Addition of purified agrin to T cell cultures or enhancement of clustering with an antibody to agrin significantly lowered the threshold of antigen responsiveness by T cells. Thus, the modes for information delivery between lymphocytes and neurons may be more closely related than previously anticipated.

    CREDIT: A. A. KHAN ET AL.
  9. Sorting Sorted Out

    When lysosomal enzymes travel from the Golgi complex to the lysosome, they are packaged into clathrin-coated vesicles at the exit (trans) side of the Golgi complex. In this sorting step, lysosomal enzymes bind to the mannose 6-phosphate receptor (MPR), which in turn is packaged into vesicles for onward trafficking. It has been thought that the adaptin family of proteins mediated the packaging process. Puertollano et al. (p. 1712) and Zhu et al. (p. 1716) now show that it is likely that the so-called GGA family of proteins are the key to this sorting event (see the Perspective by Tooze). The GGAs interact with the mannose 6-phosphate receptor in the trans-Golgi network, and a mutant form of one of the GGA proteins could block Golgi export of the MPR. Conversely, mutant receptors that could not bind GGAs did not correctly sort to lysosomes.

  10. Building Blocks of Breakout

    Mammals are an assemblage of organisms, many of which are harbored in the gut and remain confined there, usually to mutual benefit. Occasionally, some armed with virulence factors escape containment and cause life-threatening infections, such as the notorious Listeria monocytogenes. In an elegant in vivo model for listeriosis, Lecuit et al. (p. 1722; see the Perspective by Finlay) show how the breakout occurs. Listeria bears a molecule on its surface known as internalin, which binds in a species-specific manner to a complementary host molecule known as E-cadherin. Listeria does not cause invasive infections in mice unless the mice are engineered to express the human form of E-cadherin. This molecule then binds Listeria's internalin and permits the bacteria to enter the cells lining the gut wall, cross the cells, exit, and then start a systemic infection.

  11. The Act of Glucose Uptake

    Insulin is essential for proper control of the concentration of glucose in the blood, and failure of insulin signaling contributes to type 2 diabetes mellitus. The insulin receptor produces multiple intracellular signals, however, and it has not been clear which of these are important for regulation of metabolism in liver and muscle and thus of blood glucose concentrations. Cho et al. (p. 1728) report that animals lacking the protein kinase Akt2 (also known as protein kinase Bβ) were mildly hyperglycemic and that insulin appeared to be less effective in these animals than in wild-type animals in causing glucose uptake from the blood. Insulin's effect to suppress glucose production from the liver was strongly decreased. Thus. signaling through Akt2 appears to account for physiologically relevant metabolic effects of insulin.

  12. Magnetic Ordering in Cuprates Below Tc

    How the electronic behavior of the high-temperature cuprate superconductors varies as the temperature is decreased below the transition temperature Tc has been controversial. Outstanding issues include the temperature and doping dependence of the transition from the pseudogap phase and the superconducting phase as well as the role that antiferromagnetic ordering of the host material may play. Sonier et al. (p. 1692) introduce detailed muon spin measurements in high-quality YBa2Cu3O6+x crystals as a function of doping and temperature. Their results reveal the onset of spontaneous static magnetic fields at temperatures that depend on the doping level and that are well below Tc.

  13. Catching the Attosecond Train

    Subfemtosecond optics offer the possibility to explore the fast dynamics of electronic excitations, so far inaccessible to present probes. However, the problem of generating such short pulses leads to that of measuring and characterizing them. Paul et al. (p. 1689; see the news story by Service) describe a technique that produces a train of short pulses, formed through the nonlinear optical process of higher harmonic generation of the fundamental wavelength of an intense laser through its interaction with a jet of helium atoms. They compare pairs of higher harmonics and find that the pulses are in-phase, a necessary condition that confirms that the pulses are indeed 250 attoseconds in duration.

  14. Frizzled Proteins Find G Proteins

    Wnt proteins play critical roles in the control of development and appear to act by binding to receptors known as Frizzled proteins. Although the Frizzled proteins look like G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors, there is little evidence to support such a signaling mechanism. Analysis of the receptor function has been limited by the lack of availability of purified Wnt ligands. Liu et al. (p. 1718) have created a chimeric Frizzled receptor in which the extracellular and transmembrane domains of the hamster β2-adrenergic receptor (β2AR) were fused to the intracellular domain of rat Frizzled-1. Mouse F9 teratocarcinoma cells expressing the modified receptor then responded to the β2AR agonist isoprenaline as though they had been treated with Wnt. They showed typical stabilization of β-catenin, which then acts in the nucleus to promote transcription of Wnt-regulated genes in cooperation with the Tcf and Lef transcription factors. In cultured animal caps from Xenopus embryos, pertussis toxin (an inhibitor of G protein signaling) blocked the transcriptional response to transfected Wnt8. Together, the results indicate that Frizzled proteins do indeed produce their classic developmental effects through β-catenin and Tcf-Lef by coupling to G proteins.

  15. Complement Complex

    Complement component C3d enhances the adaptive immune response to foreign antigens by simultaneously binding to the antigen and to the complement receptor type 2 (CR2) on B lymphocytes. Interaction with C3d requires the first two of 15 or 16 short consensus repeats (SCR) of CR2. Now Szakonyi et al. (p. 1725) have determined the crystal structure of the SCR1 and SCR2 domains of human CR2 in complex with C3d at a resolution of 2.0 angstroms. The interface is between C3d and SCR2 and involves mostly main-chain interactions. The SCR domains form a V-shape with side-chain packing. This structure is likely to facilitate targeted drug design.

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