Editors' Choice

Science  16 Mar 2012:
Vol. 335, Issue 6074, pp. 1280
  1. Paleontology

    Preserving the Burgess Shale

    1. Nicholas S. Wigginton
    CREDIT: GAINES ET AL., PROC. NATL. ACAD. SCI. U.S.A. 109, 10.1073/PNAS.1111784109 (2012)

    The Burgess Shale in British Columbia, Canada, is one of the most famous fossil fields in the world, containing remarkably preserved soft-bodied marine life from just after the Cambrian explosion. In most other fossil locations, these soft, fragile tissues usually degrade in sediments soon after they are deposited. By examining the composition of the overlying sediments and measuring the sulfur isotope composition of minerals surrounding the fossils from the Burgess Shale, the Chengjiang in China, and five other Burgess Shale–type deposits, Gaines et al. propose that the unique depositional environment and seawater chemistry composition were the primary factors that facilitated preservation. The seafloor at this time in Earth's history was probably anoxic and also deprived of other oxidants such as sulfate, which inhibited initial bacterial degradation of recently deposited organic remains on the seafloor. Soon after deposition, the fossils were entombed in fine-grained clay sediments and then capped by a thick carbonate cement—a result of increasingly alkaline seawater. The cement further prevented the exchange of oxidants into sediment pore water and allowed the organic matter to remain, eventually transforming into the carbonaceous compressions seen today.

    Proc. Natl. Acad. Sci. U.S.A. 109, 10.1073/pnas.1111784109 (2012).

  2. Physics

    Comeback Superconductivity

    1. Jelena Stajic

    The properties of materials at low temperatures are often tuned by varying chemical composition or pressure. In iron-based superconductors, the transition temperature Tc at which their electrical resistivity plummets to zero exhibits a maximum as a function of both composition and pressure; beyond these optimal settings, Tc monotonically falls to zero. Sun et al. report radically different behavior in some representatives of a family described by the formula A1-xFe2-ySe2 (where A = K, Rb, or Cs, with possible Tl substitution). In these materials, an increase in pressure beyond the optimal pressure is accompanied by the usual monotonic decrease of Tc down to zero; however, superconductivity unexpectedly and abruptly reappears at even higher pressures and then disappears just as abruptly. Notably, the reemergent Tc is substantially higher than the maximum Tc in the conventional part of the phase diagram (48.7 K for the compound K0.8Fe1.7Se2, approaching the highest Tc for an iron-based superconductor of 55 K). The explanation for this unusual effect awaits further structural and magnetic characterization of the samples under pressure, although preliminary results indicate that the tetragonal crystal structure is preserved throughout.

    Nature 483, 67 (2012).

  3. Medicine

    Bone of Contention

    1. Paula A. Kiberstis
    CREDIT: FUJITA ET AL., NAT. MED. 18, 10.1038/NM.2659 (2012)

    Vitamin E is a widely used dietary supplement because its antioxidant activity is thought to benefit cardiovascular health. As is true for many supplements, vitamin E's health effects are complex; its role in bone metabolism has been particularly controversial. Two independent studies support the view that the form of vitamin E used in most supplements (α-tocopherol) may adversely affect bone. Fujita et al. found that mice lacking α-tocopherol transfer protein, a model of vitamin E deficiency, had a higher bone mass than controls, because of reduced activity of bone-resorbing cells called osteoclasts. Wild-type mice fed a diet supplemented with α-tocopherol showed a 20% reduction in bone mass as compared to controls; however, the mice were young and undergoing rapid bone growth, so the relevance to effects in adult humans is uncertain. However, data derived from a cross-sectional study of postmenopausal women, about half taking vitamin E, are broadly consistent with the mouse work. Association between serum levels of α-tocopherol, γ-tocopherol, and markers of bone formation and turnover in the women led Hamidi et al. to postulate that vitamin E supplements may negatively affect bone formation.

    Nat. Med. 18, 10.1038/nm.2659 (2012); J. Bone Miner. Res. 27, 10.1002/jbmr.1566 (2012).

  4. Biomedicine

    Modeling Ovarian Cancer

    1. Beverly A. Purnell

    Among the four types of epithelial ovarian cancer, which affects about 1.4 % of women, the serous type is found in 70% of ovarian cancer deaths. Studies have reported different origins for ovarian cancer—the ovary or fallopian tube. Kim et al. generated a double knockout mouse, eliminating the genes for Dicer, the enzyme that converts pre-microRNAs to mature microRNAs, and Pten, a tumor suppressor, in the reproductive tract, and found that serous carcinomas developed from the fallopian tube and then metastasized elsewhere. All of these mutant mice died within about 6 to 12 months. Single mouse knockouts did not show tumor development in the reproductive system. Abnormal cell proliferation started in the stromal compartment of the fallopian tube rather than the epithelial layer, and the cells underwent a stromal-to-epithelial transition. The cancers in the double mutant mice were similar phenotypically, histologically, and at the molecular level to those seen in humans. This work supports the idea of a fallopian tube origin of human high-grade serous ovarian carcinoma and provides a model system for future analyses of this deadly cancer in women.

    Proc. Natl. Acad. Sci. U.S.A. 109, 10.1073/pnas.1201029109 (2012).

  5. Cell Biology

    Seamlessly Hemeless

    1. Caroline Ash

    Hemoproteins are ubiquitous and are required, for example, in respiratory chains, redox reactions, signal transduction, and oxygen transport and sensing. Until now, no eukaryote has been identified that can survive without heme. Kořený et al. have discovered a group of evolutionary outliers that defy the omnipresent heme. Phytomonas serpens is a plant-parasitic kinetoplastid that has no essential heme-containing macromolecules and a rather odd physiology as a result. The only heme biosynthesis gene Phytomonas possesses is ferrochelatase, and it has no respiratory cytochromes, even though it can reoxidize NADH produced during glycolysis, and succinate dehydrogenase still assembles into a functional entity able to reduce ubiquinone. The sugar-rich environment of plant sap allows the organism to bypass the heme requirement for proton pumping, via the non-heme iron of the alternative terminal oxidase. Even in this extreme example, there is enough functional redundancy in the range of cellular processes to allow life to proceed uninterrupted, and Phytomonas throws considerable light on potentially undiscovered links and pathways.

    Proc. Natl. Acad. Sci. U.S.A. 109, 10.1073/pnas.1201089109 (2012).

  6. Evolution

    Bottleneck Dynamics

    1. Sacha Vignieri

    Natural or anthropogenic disturbance can reduce the number of individuals in a population very rapidly. When these populations subsequently recover, the individuals in the population will be entirely descended from those that survived the previous disturbance. This process is referred to as a population bottleneck and is a commonly recognized reason for declines in genetic diversity. Long-term monitoring of a small water vole population on an isolated Scottish island allowed Oliver and Piertney to document the impacts of a bottleneck driven by the introduction and subsequent removal of sheep. Every individual water vole on the island was identified before, during, and after the sheep-induced population bottleneck, which reduced the population to just five individuals. Monitoring both neutral genetic diversity (in the form of microsatellite DNA) and adaptive diversity (of an MHC allele known to confer resistance to parasites in the population) showed that although genetic diversity, in general, was greatly reduced by the bottleneck, adaptive diversity loss was countered by apparently strong selection for heterozygotes.

    Mol. Biol. Evol. 29, 10.1093/molbev/mss063 (2012).

  7. Astrophysics

    Magnetic Radio Stars

    1. Maria Cruz

    Magnetars are rapidly spinning neutron stars that are powered by extremely strong magnetic fields. Observationally, they are characterized by x-ray outbursts and by the lack of radio emission. As such, they are widely believed to differ from radio pulsars, which have much weaker magnetic fields and are instead powered by stellar rotation. Over the past 6 years, however, a few magnetars have been observed to show transient, pulsed radio emission after their x-ray outbursts. Rea et al. compared the radio emission from these magnetars with that from radio pulsars and concluded that, despite the differences, radio-emitting magnetars might also be powered by the neutron star's rotational energy, rather than the magnetic energy. The differences in the radio properties of magnetars and radio pulsars might be explained by the differences in the geometry of their magnetic fields. The topology of the magnetic field of magnetars is more complex and is related to their x-ray activity. The knowledge of a magnetar's rotational parameters and x-ray luminosity might thus suffice to predict whether it will emit radio waves.

    Astrophys. J. 748, L12 (2012).