This Week in Science

Science  02 Jun 2006:
Vol. 312, Issue 5778, pp. 1273
  1. One-Dimensional Ferromagnetism?


    As a wire gets narrower, its conductance can become quantized in multiples of 2e2/h, where e is the electron charge, h is Planck's constant, and the factor of 2 accounts for the two possible spin channels. Only when the degeneracy is lifted, for example, by a magnetic field, would the conductance be expected to show a value of e2/h. However, Crook et al. (p. 1359) present evidence that this scenario may not be complete. They find an e2/h plateau without applying a magnetic field and interpret the results in terms of a spontaneous spin polarization, or ferromagnetism, in a one-dimensional GaAs wire.

  2. Sizing Up Aerosols

    Determining which aerosol particles will act as cloud condensation nuclei (CCN) is vital for understanding the interaction of aerosols and clouds and the resulting climatic impacts. However, the formation of CCN is thought to occur through a complex series of processes that includes many chemical and physical pathways, and has always been difficult for models to parameterize. Dusek et al. (p. 1375; see the Perspective by Rosenfeld) show that measured CCN concentrations can be approximated quite well for a number of classes of aerosols by using mainly size-distribution measurements and only a crude parameterization of the chemical effects on CCN activation. This result, if general, has important implications for the fields of cloud and climate modeling in that it would greatly simplify the treatment of aerosol effects on cloud physics in regional and global models and allow CCN abundances to be estimated from remote-sensing data.

  3. Beats of Heat

    The Sun's atmosphere rotates with a period of about 25 days near the equator and 35 days near the poles, and the resulting twist of magnetic field lines causes the output of solar energy pulses on similar time scales. The extreme ultraviolet radiation from the Sun is the heat source for the upper atmospheres, or thermospheres, of planets, and Forbes et al. (p. 1366; see the Perspective by Müller-Wodarg) have spotted a 27-day periodic fluctuation in Mars' thermosphere, which they compared with simultaneous measurements at Earth. The beating of Earth's upper-atmosphere temperature changes is twice as strong as the signal for Mars; the difference arises from a combination of distance from the Sun and the effects of cooling by CO2. These joint observations constrain CO2 cooling rates in basic models of planetary atmospheres.

  4. Firing Photoelectrons from STM Tips

    The high spatial resolution afforded by the scanning tunneling microscope (STM) has been used to transfer a photoexcited electron from the STM tip to a molecule. Wu et al. (p. 1362, published online 20 April) adsorbed magnesium porphine on a thin oxide film grown on a metal and then varied the incident radiation at a nearby STM tip from near-infrared to green. The molecules accept the electron through a two-step, photon-assisted resonance tunneling pathway. This method allows the excited and charged states to be mapped out as a function of position and photon energy.

  5. Daughter Confirmation of Early Differentiation

    The short-lived isotope 142Nd can be used to date events in the first 50 million years of the history of the solar system. Samples from Earth evidently have a different 142Nd/144Nd ratio from that of early meteorites, implying that there was an early differentiation event in Earth's mantle that removed a complimentary reservoir from later geological processes that we can sample. Rankenburg et al. (p. 1369) show that samples from the Moon are isotopically like meteorites and not Earth. This result supports the interpretation of an early differentiation on Earth and implies that widespread melting of the Moon may have persisted to about 220 million years after formation of the solar system.

  6. Recognizing a Good Thing Growing


    Remains of figs appear in several archaeological sites in the Jordan Valley as early as about 11,400 years ago. Kislev et al. (p. 1372; see the news story by Gibbons) describe these samples and show that they represent a variety of fig in which the fruit forms and ripens without pollination. This mutation arises on some fig trees, but the abundance of the remains implies that humans recognized these rare trees and propagated them by planting branches. Evidence of such activity may mark one of the earliest forms of agriculture.

  7. Getting to Grips with Gut Flora

    At least 10 trillion microorganisms inhabit our lower intestinal tract; without them, we could not process the bulk of our food, and we would be vulnerable to the damaging effects of ingested toxins. Gill et al. (p. 1355) present a detailed metagenomic analysis of human intestinal microflora. Colonic bacteria and archaea not only help to keep the gut wall intact and healthy; they supply us with a suite of glycoside hydrolases to digest plant carbohydrates, trophic chains of organisms for fermentation of fiber to short chain fatty acids, methanogenesis for hydrogen scrubbing, the means to synthesize amino acids and vitamins, and pathways for the transformation of xenobiotic compounds from plant phenolics to tetrachloroethene.

  8. More on the Highland Mangabey

    In 2005, a description of the highland mangabey Lophocebus kipunji from southwest Tanzania was published. At that stage, the species was known only from photographs. More recently, a specimen became available that enabled an assessment of a range of morphological and molecular parameters. Davenport et al. (p. 1378, published online 11 May) provide molecular and morphological evidence that kipunji is actually more closely related to Papio than it is to Lophocebus. Thus, they name and describe Rungwecebus, Africa's first new extant genus of primate in 83 years, and provide results from ecological studies carried out on this endangered monkey.

  9. Development, Stress, and Life Span

    Cell-cycle checkpoint proteins arrest cell division in response to genomic damage and are important in development, but in nondividing cells, these proteins may play a further role in cell maintenance. Olsen et al. (p. 1381) show that decreased function of checkpoint proteins in postmitotic, somatic cells of the adult worm triggered increased expression of genes that allow the organism to resist stress. This adaptive response increased organism survival and extended life span by up to 25%. Thus, checkpoint proteins may control whole organism susceptibility to stress, survival, and normal aging.

  10. The Making of an Insect


    In insects, imaginal discs control the transition from larva to adult. The discs must grow and differentiate in order to form an adult of reasonable size with all of its normal legs and wings. Truman et al. (p. 1385; see the Perspective by Léopold and Layalle) now analyze the processes controlling disc growth and differentiation in the Manduca larva. Cellular proliferation in the discs depends on how well fed the larva was, whereas differentiation into adult structures is managed by juvenile hormone, the presence of which represses differentiation.

  11. Getting Across the Membrane

    Bacteria obtain essential nutrients such as vitamin B12 and iron through a family of outer membrane proteins that sequester these compounds and transport them into the periplasmic (intermembrane) space. The members of this family all adopt a barrel-like architecture and have one domain that serves as a plug. How the nutrient is moved through the barrel is unclear, although an inner membrane protein called TonB is known to participate and to supply the energy to unplug the transporter. Shultis et al. (p. 1396) and Pawelek et al. (p. 1399) have determined the structures of the complexes formed by the vitamin transporter BtuB and the iron transporter FhuA, respectively, with the C-terminal domain of TonB. In both cases, TonB induces a portion of the plug to form a β strand, which is then co-opted into a β sheet.

  12. March of the Locusts

    Locust swarms can invade large areas of Earth's land surface and are estimated to affect the livelihood of one in ten people on the planet. The key to effective management of locust outbreaks is early detection of the marching juveniles (bands), because control of flying swarms is costly and ineffective. Buhl et al. (p. 1402; see the Perspective by Grünbaum) reveal that there is a critical density at which locusts will begin collective motion. The onset of this behavior is characterized by a sudden switch from disordered movement of individuals in the group to highly aligned collective motion. The nonlinearity of this transition means that small increases in density can result in abrupt changes in collective motion. The results match predictions from models of phase transitions from disorder to order in statistical physics. These models can permit scaling from laboratory experiments to large populations in the field and hence inform plans for controlling locust outbreaks.

  13. Early and Late Effects of SOD in ALS

    Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease primarily affecting upper and lower motor neurons and is commonly caused by toxicity resulting from dominant mutations in Cu-Zn-superoxide dismutase (SOD1). Although it appears that neuronal toxicity of SOD1 mutants is non-cell autonomous, it has yet to be established which cell types mediate damage and in which phases of the disease this takes place. Boillée et al. (p. 1389) used selective removal of a mutated SOD1 in two cell types, motor neurons or microglia-immune cells of the spinal cord, to show that the former affects timing and early disease progression, whereas the latter dictates the rate of disease progression after onset.

  14. Rice Domestication, Step by Step

    Rice, one of the most important crops of today, is also one of the oldest. A major factor in rice domestication is the retention of the grain by the plant after ripening that allowed for easy harvest by humans. Recently, a putative transcription factor was identified in which a single base change controls grain retention. Konishi et al. (p. 1392, published online 13 April; see the Perspective by Doebley) identify a second gene regulating grain retention that is also a putative transcription factor. This gene is a homolog of a known transcription factor in Arabidopsis involved in the regulation of floral developmental genes. The release of the rice grain is under the control of a single base-pair change in the upstream regulatory region. Further studies of these genes could help pinpoint the processes and timing of domestication of this important crop.

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