This Week in Science

Science  05 Feb 2010:
Vol. 327, Issue 5966, pp. 621
  1. Development of Myeloid Immune Cells

      As leukocytes develop to maturity, they proceed through an array of phenotypically distinct intermediates. For T and B lymphocyte populations, the different developmental stages, anatomical locations, and cell signals required for progression are well established. However, until recently, much less has been known about how development proceeds in the myeloid lineage, which includes monocytes, macrophages, and dendritic cells. Geissmann et al. (p. 656) review our current understanding of myeloid lineage development and describe the developmental pathways and cues that drive differentiation.

    1. Freezing Supercool Water

        Under equilibrium conditions, water will freeze at 0°C, but, under certain conditions, it can be kept in a supercooled liquid form below this temperature. Ehre et al. (p. 672) present a careful and detailed study of the freezing of water drops on both positively and negatively charged pyroelectric surfaces using a combination of optical microscopy and x-ray diffraction: Supercooled water froze at different temperatures, depending on the charge of the substrate with the initial freezing occurring at the liquid-substrate interface on a positively charged substrate and at the air-water interface on a negatively charged substrate. Thus, freezing could be induced upon heating when the substrate charge also changed from negative to positive.

      1. Pulsar Wind Nebula

          Pulsar wind nebulae are the result of the interaction between the wind of relativistic charged particles emitted by pulsars and their surrounding interstellar medium. Using the AGILE satellite, Pellizzoni et al. (p. 663, published online 31 December) detected extended gamma-ray emission between 100 megaelectron volts and 3 gigaelectron volts from the 10,000-year-old Vela pulsar wind nebula. This detection sets constraints on the relativistic particle wind of pulsars, its energetic content, and its interactions with the surrounding medium. It also suggests that pulsar wind nebulae could form a subset of the yet unidentified galactic gamma-ray sources.

        1. Splitting Quantum States

            The ability to manipulate the spin and charge states in quantum dots electrically make them attractive as the building blocks (qubits) for quantum information processing systems. Petta et al. (p. 669; see the Perspective by Burkard) now show that the avoided crossing of a spin-singlet state and spin-triplet state of a double quantum dot can operate as a state splitter. By controlling the rate at which an incoming state is swept across the crossing region, it can be separated into either the singlet or triplet states. The ability to manipulate the quantum states without needing high or local magnetic fields should prove useful in integrated quantum circuits.

          1. Metal-Insulator Transition


              At near-zero temperatures, some materials undergo a metal-insulator transition and their electronic properties change from conducting to insulating. In the dilute magnetic semiconductor Ga1−xMnxAs, a promising spintronics material, the metal-insulator transition is driven by the substitution of Ga atoms with Mn. While disorder clearly plays a key role in this transition, the influence of electron-electron correlations has been far from clear. Richardella et al. (p. 665; see the Perspective by Fiete and de Lozanne) used scanning tunneling microscopy to study the electronic states of this system. The autocorrelation function of the local density of states exhibited a power law (rather than an exponential) decay at Fermi energy. Thus, electron-electron interactions are indeed crucial for understanding dilute magnetic semiconductors.

            1. Ironed Out

                In large regions of the ocean, low levels of the essential nutrient, iron, limits primary productivity. Iron's chemistry and bioavailability are highly dependent on pH. Rising concentrations of atmospheric CO2 is leading to the acidification of the ocean. Shi et al. (p. 676, published online 14 January; see the Perspective by Sunda) show that the bioavailable fraction of iron dissolved in the ocean may decline as a result of the decrease in ocean pH, which affects the rate of iron uptake by diatoms and coccolithophores. Unless iron input to the oceans increases, these changes may lead to an increase in the iron stress of phytoplankton.

              1. Not at the Mercy of the Wind

                  How can insects that migrate at high altitudes on fast-moving winds influence their direction of migration, when wind speeds typically exceed their self-propelled air speeds by a factor of three or four? Using automated vertical-looking entomological radar systems, Chapman et al. (p. 682) show that compass-mediated selection of favorable tailwinds, and partial correction for crosswind drift, are widespread phenomena in migrant insect species. Specialized flight behaviors have decisive influence on the migration pathways achieved by insects. Thus, contrary to popular belief, migrant insects are not at the mercy of the wind.

                1. Evolutionary Hops

                    CREDIT: BERT WILLAERT

                    In order for animals to colonize new environments and disperse, specific traits may be required. Toads form a large and ecologically diverse group and have achieved extremely wide distribution in a very short time frame. To test what traits may be associated with species range in modern toads, Van Bocxlaer et al. (p. 679; see the news story by Pennisi) phylogenetically reconstructed the evolutionary history of morphological and physiological traits linked to the toads' geographic ranges. Toads have evolved an optimal dispersal phenotype comprising many traits, including those affecting reproductive choices and water retention, which preceded their expansion and radiation.

                  1. Complex Cooperativity

                      Cooperativity in multisubunit protein complexes is classically understood in terms of either a concerted model, in which all subunits switch conformation simultaneously, or a sequential model, in which a subunit switches conformation whenever a ligand binds. More recently, a “conformational spread” model has suggested that a conformational coupling between subunits and between subunit and ligand is probabilistic. Using high-resolution optical microscopy, Bai et al. (p. 685; see the Perspective by Hilser) observed multistate switching of the bacterial flagellar switch complex that was previously understood in terms of a concerted allosteric model. The conformational spread model gives quantitative agreement with the data.

                    1. VSV in 3D

                        CREDIT: GE ET AL.

                        Rhabdoviruses are a family of negative-stranded RNA viruses that includes rabies virus, which have a characteristic bullet shape. Though structures of individual rhabdovirus proteins have been reported, how these are organized into a bullet shape has remained unclear. Now, Ge et al. (p. 689) report a cryo-electron microscopy structure of a model rhabdovirus, vesicular stomatitis virus. The structural data and examination of mutants allows modeling of virion assembly.

                      1. Protective Abundance

                          Massively repeated sequences are generally dangerous to genomes because they promote recombination and, potentially, genome instability. Eukaryotic ribosomal RNA genes (rDNA), which are highly transcribed, are organized into large arrays of repeats and have a system that actively maintains these large arrays. In getting to the bottom of this apparent contradiction, Ide et al. (p. 693) found that reducing the number of rDNA repeats in yeast resulted in a marked sensitivity to DNA damage, owing to heavy rDNA transcription preventing repair of compromised DNA replication forks. It appears that the additional copies of rDNA both reduce the ability of transcription to interfere with DNA repair and provide, through the action of condensin, templates for the recombination-based repair of replication-induced damage.

                        1. Detection Versus Localization

                            Active-sensing systems allow an animal some control over the information acquired from the environment. For example, echolocating bats are known to control many aspects of their sonar signal design. However, their sensory data-acquisition strategies are still not fully understood. Yovel et al. (p. 701) found that Egyptian fruit bats (Rousettus aegyptiacus) did not aim their sonar beams directly on target in a standard localization task. Instead, the bats pointed the main lobe of their sonar beam off-axis. Bats approaching the target emitted sounds to the left and right of its localization, in an alternating manner, so that the maximum slope of the change in intensity of the sonar beam was often positioned close to the target. This echolocation strategy is ideal for target localization but occurs at the cost of target detection.

                          1. Centrosome-Free Axonal Regeneration

                              Neuronal axon growth is thought to depend on microtubules that are assembled at the centrosome, the classical microtubule organizing center (MTOC), which may even dictate axon growth. However, such a local trigger of neuronal polarization and focal microtubule nucleation in the cell body appears difficult to reconcile with the sophisticated microtubule arrays observed in neurons. Now, Stiess et al. (p. 704, published online 7 January) provide evidence, through physical ablation of the centrosome in mammalian neurons, that axon growth is regulated by acentrosomal microtubule nucleation. The findings suggest that the centrosome loses its function as an MTOC during neuronal development, that axon growth depends on decentralized microtubule assembly, and that neuronal differentiation takes place in the absence of a functional centrosome.

                            1. Transmission of Drug Resistance

                                Understanding the dynamics of drug-resistant strains of HIV and the key determinants affecting their evolution and spread is crucial for predicting future effects of drug treatment. Current models can only track one resistant strain, so Smith? et al. (p. 697, published online 14 January) used empirical data from San Francisco to parameterize models that consider the transmission of single-, double-, and triple-resistant HIV strains. Many people who are infected with a resistant strain are capable of infecting more than one other person; a scenario that could trigger an epidemic wave of drug-resistant virus. At a time when the World Health Organization's strategy for universal testing and treatment is being rolled out, the insights gained from this work are not restricted to HIV transmission and treatment in resource-rich countries, but are widely applicable.