This Week in Science

Science  14 Jan 2000:
Vol. 287, Issue 5451, pp. 189
  1. Chilling Down Deep

    During the past 100 million years, Earth's climate has evolved from the warm “greenhouse” conditions of the Cretaceous period in the late Mesozoic to the cold “icehouse” conditions of the late Cenozoic when large amounts of permanent polar ice formed. Understanding when ice began to accumulate requires a detailed record of global temperature. Lear et al. (p. 269; see the Perspective by Dwyer) have applied a technique that has been used for the Quaternary period, magnesium-calcium paleothermometry, to determine the temperature history of the deep ocean for the last 50 million years and interpret the existing oxygen isotopic record of benthic foraminifera in terms of ice sheet growth. They find that ice sheet growth began 34 million years ago, and that the deep ocean cooled in four major steps by 12°C since then.

  2. Rice Gets an A

    Vitamin A deficiency exacerbates many health problems and in extreme cases is a common cause of blindness in certain areas of the world. Provision of adequate supplies of vitamin A through commonly available staples could alleviate considerable suffering. Ye et al. (p. 303; see the Perspective by Guerinot) have now bioengineered rice to synthesize the precursors to vitamin A. It is hoped that these yellow-tinged rice grains may serve to bring adequate amounts of vitamin A to those with diets highly dependent on rice.

  3. Watered Down

    Many toxic metals enter food chains through intake by benthic filter-feeding animals such as clams and oysters. It has been thought that porewater concentrations of many metals in sediments are controlled in a form known as acid volatile sulfides (AVS), and that most of the metals in benthic organisms were from uptake of these species. Their characterization has thus been used in developing regulations on metal pollution of sediments. Lee et al. (p. 282) show instead that uptake of metals by several benthic species primarily reflects the metal concentration in the sediment, not that of the AVS in the porewater.

  4. Busier Signals?

    There are currently 400 million mobile telephones in use globally and that number is projected to increase by 250,000 daily and overtake landline connections by 2004. With landlines, connections between callers are made directly and so interference between callers is not a problem. In contrast, interference effects are expected to increase as the number of mobile units increase, especially in densely populated areas. Moustakas et al. (p. 287) combine ideas from information theory with those from electromagnetic wave propagation theory and show that multiple scattering may actually increase, rather than decrease, the information capacity carried between transmitter and receiver.

  5. Oxidizing Plutonium

    In air, plutonium forms an oxide that affects its reactivity in the environment. Haschke et al. (p. 285; see the Perspective by Madic) now show that the stable binary oxide in air is not PuO2 as long believed but actually the more oxidized PuO2+x (x is typically 0.27) that forms through reaction with water. This result has implications for all aspects of plutonium chemistry—from environmental safety to its stability in reactors.

  6. Making Cuts Through a Glass

    When liquids cool and form glassy states instead of crystals, the relaxation processes of their components become nonexponential. Theoretical studies and indirect experimental evidence suggest that particle motions become heterogeneous, with some particles moving much more slowly than expected for diffusion and other much faster particles moving together in clusters. Kegel and van Blaaderen (p. 290) now provide direct evidence for such “dynamical heterogeneity” through confocal microscopy studies of glasses made of colloidal particles. They followed particle motions in optical two-dimensional slices and show that particle displacements do not follow a Gaussian distribution.

  7. Cold Control of Water Clusters

    One way to isolate a less stable chemical species is to trap it under cold conditions. Nauta and Miller (p. 293) have taken this approach to extremes in order to isolate cyclic clusters of six water molecules which have been predicted theoretically but have been elusive in experiments. They formed the higher energy cyclic hexamer by sending liquid helium droplets through water vapor. Infrared spectroscopy revealed that the cyclic isomer formed by insertion of water molecules into smaller cyclic species. The ultracold superfluid environment of the droplet quenched the cyclic hexamers and prevented rearrangement into the more compact and stable “cage” structure.

  8. Rerunning Evolution

    Natural tests of some central tenets of evolution are the subject of two reports (see the news story by Pennisi). Speciation should result from divergent natural selection, but direct field evidence has been lacking until now. Rundle et al. (p. 306) show that in replicate populations of threespine sticklebacks in western Canadian lakes, reproductive isolation between populations in individual lakes has evolved in correlation with the environment. The outcome is that independently evolved populations in the same environment, but in different lakes, show no reproductive isolation, whereas populations in different environments within the same lake show substantial isolation. The introduction of the fly Drosophila subobscura from Europe to the New World in the 1970s has been called a “grand experiment in evolution” because it offers the opportunity to study the rates and predictability of evolution on a continental scale. Huey et al. (p. 308) show that the invading flies have quickly evolved a gradation in wing size that is remarkably convergent on that seen in the ancestral European populations; on both continents, wing size increases with increasing latitude.

  9. Interneuron Anatomy and Activity

    Small groups of inhibitory interneurons can exert enormous influence over large numbers of excitatory nerve cells in the neocortex (see the Perspective by Miles). By using triple and sometimes quadruple recordings of synaptically connected neurons, Gupta et al. (p. 273) have unraveled some of the complexity and diversity of synaptic inhibition in the cortex. On the basis of axonal morphology, synaptic dynamics, and general firing properties, they find a number of distinct cell classes and clearly defined groups of synaptic connections. In their connections with pyramidal cells, each defined inhibitory cell group shows one of the distinct patterns of temporal depression or facilitation. However, when the same interneuron makes connections with one of the other inhibitory cell types, a different temporal dynamic may emerge. Martina et al. (p. 295) have analyzed the mechanisms that underlie the remarkable speed and efficacy of interneuron activation. By simultaneously recording from dendritic and somatic membranes from unambiguously identified somatostatin-containing hippocampal interneurons, they show that these dendrites have high densities of voltage-gated sodium and potassium channels. Active conductance opens the possibility that action potentials may be initiated near the synaptic input instead of only at the axon hillock. Once initiated, these action potentials propagate rapidly and reliably through the dendrite and the soma of the cell, even during high-frequency trains.

  10. Binding Directly

    Two related nonclassical major histocompatibility complex (MHC) class I molecules, T22 and T10, bind to γd T cell receptor (TCR) in the absence of any peptide ligand. Wingren et al. (p. 310) have determined the structure of T22 at 3.1 angstroms. The overall fold is similar to that of MHC class I molecules, but the peptide-binding groove is severely truncated, which exposes surfaces on T22 that may bind directly to γd TCRs. The structure indicates that γd and αb TCRs bind differently to their respective MHC ligands and provides a basis for their different functions in the immune system. Crowley et al. (p. 314) investigated the functional relevance of the interaction of T22 and T10 interaction with γd T cells. They provide evidence that T22 is a physiological ligand for murine γd T cells by identifying a population of γd T cells that bind T22 in normal mice and show that this population is activated by T22 or the closely related molecule T10.

  11. Primate Clones Through Embryo Splitting

    The ability to produce genetically identical nonhuman primates would be a significant advantage for medical research. Chan et al. (p. 317) have dissected embryos at the 8-cell stage into components containing two blastomeres and, after culture in vitro, transferred these into surrogate mothers. This procedure has been performed on 107 rhesus embryos, with resultant “multiples” showing varying developmental capacities. Apoptosis occurred at a higher rate in the inner cell masses of the split embryos than in controls. One live birth has been achieved.

  12. CD9 and Female Fertility

    The tetraspanin CD9 is an integral membrane protein that associates with other membrane proteins. Miyado et al. (p. 321) and Le Naour et al. (p. 319) have independently examined CD9 knockout mice. When CD9 was eliminated by homologous recombination, the mice developed normally, and CD9-/- males were fertile when mated to wild-type mice. CD9-/- females, on the other hand, were infertile. This infertility is not due to defects in ovulation or oocyte maturation, but instead resulted from the failure of sperm and oocyte to fuse. It is plausible that the role of mouse CD9 in oocyte-sperm fusion may be related to incidents of infertility in human females.

  13. Making Memories that Last a Lifetime

    Long-lasting memories usually take considerable time to form—it has long been recognized, for example, that cerebral trauma can wipe out recently formed memories. McGaugh (p. 248) reviews the processes that control and influence this process of memory consolidation.

  14. Buried Deep

    The continental crust was formed from Earth's upper mantle; thus elements that are enriched in the crust should be depleted in the mantle, and vice versa. This mass balance, along with knowledge of the starting composition of Earth's mantle, can be used to evaluate crustal growth through time and the degree of homogeneity in the mantle. This mass balance seems to hold for many elements and element ratios, for an assumption that the Earth's mantle had an initial composition similar to that of chondrites. However, a few elements, notably niobium, titantium, and tantalum, seem to be depleted in all samples of both crust and mantle. Rudnick et al. (p. 278) propose a solution by identifying that rutile, a trace phase in eclogite that forms during subduction of oceanic crust, may provide the missing reservoir for these elements. Sequestration of rutile-bearing eclogite near the base of the mantle would provide a solution to the mass balance.

  15. Guard Cell Response

    Of the many physiological processes that abscisic acid affects in plants, movement of guard cells is a particularly important one. By opening and closing stomata, the guard cells regulate water loss or retention through transpiration. Li et al. (p. 300) have now identified a component of the abscisic acid signaling cascade that is specific to guard cell action. The protein AAPK (abscisic acid-activated serine-threonine protein kinase), the gene for which has now been cloned from Vicia faba, responds through auto- and transphosphorylation to abscisic acid and is required for guard cell response to abscisic acid signals.

  16. Linking Src and JNK

    Analyses performed with mammalian cell cultures have indicated that the Src oncoprotein activates Jun NH2-terminal kinase JNK. However, strong in vivo data have been lacking. Tateno et al. (p. 324) now use Drosophila mutants to support this cell culture observation. A signaling pathway that includes the Drosophila JNK homolog Basket (Bsk) and the factor hemipterous (Hep) is required for the fusion of two epithelial sheets at the dorsal midline of the embryo. Like the known Bsk/Hep pathway components, a c-Src homolog DSrc42 is required to allow cell shape changes for successful embryonic closure. Hence, a physiological link is established between Src and JNK.

  17. Control of Autoimmune Diabetes

    Destruction of pancreatic β cells is involved in autoimmune diabetes, although the specific causes of the destruction have been unclear. Yoon et al. (Reports, 14 May, p. 1183) presented experiments in mice in which the βcell suppression of glutamic acid decarboxylase (GAD) expression seemed to prevent the development of autoimmune diabetes. They concluded that modulation of GAD might have therapeutic value.

    Separately, Atkinson et al. and Tian and Kaufman question whether other factors involved in their experiments could have suppressed diabetes in the mice. Yoon et al. provide additional details of their experiments, analysis, and controls, but agree with Atkinson et al. and Tian and Kaufman that “caution must still be exercised about the relative importance of GAD in diabetes.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/287/5451/191a

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