Editors' Choice

Science  13 Aug 2010:
Vol. 329, Issue 5993, pp. 729
  1. Evolution

    A Hop, Skip, and a Jump

    1. Laura M. Zahn
    CREDIT: ISTOCKPHOTO.COM
    CREDIT: JOSÉ LUIS BARTHELD

    Marsupial mammals bear their young much earlier than placental mammals and then nurture them in a maternal pouch. They split from the placental line more than 100 million years ago, and the evolutionary relationships among the Australasian (left) and South American (right) species have been debated. Nilsson et al. have performed genome-wide scans for retroposon integrations in the opossum and kangaroo genomes. A subset of the resulting loci was validated in 20 species from the seven marsupial orders, yielding 53 phylogenetically informative retroposon markers that root modern marsupials and demonstrate a clear separation between the South American and Australasian denizens. Furthermore, they support the view that the marsupial fauna of Australia was populated by a single Gondwanan migration event originating from what is today South America.

    PLoS Biol. 8, e1000436 (2010).

  2. Immunology

    Too Much Recognition

    1. Kristen L. Mueller

    Multiple sclerosis is an autoimmune disease of the central nervous system; normally tolerant T cells attack the myelin sheaths that surround nerve fibers. Although some viruses have been fingered as environmental triggers of the disease, how they would cause multiple sclerosis has not been established. Ji et al. show that CD8+ T cells that express two antigen receptors with distinct specificities can drive the development of a virus-induced, multiple sclerosis–like condition [experimental autoimmune encephalitis (EAE)] in mice. The authors used mice whose CD8+ T cells expressed a transgenic antigen receptor that specifically recognized myelin basic protein. These mice did not spontaneously develop autoimmunity, but infection with vaccinia virus that had been engineered to express recombinant myelin basic protein did trigger the onset of EAE. Surprisingly, viral expression of myelin basic protein was not required because mice infected with wild-type vaccinia virus also developed EAE, and the authors found that EAE only developed in mice whose T cells were of dual specificity. That is, the engagement of the endogenous T cell receptors by virus leads to T cell activation, but because these T cells can also recognize myelin basic protein, they then promote EAE development. Similar results were seen when mice were infected with adenovirus, indicating that viral infections generally could promote the development of multiple sclerosis in some of the infected individuals.

    Nat. Immunol. 11, 628 (2010).

  3. Climate Science

    Antarctica's Adolescence

    1. Brooks Hanson
    CREDIT: DON BLANKENSHIP/UTIG

    Antarctica, particularly East Antarctica, harbors much of the world's ice. Thus, accurate knowledge of its sensitivity to melting from global warming is critical for assessing likely sea-level rise in the years ahead. To this end, two recent studies examine the region's dynamics during past glacial cycles and the period when it first grew. Lilly et al. use cosmogenic isotopes, which accumulate in exposed bedrock, to show that the height of the East Antarctic ice sheet fluctuated by only about 100 m through at least the last glacial cycle; the best model fit to the data includes a slow decline in height over longer times. Thus, the East Antarctic ice sheet has had a stable height during past glaciations. In exploring its stability during warmer times, Peters et al. show that an incised coastal valley in Egypt records a sudden and rapid change (within 2 million years) in sea level of about 40 m, at the time when global records imply that major ice sheets first grew in Antarctica, about 35 million years ago. Several smaller fluctuations in sea level (of about 10 m) are also recorded. The rapid changes correlate with estimated fluctuations in atmospheric CO2 levels and occurred when levels dropped below about 750 ppm.

    Geology 38, p. 703; 723 (2010).

  4. Physics

    An Orderly Transition

    1. Jelena Stajic

    Classical phase transitions, such as boiling or freezing, are usually driven by heat exchange, which introduces the order or disorder necessary for rearrangement. In contrast, quantum phase transitions occur at zero temperature as a result of quantum (rather than thermal) fluctuations and are controlled by parameters such as pressure or chemical composition. Most quantum phase transitions have been observed in compounds with carefully tuned chemical compositions, which are inevitably subject to disorder. Jaramillo et al. now induce such a transition in elemental chromium, which presents an unusually clean system. Chromium is antiferromagnetic at ambient pressure with a transition (Néel) temperature TN close to room temperature; as the authors increase the pressure, TN decreases and eventually disappears at a quantum critical point close to 10 GPa. Along the way, the antiferromagnetic phase changes character from mean-field–like to quantum critical. This change is reflected in the value of a critical exponent for resistivity, which approaches 0.25 at very low temperatures, easily distinguishable from the mean-field value of 0.5. Comparison with a related nonstoichiometric system indicates that pressure and chemical doping lead to distinct phase transitions.

    Proc. Natl. Acad. Sci. U.S.A. 107, 13631 (2010).

  5. Cell Biology

    Splitting Image

    1. Helen Pickersgill
    CREDIT: ADAPTED FROM PTACIN ET AL., NAT. CELL BIOL. 12, 791 (2010)

    Occasionally, complex cellular structures previously thought to be devised by eukaryotes turn out to have functional counterparts in prokaryotes. In eukaryotes, mitosis is an intricate process regulated by many different proteins. It requires the assembly of a filamentous structure composed of microtubule polymers known as the mitotic spindle, which is used to segregate homologous chromosomes into daughter cells during cell division. Many bacteria use simpler approaches known as DNA partitioning systems to segregate their chromosomes.

    Using super-resolution microscopy, Ptacin et al. have captured images of a mitotic spindle-like structure in the bacterium Caulobacter crescentus. Fluorescent labeling revealed that the ATPase ParA formed a narrow linear structure (green) stretching from pole to pole during segregation of the newly replicated DNA (purple) in C. crescentus. Additionally, electron micrographs of purified ParA confirmed that the protein could form linear polymers in vitro. The authors also identified another component of the partitioning system, TipN (yellow), which binds to ParA and is required for its function. Previous analogies have been made between bacterial chromosome segregation and eukaryotic mitosis; these findings narrow the gap between these two domains of life.

    Nat. Cell Biol. 12, 791 (2010).

  6. Chemistry

    A Peek at Polymerization

    1. Jake Yeston

    The properties of plastics depend intimately on the dynamics of the polymerization reactions used to produce them. Because the reactions are often quite rapid, though, they can prove hard to track. Christianson et al. have adapted a nuclear magnetic resonance (NMR) spectrometer in order to monitor polymerization reactions on a millisecond time scale. Using a stopped-flow approach more common in optical spectroscopy, they set up the probe to acquire data just after pneumatically driven mixing of a catalyst solution stream with a stream of reactive monomer. With this system in hand, they examined the room-temperature polymerization of 1-hexene by a borane-activated zirconium catalyst. They were able to observe initiation and chain propagation steps directly, and also monitor buildup of a transiently deactivated form of the catalyst, resulting from borane binding to a zirconium hydride ligand after chain elimination from the metal center. The results bolster previous mechanistic hypotheses surrounding this catalyst's behavior and highlight the utility of stopped-flow NMR.

    J. Am. Chem. Soc. 132, 10.1021/ja105107y (2010).

  7. Psychology

    Mindless Reading

    1. Gilbert Chin

    Having gotten this far, do you remember what you read just a minute ago? Or did your eyes scan the previous five or six sentences without any of that information having been absorbed? And if you did zone out, were you aware of having done so? Reichle et al. have studied precisely this kind of behavior by developing a protocol for interrogating subjects who were reading Sense and Sensibility. One theoretical framework posits that a meta-processor oversees cognitive components that support reading (including oculomotor systems and lexical lookup) and also a default or task-decoupled network that mediates mind wandering. The authors found that people were less likely to fixate for longer durations and that their eye movements were more sensitive to parameters such as word length and frequency when they were reading attentively. On the other hand, when people were reading mindlessly, their awareness of having become decoupled from the text surfaced only after a few minutes.

    Psychol. Sci. 21, 10.1177/0956797610378686 (2010).

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