This Week in Science

Science  07 Jan 2011:
Vol. 331, Issue 6013, pp. 8
  1. Ammonite, Reconstructed

      CREDIT: KRUTA & LETHIERS, UMR 7207

      Ammonites were an abundant marine organism that went extinct about the same time as the dinosaurs—roughly 65 million years ago. Although their shells make good fossils, other ammonite structures are rarely discerned. Kruta et al. (p. 70; see the Perspective by Tanabe) have used synchrotron-based x-ray microtomography to visualize and reconstruct the mouthparts of three specimens. The morphology of the jaws and radula suggests that ammonites fed on small marine invertebrates—indeed, tiny crustaceans and snail-like gastropods were found among the jaws of one specimen.

    1. Heating the Solar Atmosphere

        The question of why the Sun's outer atmosphere, or corona, is much hotter than its surface is one of the main unresolved issues in solar astrophysics. By combining measurements from NASA's Solar Dynamics Observatory and the Japanese Hinode satellite, De Pontieu et al. (p. 55) show that jets of plasma propelled upward from the region immediately above the Sun's surface are implicated in the heating of the solar corona. The results challenge current models for coronal heating and show that the interface region between the surface of the Sun and its corona plays a crucial role in energizing the solar atmosphere.

      1. Spinning the Unspinnable

          CREDIT: LIMA ET AL.

          Weaving and spinning can take a weak material like straw or yarn and turn it into a much tougher rope. Lima et al. (p. 51) found that by using carbon nanotubes as a support material, they could spin and weave a range of materials that otherwise are considered intractable to such manipulation, ranging from superconductors to sutures containing biomedical agents. The desired materials were deposited onto a web of multiwalled carbon nanotubes, using an electrostatic powder coating gun, and then twisted into yarns, which could be knotted and sewn, and showed excellent retention of the guest particles when subjected to solvents or a mechanical washing cycle.

        1. Toward Perfection?

            When physicists tried to re-create the conditions believed to have existed microseconds after the Big Bang, they found, to their surprise, that the resulting “soup” of quarks and gluons behaved not like a gas, but like a perfect (frictionless) liquid. Cao et al. (p. 58, published online 9 December) have studied one such candidate for a perfect liquid at a convenient scale—a dilute gas of fermionic Li-6 atoms—and measured its viscosity in a wide temperature range. The results were consistent with expectations that a resonant Fermi gas would have properties dependent only on density and temperature. Although the estimated viscosity/entropy ratio approached the perfect fluid limit, it still exceeded it by fivefold. Nevertheless, these measurements can now be compared with advanced theoretical models.

          1. Spinning for Naught

              Large-scale structures or discontinuities in Earth's interior are typically caused by transformations in the physical or chemical properties of minerals that occur when pressure increases with depth. For example, an electronic spin transition in iron atoms within minerals that are stable at high pressures and temperatures has been predicted to influence some minerals' compressibility and, hence, the speed of sound waves passing through the lower mantle. Using an inelastic x-ray scattering technique at high pressures, Antonangeli et al. (p. 64) show that the spin transition in fact does not influence how ferroperriclase (a major lower-mantle mineral) is compressed, but it does appear to affect anisotropy (i.e., directionally dependent properties) within ferroperriclase, which may account for the observed directional dependence of some seismic waves in the lower mantle, even though the spin transition itself, which should occur at a defined depth, does not correspond to any specific structure or anomaly in the lower mantle.

            1. Seasonal Behavioral Plasticity

                The African butterfly Bicyclus anynana shows a sex-role reversal in courtship behavior, which is set during larval development and controlled by larval rearing temperature. In the wet season form, the males court and the females choose, while in the dry season form, females court and males choose. Prudic et al. (p. 73) show that these changes in mating behavior correlate with a cryptic change of the sexual ornament in both sexes. In the wet season, males have brighter sexual signal in the ultraviolet (UV) range, and in the dry season, females have a brighter sexual signal in the UV range. These changes in both sexual roles and signal are also correlated with a change in costs and benefits to mating among the different seasonal forms. Females have both increased longevity and reproductive output if they mate with dry season males, but dry season males have a reduced life span when mated, while wet season males do not. Thus, reciprocal patterns of sexual selection through the seasons result in mutual ornamentation.

              1. Steady As She Goes

                  Most of the chemical oxidation of atmospheric gases and particles is done by the hydroxyl radical (OH). However, the concentration of atmospheric hydroxyl radicals is extremely difficult to measure, so it must be inferred from measurements of the abundance of other species, like methyl chloroform. Past studies of atmospheric methyl chloroform have indicated that OH can undergo large annual shifts in concentration, although atmospheric models predict a less variable history. Taking data since 1998 and advantage of the Montreal Protocol in limiting methyl chloroform emissions, Montzka et al. (p. 67; see the Perspective by Isaksen and Dalsøren) calculated a more precise estimate of OH variability and find that it varies in concentration less than has been assumed. The results help resolve differences between earlier results, other proxies, and global photochemical models.

                1. Chill Wind

                    For many plants, the chilly temperatures of winter act to coordinate flowering with the more favorable growth environment that follows in springtime. This process of vernalization translates environmental temperatures into developmental responses through a cascade of molecular responses that depend on epigenetic regulation of the floral repressor. Heo and Sung (p. 76, published online 2 December; see the Perspective by Turck and Coupland) have identified an RNA that is transcribed from an intron of the repressor gene, but that itself does not encode a protein. Instead, this noncoding RNA, called COLDAIR, adds a histone-methylating complex onto the repressor locus. With the repressive gene itself repressed, the stage is then set to allow flowering.

                  1. Nitrogenase Assembly Pathway

                      CREDIT: KAISER ET AL.

                      A key step in the global nitrogen cycle is the reduction of atmospheric dinitrogen to ammonia by nitrogenase, a complex metalloenzyme. The catalytic component of this enzyme, the molybdenum-iron protein NifDK, comprises two unusual metalloclusters, the [8Fe-7S] P cluster and the [Mo-7Fe-9SX-homocitrate] M cluster. Likewise, the protein NifEN displays sequence similarity to NifDK, but it plays a role in nitrogenase assembly: It converts an iron-only precursor form to the mature molybdenum cluster and delivers this to NifDK. Kaiser et al. (p. 91) describe the crystal structure of NifEN. By making a structural comparison with apo and holo NifDK, the pathway of cluster insertion can be inferred, which indicates that it is similar in both NifEN and NifDK proteins.

                    1. The Mechanics of Movement

                        Within cells, formin proteins promote the elongation of cytoskeletal actin filaments by associating with filament tips. This activity has the potential to harness actin-generated pushing forces to change cellular architecture. Mizuno et al. (p. 80, published online 9 December; see the Perspective by Pollard) have devised a simple method to analyze the movement of single molecules of a member of the formin family, mDia1, along the growing actin filaments; they discovered that formin molecules mechanically rotate on the end of actin filaments, both during growth and depolymerization of the filament, and can thus influence cell shape during important stages in the cell cycle.

                      1. Mind the Gap (Junction)

                          The role of chemical synapses in learning and memory in the adult mammalian brain is well established. In contrast, the more rapid neuronal transmission that is mediated by electrical synapses is not fully understood. Using a variety of different manipulations—including fear conditioning, drugs, and electrophysiological recordings—in freely moving rats, Bissiere et al. (p. 87) discovered that blocking neuronal gap junctions containing connexin 36 controls the acquisition and the consolidation of fear memories within the dorsal hippocampus. It appears that interfering with gap junction–mediated neuronal transmission selectively prevented the formation of a memory during aversive experiences such as fear conditioning.

                        1. Snapshot Electron Holography

                            Holographic imaging using laser light is a powerful technique that can provide a three-dimensional rendering of an object. By using short pulses of light, a four-dimensional view can be obtained and provide snapshots of dynamic processes. The photoionization of electrons using intense laser beams is a fast and coherent process that has been exploited by Huismans et al. (p. 61, published online 16 December) to develop a technique for electron holography. High time resolution of the photoionzation process indicates that it could be used to probe other processes occurring on ultrafast time scales.

                          1. Modeling the World in the Brain

                              There is a wide consensus in neuroscience that the brain uses internal models to interpret external stimuli and to make predictions about future events. Despite this consensus and a rich history of studies providing ample behavioral evidence about optimal internal models in the brain, it has been difficult to identify the neural signatures of these internal models. Using statistical methods to analyze recordings from the visual cortex of ferrets, Berkes et al. (p. 83) found that neuronal firing patterns during spontaneous activity were similar to those during evoked activity. During development, it seems that the internal model in the visual cortex gradually adapts to the properties of natural visual scenes.

                            1. NK Cells in the Middle

                                Since their discovery more than 30 years ago, natural killer (NK) cells have been predominantly thought of as immune cells with an immediate cytolytic function. Engagement of a variety of cell surface receptors on NK cells can trigger direct killing of virus-infected and tumor cells. Their rapid response time, immediate target cell killing, and lack of rearranged antigen receptors have led to their categorization as a cell of the innate immune system; however, recent studies have shown that NK cells also exhibit several characteristics of adaptive immunity, including immunological memory, which is typically only associated with T and B lymphocytes. Vivier et al. (p. 44) review recent advances in NK cell biology and discuss where NK cells fit into the immune response paradigm.

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